VHF mobile antenna with gain - homebrew Forum at eHam.net

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VHF mobile antenna with gain	Reply
by KC0SHZ on July 9, 2006	Mail this to a friend!
Any ideas about a VHF antenna suitable for mobile use that would have gain over a 5/8 wave vertical end fed dipole that could be homebrewed in 5 evenings or less? Other than a yagi type, I can't really think of any.

RE: VHF mobile antenna with gain	Reply
by W3LK on July 9, 2006	Mail this to a friend!
The question becomes why? Except at the very fringe areas, the so-called "high gain" antennas are no better performers than a standard 5/8 wave or, in many cases, a plain 1/4 wave. Under some circumstances they can be worse. Be aware that the gain figures of Comet, Diamond, Maldol, Opek and all the other imports are pure BS. They are measured, not against the standard isotropic antenna, but against a typical HT rubber duck which is already several dB down. 73, Lon - W3LK Baltimore, Maryland

RE: VHF mobile antenna with gain	Reply
by DROLLTROLL on July 9, 2006	Mail this to a friend!
Way back when...... I used to install data terminals in public safety vehicles and taxis etc. We would always use quarter wave whips as they always outperformed gain antennas. With "gain antennas" we would always have data dropouts due to the inherent lobes of the "gain antenna". It was so bad that even going up a hill or being parked on an incline would be enough to put a null in any signal thus losing information. For voice, the dropouts and flutter might be acceptable, but it's for this reason I always run quarter wave antennas at VHF/UHF.

RE: VHF mobile antenna with gain	Reply
by KC0SHZ on July 10, 2006	Mail this to a friend!
Thanks, I thought that I was going crazy. I have a 5/8 wave vertical and will be needing all the gain I can get as we will be working simplex frequencies on a bike ride this weekend. I kept looking and looking for more gain, but either found directional beams or nothing. Thanks for the feed back, I will stop worrying about boosting gain, and start worrying about how to keep the 150 riders from getting heat stroke during our triple digit temps this weekend.

RE: VHF mobile antenna with gain	Reply
by K4JSR on July 10, 2006	Mail this to a friend!
Hey, Lon; Where can I buy a standard Isotropic Antenna? Better yet, two. I would love to co-phase them! ;-D As far as models go, I would rather have a Christie Brinkley than a mathmatical model! ;-P Wise Acre button <OFF>. The channel is now returned for "NORMAL" use. 73, Cal K4JSR Tongue-n-Cheek, Ga.

RE: VHF mobile antenna with gain	Reply
by WB6BYU on July 10, 2006	Mail this to a friend!
The main problem is that omnidirectional gain requires greater antenna height. To get 3dB of gain requires more than doubling the antenna length. (And a 5/8 wave really doesn't do that much better than a quarter wave because the roof of any common vehicle is too small to achieve the theoretical gain over an infinite groundplane.) If you mounted an 8' antenna on top of your car you could get some more gain, but it probably would be impractical for driving around town. (And, unless the bottom of the antenna was at least 8' off the ground, you may do as well with a lower gain antenna on a mast so the top is at the same height.) What you could do, however, is what I do for transmitter hunting: I put a 2- or 3- element quad on a mast so it sticks up above the roof and I can rotate it from inside the car. That gives you more gain than an omni but you have to point it in the right direction.

RE: VHF mobile antenna with gain	Reply
by W3LK on July 10, 2006	Mail this to a friend!
Cal: CB shops have the co-phasing harnesses. You can have Christie; too young for me. :) Enjoyed the post! Lon - W3LK Baltimore, Maryland

RE: VHF mobile antenna with gain	Reply
by KE3HO on July 11, 2006	Mail this to a friend!
Lon, >> Be aware that the gain figures of Comet, Diamond, Maldol, Opek and all the other imports are pure BS. They are measured, not against the standard isotropic antenna, but against a typical HT rubber duck which is already several dB down. << I don't know about the other manufacturers, but the ONLY antennas that Diamond quotes gain versus an HT antenna are their own HT antennas. ALL, without exception, of their mobile and base antennas are compared to either an isotropic radiator or a dipole and their data is marked either dBi or dBd. Now, whether or not those numbers are accurate is a separate issue. I am inclined to believe that they are inflated, but not to the extent that they would be if a stock HT antenna were the reference. 73 - Jim

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