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Homebrew high gain FM broadcast antenna ??
I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 -
20 feet). I can get the formulas for the spacing of the reflectors and directors from literature, but it always says further optimization will be necessary. This 'further optimization' might prove difficult and I wonder if anybody here has experience with this or has built one of these antennas?? Uwe |
UWE wrote:
"I would like to put up a big hi gain FM broadcast band antenna." 1.5 wavelengths is about 15 feet near the center of the FM broadcast band. Kraus gives an example of this 6-element Yagi. It has a radiator, reflector, and 4 directors in Fig. 8-29 on page 248 of his 3rd edition of "Antennas". Directivity is about 12 dBi and bandwidth is 10%. Construction is not too critical because the space between its elements is fairly wide. I have built Yagis and been pleased with their performance. Best regards, Richard Harrison, KB5WZI |
Hi Uwe
I'd suspect that the optimisation you are talking about is not going to make a lot of difference to your end use. You tend to be able to tune yagi's for best forward gain or best front to back ratio but not both together. You have the added complication that if you want to cover the entire FM band a single yagi would be a major compromise and it might be better looking into a different kind of design like a log periodic or axial mode helix. As a general rule too, the longer the yagi the narrower bandwidth its operation is. My gut feel for a 20ft FM band yagi antenna would be only for use over maybe 200kHz. It would of course still work further from the design frequency but at reduced performance. As an aside some of the optimisation will be due to slight material differences and deviations from original mounting techniques (eg through boom vs on top, proximity to buildings etc) Would you care to share you reasons for the high gain design? Is it one distant station you want or just for general DX use? Might you be trying to reduce interference effects? Cheers Bob VK2YQA Uwe wrote: I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 - 20 feet). I can get the formulas for the spacing of the reflectors and directors from literature, but it always says further optimization will be necessary. This 'further optimization' might prove difficult and I wonder if anybody here has experience with this or has built one of these antennas?? Uwe |
On Mon, 15 Nov 2004 23:53:47 GMT, Uwe wrote:
I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 - 20 feet). I can get the formulas for the spacing of the reflectors and directors from literature, but it always says further optimization will be necessary. Uwe, Check out www.cebik.com he has an article "some notes on FM BC antennas". Or at least something like that. He does extensive antenna modeling. One of the problwms with FB broadcast antennas is that 88-108 (US FM allocation) is a very wide band percentage wise to make a good yagi for. Programs like YO, QY4, are likely inadaquate for wide bandwidth antennas so you may end uo with EzNEC or one of the NEC varient calculation programs for antenna design. If you were to take a 13b2 and lengthen the elements they would be too closely space to get the same performace as the origional. You have to scale length, spacing (and boom length) and diameter by the same factor to get the same antenna at a different frequency. IE: everything would be about 1.45 larger to optimize for 100Mhz. Good luck. Allison |
Uwe wrote:
CLIP.. The physical laws are the same, be it for a 6m antenna, for example, or a FM broadcast receiving antenna. In fact I was thinking in stripping my Cushcraft 13 element 2 m antenna for the purpose and just lengthen all the elements. It would probably be a poor idea to modify the 2M antenna. As someone else mentioned, your spacing would not be correct. A 15 foot long boom would support 1 Reflector, 1 Driven Element, and 5 Director elements. This would give you a estimated gain of almost 10 dBd. Normally the bandwidth of these antennas is about 7% at the design frequency. For your use this would be about 6.5 MHz. If you are only interested in a small portion of the band, this will work fine. Getting higher gains is a case of diminishing returns. Typically, to obtain a 3 db increase in gain you need to double the number of director elements and effectively double the boom length. So, going from 15 to 20 feet will only gain you another 1 or 2 db. In cases of extreme fringe operation, this may make a difference, but mostly you won't notice it. If you want to play with boom lengths and element spacing try one of my web pages at: http://www.k7mem.com/Electronic_Note.../yagi_vhf.html Optimization is nice to do if you have the equipment, but with careful construction, you should be able to build a antenna that is very close to the intended specifications and not require any tedious tweaking. For other antenna designs you might try Ian White's (G3SEK) web site at: http://www.ifwtech.co.uk/g3sek/diy-yagi If your intent is to cover the entire FM band, you may want to look for information on Log-Periodic antennas. Gain will not be big, but the bandwidth will be. -- Martin E. Meserve - K7MEM http://www.k7mem.com |
Bob, the reason to want the high gain is as follows.
I live in Maine south of the Canadian border, close but not quite close enough to receive Canadian FM stations. I can receive one station faintly using a Radio Shack BC band antenna and wanted to expand on that with a better antenna and possibly a change in IF filters in the tuner I use. And I thought that a very high gain antenna with its narrow pattern would also aide in adjacent channel rejection. I realize that it could be a problem covering the entire BC band and I am not set on any particular design, I first wanted to see what people here had to say. Regards Uwe KB1JOW From: Bob Bob Newsgroups: rec.radio.amateur.antenna Date: Tue, 16 Nov 2004 19:18:13 +1100 Subject: Homebrew high gain FM broadcast antenna ?? Hi Uwe I'd suspect that the optimisation you are talking about is not going to make a lot of difference to your end use. You tend to be able to tune yagi's for best forward gain or best front to back ratio but not both together. You have the added complication that if you want to cover the entire FM band a single yagi would be a major compromise and it might be better looking into a different kind of design like a log periodic or axial mode helix. As a general rule too, the longer the yagi the narrower bandwidth its operation is. My gut feel for a 20ft FM band yagi antenna would be only for use over maybe 200kHz. It would of course still work further from the design frequency but at reduced performance. As an aside some of the optimisation will be due to slight material differences and deviations from original mounting techniques (eg through boom vs on top, proximity to buildings etc) Would you care to share you reasons for the high gain design? Is it one distant station you want or just for general DX use? Might you be trying to reduce interference effects? Cheers Bob VK2YQA Uwe wrote: I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 - 20 feet). I can get the formulas for the spacing of the reflectors and directors from literature, but it always says further optimization will be necessary. This 'further optimization' might prove difficult and I wonder if anybody here has experience with this or has built one of these antennas?? Uwe |
Ewe wrote:
"I realize that it could be a problem covering the entire BC band and I am not set on any particular design." Kraus` 6-element Yagi on page 248 of the 3rd edition of "Antennas" has 12 dBi doirectivity and a 10% bandwidth. FM channels are 200 KHz wide. 10% of 100 MHz is 10 MHz. That is 50x what you need for a single channel. If you want to cover the entire FM band with an antenna, you can do it easily with a rhombic. Arnold B. Bailey`s rhombic in "TV and Other Receiving Antennas" scales to: two #10 wires 72 feet long, separated by 36 feet at the midpoint. Overall length is 61 feet. Surge resistance is 600 ohms, and 600 ohms is used as the terminating resistor value. Bailey uses 300-ohm twinlead as a transmission line. Gain is 14.5 dBd (or less). Bandwidth is 30% for 1 dB down. That`s 1.5x the width (20 MHz) of the U.S. FM band. What you probably need is height. Signal drops fast in the shadow of the horizon or an obstruction. Best regards, Richard Harrison, KB5WZI |
Hi Uwe
I cant resist... Brrrr COLD! I visited Toronto early this year and havent gotten over the throat irritation yet! It was about 33C when I left Sydney and -25C when I got to Toronto! Well then I can now see the use of employing the narrowest band design possible, assuming you only want the Canadian station. Personally I wouldnt want to limit its bandwidth because I *might* one day want to use it for another distant station. Is it worthwhile doing an analysis of those stations that might interfere? Like if the interfering source is at 90 degrees heading from the station you want, anything that starts as a dipole element will help. If its directly behind you may want to adjust for best front to back, even to the point of tuning the reflector with some C. This BTW is much easier on a quad reflector than a yagi. (ie make a "Qagi") My theoru isnt good on this, but I can see you want a design that has got low sidelobes and good front to back. Yagis can be optimised for all manner of things so one design you see may work great for f/b but bad in sidelobes. Is a phased array better? (ie mount your boom at right angles to the station and use 2 element designs) How does a quad or helix compare? Does choosing circular polarisation help? This is actually a more difficult problem than first thought. My path would be not to think too much, but put up a helix and see how it performs. I use to have one on 2M for satellite work that I also used for terrestrial TV and FM. (Very wide band) I was however in a zero interference area. Given though that I guess you want to go with a yagi design, my thought is to build it to the Canadian station frequency with a tuned refelector and also see what happens. When you have built it plot the pattern at various frequencies. Have a look at Mr Cebiks web page on OWA yagi's. (http://www.cebik.com/2mowa1.html) He often shows radiation patterns and lots of other data. Well worth a look. Apologies for the length! Cheers Bob VK2YQA Uwe wrote: Bob, the reason to want the high gain is as follows. I live in Maine south of the Canadian border, close but not quite close enough to receive Canadian FM stations. I can receive one station faintly using a Radio Shack BC band antenna and wanted to expand on that with a better antenna and possibly a change in IF filters in the tuner I use. And I thought that a very high gain antenna with its narrow pattern would also aide in adjacent channel rejection. I realize that it could be a problem covering the entire BC band and I am not set on any particular design, I first wanted to see what people here had to say. Regards Uwe KB1JOW |
"Uwe" wrote in message ... I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 - 20 feet). I can get the formulas for the spacing of the reflectors and directors from literature, but it always says further optimization will be necessary. This 'further optimization' might prove difficult and I wonder if anybody here has experience with this or has built one of these antennas?? Uwe Uwe, I suspect you can buy a commercially made FM antenna for the cost of materials that it would cost to build your own (excluding the rhombic). Optimization is nor very practical unless you want to spends 100's of $$$ on test equipment. The large Radio Shack FM antenna, if they still make it, should do the job. An ANTENNA MOUNTED preamp might also help. Since you are trying to pick up distant stations, antenna height is very important. Tam/WB2TT |
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