Antenna Spacing

[Andrew Pritchard]

I've got all my antennas mounted on the trunk lid and have never had a single instance with one blowing over. I've got two magmount antennas and my big ham antenna is a trunk lip mount, not one has ever blown off. Never had any problems with them being too close either, each is about a foot apart. (I've got 3 on the trunk)

 

[Jesse Risley]

Get your scanner/receiver antennas as far away from transmitting antennas as possible. You can desensitize your scanner/receiver ("blow" the front end) if you have it on while you are transmitting via ham/two-way radio from an antenna that's simply too close. Be careful.

 

[J West]

Space all two-way radio antennas at least 1/4w apart from each other, based on the lowest frequency in use. Below is a quick guide for spacing to give you a general idea:

VHF High (136-174 MHz) ~19 inch spacing
UHF (430-470 MHz) ~6 inch spacing
700/800 MHz ~3 inch spacing

When scanners are used in the same vehicle as two-way gear, get the scanner antennas as far away from the other antennas as you can.

In the case of CB antennas, other HF antennas and the like...It's impractical to try and maintain 1/4w spacing from other antennas, so get them as far away as you can. In my case, I've got a CB antenna spaced 19" from a VHF 1/4w antenna (hooked to a 110 watt radio) and there is no interaction between the two.

YMMV.

 

[Jesse Risley]

Here's my setup

I have a 99 Silverado Z-91 and a 2005 Impala as chase vehicles. You can see that even my antenna patters are a bit close for comfort, so I always turn off the scanner when transmitting just to be safe.

 

[David Drummond]

That's interesting. I've been blowing out 50watts for years less than 12" from my scanner antenna and have yet to blow one out. It blocks reception when transmitting, but it comes right back when you unkey. Maybe Uniden scanners are just good like that.

 

[Hans Schroeder]

I've had my 45w 2m antenna as close as 12" to my scanner antenna, and have never had issues, save for what David was describing above. The past several years, I've had the scanner antenna on the trunk, and the 2m on the roof though. I have had a boomerang near the back of the roof, and before it was permanent, it did fly off once. You'll have to ask Steve Miller (OK) what a flying boomerang looks like... It did hit the rear window (loud thud), but didn't break it. It also put a nice size dent in my rear quarter panel when it finally came down.

 

[Jesse Risley]

It's not that you're going to physically burn up or blow out the scanner, but transmitting too close to a receiving (scanner) antenna could overload your radio and desensitize it across the board....one of the "symptoms" of this problem would be major intermod in certain areas...nothing terriblty serious but extreme cases can render the scanner basically unusable without an attenuator. I would just recommend that you turn the scanner off while transmitting, at least to be on the safe side.

If your current mode of operations works find and you are satisfied then disregard my suggestions. Some people have no problems..but just beware it can happen if the two antenna are less than 1/4 wave apart at the least.

 

[david diehl]

That's interesting. I've been blowing out 50watts for years less than 12" from my scanner antenna and have yet to blow one out. It blocks reception when transmitting, but it comes right back when you unkey. Maybe Uniden scanners are just good like that.

The rason that the scanner is cutting out when you transmit your 2M radio is because the scanner is getting overloaded with the signal that is coming out of your VHF.
I am extreamly suprised that you have blown your scanner yet.

 

[David Drummond]

The rason that the scanner is cutting out when you transmit your 2M radio is because the scanner is getting overloaded with the signal that is coming out of your VHF.
I am extreamly suprised that you have blown your scanner yet.

umm, yeah I knew that. Point was that I've done it literally for YEARS with Uniden scanners and haven't blow one out yet. Why, I don't know, but it has never happened.

 

[Tom Stefanac]

It's not that you're going to physically burn up or blow out the scanner, but transmitting too close to a receiving (scanner) antenna could overload your radio and desensitize it across the board

umm, yeah I knew that. Point was that I've done it literally for YEARS with Uniden scanners and haven't blow one out yet. Why, I don't know, but it has never happened.

In the very first post I mentioned that of all the scanners, only the Uniden 250D (without APCO card) was zapped. The VHF side did not suffer that badly but the UHF side lost all sensitivity. A diode within the scanner between the BNC pin and pre-amp was cooked to crisp.

The thing is I had keyed up on VHF many times using 50+ watts and never killed the 250D, it was only when I keyed 50 watts on UHF (440 MHz) that I cooked the scanner. Desensing equipment temporarily was not the concern; the main concern is cooking it permanently.

Front end overload is an obvious problem and issue anytime, even adding an antenna with too much gain in a high noise environment is an issue just like putting an aftermarket 17 inch whip on a portable unit will often increase the front-end and with that noise will become more of an issue.

Getting back to the scanners after doing some research and making some calls I have discovered there are several Uniden scanners which are prone to being cooked due to the relationship that exists between the BNC feed pin and the diode within the radio before the pre-amp. Most don't have a problem unless you really use a lot of power (100+ watts) but the 250D, 246T, BC350A, BC72XLT, BR330T and BCT8 are the main ones to watch. Most of them are portable and are not built with overload in mind unlike the base/mobile models which more or less are.