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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 08-20-2010, 9:24 PM
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Default Antenna Spacing

I have a question. I have a VHF mobile antenna NMO mount on the truck. I recently added an 800 MHZ Magnetic Mounted Antenna To Use With My OEM State Radio...I have them right next to one another. Can anyone tell me if there is an exact spacing they need to be apart without interferring with one another? As of now the 800 is in front of the VHF and is like 6 inches from it. Is there a space apart they should be? Can anyone tell me?
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Old 08-20-2010, 10:00 PM
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a foot apart is a good safe distance, of course the further the better.
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Old 08-20-2010, 10:08 PM
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a foot apart is a good safe distance, of course the further the better.
I've only got so much room on top of a single cab pickup.. lol But I will move the other farther apart, That is what I wanted to know.. I'll take care of that asap. Thanks davidgcet... Appreciate That..
I got a foot, When I hit a foot and a half it started slanting and wouldn't set straight. But A foot is better than the 4 inches they were before. We'll try this.. thanks again!
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Old 08-21-2010, 1:39 PM
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For RX only antennas, what would anyone recommend for spacing between. I want to set up my 2M/70cm J-pole as far away as possible from my RX only, I know that much, but I have a 2M/70cm, CB, marine whips which are on RX only radios, plus plan to put something up for air, milair, satcom and something for 700-900 MHz. Essentially I would run all of these to some sort of switching box to make it easy for me to select the proper antenna for the band. I'm looking for suggestions on some proper spacing so they don't interfere whit one another. My mast will be approx. 25 ft in height once all is said and done. Any input would be greatly appreciated.
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Old 08-21-2010, 1:45 PM
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as far as RX only, they won't interfere with each other as long as they are not touching. your 2m should be out the top and then the rest put at least 5' below it. is this a mast pole or a tower? with a tower or wide pole, you wnat to stand them off from the face at least 18" to eliminate shading from the tower. or at least put them on the side facing your main listening area. shading is not as bad in RX as TX, but still can have a noticeable affect in some instances.
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Old 08-21-2010, 1:56 PM
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If you are only receiving, a few inches should do, but note that you may not get the same quality of coverage that you would on the side of the antenna that isn't blocked. The further is better.

Now for TXing, it depends what bands you are operating on. If you are on 2M and listening to the 15X.XXX MHz Public Safety band, you may have some problems. If you are on 2m and listening to the 800MHz Public Safety band, you shouldn't have a problem with even the highest power. It will depend on your output power also.
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Old 08-23-2010, 10:52 PM
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It's all a compromise. Just space them as far apart as you can. If you're transmitting, the roof of a vehicle simply doesn't have enough room to separate them enough to avoid some sort of interference. Just as an example, mountain top systems using split antennas can have interference between antennas spaced 50 feet apart, so forget that notion on a vehicle roof.

Just keep them as far apart as you can, and that's simply the best you can do.
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Old 08-24-2010, 11:37 PM
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Originally Posted by zz0468 View Post
It's all a compromise. Just space them as far apart as you can. If you're transmitting, the roof of a vehicle simply doesn't have enough room to separate them enough to avoid some sort of interference. Just as an example, mountain top systems using split antennas can have interference between antennas spaced 50 feet apart, so forget that notion on a vehicle roof.

Just keep them as far apart as you can, and that's simply the best you can do.
Having two on a vehicle roof and keeping them seperate, Still having interference between the two. Should I turn one off to transmit on the other to keep from blowing the recieve out on the other or will it be ok to have them both on? I've heard both.
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Old 08-26-2010, 2:06 AM
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Having two on a vehicle roof and keeping them seperate, Still having interference between the two. Should I turn one off to transmit on the other to keep from blowing the recieve out on the other or will it be ok to have them both on? I've heard both.
Turning one off to prevent blowing the receiver is not likely to help anything. The same high level of RF is entering the receiver, whether it's on or off. Some may argue that the receiver front end is more vulnerable when it's powered on, but I have never seen any evidence of that. The most likely reason to shut one down when transmitting on the other is keeping the receiver from squawking at you ever time you transmit.

I routinely run multiple radios and multiple antennas on my vehicles, at power levels up to 100 watts from 4 to 450 MHz, and NEVER have experienced any damage.
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Old 12-04-2014, 1:09 AM
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I'm going to resurrect this post...

I just purchased a car (2012 Ford Fusion) I have a Kenwood TK-8180 (450-470mhz, 30watts) for local and county systems, and a Uniden BCD996XT scanner used for monitoring state P25 system.

I had planned to mount the antennas on the trunk using "L" style brackets that i have from the old car: http://www.theantennafarm.com/catalo...3400-1869.html

I am using the Laird Tech TRAB Series Phantom antenna for the Kenwood: http://www.theantennafarm.com/catalo...eries-856.html

and the Laird Tech ASCANC antenna for the 996XT:
http://www.theantennafarm.com/catalo...canc-1049.html

I am curious as to the minimum antenna spacing i would need? Will mounting both antennas on either side of the trunk (driver and passenger sides) work?

Thanks for the help!
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Old 12-04-2014, 1:16 AM
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Quote:
Originally Posted by WQOC472 View Post
I'm going to resurrect this post...

I just purchased a car (2012 Ford Fusion) I have a Kenwood TK-8180 (450-470mhz, 30watts) for local and county systems, and a Uniden BCD996XT scanner used for monitoring state P25 system.

I had planned to mount the antennas on the trunk using "L" style brackets that i have from the old car: http://www.theantennafarm.com/catalo...3400-1869.html

I am using the Laird Tech TRAB Series Phantom antenna for the Kenwood: http://www.theantennafarm.com/catalo...eries-856.html

and the Laird Tech ASCANC antenna for the 996XT:
http://www.theantennafarm.com/catalo...canc-1049.html

I am curious as to the minimum antenna spacing i would need? Will mounting both antennas on either side of the trunk (driver and passenger sides) work?

Thanks for the help!
Best would be to mount one on the passenger side hood and the other on the driver's side trunk. That would maximize distance (and the passenger compartment would further help to isolate them). Next would be (based on the L bracket mounts you indicate) one on one side of the trunk and the other on the opposite side (giving the maximum distance from that limited space). Just make sure that your scanner isn't scanning any UHF frequencies when you transmit on the Kenwood.
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Old 12-04-2014, 1:27 AM
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Thanks for the input.

The scanner is only receiving on 800 to monitor my state's P25 system. I will experiment with one on the hood and the other on the trunk and see if it looks good. If not i will put them on either side of the trunk.

The UHF is mostly for monitoring as well. I transmit on the radio maybe once or twice a month at the most (i'm a volunteer, so it gets limited use on the TX side) its a RX only 90% of the time.
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Old 12-04-2014, 1:32 AM
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Quote:
Originally Posted by WQOC472 View Post
I am curious as to the minimum antenna spacing i would need? Will mounting both antennas on either side of the trunk (driver and passenger sides) work?
Here you would have to define what you mean by 'will it work?'.

If you mean no mutual interference, no. Almost no amount of spacing available on a vehicle will work, without some sort of extra help.

If you mean will the scanner be safe from damage from the transmitter? Yes, it should be fine. If the scanner makes rude noise at you when you transmit, turn it off.

The two radios will receive just fine and will hardly know the other exists.
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Old 12-04-2014, 1:36 AM
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Should be fine if you put one on each side of the trunk.
Quarter wave length is often used as a guide line for separation of antennas, however more is better. 30 watts on UHF shouldn't be a big deal.
I've got a 50 watt VHF radio and a 15 watt 800MHz radio with about 20 inches of spacing between the antennas with no issues and no de-sense.
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