Tuning mobile whip with no ground plane

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LakeMan2

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I have a Larson NMOQSPEC mobile antenna that I would like to tune for UHF (440 for example). Due to mounting options I will have either a poor or no ground plane. I realize that there are 1/2 wave base loaded NGP available. But due to the mounting location, the "higher" thick base may not be an option. I am wondering if I could get a thin, low base profile like the QSPEC to work. I know that technically those are supposed to be 1/4 wave, but performance being primarily a function of freq/wavelength, and antenna length, I was thinking it might work. Unity gain is just fine (mobile+ higher mounted unity gain should be better than HT).

I think I understand inductance, reactance, VSWR, etc. But thinking I do and really understanding may be two different things. So to get a handle on things and test the feasibility, I did some testing before I even attempted to cut anything. I have a older NMO magnetic mount with 17' of RG58. Unfortunately I have to measure everything at source end of the 17' cable.

I put the uncut antenna (~21") on a 3'x3' piece of sheet metal as a ground plane to get a baseline. According to Larson that should be good for ~136MHz. I have an antenna analyzer (Rig Expert) and did R/X and SWR sweeps. It appears to be resonant (reactance X=0) at about 133MHZ with an inductance of 35ohms and a SWR of 1.43. Good enough and in the ball park.

I then removed the sheet metal and have the antenna just on a wood table. Of course at 133MHz it looks bad as expected. I swept up at UHF and it appears that it is resonant at 427 (exact freq unimportant), again with reactance zero, inductance 37 ohms, and a SWR of 1.36. The lower SWR probably due to the higher coax losses at UHF vs VHF. I assume this appears to tune here because the length is around a full wave length at 427MHz?

Am I missing something here? It looks like I could tune this NMOQSPEC whip for 440 (or higher) and it work just fine without a ground plane. I don't know what the radiation pattern would be, but I assume it would be fairly normal?

Thanks
 

LakeMan2

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I will be mounting in on a boat ski/wakeboard tower so basically a cross tube. A horizontal ground plane is not feasible. . UHF not VHF, since I need to be able to transmit from shore (inland lake) to the boat.
 

LakeMan2

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I answered part of my question. I found an antenna modeling program. Messed with it some and modeled a full wavelength vertical. It has a very high incident angle such that the primary radiation is around 45 deg up from the horizon. It does have some slight gain on the order of 3 to 4 dBi (according to the model, but there is a good chance it was not set up well enough to extract meaningful numbers from). At relatively low angles (13 deg) the gain was around unity. I still think it might work though unless there is still some issue I don't see yet.
 
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Rred

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If you have no ground plane or counterpoise and you are mounting the antenna on top of some vertical "pipe" structure, the "pipe" can be used as a vertical counterpoise, basically giving you a vertical dipole antenna. If you tie that tower into the boat's bonding system and that is also grounded (to the props or otherwise) it might even be good enough to be a good ground.

But if you have no ground and no counterpoise? Then the outer jacket of your coax cable BECOMES the counterpoise, and performance, SWR, and safety (from radio energy burns) all depend on how many wavelengths long the cable is, and how the cable happens to affect the signal that is being reflected back and carried in it.

As soon as you say "Boat" and "tower" all bets are off, the individual installations on boats of all kinds are extremely dependent on the particulars for each boat. Best bet is to just do it as best you can. and presume you may have to change or modify something later if it can't be made to work well enough.
 

lmrtek

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what you made was a 3/4 wave UHF antenna which still needs a ground plane and has a poor takeoff angle

1/2 wave is the only way to go she the go is small or non existant

don't trust cheap antenna analyzers

trust the rf engineers at Larsen
 
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