Antenna config for mobile setup?

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lexmedic157

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I'm looking to install dual band mobile with a single antenna output (something like a FT-7900). My question is what sort of antenna is required? Currently I have a scanner with a Larsen tri-band antenna on a lip mount and a commercial UHF radio with a 1/4 wave antenna on a lip mount. I'd like to leave both in place and add the new dual-band, but don't want to add another antenna. Do I need to diplex, or can I splice into the tri-band antenna? If I need to do either what is the suggestion for the components needed to do that? Is it sufficient to add a BNC "T" to the scanner line to use the dual band radio?
 

Sarge43

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you really will need another dual band antenna. One diplexer won't do it. BNC T is out of the question. Think what will happen when you transmit right into another radio. Also you want your ham dual band at 19" away others.
 

lexmedic157

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This is what the trunk looks like currently:



Forgive my ignorance with this. So there isn't a way to split the dual band antenna for use in both applications? Guess I don't know what can happen if I transmit with a scanner in-line. How is it possible to run two separate radios into one antenna? No sarcasm intended, looking for an education.
 

Sarge43

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you'd blow out your scanner real quick and maybe fry the transmitter too. I'd add a dual band in the center there and spread the other two out to the edges. What UHF radio are you using, GMRS or something?
 

gewecke

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Try a Larsen NMO 2/70 trunk mount for your dual band radio. They work quite well!
n9zas
 
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A diplexer allows 2 radios of different bands to use a single dual-band antenna. A diplexer is actually a filter that passes 2 spearate band of frequencies such as 140-155 MHz and 440-460 MHz then combines them to a single port that the antenna coax connects to, not two radios in the same band.
 

lexmedic157

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you'd blow out your scanner real quick and maybe fry the transmitter too. I'd add a dual band in the center there and spread the other two out to the edges. What UHF radio are you using, GMRS or something?
The UHF is local emergency services radio
 

gewecke

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The ideal way is a separate antenna for each band that you operate,but I know that's not always feasible depending on what type of vehicle you're using.
n9zas
 

lexmedic157

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Hmm, not really wanting to add more antennas. Any other options? Any sort of device that prevents back-feed into the scanner by transmitting on the dual-band? Just throwing it out there...
 

elk2370bruce

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Hmm, not really wanting to add more antennas. Any other options? Any sort of device that prevents back-feed into the scanner by transmitting on the dual-band? Just throwing it out there...
That level of filtering would take a pretty thick prayer rug and only thing that you'll be throwing out (realistically) is the fried/scrambled scanner.

The Yaesu 7900/7800 was designed with a dual band antenna in mind (only one SO239 on the back). There are a number of dual band antennas out there from Diamond, Optic, etc that will do the job nicely and for a reasonable price.
 

lexmedic157

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So is it feasible to solely use the Larsen tri-band antenna for the Yaesu? Any suggestion for a hidden scanner antenna?
 

robertmac

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Amateur radio?

Is this a question about using the 7900 in band with a dual/tri band antenna? Or is it a question about using the 7900 illegally out of band for UHF coms.? If so, a better answer might be obtained by moving this question from amateur related topics to the Antenna/Coax or some other appropriate thread
 

lexmedic157

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Well maybe a move to antenna/coax is in order, but I'm asking for a way to use a scanner, an amateur radio, and a UHF radio attached to a tri-band antenna and a 1/4 wave UHF. (if it's possible) Not sure what illegally out of band for UHF coms means, but I am not attempting to do anything illegal. Maybe just a FT-2800 or an IC-2200?
 

SCPD

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Third antenna

Hmm, not really wanting to add more antennas. Any other options? Any sort of device that prevents back-feed into the scanner by transmitting on the dual-band? Just throwing it out there...
Myself, I would just go with a dual band third antenna, rather than spend money on trying to do what you want to do. You already got two antennas on the deck, what the difference between two and three, as far as ascetically pleasing to the eye? The other option is to decide if you really need what both of whatever the existing two antennas are hooked to. Put the dual-bander in the middle, separate all three by a foot and you'll be fine. I know folks will say that's not far enough apart, but from personal experience, a foot apart is fine.
I run a Larsen dual band, a UHF 5/8, and a 10 meter antenna all about a foot apart on top of a SUV, I get out just fine on all three, and the ham and UHF radio don’t affect each other’s receive.
Heck, look at the top of your typical TV station van. Separation?
 
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stevelton

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Ive seen people use a T inline with their car stereo antenna to have the scanner receive off of that. You lose A LOT a signal that way, but you would not add more antennas. Any way you go, if you try to combine any antenna, you are going to lose signal strength on what ever is combined. But choose the 2 most important radios, and and maybe have the scanner hooked to one of those suction cup wire antennas that go along the top of the inside windshield/headliner.

Doing what you are wanting to do, you ARE going to lose signal on something, you just need to decide which ones are more important.
Steven
 
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