How close is TOO close

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AronDouglas

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I just want to make sure that I'm not going to burn out my scanner by having it next to (within 4 feet) of my CB (actualy CB and scanner are about 2 feet apart bu the antennas are with 4 feet of each other). I dont scan the CB range when I key up, but both CB and scanner run all the time. I know sometimes when I key up the scanner stops in the 400something band range (a public works channel), but I half expected it to do that. So should I invest in an RF shield for the scanner or is there anything I can really do other than not making it a habit of keying up with the scanner running.
 

kb8rvp

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I just want to make sure that I'm not going to burn out my scanner by having it next to (within 4 feet) of my CB (actualy CB and scanner are about 2 feet apart bu the antennas are with 4 feet of each other). I dont scan the CB range when I key up, but both CB and scanner run all the time. I know sometimes when I key up the scanner stops in the 400something band range (a public works channel), but I half expected it to do that. So should I invest in an RF shield for the scanner or is there anything I can really do other than not making it a habit of keying up with the scanner running.
Are we talking about a standard 4watt CB radio? Also are you using a desktop or handheld scanner? I have noticed most of the desktop scanners have some built in protection at the antenna jack inside the scanner but many handheld scanners do not... If it's a standard 4watt CB it shouldn't hurt the scanner but if your using a modified CB or using an amp putting out 150 watts then it could damage the scanner if it's that close.

Mike
 

wtp

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on or off

i was told by my electronics teacher years age that with solid state receivers it does not matter if it is on or off.
the problem is the amount of energy that the antenna receives. so turning it off and putting it right next to a transmitter will probably do it in. so the father you can get them apart the better.
 

jonwienke

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I just want to make sure that I'm not going to burn out my scanner by having it next to (within 4 feet) of my CB (actualy CB and scanner are about 2 feet apart bu the antennas are with 4 feet of each other).
It is possible to fry the scanner by having the scanner antenna too close to another antenna that is transmitting.

I dont scan the CB range when I key up
That doesn't matter. The energy hitting the scanner's RF amp from the antenna is the same regardless of the frequency the scanner is set to receive. It also doesn't matter whether the scanner is on or off.

or is there anything I can really do other than not making it a habit of keying up with the scanner running.
You have 2 options:

1. Move the antennas farther apart.
2. Disconnect the scanner antenna when transmitting with the CB.

With a 4-watt CB, a few feet of separation is probably enough. If it wasn't, you'd have already fried your scanner. But there have been several documented incidents where a scanner with an external antenna mounted on the same vehicle as a 50-75-watt radio have been damaged when the big radio keyed up. My 436HP survived being about 5 feet from a ham antenna on a pickup when a 75-watt radio keyed up at full power, without any apparent damage. IMO I got lucky because it was wearing the stock antenna at the time, and was below the transmitting antenna and out of the main emission path.

I don't know that there is any hard data about the size of the danger zone, and it would vary depending on the transmitting antenna, the scanner's receiving antenna, and whether there were any reflective objects in the vicinity. But I'd prefer not to volunteer a $500+ scanner for destructive testing.
 

AronDouglas

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I wanted to keep this as vague as possible hoping there was a general rule of thumb I could follow, but I guess some specifics are needed. I am scanning with a BCT15X and I have a Galaxy 595 modded by SnakeRadioCustoms, it can push 15-18 watts on AM and upwards of 20 watts on SB. I do plan on getting an amp some time soon (either a KL203 or a 175watt power cabinet). I will obviously take precautions when running an amp.

I can really move my scanner antenna since I drilled a hole through the roof to mount it :), so I'll just be careful when keying up.
 

k3cfc

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Are we talking about a standard 4watt CB radio? Also are you using a desktop or handheld scanner? I have noticed most of the desktop scanners have some built in protection at the antenna jack inside the scanner but many handheld scanners do not... If it's a standard 4watt CB it shouldn't hurt the scanner but if your using a modified CB or using an amp putting out 150 watts then it could damage the scanner if it's that close.

Mike
I run an antron 5 ft above my tower with my scantenna right below it. i run a 40 ch ssb with a 50 watt amp and have done so for 20 years and never had a problem. just this simple.
 

pinballwiz86

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I just want to make sure that I'm not going to burn out my scanner by having it next to (within 4 feet) of my CB (actualy CB and scanner are about 2 feet apart bu the antennas are with 4 feet of each other). I dont scan the CB range when I key up, but both CB and scanner run all the time. I know sometimes when I key up the scanner stops in the 400something band range (a public works channel), but I half expected it to do that. So should I invest in an RF shield for the scanner or is there anything I can really do other than not making it a habit of keying up with the scanner running.
4 watts at 4 feet? You should be okay.
 

canav844

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Option 1: Switch to a 4watt CB

Option 2:
Aron, what kind of vehicle do you drive?

Vertical separation may help, but that applies much more to a tower than a car, however the roof of a truck vs off the bed or hood may breakup the path so the antenna isn't taking a direct hit. There's one scanner for me that sees primarily 800mhz use for me, with it's expense if you're in that boat, maybe consider keeping to a duck antenna in the car and using the metal roof for separation (a rain cap for the mount on the roof may be cheaper than a new scanner).

You also need to take a look at the whole system; make sure that if the coax for one antenna goes up to the roof on the right, but the coax for the other antenna on the left, or go with expensive highly shielded coax (or do both). If they're run up together and not really well shielded, you may well be bleeding one into the other long before it gets to the antenna. But the more leaky RG-58 is also much easier to bend and route through cars tight spaces and odd shapes.

And also though, probably less so, consider placement of the radios in the vehicle; if the CB shop opened one of the radios up and cranked the power on it, their may be less or improperly restored shielding and that shielding may be too weak when the RF inside the box is beyond design limits. Not familiar with the amps you're looking at, but I can usually spot the guys running amps on the highway as I get a burst of static on the highway around their cab, even if they aren't talking (but take a look and you find the big antennas with the coil, and sometimes hear them come on and play their roger beeps on 19 for all to hear). Due to their illegal status some of the amp manufacturers put out crap products that cause such issues for others (not saying all cause I know some of it runs very clean, but it's something to be aware of, wrong choice and you may fry your scanner in much less time). And I want to be clear I'm not flaming, and think that if there was a 25-50 watt limit on CB, many of the shenanigans would be avoided and the playing field would be quite level; but there are a few more factors you may need to look into ruling out.

CB for me is more ears than mic so on my last car the CB was about 3 ft from my ham antenna and 5 foot from my scanner antenna, the cheap 4w CB didn't bother the other two, I could watch the RX signal needle jump on the CB when keying the 50 watt ham radio, never did fry it, but stopping by the next walmart or truck stop and throw in a $35 cobra or uniden wasn't a big deal to replace it for the mostly listening use. Just changed vehicles earlier this month, initially at least, I'm looking like it will be a minimum 6ft separation for just about everything, at least until I find an excuse to cultivate an antenna farm.

I know a lot of the commercial installers I've met use 1/4 wave separation as the absolute minimum rule of thumb, and that's usually with far more selective radios designed for commercial and public safety use, and generally in the 50 watt range. If it were a 4 watt CB, that would be so little power that it shouldn't be as much an issue. There can be some trial and error to eliminate the mixing, a few mag mounts and moving things around until the desired separation is there, then drilling holes is something to tuck away in your mind when it comes to the next vehicle.
 

AronDouglas

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Thanks for the replys everyone, its much appreciated.

@canav844, your first choice is not an option I'm afraid. This was a custom made radio as a gift and its staying with the truck. I cannot for the life of me find the video, but just look up SnakeRadioCustoms on YouTube as he post all his builds there.

The truck that its mounted in is a '98 dodge 3500. http://forums.radioreference.com/pi...0-w-bct15x-austin-spectra-mobile-antenna.html There is a BCT15X in place of the truck stereo and its antenna wire (the coax that comes with an Austin Spectra, think its micro line, low loss coax) runs behind the dash, up the left A-pillar, over the headliner and to the centrally mounted roof antenna (connected to a NMO connector).

The CB is mounted directly below it on a custom built laptop mount and it feeds two 5foot Wilson silverload top load fiberglass antennas mounted on a set of WestCoast mirrors (supported with medium weight springs). Each antenna is fed with equal lengths of dual shielded RG6 coax from the back of the radio (no splitter was used, I ran both coax lines into a single PL259 that plugs into the back of the radio). Each antenna has a dedicated ground wire that runs down and hard mounts to the truck frame.

The CB is used mostly for listening right now, but hopefully soon the other work trucks will have radios i them and the radio traffic should increase. I have heard nothing but good things about the KL 203 and the power cabinets as they put out the most 'true' power.

KL203 KL 203 P

175 power cabinet Magnum SX-175 Power Cabinet

I've seen both of these put on the 959 before and they put out some serious power. They will see seldom use however, but I want the power when its needed.
 

jonwienke

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I run an antron 5 ft above my tower with my scantenna right below it. i run a 40 ch ssb with a 50 watt amp and have done so for 20 years and never had a problem. just this simple.
Not really. Having the scanner antenna directly below the other antenna means it's out of the main radiation path, especially if the Antron is has a ground plane and the scanner antenna is under that. Having them side by side with <3ft separation would be a totally different kettle of fish.

If the scanner antenna is not resonant at CB frequencies, that will help.

Any vertical separation to get the antennas out of each others' gain patterns will help.

Physically separating the antennas as much as possible will help.

A high-pass filter on the scanner antenna line would eliminate any possibility of a problem, but at the cost of not being able to scan anything below the cutoff frequency of the filter. If you're going >100 watts this is your best bet to avoid damaging the scanner.
 

k3cfc

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Not really. Having the scanner antenna directly below the other antenna means it's out of the main radiation path, especially if the Antron is has a ground plane and the scanner antenna is under that. Having them side by side with <3ft separation would be a totally different kettle of fish.

If the scanner antenna is not resonant at CB frequencies, that will help.

Any vertical separation to get the antennas out of each others' gain patterns will help.

Physically separating the antennas as much as possible will help.

A high-pass filter on the scanner antenna line would eliminate any possibility of a problem, but at the cost of not being able to scan anything below the cutoff frequency of the filter. If you're going >100 watts this is your best bet to avoid damaging the scanner.
Okay I'll buy that for a dollar. on my van i run an i com 2200h 65 watts with a mag mount. less then three feet i have my ten meter.with a mag mount in the middle i have a uniden MR 8100 scanner. i have run this setup for more then five years with NO problems. just saying.
 
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