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#1
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Dual rhombic antenna
I picked up a Oct 1973 issue of 73 & noticed a interesting article on
a mast mounted rotatible very hig gain 435mhz antenna, cheap and easy to built. Ive been trying to build a high gain antenna to recieve ATV in the 903-928 band & seems this maybe the one. The claims for this antenna seem streached abit, they claim 26dB gain from just wire and a few resistors and claim its 435mhz design can work up to 890mhz, theres even a quad rhombic shown also. Sounds to good to be true, so i wandering if anyone has had any experience with actually building & using one of these antennas. I would like to build some kind of high gain recieve only antenna to see if its possible to get a decent signal from some of the ATV repeaters. Seems here in western N.C. the ATV activity is located mostly in this located band. Thx N4aeq |
#2
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Dual rhombic antenna
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#4
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Dual rhombic antenna
wrote:
I picked up a Oct 1973 issue of 73 & noticed a interesting article on a mast mounted rotatible very hig gain 435mhz antenna, cheap and easy to built. Ive been trying to build a high gain antenna to recieve ATV in the 903-928 band & seems this maybe the one. The claims for this antenna seem streached abit, they claim 26dB gain from just wire and a few resistors and claim its 435mhz design can work up to 890mhz, theres even a quad rhombic shown also. Sounds to good to be true, You guessed it. This antenna has been thoroughly debunked in the 33 years since the original article. I don't have detailed references, but the story goes that ground gain of 6dB was added on *twice* to the claimed original free-space gain figure... which was optimistic in the first place. -- 73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB) http://www.ifwtech.co.uk/g3sek |
#5
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Dual rhombic antenna
Stargatesg1 wrote:
Also I would check to make sure the gain is listed as 26 DBd instead of DBi. Back in the day, allot of companies inflated their specs by stating the DBi gain instead of DBd. DBi being gain over an isotropic and DBd being gain over a dipole. At 26Db there isn't enough difference between dipole and isotropic to matter. Dave WD9BDZ |
#6
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Dual rhombic antenna
To get 26 dB over a dipole would require rhombic leg lengths of about
125 wavelengths, or about 275 feet at 450 MHz. The overall length would be around 500 feet. The beamwidth would be so narrow you'd have difficulty getting it aimed toward one station, even if you were able to turn it somehow. Roy Lewallen, W7EL David G. Nagel wrote: Stargatesg1 wrote: Also I would check to make sure the gain is listed as 26 DBd instead of DBi. Back in the day, allot of companies inflated their specs by stating the DBi gain instead of DBd. DBi being gain over an isotropic and DBd being gain over a dipole. At 26Db there isn't enough difference between dipole and isotropic to matter. Dave WD9BDZ |
#7
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Dual rhombic antenna
This gain figure sounds very much like the 144MMz EME double rhombic
that was in the ARRL handbook for years. As I remember it was "attacked" some years ago for claiming such a high gain. I built one for 70cm some years ago and the results were disappointing, or rather not as good as a 22 el loop quad. I didnt do much experimenting with it sorry. Cheers Bob VK2YQA wrote: I picked up a Oct 1973 issue of 73 & noticed a interesting article on a mast mounted rotatible very |
#8
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Dual rhombic antenna
Exactly what i wanted to know, i may hold off on this one and search
for a different type high gain uhf antenna. Only problem is all the design programs i have found on the web for yagi or log perodic antennas are for 6-8 elements maximum, @ 900mhz i would like to have as many elements as a boom would support. So my next question would be where to look for a multi element beam using atleast a minimum of twelve elements? |
#9
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Dual rhombic antenna
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#10
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Dual rhombic antenna
wrote in message oups.com... I picked up a Oct 1973 issue of 73 & noticed a interesting article on a mast mounted rotatible very hig gain 435mhz antenna, cheap and easy to built. Ive been trying to build a high gain antenna to recieve ATV in the 903-928 band & seems this maybe the one. The claims for this antenna seem streached abit, they claim 26dB gain from just wire and a few resistors and claim its 435mhz design can work up to 890mhz, theres even a quad rhombic shown also. Sounds to good to be true, so i wandering if anyone has had any experience with actually building & using one of these antennas. I would like to build some kind of high gain recieve only antenna to see if its possible to get a decent signal from some of the ATV repeaters. Seems here in western N.C. the ATV activity is located mostly in this located band. Thx N4aeq Hi Tucker I had to re-look at the month that article was published to be sure it wasnt an April 1 issue. The antenna's beamwidth, with a circular cross section, will be close to 10 degrees maximum. It is likely that a 12 foot diameter dish would be the minimum size that would provide that 26 dB Gain at 900 MHz. That gets pretty difficult to make stable when also rotateable. I would suggest that you start with a Yagi and try it. You'll soon know if that gain is adequate. And the money spent on parts wont hurt your pocketbook. I sure wouldnt expect you to jump into antenna design with a beginning project of building a 26 dB Gain rotateable antenna. Jerry |
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