Whip Length On Mobile Antenna

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willgrah

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Hey all, a quick question. I have a Tram 1170 5/8 over 1/2 collinear mag mount on my car now. Not a bad antenna except that it sways in the breeze a little too much for me. I would like to just take the 5/8 whip portion and put it in the loaded base eliminating the 1/2 wave whip and coil. I don't see a problem doing that (correct me if I am wrong) I just need to know where to measure from for the proper length of the 5/8 whip.

BTW this is used on GMRS, 465 mhz is what I use for my calculations. Thanks. Bill

Just to clarify, I know what the proper whip length is supposed to be for a 5/8 at that freq, I just don't know if I need to include the base in the mix of just cut the whip and go. The rod is 15" now but seems too long based on the lengths advertised by other manufacturers for a 5/8" base loaded whip. SWR's are 1.25:1.
 
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mmckenna

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The 5/8's wave portion requires a coil to properly match the impedance. You won't be able to attach it directly to the base without having matching issues.
 

willgrah

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The 5/8's wave portion requires a coil to properly match the impedance. You won't be able to attach it directly to the base without having matching issues.
OK, that makes sense. Now if this is a 5/8 over 1/2 wave antenna, could you put a 1/2 wavelength whip on the base coil? Trying to shorten it up and really do not need all the gain of the 5/8 over 1/2. Thanks, Bill
 

mmckenna

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Yes, the base is set for a half wave radiator, but you would still need to go through and retune it all.

Unless you have a good SWR meter and the time to do it, you are quickly getting into the area where it would be cheaper to just purchase a new antenna of the correct design.
 

willgrah

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Thanks, now the question is, a half wave at 465 is roughly 12 inches, but is that measured just for the whip or do you count the base in the formula.
 

prcguy

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I would say the coil is not set up for a half wave antenna which is a very high impedance. There should be a center coil and I suspect you could remove the center coil and top whip then adjust the remaining bottom whip for a good match and be done.
prcguy
 

willgrah

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SWR's with 5/8 whip in place after a little adjustment 1.2:1. With 1/2 wave whip in place 3:1. Put it back together like factory 1.2:1. Gonna try with 5/8 wave whip in the morning to see how it sounds and hears. We will see!
 

willgrah

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The 5/8 whip transmitted fine locally but reception was lacking. I put it back to the factory configuration and its "ears" were noticably better. Worth the shot. I ordered a magnetic NMO mount and a 5/8 whip. Now I can change antennas to see which one performs the best.

My problem is that I travel to the big city daily, repeater is very high in the sky, but local terrain is suburban and flat. Whatever antenna that compromises the best will be the winner. I figured the 5/8 might split the difference between a 1/4 wave and the collinear at 5.5 db.

Thanks for the help! Bill
 

willgrah

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A quick update. I liked the 5/8 Larsen Whip and the NMO Mount much better. Audio is better and reception is better. It is also shorter and more stable, we have a winner. I mounted my Ritron RPM450 in the car last week. 25 Watts and the new antenna I can hit the repeater from the front of my house. Again, this is a high very well done repeater but the job is getting done. The base antenna is going to get a coax upgrade (RG8X to LMR400) and a slight height increase (another 5 foot). After I do that this weekend I will do some Simplex testing mobile to base, HT to base and hopefully call it done for a while. Thanks for everyones input. Bill
 
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