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Author Topic: Dual-band antenna issue?  (Read 2806 times)
AD0DJ
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Posts: 4




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« on: April 28, 2013, 01:55:43 PM »

Hi, folks,

Question about a possible mobile antenna issue - I'm running a Kenwood TM-D710A, and was until recently using a mag mount on the roof of a Mercury Mariner and an inexpensive antenna and getting decent performance on both 2 meters and .7 with that.

Then I switched to a Comet lip mount (CP-5NMO) mounted on the edge of the hood (driver's side, about 8" from the rear edge of the hood) and my performance on .7 meters (specifically with regard to a local repeater) got really bad - receive has a lot of sizzle in it and I'm usually unreadable on transmit.

I checked the SWR using a cross-needle meter and a small dummy load attached with an adapter to the NMO connector on the mount to check the feedline SWR, and I get a great SWR on 2 meters (about 1:1 plus a bit, about the what the dummy load alone is supposed to be) and a so-so/fair SWR on .7, that being about 2:1.  Then with the antenna back on, I get about 2:1 on 2 meters and 5:1 on .7 meters.

I then went back to putting the antenna back on the mag mount for comparison, locating the mag mount both on the roof and next to the lip mount on the hood, and got the same SWR readings in both locations (2:1 and 5:1 on 2 and .7 respectively).  I don't have an SWR measurement from when everything worked great with the mag mount, but there is a clear performance difference between now and when I first set this rig up in the car, and that 5:1 SWR on .7 can't be good.

I'm pretty new to all this, only having had my ticket since March 1st of this year, but I've been doing a lot of research and am starting to think the perhaps the antenna is the issue - might the loading coil be the issue here?  As mentioned earlier, 2 meters is still working fine, and I THINK I've ruled out feedline issues with the above steps I've taken.  Have I missed some other potential issue besides the antenna?  I'm right about to the point of getting a Larsen NMO2/70B to see if that helps, but thought I'd see if anyone had anything else to suggest here.   

Thanks in advance for any tips -

Joe AD0DJ   
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N2LWE
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Posts: 104




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« Reply #1 on: April 28, 2013, 05:18:26 PM »

I recently installed a Comet lip mount NMO too and thought when I tightened the allen screws that this made a good ground but my performance was poor also. I took the lip mount off and with a razor knife I scraped the under side of the edge of the tailgate to make sure I was getting a good ground. That was my problem, it's working great now. I didn't realize when I originally installed it that the screws weren't penetrating through the clear coat on the paint. I hope this helps.
« Last Edit: April 28, 2013, 05:21:49 PM by N2LWE » Logged
AD0DJ
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Posts: 4




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« Reply #2 on: April 28, 2013, 06:06:43 PM »

Aha, that could be my problem - I'll try adding a ground to the chassis and see if that helps.  Thank you very much!
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N2LWE
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Posts: 104




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« Reply #3 on: April 28, 2013, 06:50:34 PM »

That's what my problem was. It's amazing that such a small issue, two small screws not penetrating through to the bare metal. Once I corrected this, it's been great since. Let me know if that's what it was. I'm curious. Good luck.
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AD0DJ
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« Reply #4 on: May 01, 2013, 07:19:23 PM »

Hi, N2LWE - reporting back with my problem fixed, which turned out to be (as I'd suspected) the antenna.  The big brown truck brought me a Larsen NMO2/70B today, which I installed right away, on the new lip mount.  I powered up and found that my receive was about 100% cleaner on 440 than it had been (and about the same as it used to be before I had the problem).  I hooked up the SWR meter and got a NICE low SWR on 440, like 1.15 or so, compared to the 5:1 I'd been seeing with the old antenna.

I made no grounding changes, and have done nothing special for RF or DC ground, and I haven't done any bonding either.  It's just red and black to the battery, lip mount antenna feed to the box, lip mount on the edge of the hood with shims, no scraping of paint for the setscrews.

I have to think that this confirms that the problem was the old antenna, which did perform all right for a while on the mag mount, but I think the loading coil in the antenna must have packed up right around the time I switched to the lip mount, as 2-meter performance didn't suffer after that (or didn't suffer enough to notice, anyway).  And when I switched the old antenna back to the mag mount, the problem didn't go away.  Seems like an odd coincidence that the antenna would go south at the same time I made the mount change, but funny things happen sometimes...  At any rate I'm back on the air.  Thanks for the tip!

73, Joe AD0DJ 
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N2LWE
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Posts: 104




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« Reply #5 on: May 03, 2013, 05:37:37 PM »

I'm glad it worked out for you and wasn't anything serious like the radio. If the antenna was old, well it's always nice getting "new", lol. So you're all set. New antenna and new mount. Enjoy the airwaves, 73, Joe N2LWE.
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N7WR
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« Reply #6 on: May 03, 2013, 06:39:39 PM »

The Larson NMO2/70 is THE BEST 2/440 mobile antenna made.  The fact that so many public safety agencies use Larson mobile antennas is a testament to their performance and durability.  I use Larson exclusively on all of my mobile installations.
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K1CJS
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Posts: 6055




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« Reply #7 on: May 04, 2013, 09:20:40 AM »

Yeah, it isn't surprising.  Comet quality control wasn't--and still isn't, it seems--the best.  I had one which I had to exchange to get a good antenna.  Still a Comet and the same model, but the difference between the two was like night and day.
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