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Shortened radials: which length?
I was wondering if it's possible to shorten the radials from the standard
1/4 wavelength to another value, a smaller one. For example, in a 20 meters antenna, which is the best value of length for shortened radials or, in alternative, until which value is possible to go to without superlosses? Thanks in advance E il marconista sulla sua torre, le lunghe dita celesti nell'aria, riceveva messaggi d'auguri per questa crociera straordinaria. E trasmetteva saluti e speranze in quasi tutte le lingue del mondo, comunicava tra Vienna e Chicago in poco meno di un secondo. E la ragazza di prima classe, innamorata del proprio cappello, quando la sera lo vide ballare lo trovò subito molto bello. Forse per via di quegli occhi di ghiaccio così difficili da evitare, pensò "Magari con un po' di coraggio, prima dell'arrivo mi farò baciare". Francesco DeGregori |
Shortened radials: which length?
Francesco L. wrote:
I was wondering if it's possible to shorten the radials from the standard 1/4 wavelength to another value, a smaller one. For example, in a 20 meters antenna, which is the best value of length for shortened radials or, in alternative, until which value is possible to go to without superlosses? If radials are buried, their velocity factors may be something in the ballpark of 0.5. That's certainly a shortening effect. With elevated radials, the radials can be shortened and the vertical element lengthened to maintain resonance. Radials can also be shortened by loading each radial with a coil. -- 73, Cecil http://www.w5dxp.com |
Shortened radials: which length?
Francesco L. wrote:
I was wondering if it's possible to shorten the radials from the standard 1/4 wavelength to another value, a smaller one. For example, in a 20 meters antenna, which is the best value of length for shortened radials or, in alternative, until which value is possible to go to without superlosses? Thanks in advance =============================== It is my understanding that when radials are on the ground or buried ,their length can be any hence also shorter than 1/4 wavelength ,but when very they are very short ,more of the same will give an improvement especially when on dry ground. When radials are above ground like when part of a vertical antenna system fitted on a roof the quarter wave radial(s) ,at least 1 for each band , are required ,since then they are 'the other half' of a dipole. |
Shortened radials: which length?
On Sat, 23 Feb 2008 22:54:37 +0100, "Francesco L."
wrote: I was wondering if it's possible to shorten the radials from the standard 1/4 wavelength to another value, a smaller one. For example, in a 20 meters antenna, which is the best value of length for shortened radials or, in alternative, until which value is possible to go to without superlosses? Thanks in advance Hi Francesco, On the ground, or elevated? 73's Richard Clark, KB7QHC |
Shortened radials: which length?
Hi Francesco,
On the ground, or elevated? Both cases please. I did a reasearch on arrl antenna book, low band dxing and newsgroups and got a bit confused, so I need more tips. There are too many options, I mean: at a certain height above the ground, atop the roof, on the ground and so on, so I'm trying to collect as much info as possible in order to get the overall picture. Thanks |
Shortened radials: which length?
On 24 feb, 15:15, "Francesco L." wrote:
Hi Francesco, On the ground, or elevated? Both cases please. I did a reasearch on arrl antenna book, low band dxing and newsgroups and got a bit confused, so I need more tips. There are too many options, I mean: at a certain height above the ground, atop the roof, on the ground and so on, so I'm trying to collect as much info as possible in order to get the overall picture. Thanks Hello Francesco, It depends heavily on the antenna. When your antenna is an electrically half wave radiator, the input impedance is in the kOhm range and "ground" becomes less important. Many CB 27 MHz antennas are half wave antennas (without any radials). For other lengths, "ground" is important. The smaller your antenna the lower the radiation resistance (=higher feed current), and the more important the "ground" system. For fertile wet ground and buried radials, the 0.25lambda is no longer a magic number as the attenuation is very high. For dry Rock/sand, dielectric properties dictate and some standing waves appear in the ground conductor. As long as the real part of the ground impedance is less then the real part of the input impedance of the radiator, it is OK. If possible, I prefer elevated radials. In many cases, they can be shorter than 0.25 lambda (add more of them). The disadvantage is that your floating ground becomes capacitive and you get a common mode voltage on the cable screen, so you should add a common mode choke. When you make them very short, the electrical situation is upside down: your radiator functions as ground and your floating radial network is the antenna. An advantage of floating radials and the high end of HF is that your radiation center is higher and ground properties become less important, so you can have less loss (also in nearby structures) and less noise from electrical equipment. To give a more precise answer, one need to know your local conditions, structural limitations, frequency and antenna type to be used, etc. For low frequency, "Ground systems as a factor in antenna systems" (Brown, Lewis Epstein, 1937) maybe interesting for you in case of buried radials. I know this doesn't answer your question, but I hope it will help you a bit. Best regards, Wim PA3DJS www.tetech.nl (Dutch). |
Shortened radials: which length?
On Sun, 24 Feb 2008 15:15:34 +0100, "Francesco L."
wrote: Hi Francesco, On the ground, or elevated? Both cases please. I did a reasearch on arrl antenna book, low band dxing and newsgroups and got a bit confused, so I need more tips. There are too many options, I mean: at a certain height above the ground, atop the roof, on the ground and so on, so I'm trying to collect as much info as possible in order to get the overall picture. Thanks Hi Francesco, In the air: You are going to need a tuner for a standard height antenna with shorter radials. Instead, you can try adding loading coils to each radial (about midpoint). This will take experimentation to achieve resonance. So plan on putting it up and taking it down many times. You will also need a good choke at the feed point (aka 1:1 W2DU BalUn); and another one a quarter wave down the line. Some who report here say you will need more radials the lower the antenna is, and the closer to ground. On the ground: As many small radials as you can make, as long as the radiator. If that is too long (will not fit in garden), then simply fill the area you can. "Fill?" For argument's sake (a starting point) a dozen or more 0.1 wavelength radials. If this is too long, increase the count and make them the maximum length you can. Read Wim's advice for halfwave radiators. They have the reputation of being ground free, but it will ease tuning if you build some radials - whatever length, whatever count (not critical). If you don't build these; then matching becomes a function of line placement. If the line changes, so does the match. 73's Richard Clark, KB7QHC |
Shortened radials: which length?
I believe other folks have mentioned that when the radials are buried,
no specific length is required for the antenna to be resonant. The more radials you use and the longer they are, the lower the loss. There's a point of diminishing returns for both length and number, and it turns out that if you have only a few radials, making them very long doesn't help much. Elevated radials normally have to be close to a quarter wavelength long for resonance unless they're very close to the ground in which case they need to be somewhat shorter. You can use the same techniques to shorten elevated radials, though, as you do a vertical radiating element. You can add a loading inductance at the feedpoint or farther along each radial, you can use a capacitive "hat" at the end, or some combination of the two. And just as happens with loaded verticals, the result will be narrower bandwidth, lower feedpoint resistance at resonance, and potentially higher loss. Roy Lewallen, W7EL |
Shortened radials: which length?
Thanks to all!
ik8vwa |
Shortened radials: which length?
Francesco L. wrote:
"I was wondering if it`s possible to shorten the radials from the standard 1/4 wavelength to another value, a smaller one." Elevated radials are resonant, but the 20th edition of the ARRL Antenna Book has Fig 47 on page 6-26 showing radials shortened by capacitive or inductive loading. Buried radials have their resonance swamped by earth conductance. Such radials are surely effective when the greatest current drops nearly to zero at the ends, farthest from the vertical radiator, No interconnections far from the origins of the radials are desirable as they only encourage circulating current which only adds to loss. E.A. Laport has Figs. 217 and 218 on page 119 of "Radio Antenna Engineering" taken from Brown, Lewis, and Epstein which show field strength variies versus 2, 15, 30, 60, or 113 ground radials of 0.412 or 0.137 lambda, at antenna heights up to 100 degrees, so that you can see what the shorter radials cost you. Best regards, Richard Harrison, KB5WZI |
Shortened radials: which length?
On Sun, 24 Feb 2008 15:15:34 +0100, "Francesco L."
wrote: Hi Francesco, On the ground, or elevated? Both cases please. I did a reasearch on arrl antenna book, low band dxing and newsgroups and got a bit confused, so I need more tips. There are too many options, I mean: at a certain height above the ground, atop the roof, on the ground and so on, so I'm trying to collect as much info as possible in order to get the overall picture. Thanks If you are calling 'low band' those frequencies below the AM broadcast band, any antenna you use will be of compromise size (less than 1/4 wave tall, less than 1/4 wave ground radials and less than 1/2 wave high.) A full length dipole is probably a challenge. That being said, I operate mobile HF mostly. The car body is generally a poor, but raised ground plane. On 40 and up, communications are competitive enough to make contacts and communicate during normal open band hours (give or take SW Broadcast stations.) Because of power restrictions, 60 Meters works well mobile as competition is restricted to ERP 50 watts as radiated from a dipole antenna. 75/80 Meters is tough due to band conditions and antenna size in wavelengths and the number of high power stations on the air. Your ground plane antenna whether on the ground or in the air will likely be very short. Even though its height and the length of the ground radials (raised or on the ground) will be longer, think of the effect of the mobile. Even though it has a shortened ground radial system, it does work. If you can match the antenna, you can probably work with it. My experience is that the better the ground and the longer the vertical element (within limitations), the better the antenna works. I doubt there is much data reflecting various lengths of shortened ground radials as there could be too many variations. However, there is a manufacturer who has a vertical trap-multiband antenna that uses one tuned ground radial for each band made up of helical wire and/or a loading coil. If you can't make a full 1/4 wave vertical, then do your best to make it as tall as possible, as many, and as long radials as you can, or make the antenna as high as you can with four radials as long as you can, maybe loaded by helical or loading coil windings. I hope this helps Buck N4PGW -- 73 for now Buck, N4PGW www.lumpuckeroo.com "Small - broadband - efficient: pick any two." |
Shortened radials: which length?
If you can't make a full 1/4 wave vertical, then do your best to make it as tall as possible, as many, and as long radials as you can, or make the antenna as high as you can with four radials as long as you can, maybe loaded by helical or loading coil windings. My referral to "low band dxing" was just incidental. That was because info about ground radials are scattered here and there. My target is to realize a 2 element phased array (which I'm already assembling) completely homemade and was wondering about the impact that a few or a lot of radials can have on the system. Obviously I'm talking about a fixed station, not mobile. Anyway thanks for the contribution, much appreciated, of you all. Francesco, ik8vwa |
Shortened radials: which length?
Buck, N4PGW wrote:
"---or make the antenna as high as you can with four rafials as long as you can, maybe loaded by helical or loading coil windings." Yes, a ground-plane antenna needs resonant radials or at least with its vertical radiator it needs to make a resonant system. According to J.D. Kraus, radial conductor ground-plane antennas were originated by G.H. Brown. Story is that Brown said only two radials were needed (for equilibrium?). RCA`s marketing department insisted on four and that`s how it`s been ever since. Best regards, Richard Harrison, KB5WZI |
Shortened radials: which length?
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Shortened radials: which length?
Richard Harrison wrote:
. . . According to J.D. Kraus, radial conductor ground-plane antennas were originated by G.H. Brown. Story is that Brown said only two radials were needed (for equilibrium?). RCA`s marketing department insisted on four and that`s how it`s been ever since. That's a true story, also related in Brown's autobiography _And Part of Which I Was_. Two radials are necessary for near cancellation of their fields, to allow a circular pattern. The marketeers felt that it didn't look symmetrical enough to be marketed as an omnidirectional antenna so insisted on adding two more. Two radials produce a very circular pattern in a directly horizontal direction. The pattern becomes non-circular above and below horizontal, but only very slightly for moderate angles above and below. I took advantage of knowing that story when I was asked not long ago to design an omnidirectional antenna to go in a small essentially flat volume. It ended up as a variation of a ground plane with two radials, implemented as flat traces on a PC board substrate. I built a prototype with copper tape and had it measured in an anechoic chamber at a test lab. It had the most circular pattern the lab technicians had ever seen, better than their very expensive reference antenna. The extreme quality of the circularity was actually a lucky coincidence because there are factors such as feedline coupling which cause some variation even in a lab environment. But it left no doubt that the pattern circularity was in fact very good in spite of the apparently asymmetrical construction. Brown was right. George Brown, one of the pioneers of television, was also the inventor of the turnstile antenna, widely used for TV broadcasting. And he's the Brown of Brown, Lewis, and Epstein's seminal paper on radial ground systems. He also had a legendary sense of humor. One of his most famous stunts was substituting a blue-dyed banana for the yellow one in a bowl of fruit used to test the first color TV broadcast, causing a great deal of head-scratching among the engineers at the other end trying to figure out what was wrong with the color transmission. Walt Maxwell, W2DU, worked with Brown at RCA. Roy Lewallen, W7EL |
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