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
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