Amateur Radio Wiki - User contributions [en]

Loop

2009-06-20T10:10:48Z

Toowoomba4350: New page: Magnetic loop From Wikipedia, the free encyclopedia see: http://en.wikipedia.org/wiki/Magnetic_loop (for pictures etc) Jump to: navigation, search HF RX Loop, diam. 100 mm, 70 k-120 MHz,...

Magnetic loop
From Wikipedia, the free encyclopedia see: http://en.wikipedia.org/wiki/Magnetic_loop (for pictures etc)
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HF RX Loop, diam. 100 mm, 70 k-120 MHz, 1 turn, TransformerMagnetic loop antennas (also known as small transmitting/receiving loops) are small compared to other antennas for the same wavelength. A magnetic loop is one in which the current amplitude is constant round the circumference, and it is therefore small enough to avoid a significant standing wave in the current distribution. This implies a circumference of 1/10 wavelength or less. Small transmitting loops are typically smaller than 1/4 wavelength in circumference at the intended frequency of operation. Since full-size antennas for shortwave communication can be very large -- sometimes several hundred meters in size -- the loop's ability to operate with smaller size albeit reduced radiation efficiency gives it some advantages, especially for mobile use and military use. Conceptually, the magnetic loop can be thought of as a high Q tank circuit having very large diameter to length ratio to facilitate the magnetic "leakage" that makes it effective as an antenna.

Analysis of the design by antenna professionals, confirmed by controlled experiments, has shown that high radiation efficiency is not obtained, and that the main advantage of the antenna is its compact size. These antennas are usually operated close to ground and therefore produce high-angle radiation at the lower frequencies, which is well suited to NVIS (near-vertical incidence skywave) propagation.

Usually a capacitor is used to "enlarge" the antenna by tuning it to resonance in a parallel L-C circuit. The disadvantage of this method is the low bandwidth of the antenna (high Q) which limits operation to a narrow frequency range. A high Q can also be advantageous, however, since tuned magnetic loops work within a narrow frequency range, when used for reception they reject noise produced in the receiver from intermodulation products. This reduces the noise level as compared with wider-bandwidth antennas. However, because all transmitted signals require a finite bandwidth, the high Q of magnetic loop antennas means that they cannot be used for higher-bandwidth applications.

A further advantage of magnetic loops used for receiving is that they respond to the magnetic-field component of the arriving signal; locally generated rf noise (within 1/6 wavelength) has a generally weak magnetic component so the noise tends to be rejected. This effect is more marked at lower frequencies.

As a result of the narrow operating bandwidth of the antenna, if the frequency of operation is changed, the antenna must be retuned by changing the value of the antenna's tuning capacitor. Bandwidth is the usable frequency range of an antenna. When the antenna is operated outside of its bandwidth, energy from the transmitter is reflected back from the antenna, down through the feedline, and back to the transmitter, and so the antenna obviously fails to radiate correctly.

In addition to narrow bandwidth, magnetic loops have very low radiation resistance, often one or two orders of magnitude less than a full size antenna such as a dipole, and only a fraction of an ohm. Efficiency thus depends on low-loss construction, typically use of thick conductors, and low loss air, mica, or vacuum dielectric capacitors to raise the Q to as high a value as possible consistent with the required bandwidth.

In addition to the high currents resulting from the low radiation resistance, high voltages appear across the tuning capacitor when the loop is used for transmission; a kilowatt transmitting loop can have currents of the order of 100 Amperes and voltages at the capacitor of several tens of kV.

Magnetic loops are often fed with 50 ohm coaxial cable connected across a smaller coupling loop that is 1/5 to 1/8 the size of the antenna. This feed loop provides an impedance match to the loop's low feed resistance over the widest frequency range when it is located on the side of the antenna opposite the tuning capacitor. A less common feed system breaks the tuning capacitor into a two series capacitors with the feed across one of them.

The magnetic loop antenna is an old design which in limited use because of its low radiation efficiency and narrow bandwidth. However many military, commercial, and amateur radio operators still use them today because of the advantages conferred by small size, high-angle radiation and easy transportability. The magnetic loop was widely used in the Vietnam war due to its high portability.

Contents [hide]
1 See also
2 External articles and further reading
2.1 General references
3 External links

[edit] See also
Antenna (radio)

[edit] External articles and further reading
"Small Transmitting Loop Antennas - AA5TB"
"Theory of operation of Mobile HF NVIS Magnetic Loop Antenna"
"W2BRI's Magnetic Loop Site"
"Portable magnetic loop Construction"
"Small non magnetic loop construction"
"Antenna design software"
"SWDXER.CO.NR" Receive only loops for shortwave that work.
"[1]" Elevated Magnetic Loop for 160 & 75 Meters
"[2]" Magnetic Loop Antenna Experiments
"Performance of a small loop antenna in the 3-10 MHz band", Boswell, Tyler and White, IEEE Antennas and Propagation Magazine, Vol. 47, No. 2, April 2005.

[edit] General references
The ARRL Antenna Book (15th edition), ARRL, 1988, ISBN 0-87259-207-5

[edit] External links
Small Transmitting Loop Antenna Calculator - Online calculator for magnetic loops that performs the "Basic Equations for a Small Loop" in The ARRL Antenna Book, 15th Edition
Retrieved from "http://en.wikipedia.org/wiki/Magnetic_loop"
Categories: Radio frequency antenna types | Antennas (radio) | Radio electronics

Quad antenna

2009-06-20T10:08:23Z

Toowoomba4350: New page: Quad antenna From Wikipedia, the free encyclopedia: see: http://en.wikipedia.org/wiki/Quad_antenna (for pictures etc) Jump to: navigation, search This article may require cleanup to meet...

Quad antenna
From Wikipedia, the free encyclopedia: see: http://en.wikipedia.org/wiki/Quad_antenna (for pictures etc)
Jump to: navigation, search
This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (February 2008)

patent US2537191
A quad antenna.The Quad antenna is an antenna which is frequently used by amateur radio operators. It consists of a driven element and parasitic elements, like a Yagi; however, the elements are square loops. If there is only one parasitic element it is referred to as a 'Cubical Quad' (since it will be an approximate cube).

Contents [hide]
1 History
2 Advantages over a Yagi
2.1 Higher Gain
2.2 Radiation Resistance approximately 50Ω
2.3 Lower Boom Height
2.4 Shorter Boom
2.5 May be Internally Stacked to form Multi-Band Arrays
2.6 Lower Angle Radiation
2.7 Broader Bandwidth
2.8 Quieter to RF Noise
3 The E-Z-O variation
4 Patents
5 References

[edit] History
It was developed by Clarence C. Moore (patent no. 2,537,191), W9LZX, an engineer at HCJB (a shortwave missionary radio station high in the Andean Mountains). He developed it to resolve issues with large coronal discharges in the thin air with a beam antenna.

Moore describes his antenna as "a pulled-open folded dipole" and describes the time when it was developed:

“ We took about one hundred pounds of engineering reference books with us on our short vacation to Posoraja, Ecuador during the summer of 1942, determined that with the help of God we could solve our problem. There on the floor of our bamboo cottage we spread open all the reference books we had brought with us and worked for hours on basic antenna design. Our prayers must have been answered, for gradually as we worked the vision of a quad-shaped antenna gradually grew with the new concept of a loop antenna having no ends to the elements, and combining relatively high transmitting impedance and high gain. ”

Moore's design eliminated coronal discharge. "End effect", which is inherent with the Yagi, is absent in a quad because its elements have no ends. But other advantages appeared. The higher impedance, mentioned in the quote above, translates to lower current and thus lower loss on the transmission lines. And gain was found to be higher than that of a Yagi.

[edit] Advantages over a Yagi
Rigorous testing of the quad antenna, reported in William Orr's authoritative "All About Cubical Quad Antennas", and others, have illustrated the following advantages over a Yagi antenna.

[edit] Higher Gain
The 2-element Quad has almost the same gain as a 3-element Yagi: about 7.5 dB over a dipole. Likewise, a 3-element Quad has more gain than a 3-element Yagi. However, adding quad elements produces diminishing returns. Quoting from William Orr, "Whereas parasitic beams having twenty or thirty parasitic directors are efficient, high gain antennas, it would seem... that maximum practical number of parasitic loop elements for the Quad array is limited to four of five." (Orr, Pg. 48)

[edit] Radiation Resistance approximately 50Ω
Radiation resistance is affected by antenna height above ground, element spacing, and environmental conditions. However, values will be higher than Yagi's and more closely matched to a 50 Ω coaxial feed line.

[edit] Lower Boom Height
"A two-element, three-band quad, with elements mounted only 35 feet above ground, will give good performance in situations where a triband Yagi will not." (Page 12-3 of the 15th edition of The ARRL Antenna Book.)

[edit] Shorter Boom
William Orr's book shows a 10-15-20 meter, 2-element Quad with boom length of 6'10".

[edit] May be Internally Stacked to form Multi-Band Arrays
As Orr states, interaction between antennas of a multi-band Quad are quite low, even when fed with a single feed line. (Orr, Pg. 63)

[edit] Lower Angle Radiation
According to K0SR, (pg. 5 of the ARRL's Jan/Feb 2008 National Contest Journal) the claim that quads "open the band earlier", which suggests that they exhibit a lower angle of radiation than Yagis, has persisted for 50 years in spite of the fact that computer models disagree. He posits that the vertical sides of each element actually radiate the low angle component.

[edit] Broader Bandwidth
The bandwidth for a 3-element Quad antenna that is tuned for maximum gain is limited because excursions from the design frequency unbalance the near resonance condition of the parasitic elements. However, by lengthening the reflector and director elements, thus sacrificing approximately 1 dB gain, the entire 10 meter band may be worked with an SWR below 1.75:1.

[edit] Quieter to RF Noise
This is an unsubstantiated, subjective note.

[edit] The E-Z-O variation

3-Element E-Z-O Antenna PrototypeAn improvement over the Quad design has been claimed by N8PPQ. In 2008 he introduced the E-Z-O antenna which uses flexible dielectric tubes, rather than rigid poles, to support the electrical elements. He claims slightly higher gain over the Quad due to its circular form; circles being the shape that the Quad only roughly approximates. Here is his list of reported improvements over the Quad antenna:

Lower Cost
Higher Gain
Lighter in Weight
Less Wind/Ice Loading
Easier to Install and Maintain
Esthetically Pleasing
The magnitude of the dielectric effect on the outside band elements was surprising. Experimentation was required to establish optimum element lengths. Referencing literature was not to be found. As reported at http://personal.ee.surrey.ac.uk/Personal/D.Jefferies/antennexarticles/diecon.htm, "As far as we are aware, there has been no reported work on encasing loop antennas in dielectric." It appears that "near field" interaction between parallel components increases the magnitude of the dielectric effect. Near field analysis is too complicated for this author.

[edit] Patents
Patent 2,537,191

[edit] References
All about Cubical Quad Antennas by William I. Orr

Quagi

2009-06-20T10:04:46Z

Toowoomba4350: New page: The VHF Quagi from an article by Wayne Overbeck, K6YNB in QST April 1977 see: http://www.geocities.com/garyntricia/quagi.html for photos etc There have been many half-hearted attempts t...

The VHF Quagi
from an article by Wayne Overbeck, K6YNB in QST April 1977

see: http://www.geocities.com/garyntricia/quagi.html for photos etc

There have been many half-hearted attempts to combine a Yagi and a Quad. Most have produced questionable results. Well, here are the "true" facts, the real dope, the hot scoop.

As its name suggests, the quagi combines the best features of the cubical quad and the linear Yagi-Uda beam antenna. This article describes an eight element quagi design with quad type driven element1 and reflector plus six Yagi type parasitic directors.

The result is an antenna that has outperformed all similar size conventional Yagis and a number of bigger ones at three vhf conferences where antenna gains were measured. At frequencies above 144 MHz, only larger and very well tuned antennas tend to out perform the eight element Quagi. But equally important, the quagi can be built by the average amateur with simple materials at a fraction of the cost of a commercial antenna.

The Quagi requires neither fine tuning nor careful handling. Many of these antennas have survived several years of bouncing up and down mountain roads that are little more than goat trails. The elements can be bent and restraightened without affecting the gain or VSWR.

The antenna's secret, if there is one, is its hybrid character. With Yagi type directors, the quagi retains the simplicity and gain of a long boom Yagi without the Yagi drawback at VHF - a dipole driven element. As many antenna builders have learned, matching a Yagi at VHF (and especially at UHF) is not easy. Gamma matches tend to become less than effective at these frequencies, and other feed methods such as the delta with a balun and universal stub lead to cumbersome antennas that may not endure rough handling or wet weather.

In fact, some of the best VHF-UHF antennas these days, both homemade and commercial, are hybrid designs that avoid the problems inherent in a Yagi type dipole driven element by not using one! Notable commercial examples of this trend are the log-periodic Yagi (using a log-periodic type broadbanded feed) and a 20 element colinear with yagi type directors. Both are effective antennas, sans dipole feed problems.

The Quagi solves the feed problem in a way that is more practical for the amateur by using a quad type driven element that requires no tuning at all for good performance, even at 432 MHz. The Quagi can be fed directly with RG-8/U (or if necessary, RG-11/U, since characteristic impedance of the loop is about 60 ohms at resonance). Anyone who can measure off some lengths of wire can build one and make it work, with no test equipment at all. The quad loops don't even have to be very "square" for the antenna to work correctly. Besides the simplicity, the quad loops offer a fringe benefit of a little extra gain over linear half-wavelength elements. More about that in a moment.

Some Quad - Yagi Theory

Although this is a practical construction article and not a theoretical treatise on antenna design, it would violate the spirit of amateur radio to merely present the dimensions and send people racing off to build a new type of antenna without any discussion of its theoretical basis. The Yagi-Uda antenna with its half-wavelength linear elements was very popular before amateurs began experimenting with parasitic beams made of full-wavelength wire loops. However, it became apparent in the 1940's and 1950's that the cubical quad rivalled the performance of a conventional Yagi.

An excellent history and theoretical explanation of the quad appears in Orr's "Quad Antennas"1. In fact, Orr provided some of the inspiration for the quagi when he suggested that a two-element quad had an edge over a two-element Yagi because of the loop gain (perhaps an extra 1.5 dB). However, he found that quads seemed to lose this advantage as more elements were added.

Others have disputed that conclusion, notably Bergen2, Lindsay3 and more recently Harrison4. Lindsay's studies at the Denver University antenna range in the 1960s suggested that, for any given boom length, a quad would outperform a Yagi by about 2 dB. Harrison summarized two scholarly antenna studies and concluded that a loop-Yagi (i.e., a quad type antenna) with any number of elements probably outperforms a similar size Yagi by 1 dB or more.

Lindsay reported that a Yagi must have nearly twice the boom length of a quad to achieve the same gain. This author took advantage of the high gain per foot of boom of the quad to build and extremely compact and lightweight moonbounce array of 16 three-element quads for a recent 2-meter DXpedition to Alaska. A description of this small high-gain antenna appears in a moonbounce application note published by Eimac5. The Alaskan moonbounce expedition itself was recently described in QST6.

The Birth of the Quagi

This type of thinking led to the inspiration for the quagi. More immediately, though, the quagi was born because a commercial Yagi for 432 MHz performed poorly. It's a story worth reading if you have ever believed the exaggerated catalog gain figures for some commercial antennas.

In 1970, I took a brand-new commercial Yagi which had a claimed gain figure of 13.5 dBd to a measuring session at the West Coast VHF Conference. It measured 6.4 dB over a dipole! A quick check revealed no errors in assembly, and an identical Yagi was soon measured - also around 6 dB gain!

"It's the gamma match. They don't work at 432 MHz," a veteran of many vhf antenna measuring sessions said knowingly. "In fact, few amateurs can put together any kind of feed system that works well at that frequency," he added.

That was a bit of an overstatement, of course. Several true Yagi designs have now been published that work well at 432 MHz - including the W1HDQ classic "Tilton Yagi,"7 and newer designs by Knadle8 and Hilliard9. However, all three use matching systems that require considerable skill to tune. Having read what Orr, Lindsay and others have said about quads and Yagis, the author wondered if a quad type of feed might be the answer.

Before the next West Coast VHF Conference, the ill-performing, store-bought Yagi was modified in only one way. the driven element was removed and replaced with a quad type loop of no. 12 wire. There was no matching of any kind. A type-N connector was soldered in the center of the quad loop bottom side, and it was fed directly with RG-8/U. Now the SWR and gain both looked better. In the gain contest at the next VHF Conference, this antenna was measured at 9.8 dB over a dipole. Simply eliminating the gamma-matched dipole and adding a quad loop had increased the gain 3.4 dB!!!

That led WB6RIV and the author into a summer of antenna design work on a backyard antenna range with a signal source, a remote dipole and a field-strength meter. If just replacing the driven element would get us an extra 3.4 dB, what could be done with a whole new design optimized for a mix of quad and Yagi type elements?

Many, many antennas later, we had an eight-element quagi whose element lengths and spacing seemed about right, with an overall antenna size that seemed to be a good compromise between bulk and gain.

The rest is history. The quagi has won three consecutive 2-meter antenna-gain contests at West Coast VHF Conferences with measured gains up to 14.2 dB over a dipole10. The 220-MHz version has won two out of three measurements (losing once to a much bigger log-periodic Yagi). Even on 432 MHz, where the quagi with it's four-foot 10-inch (1.47M) boom is usually one of the smallest parasitic antennas measured, it finishes high in the standings, not far behind the 10-foot (3.0M) Yagis.

In presenting this information, the author does not mean to imply that the 2-meter version of the eight-element quagi will outperform some of the largest antennas, such as the popular 16-element collinear. Both are a bit big to enter in gain contests, but bother probably outperform the quagi by 1.5 to 2.0 dB on 144 MHz. On 432 MHz a 12-foot (3.66M) log-periodic Yagi described by Holladay11 and made commercially by KLM Electronics tops the four-foot 10-inch (1.47M) quagi by about 2.8 dB. (But it costs and weighs at least 6 dB more!!!!)

New Ideas, Anyone?

As far as we know, no quagi of any size has ever been built for a frequency below 144 MHz. A 20-meter quagi would probably be a very good antenna, but it would be 140 feet (42.6M) long!!! Moreover, gamma-matched dipoles work well at 14 MHz; this design was created to solve problems unique to the vhf-uhf world. A conventional Yagi may well deliver more gain per pound or per square foot of windload at 20 meters. Even at 50 MHz, a quagi of this design would be too big for the author's portable expeditions and contest work.

If what you want is a good, easy-to-build antenna for 144, 220, or 432 MHz, the quagi may be your answer. Here are the construction details.

How to Build a Quagi

There are few tricks to quagi building. The author has mass produced as many as 16 in one day. Table 1 gives the dimensions for various frequencies.

The boom is wood or any other nonconductor (e.g., fiberglass). If you use a metal boom, you'll have to redesign the whole thing and come up with new element lengths. Many vhf antenna builders go wrong by failing to follow this rule: If the original uses a metal boom, use the same size and shape metal boom when you duplicate it. If it calls for a wood boom, use a nonconductor. Many amateurs dislike wood booms, but in the author's salt-air environment they outlast aluminum, ( and surely cost less). Varnish the boom if you wish.

The 2-meter version is usually built on a 14 foot (4.27M) 1" (25mm) x 3" (75mm) boom, with the boom cut down to taper it to one inch (25mm) at both ends. Clear pine is best because of its light weight, but construction grade Douglas fir works well. At 220 MHz, the boom is under 10 feet (3.05M) long and most builders use 1 (25mm) x 2 (50mm) or (preferably) 3/4 (20mm) by 1-1/4 inch (30mm) pine molding stock. On 432 MHz the boom must be 1/2-inch (12mm) thick or less. Most builders use strips of 1/2-inch (12mm) exterior plywood for 43212.

Element Lengths
144.5 MHz
147 MHz
222 MHz
432 MHz
446 MHz

Reflector (all No 12 TW wire, closed
2200 mm loop
2159 mm
1432 mm
711 mm
689 mm

Driven Element (No 12 TW, fed at bottom)
2083 mm loop
2032 mm
1359 mm
676 mm
657 mm

Directors
913 mm to 889 mm in 5 mm steps
897 mm to 873 mm in 5 mm steps
594 mm to 568 mm in 3 mm steps
298 mm to 291 mm in 1.5 mm steps
289 mm to 280 mm in 1.5 mm steps

Spacing

R - DE

533 mm
521 mm
346 mm
178 mm
173 mm

DE - D1

400 mm
391 mm
260 mm
133 mm
130 mm

D1 - D2

838 mm
826 mm
546 mm
279 mm
272 mm

D2 - D3

445 mm
435 mm
289 mm
149 mm
144 mm

D3 - D4

663 mm
651 mm
432 mm
222 mm
215 mm

D4 - D5

663 mm
651 mm
432 mm
222 mm
215 mm

D5 - D6

663 mm
651 mm
432 mm
222 mm
215 mm

The quad elements are supported at the current maxima (the top and bottom, the latter beside the feed point) with Plexiglas or small strips of wood. The quad elements are made of no. 12 (2.8mm) copper wire, commonly used in house wiring. Some builders use no. 10 (3mm) wire on 144 MHz and no. 14 (2.5mm) wire on 432 MHz, although this changes the resonant frequency slightly. Solder a type-N connector (an SO-239 is often used at 2 meters) at the midpoint of the driven element bottom side, and close the reflector loop.

The directors are mounted through the boom. They can be made of almost any metal rod or wire about 1/8-inch (3mm) diameter. Welding rod or aluminum clothesline wire will work well if straight. The author uses 1/8-inch (3mm) stainless-steel rod secured from an aircraft surplus store.

A TV type U-bolt mounts the antenna on a mast. The author uses a single machine screw, washers and nut to secure the spreaders to the boom so the antenna can be quickly "flattened" for travel. In permanent installations two screws are recommended.

Construction Reminders

Here are a couple of hints based on the experiences of those who have already built the quagi. first, remember that at 432 MHz even a 1/8-inch (3mm) measuring error will deteriorate performance. Cut the loops and elements at carefully as possible. No precision tools are needed but be careful about accuracy. Also, make sure you get the elements in the right order. The longest director goes closest to the driven element.

Finally, remember that you are feeding a balanced antenna with an unbalanced line. Every balun the author has tried introduced more losses than the feed imbalance problem. Some builders have tightly coiled several turns of the feed line near the feed point to limit radiation further down the line. In any case, keep the feed line at right angles to the antenna. Run it from the driven element directly to the supporting mast and then up or down perpendicularly for best results.

Phasing Quagis

Like other antennas, quagis can be phased for additional gain. Arrays of two, four, eight, and sixteen have been phased successfully for tropospheric, meteor scatter and moonbounce work. Table 1 gives suggested stacking distances. When phasing quagis, make sure each bay is fed in the same sense. Each bay must have its driven element in the same relative position and must fed with the center conductor going to the same side.

For those not wishing to calculate phasing-line lengths, here is a phasing plan for two quagis on 144 MHz that should achieve at least 2.5 dB stacking gain. Secure 25 feet (7.62M) of Belden no. 8224 feed line, an 80-ohm foam cable widely used in CATV applications, and mount PL-259 connectors (with UG-176 adaptors) on each end. Cut the cable into two 148-inch (3.76M) lengths, including the length of the connectors. You'll have a few inches of cable left over. Strip the two open ends back 3/4-inch (20mm) and solder both outer braids into the grounded mounting holes on an SO-239 at the junction of the two phasing lines. If you use any other brand or type of coax than Belden no. 8221, the line lengths will probably be different.

Conclusion and Acknowledgement

The eight-element quagi has consistently performed well in many kinds of vhf-uhf service. It delivers high gain in an inexpensive and easy-to-reproduce design, even at 432 MHz. We hope this article will not only inspire others to build quagis, but also to spend more time working DX on the exciting frequencies above 144 MHz.

The author wishes to thank Wilson Anderson, Jr., WB8RIV, for his valuable assistance in the development of the quagi antenna.

References:
1 Orr, Quad Antennas, 1st Ed., Wilton, CT. Radio Publications, Inc.
2 Bergren, "The Multielement Quad," QST, May 1963, Page 11.
3 Lindsay, "Quads and Yagis," QST, May 1968, Page 11
4 Harrison, "Loop-Yagi Antennas," Ham Radio, May 1976, Page 30
5 The EIMAC moonbounce notes are available from Robert Sutherland, EIMAC Division of Varlan, 301 Industrial Way, San Carlos, CA 94070.
6 Overbeck, "Moonbounce Boondoggle," QST, February 1977.
7 Tilton, "Yagi Arrays for 432 MHz," QST April 1966, Page 19.
8 Knadle's 432 MHz Yagi is described in the ARRL Antenna Book, 13th Edition, (1974), beginning on page 243.
9 Described in Smith, "The World above 50 MHz," QST January 1972, Page 96.
10 About measured gains in antenna gain contests: except on 432 MHz, where a National Bureau of Standards reference antenna is often
used, the actual gain figure of a tested antenna means less than its relative performance compared to other antennas measured at the same
time. On 144 and 220 MHz, there is rarely a reference antenna of known gain, so the actual gain figures may vary from one VHF conference
to the next. One 144 MHz quagi received absolute gain figures ranging from 11.5 top 14.2 dBd at three measurements, winning each time
with a different gain. Whatever its true gain, the merit of this quagi is best shown by its relative superiority to other antennas against which
it has been measured.
11 Holliday, "High Gain Yagi for 432 MHz," Ham Radio, January 1976, Page 46.
12 Note: The elements are intended to be mounted through the shorter dimension of the boom, Mounting the elements through the longer
dimension of the boom may affect performance. Even vertically polarised quagis are normally assembled with the elements going through
the shorter dimension, even if this means mounting the antenna "on its side" and supporting the boom with an outrigger line.

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The following are details of a 70CM Quagi I built XMAS 1999. It had a VSWR of 1.3:1 from manufacture, with no tuning.
Measurements used were those shown for the 432 MHz Quagi in the previous table.

Materials used were 6mm (1/4 inch) aluminium rod for the directors, 6mm copper tube for the quad elements, 6mm polycarbonate (LEXAN) sheet for the quad spreaders, pultruded fibreglass boom, machined aluminium bar for director mountings.

The photo shows a section of the boom material. This was sourced from Pacific Composites in Melbourne, Australia. It is used as the support section in fibreglass grate flooring used in harsh environments. (Price was approximately A$4.50 / metre early 1999). Being manufactured using the pultrusion process, it has a high ratio of glass fibres to epoxy resin. There is almost no flexing over a 3 metre length in the vertical plane and a minimal amount in the horizontal plane. The photo shows an end view of this material (on its side). This antenna has a boom length of 1950 mm.

I wanted this antenna to be portable, so the elements needed to be removeable. I did this by machining bushes for the directors from 19mm aluminium bar and glued these to the boom using epoxy resin. The following photos (hopefully) show enough detail of these bushes. The clamping screws happened to be some from the junk box, but anything with a diameter of about 3mm should do the job.

The third photo shows a bush with a director fitted. (There is a lack of definition in this one.)

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2 Meter 8 Element Quagi
all measurements in mm

145.5 MHz
mm / 100 kHz step (250 steps)
147.0 MHz

Reflector
2,200
0.164
2,159

Driven Element
2,083
0.204
2,032

Director 1
912.8125
0.00635
896.9375

Director 2
908.05

892.175

Director 3
903.2875

887.4125

Director 4
898.525

882.65

Director 5
893.7625

887.8875

Director 6
889

873.125

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This page was created by Gary Bonnor - VK4ZGB.
Updated 11 May 02

Doppler effect

2009-06-20T06:24:28Z

Toowoomba4350: from wikipedia

Doppler effect
From Wikipedia, the free encyclopedia

This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (January 2008)

A source of waves moving to the left. The frequency is higher on the left than on the right.The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is the change in frequency of a wave for an observer moving relative to the source of the waves. It is commonly heard when a vehicle sounding a siren approaches, passes and recedes from an observer. The received frequency is increased (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is decreased during the recession.

For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in special relativity, only the relative difference in velocity between the observer and the source needs to be considered.

Contents [hide]
1 Development
2 General
3 Analysis
4 A common misconception
5 Applications
5.1 Sirens
5.2 Astronomy
5.3 Temperature measurement
5.4 Radar
5.5 Medical imaging and blood flow measurement
5.6 Flow measurement
5.7 Velocity profile measurement
5.8 Underwater acoustics
5.9 Audio
5.10 Vibration Measurement
6 See also
7 Notes
8 Further reading
9 External links

[edit] Development
Doppler first proposed the effect in 1842 in his treatise "Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels" (On the coloured light of the binary stars and some other stars of the heavens).[1] The hypothesis was tested for sound waves by Buys Ballot in 1845. He confirmed that the sound's pitch was higher than the emitted frequency when the sound source approached him, and lower than the emitted frequency when the sound source receded from him. Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effet Doppler-Fizeau"). In Britain, John Scott Russell made an experimental study of the Doppler effect (1848).[2]

An English translation of Doppler's 1842 treatise can be found in the book The Search for Christian Doppler by Alec Eden.[1]

[edit] General
In classical physics (waves in a medium), the relationship between observed frequency f and emitted frequency f0 is given by:

where
is the velocity of waves in the medium
is the velocity of the source relative to the medium
is the velocity of the receiver relative to the medium.
Both velocities vs and vr are computed so that the observed frequency is increased when either the source is moving towards the observer or the observer is moving towards the source. The frequency is decreased if either is moving away from the other.

The above formula assumes that the source is either directly approaching or receding from the observer. If the source approaches the observer at an angle (but still with a constant velocity), the observed frequency that is first heard is higher than the object's emitted frequency. Thereafter, there is a monotonic decrease in the observed frequency as it gets closer to the observer, through equality when it is closest to the observer, and a continued monotonic decrease as it recedes from the observer. When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt. When the observer is far from the path of the object, the transition from high to low frequency is gradual.

In the limit where the speed of the wave is much greater than the relative speed of the source and observer (this is often the case with electromagnetic waves, e.g. light), the relationship between observed frequency f and emitted frequency f0 is given by:

Observed frequency Change in frequency

where
is the velocity of the source relative to the receiver: it is negative when the source is moving towards the receiver, positive when moving away
is the speed of wave (e.g. 3×108 m/s for electromagnetic waves travelling in a vacuum)
is the wavelength of the transmitted wave in the reference frame of the source.
These two equations are only accurate to a first order approximation. However, they work reasonably well in the case considered by Doppler: when the speed between the source and receiver is slow relative to the speed of the waves involved and the distance between the source and receiver is large relative to the wavelength of the waves. If either of these two approximations are violated, the formulae are no longer accurate.

[edit] Analysis
The frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in a man's direction. Assume that balls travel with constant velocity. If the thrower is stationary, the man will receive one ball every second. However, if the thrower is moving towards the man, he will receive balls more frequently because the balls will be less spaced out. The inverse is true if the thrower is moving away from the man. So it is actually the wavelength which is affected; as a consequence, the received frequency is also affected. It may also be said that the velocity of the wave remains constant whereas wavelength changes; hence frequency also changes.

If the source moving away from the observer is emitting waves through a medium with an actual frequency f0, then an observer stationary relative to the medium detects waves with a frequency f given by

where vs is positive if the source is moving away from the observer, and negative if the source is moving towards the observer.

A similar analysis for a moving observer and a stationary source yields the observed frequency (the receiver's velocity being represented as vr):

where the similar convention applies: vr is positive if the observer is moving towards the source, and negative if the observer is moving away from the source.

These can be generalized into a single equation with both the source and receiver moving.

With a relatively slow moving source, vs,r is small in comparison to v and the equation approximates to

where vs,r = vs − vr.

However the limitations mentioned above still apply. When the more complicated exact equation is derived without using any approximations (just assuming that source, receiver, and wave or signal are moving linearly relatively to each other) several interesting and perhaps surprising results are found. For example, as Lord Rayleigh noted in his classic book on sound, by properly moving it would be possible to hear a symphony being played backwards. This is the so-called "time reversal effect" of the Doppler effect. Other interesting conclusions are that the Doppler effect is time-dependent in general (thus we need to know not only the source and receivers' velocities, but also their positions at a given time), and in some circumstances it is possible to receive two signals or waves from a source, or no signal at all. In addition there are more possibilities than just the receiver approaching the signal and the receiver receding from the signal.

All these additional complications are derived for the classical, i.e., non-relativistic, Doppler effect, but hold for the relativistic Doppler effect as well.

The first attempt to extend Doppler's analysis to light waves was soon made by Fizeau. However, light waves do not require a medium to propagate, and correct understanding of the Doppler effect for light requires knowledge of the special theory of relativity. See relativistic Doppler effect.

[edit] A common misconception
Craig Bohren pointed out in 1991 that some physics textbooks erroneously state that the observed frequency increases as the object approaches an observer and then decreases only as the object passes the observer.[3] In fact, the observed frequency of an approaching object declines monotonically from a value above the emitted frequency, through a value equal to the emitted frequency when the object is closest to the observer, and to values increasingly below the emitted frequency as the object recedes from the observer. Bohren proposed that this common misconception might occur because the intensity of the sound increases as an object approaches an observer and decreases once it passes and recedes from the observer and that this change in intensity is misperceived as a change in frequency.

[edit] Applications

A stationary microphone records moving police sirens at different pitches depending on their relative direction.
[edit] Sirens
The siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer John Dobson explained the effect thus:

"The reason the siren slides is because it doesn't hit you."
In other words, if the siren approached the observer directly, the pitch would remain constant (as vs, r is only the radial component) until the vehicle hit him, and then immediately jump to a new lower pitch. Because the vehicle passes by the observer, the radial velocity does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity:

where vs is the velocity of the object (source of waves) with respect to the medium, and θ is the angle between the object's forward velocity and the line of sight from the object to the observer.

[edit] Astronomy

Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left).The Doppler effect for electromagnetic waves such as light is of great use in astronomy and results in either a so-called redshift or blue shift. It has been used to measure the speed at which stars and galaxies are approaching or receding from us, that is, the radial velocity. This is used to detect if an apparently single star is, in reality, a close binary and even to measure the rotational speed of stars and galaxies.

The use of the Doppler effect for light in astronomy depends on our knowledge that the spectra of stars are not continuous. They exhibit absorption lines at well defined frequencies that are correlated with the energies required to excite electrons in various elements from one level to another. The Doppler effect is recognizable in the fact that the absorption lines are not always at the frequencies that are obtained from the spectrum of a stationary light source. Since blue light has a higher frequency than red light, the spectral lines of an approaching astronomical light source exhibit a blue shift and those of a receding astronomical light source exhibit a redshift.

Among the nearby stars, the largest radial velocities with respect to the Sun are +308 km/s (BD-15°4041, also known as LHS 52, 81.7 light-years away) and -260 km/s (Woolley 9722, also known as Wolf 1106 and LHS 64, 78.2 light-years away). Positive radial velocity means the star is receding from the Sun, negative that it is approaching.

[edit] Temperature measurement
Another use of the Doppler effect, which is found mostly in plasma physics and astronomy, is the estimation of the temperature of a gas (or ion temperature in a plasma) which is emitting a spectral line. Due to the thermal motion of the emitters, the light emitted by each particle can be slightly red- or blue-shifted, and the net effect is a broadening of the line. This line shape is called a Doppler profile and the width of the line is proportional to the square root of the temperature of the emitting species, allowing a spectral line (with the width dominated by the Doppler broadening) to be used to infer the temperature.

[edit] Radar
Main article: Doppler radar
The Doppler effect is used in some types of radar, to measure the velocity of detected objects. A radar beam is fired at a moving target — e.g. a motor car, as police use radar to detect speeding motorists — as it approaches or recedes from the radar source. Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source. As each wave has to move farther, the gap between each wave increases, increasing the wavelength. In some situations, the radar beam is fired at the moving car as it approaches, in which case each successive wave travels a lesser distance, decreasing the wavelength. In either situation, calculations from the Doppler effect accurately determine the car's velocity. Moreover, the proximity fuze, developed during World War II, relies upon Doppler radar to explode at the correct time, height, distance, etc.

[edit] Medical imaging and blood flow measurement
An echocardiogram can, within certain limits, produce accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations is that the ultrasound beam should be as parallel to the blood flow as possible. Velocity measurements allow assessment of cardiac valve areas and function, any abnormal communications between the left and right side of the heart, any leaking of blood through the valves (valvular regurgitation), and calculation of the cardiac output. Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.

Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift (when the received signal arrives).

Velocity measurements of blood flow are also used in other fields of medical ultrasonography, such as obstetric ultrasonography and neurology. Velocity measurement of blood flow in arteries and veins based on Doppler effect is an effective tool for diagnosis of vascular problems like stenosis.[4]

[edit] Flow measurement
Instruments such as the laser Doppler velocimeter (LDV), and acoustic Doppler velocimeter (ADV) have been developed to measure velocities in a fluid flow. The LDV emits a light beam and the ADV emits an ultrasonic acoustic burst, and measure the Doppler shift in wavelengths of reflections from particles moving with the flow. The actual flow is computed as a function of the water velocity and face. This technique allows non-intrusive flow measurements, at high precision and high frequency.

[edit] Velocity profile measurement
Developed originally for velocity measurements in medical applications (blood flows), Ultrasonic Doppler Velocimetry (UDV) can measure in real time complete velocity profile in almost any liquids containing particles in suspension such as dust, gas bubbles, emulsions. Flows can be pulsating, oscillating, laminar or turbulent, stationary or transient. This technique is fully non-invasive.

[edit] Underwater acoustics
In military applications the Doppler shift of a target is used to ascertain the speed of a submarine using both passive and active sonar systems. As a submarine passes by a passive sonobuoy, the stable frequencies undergo a Doppler shift, and the speed and range from the sonobuoy can be calculated. If the sonar system is mounted on a moving ship or another submarine, then the relative velocity can be calculated.

[edit] Audio
The Leslie speaker, associated with and predominantly used with the Hammond B-3 Organ, takes advantage of the Doppler Effect by using an electric motor to rotate a horn around a speaker continuously, rapidly alternating the received frequency of a keyboard note.

[edit] Vibration Measurement
A Laser Doppler Vibrometer (LDV) is a non-contact method for measuring vibration. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface.

[edit] See also
Relativistic Doppler effect
Dopplergraph
Fizeau experiment
Fading
Rayleigh fading

[edit] Notes
^ a b Alec Eden The search for Christian Doppler, Springer-Verlag, Wien 1992. Contains a facsimile edition with an English translation.
^ Scott Russell, John (1848). "On certain effects produced on sound by the rapid motion of the observer". Report of the Eighteen Meeting of the British Association for the Advancement of Science (John Murray, London in 1849) 18 (7): 37–38. http://www.ma.hw.ac.uk/~chris/doppler.html. Retrieved on 2008-07-08.
^ Bohren, C. F. (1991). What light through yonder window breaks? More experiments in atmospheric physics. New York: J. Wiley.
^ D. H. Evans and W. N. McDicken, Doppler Ultrasound, Second Edition, John Wiley and Sons, 2000.

[edit] Further reading
"Doppler and the Doppler effect", E. N. da C. Andrade, Endeavour Vol. XVIII No. 69, January 1959 (published by ICI London). Historical account of Doppler's original paper and subsequent developments.
Adrian, Eleni (24 June, 1995). "Doppler Effect". NCSA. http://archive.ncsa.uiuc.edu/Cyberia/Bima/doppler.html. Retrieved on 2008-07-13.

[edit] External links
Wikimedia Commons has media related to: Doppler effect
Doppler Effect, ScienceWorld
Java simulation of Doppler effect
Doppler Shift for Sound and Light at MathPages
The Doppler Effect and Sonic Booms (D.A. Russell, Kettering University)
Video Mashup with Doppler Effect videos
Practical Doppler flow meters- Doppler flow meters with engineering examples and applications
Wave Propagation from John de Pillis. An animation showing that the speed of a moving wave source does not affect the speed of the wave.
EM Wave Animation from John de Pillis. How an electromagnetic wave propagates through a vacuum
Signal-Processing - Ultrasonic Doppler Velocimeters for real time measurement of velocity profiles in liquids
Doppler Shift Demo - Interactive flash simulation for demonstrating Doppler shift.
[1] Excellent interactive applet, go to applet thumbnails>upcoming applets.

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