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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 09-28-2013, 10:05 AM
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Default GPS antenna location on a vehicle

On my F150 extended cab pick up truck I have 3 antennas for transmitting and receiving. One Amateur Radio dual band, VHF high band and a VHF low band. I am trying to decide where to mount 2 GPS antennas on the roof, how much will the transmitting antennas affect the GPS units? One GPS is an AVMap 6 and the other is for the GPS in an XPR5550. I plan to use Larsen NMO mounted antennas.

I am looking for comments, thanks!
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Old 09-28-2013, 10:32 AM
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I have not used the permanent mount GPS antennas on my vehicles, only the amplified mag mounts and all in one "hockey pucks". Using frequencies from HF through UHF at variable power levels and various antenna configurations. I have never noticed that transmitting has affected the receive on the GPS receivers in any way. I suppose if you get into 1.2 GHz high power you might swamp the front end on the GPSr, but for VHF you would have to have a pretty bad PA filter to put any spurs at 1.5GHz.
Long way of saying, Put em where they make the most sense ;-)

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Old 09-28-2013, 11:17 AM
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Should not have any interaction from VHF or UHF radios or antennas even if the GPS antenna is mounted very close. You could get a signal splitter for GPS antennas that passes the DC power so you would only need one antenna for both GPS receivers.
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Old 09-28-2013, 11:43 AM
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Right, many manufacturers make GPS/LMR antennas all in one package, or with the GPS antenna built into the antenna base. Many of the Mototrbo and APX radios have the GPS antenna built into the whip. Shouldn't be a problem for you.
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Old 09-28-2013, 10:21 PM
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You only need one antenna for VHF/UHF and GMRS and an antenna switch. The Comet CA 4X4 antenna is made for EMCOMM work and is very wide banded. You can adjust it so you have a 1.2 to1 SWR on VHF/UHF and you will still be way under 2 to 1 swr on GMRS.
I prefer using an antenna switch, no chance of putting power back into the front ends of the other radios. It gives you almost total isolation.
If necessary to monitor VHF/UHF and GMRS at the same time, get two of the comet antennas.
They have great range, about 3.8 db gain on 2 meter and of course more on UHF.
Look at the reviews on EHam, everyone who has one loves it. I have yet to find a better mobile antenna. Larson makes a great antenna, but the comet will out talk and out receive it and it's adjustable on VHF and UHF.
They run $60.00 to $70.00 but are worth every penny, I has a Comet SSB5 and a Larson before this one and the difference is like night and day, best mobile antenna out there.
They are made tough as nails and they fold over if necessary. I use a NMO mount in the middle of the roof. The NMO mount is worth a couple S units over a mag mount due to much better grounding to the vehicle body, just scrape down to the metal on the inside of the hole so the mount bites into the metal for a 100% connection. I get fantastic signal reports as to distance and strong signal with this antenna.
It has more great reports on EHam than any other antenna I saw, and the gain really helps, it will double the output power of your radio on VHF and UHF.

73's John
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Old 09-29-2013, 5:24 PM
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Yes, I have seen those antennas that include a GPs antenna as well as tx/rx. So I think as recommended to put them where it makes sense.

Thanks for the comments!
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Old 09-29-2013, 6:58 PM
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Right, you have some real estate to use on top of your truck, so make the most of it. Since you don't need a big ground plane around a GPS antenna, you can set them back near the rear of the cab where you likely wouldn't want to stick a transmitting antenna. Leave the wide open spaces on the roof for the antennas that need it.

I've got a GPS antenna on my work truck tied into my radio. I just stuck it up on the corner of the dashboard. Works well enough and didn't take much work to instal.
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Old 09-29-2013, 7:46 PM
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I plan to place them near the back of the cab so they will be less noticeable. I am running a FTM 350 AR with the GPS right on the back of the radio head, it is mounted to the right of the temperature controls on the dash and doesn't seem to have any problems picking up satellites.
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Old 09-30-2013, 12:53 AM
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A GPS antenna needs to 'see' the sky, meaning it should be unobstructed as much as possible. The 'shadow' a typical antenna makes isn't a biggy as far as the GPS antenna seeing the satellites. So, where can you put them that will give them a clear view of the sky? I have two on my car, one on the roof and one on the dash. Both do well enough so far as I can tell. Neither are affected by VHF/UHF or HF as far as I can tell. I've no experience with the GPS antenna being combined with another antenna, but I've seen them.
Have fun.
- 'Doc
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