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mobile antenna problem

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BigEd1314

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I have the matching pair of TK-7180 and the 8180 mobiles. The VHF is 50 watts and the UHF is 30 watts. I had the antenna's roof mounted and I noticed a problem. When I key up on the VHF radio, what ever the UHF is receiving, the signal is cut way down, and in some parts of the county, it gets cut out completly until i unkey the VHF then its fine again. I had both coax cables run together, but then I ran the UHF on one side of the vehicle, and the VHF is run to the other side of the vehicle hope to solve the porblem, this helped a very little bit. The antenna's are 15 inches apart almost in the center of the roof in a straight line. The vehicle is a 1992 Ford explorer. Anyone know what could be wrong? Thanks Ed
 

datainmotion

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Try a mag-mount antenna down on the hood (just for testing of course) as a substitute for one of the roof mounts and see if it goes away. If it does, I'd say your problem is proximity between the antennas.
 

LarrySC

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I had a ham once to tell me I could'nt put two antennas 1/8" apart using L-brackets. I told him I could as they were both scanner antennas. He said " I never thought of that". In your case you cant do that 15" thing. Cap the VHF mount and move it. Leave the UHF on top. Good Luck.
 

BigEd1314

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Well, whats is weird is, i had the mag-mounts in the exact spots i have them roof mounted, and it never did it before....
 
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BigEd1314 said:
Well, whats is weird is, i had the mag-mounts in the exact spots i have them roof mounted, and it never did it before....
They're not as efficient as permanent mount antennas.
 

OpSec

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First, the antenna's are too close to each other. Rule of thumb here is to maintain a 1/4w distance spacing between antennas, at the 1/4 wavelength of the lowest frequency used in the installation. In this case, that would mean spacing them 18-19" apart, which is a 1/4 wavelength of VHF-Hi.

Second, the Kenwoods are probably suffering some type of receiver desense regardless due to their construction. With the advent of software-controlled tuning and lack of cavities and filters in the RX section, the newer radios just aren't as well built as older units. There are many installations of older 110w radios with proper antenna spacing (and some improper) that work great without desense due to their construction.

Third, check the SWR of each antenna. If it is higher that 2.0:1, re-tune them to get below that. Anything higher than 2.0:1 is doing more harm than good to the TX'ing radio's PA section, and possibly causing spurious emissions.

Fourth, check the feedline, NMO mounts and connectors for any cracks, pinching or poor joints. Many RF related problems can be fixed with the last two items I mentioned.
 

BigEd1314

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ok, well i am going to get ahold of an SWR annalyzer tomorrow, and test them. The antennas are a motorola 1/4 VHF and a Radial Larsen 5/8 Wide Band UHF. But i'll check the SWR, and I might try lowering the TX power of the VHF also, since i don't use it as much as the UHF. Thanks for the help.
 

wesct

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BigEd1314 said:
I have the matching pair of TK-7180 and the 8180 mobiles. The VHF is 50 watts and the UHF is 30 watts. I had the antenna's roof mounted and I noticed a problem. When I key up on the VHF radio, what ever the UHF is receiving, the signal is cut way down, and in some parts of the county, it gets cut out completly until i unkey the VHF then its fine again. I had both coax cables run together, but then I ran the UHF on one side of the vehicle, and the VHF is run to the other side of the vehicle hope to solve the porblem, this helped a very little bit. The antenna's are 15 inches apart almost in the center of the roof in a straight line. The vehicle is a 1992 Ford explorer. Anyone know what could be wrong? Thanks Ed
why not run a dualband mobile antenna and a diplexer?

pull the radios out and check the receivers. you may have a problem with the front end
on the radio that does the most receiving.


wesct
 

BigEd1314

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OK, it might be the UHF, cause when i key up the UHF, the VHF is not affected or desensed. Next time i go to the radio shop, i'll have then check it, its still covered by warrenty, so it wouldn't cost me anything to fix it, if there is something wrong.

And do they make a diplexer that will cover ham along with VHF and UHF commercial? I use VHF and UHF ham along with fire dept and haz-mat team freq's. So i'd need one to cover ham, and really 400 to 470 MHz and 144 to 162 MHz.
 

kb2vxa

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Hi Ed and all,

"why not run a dualband mobile antenna and a diplexer?"

Because the frequencies are unknown to us we know not what the specs for the diplexer nor the antenna should be. In any case unless it's a ham installation both would be non-standard and would have to be custom manufactured.

There IS one possible solution, a high pass filter on the UHF side. This MAY be possible using a ham diplexer with the VHF side not connected provided the crossover point is at the proper frequency to accomplish it. You'll have to do your homework on this unless you publish the frequencies in question so someone can take an educated guess at least.

Now if this turns out to be ham I'd go with a dual band antenna and a diplexer which makes for a much neater installation at the same time.
 
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BigEd1314

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Well, I got back from the radio shop, the receiver is factory spec. I did however replace the 5/8wave UHF antenna with a regular 1/4 UHF and the problem goes away. So...could it be the gain of the antenna, and it being 15 inches apart causes the problem? And I do use ham, but thats not what I use the radios the most for.
 
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