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Comet CA712efc vs Comet GP9NCA antennas


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#1 RickW

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Posted 12 September 2018 - 03:06 PM

Do any members have experience between these two Comet antennas? Or compared with other antennas intended for fairly significant gain on GMRS frequencies?

 

As mentioned in a previous post, I have a Comet GP9 (base antenna) but the SWR is excessive since it is so far out of band and I don't think the Midland MXT400 rigs have power "fold back" for protection so we have been only running the mid power level at the most.

 

The GP9NCA may possibly be discontinued from what it looks like, but it has substantial gain for a consumer priced vertical base station antenna and several vendors indicate some inventory.

 

One other question. Has anyone rigged up an in-line tuner to flatten out the SWR on UHF antennas? I would expect that this would introduce losses, but the GP9 actually performs reasonably well at 25 feet height above ground and I could move it to 40 feet. I have never tried UHF tweaking , but, of course, there are many antenna tuners that we use routinely on the HF bands, but in that case it is more of tuning the antenna "system." 

 

Rick

 



#2 n4gix

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Posted 12 September 2018 - 06:31 PM

Please understand that an "antenna tuner" actually doesn't nothing of the sort. What it really does is present a 50 ohm impedance to the radio's final amplifier, thus fooling the radio into thinking that it sees a 1:1 SWR.

#3 kevsh

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Posted 19 September 2018 - 04:24 PM

The comet gp-9 tuned specifically for 2m and 70cm. I can imagine it is effecting you ERP cause of swr. Construct a jpole it's better than running high swr on that radio. Tram makes a Omni directional for amatuer radio and commercial, tram 1486 bought online at home Depot for 52.00 plus shipping.

#4 RickW

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Posted 20 September 2018 - 01:58 PM

Update: I moved the GP-9 to my 30 foot tower, plus 10 feet additional pipe, so it is effectively 15 feet higher than the lower tower due to higher ground plus an extra section of Rohn 25. The feedline is longer, ~ 40 feet of RG-213 type coax to the lightning arrester, then ~ 70 feet of LMR 600 to the shack. The SWR is disguised a bit more, so now on the 462 channels, it is under 1.5:1, but on the higher 467 channels it is right at about 2:1. 

 

The sensitivity seems to be quite good on either tower. Even on the lower tower I was just able to detect a Midland 5 watt Micromobile with the magnetic antenna inside another individuals house at over 20 miles away. I am at a high point as is the other station, around 1250 ASL, but I would not have expected that. If I listen on MURS frequencies, I can copy some of the transmissions from HT's used at the "nearby" Walmart at about 7 miles and similarly another business next to them that uses one of the other MURS channels.

 

I have looked for suggestions on tweaking the GP-9 to a higher frequency than the ham designed frequencies, but have never seen anything. In fact, I have never found any information on how to duplicate such an antenna. When I assembled it, I did take a peek at the construction and it is a springy wire that is folded various ways to get the phasing. I would expect that the difference for the higher frequency range antenna is slightly shorter "elements" in the wire, thus impractical to tweak without complete replacement of the wire.

 

A J-Pole antenna has similar gain to a quarterwave groundplane, so perhaps not a practical replacement. The Tram 1486 is only 63" high, so gain is not quite in the same league.

 

Maybe my question should be: What omnidirectional high gain antenna do you actually use for GMRS? And have you compared it with other antennas in an A/B comparison?



#5 n4gix

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Posted 20 September 2018 - 07:00 PM

Maybe my question should be: What omnidirectional high gain antenna do you actually use for GMRS? And have you compared it with other antennas in an A/B comparison?

I've been using a COMET CA-712EF with great success now for several years now. It's survived 90mph gusts during several winter/spring storms that blow in.




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