Yet another scanner desense discussion

[jswagchoochoo]

I've been lightly dabbling in ham radio for a couple months and the first thing I got was 30 dollar analog Uniden scanner off of eBay. Since then, I got some other antennas, built a J-pole, and have a little Baofung HT I've been using with chirpw. Probably going to get a digital trunking scanner when income tax hits.

I have a magnet mounted whip antenna for my car coming this week and I'm planning to place it directly on the top of my little hatchback. I get that keying up the HT (even at just 2 watts) right next to the scanner is a good way to eat the top end of the scanner. I'm now wondering though if near by emergency vehicles could be an issue, especially when I get a less disposable scanner. My theory goes like this:

My car is 5.475 feet wide, meaning the antenna will be placed 2.75 feet from either side, and about 5 effective feet (accounting for an 8 foot lane) from the vehicle next to it. If I'm sitting at a long traffic light, right next to an active patrol car or ambulance (The threads I read said vertical separation counts for more than horizontal, so the ambulance is probably a non issue.), and they key up their transceiver, am I going to be a risk for blowing out the scanner?

I'm not sure of the wattage of standard police transmitters, but I feel like 7 feet (5 feet to their car, plus two to get to the antenna) should be safe enough.

 

[sparklehorse]

I'm not an "expert", but I sure don't worry about this issue. I think it would take a lot more RF energy to actually damage your radio than you would get from a nearby vehicle. If this fairly common scenario you described was frying scanners left and right you would be seeing threads on this topic here at RR every day. Note that when you see the word 'desense' used around here it is generally in reference to strong RF energy causing a radio to temporarily go "deaf" to some degree. It is not a situation that causes permanent damage.

 

[jhal94]

While I can't vouch for your scanner, I have my pro-106 on the same belt as my XTS2500 doing security jobs and it has held up fine even with heavy duty cycles. They were right next to each other in holsters but I figured that was pushing it so I moved the scanner and the radio to the left and right sides of my back. I think your scanner will be fine, if you want you can try and find a filter for the 806-824 band

 

[MTS2000des]

as long as you aren't directly coupling the output of your portable into your scanner, it should not get murdered to the point of blowing out the front end.

Now in band desense may be a problem, for example, if you are listening to a VHF high band signal and key up on 2 meters, your scanner my overload and lose the signal or produce some strange mix of sounds.

Some scanners are better than others. I have a Realistic Pro-2020 from 1983 (Japanese built by GRE?) that has a bullet proof front end. I can listen to the output of a 440 repeater and talk on a portable in the same room less than 3 feet away and still hear the output of the repeater, albeit the white noise increases about 8 db.

The BC-996T I have does not have as good a front end. Keying up a portable in the same room on the same band kills whatever I'm listening to unless it's a very strong local carrier.

But none of my scanners have ever been damaged. It's a good idea to mount antennas at as far apart as practical, but at minimum a 1/4 wave apart on the operating band.

 

[jswagchoochoo]

take a lot more RF energy to actually damage your radio than you would get from a nearby vehicle. If this fairly common scenario you described was frying scanners left and right you would be seeing threads on this topic here at RR every day.

I kinda figured as much. Even bumper to bumper I still would always have at least a clear radius of 5 feet from the antenna proper, which seems like more than enough space, at least for mobile units. A quick search across eBay shows that most mobile transceivers (Naturally, the ones I've seen are all Ham variety, but given exposure regulations and such, I can't see even police ones going to far above that.) produce around 60 watts at maximum. So nothing to worry about.

Thanks for the clarification.

 

[Voyager]

Your antenna will be at least 3 feet from any other antennas, and will likely be 6 to 10 feet away, so you should be fine. You will have densense, but you won't cause any damage to your scanner.

Mobiles are usually 25W to 110W with those on systems being lower power - even lower than 25W.

 

[jswagchoochoo]

Now in band desense may be a problem, for example, if you are listening to a VHF high band signal and key up on 2 meters, your scanner may overload and lose the signal or produce some strange mix of sounds.
<SNIP>
It's a good idea to mount antennas at as far apart as practical, but at minimum a 1/4 wave apart on the operating band.

Good knowledge and I'll keep that in mind if/when I mount a transmit antenna in the car as well. VHF is my preferred band at this point, so 20 inches is more than feasible on my car's body. Anything on CB/10meters is out of the question though, since I only have 10 effective feet (Ignoring bumper lengths) from front to back.