Fine tuning a RS magmount

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737mech

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Where is the wave length computed from with this antenna? Lets say if I wanted to lengthen or shorten elements where is the ground plane measured from? The bottom of the magnet? The botom of the chrome cone? the bottom of the center element connection? What about the coils, what is the value of the coils? I've been toying all day long and would appreciate any suggestions. I'm mostly trying to fine tune into VHF airband 118-137. If you had the materials what length would be ideal for VHF airband?
 

737mech

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Rs mag mount

Which antenna specifically are you referring to? Radio Shack makes lots of mag mount antennas.
It's the one in the wiki and I tried the mod for it. Good for VHF 150-160 but not so awesome for VHF airband. I was hoping someone had it figued out for ideal airband reception.
 

737mech

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conve36

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Take a whip from another antenna and cut it to 24.86 inches. Total length should start from the bottom of the cone on the antenna base. Forget the coils. A 1/4 whip would perform better than tinkering with those coils. The coils are their for all-band reception, but the 1/4 wave whip focuses on one band. Here is a link to a calculator to determine the wavelength of a frequency. And do NOT go and make a full-wave or 1/2 wave antenna thinking its better than a 1/4, cause thats not the case. Trust me, I have bought and destroyed (over-modifying) about 4 of the RS mag-mount scanner antennas and know what works best (for me at least).

Frequency Wavelength Calculator
 

737mech

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Rs Mod

Take a whip from another antenna and cut it to 24.86 inches. Total length should start from the bottom of the cone on the antenna base. Forget the coils. A 1/4 whip would perform better than tinkering with those coils. The coils are their for all-band reception, but the 1/4 wave whip focuses on one band. Here is a link to a calculator to determine the wavelength of a frequency. And do NOT go and make a full-wave or 1/2 wave antenna thinking its better than a 1/4, cause thats not the case. Trust me, I have bought and destroyed (over-modifying) about 4 of the RS mag-mount scanner antennas and know what works best (for me at least).

Frequency Wavelength Calculator
I believe you, I'm on #2 and then some as far as whip lengths. Time to punt and try something else because I'm fourth down and 70 with anything I've tried. I'll figure something else out, thanks for the link I'll save that for scanner diy tools. It's fun to try different metal in the air and stuff, I did the OCFD, tried many others including a couple of cool aircraft antennas. The current one is great so far. The antenna is awesome on 220-400 as it should be because it is made for that. Wish the RS mag mount could fill the gap for airband VHF. I'll post a pic below. Don't laugh but yes that ground plane is the left overs from my last washer machine. :) If you see something wrong with my setup please advise and thanks in advance. I'm leaning toward DPD LP for the future. Probably order one after the holidays. I think that will get me what I want only farther into the ranges. OBTW I'm in Las Vegas south of the Nevada ranges. I figure point one of the DPD LP's toward the north and see (hear) what happens.
 
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conve36

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I would HIGHLY recommend building yourself a ground-plane antenna. I have also gone through about 4 of the Radio Shack 20-176 antennas. They work great for $26.99. Check out one of my videos on youtube (link at end of this message). I basically modified one of the 20-176 antennas for better reception on 151.385mhz (my counties fire dispatch) and also for transmitting on GMRS frequencies. It works GREAT!

So my point here is... You can actually either modify one of these antennas for your specific needs or start from scratch. Very easy to make one for very cheap. Just buy one of the chassis mount UHF connectors from Radio Shack. Then buy a few 4-foot (or close to it) welding rods from Lowes. They are the ones with the blue color code on the end. These ones solder very easily with rosin core solder! Then, cut 5 rods at exactly 24.86". Attach the radials (also knows as elements, rods, etc) to the chassis mount and tinker around with the angle, except the main vertical element which should be straight up of course. It should looking something like the picture I attached, but much longer elements.

YouTube - Radio Shack Groundplane VHF/UHF antenna project
 

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