SDS100/SDS200: How close is too close ?

So reading about these things being damaged by being too close to a transmitter ,..I have had many radios & been to many events ,..races fire scenes etc ,..Was never really concerned about it ...Is there something about sdr type radios that make them more vulnerable ? How close is too close ? At an airshow flightline as the aircraft taxis a few feet away from you while transmitting ? at a fire scene next to the mobile command post ? at a marina with boats passing by & transmitting ? I have been in all theses situations , with non sdr type radios & never had any problems ,..yet now it seems to be something coming up ?

I think too close would be less than 3 feet, and maybe a few feet farther for a very higher powered transmitter. In the past I have run a 50 watt VHF transmitter with an antenna about 3 feet from my scanner antenna with no issue.

I don't have a rule of thumb for you, but it would depend on the strength of the transmission. you are fine being quite close to a low powered transmitter or radio, and it's only going to be an issue with something larger.

There's two things to be concerned about:
Too much RF getting into the radio and damaging the receiver.
Too much RF overloading the receiver and desensing it so it won't receive (non-damaging).

Often the concern is when someone mounts a scanner antenna too close to the antenna for a higher power mobile transceiver.
The issue is that the scanners have minimal filtering on the receivers and can easily let too much RF energy into the front end of the receiver. If the two mobile antennas are very close, it can damage the scanner. At minimum, it will cause the receiver to get overloaded when the transceiver is transmitting. Overloading will cause the scanner to not be able to hear anything while the transmitter is running.
Think of trying to listen to someone talking across the other side of a room, and at the same time someone is screaming in your ear.

For mobile applications, antenna separation is key. Not quite as big an issue with transceivers as they usually have some filtering that can protect them. Scanners, not so much.

But, walking around near a fire scene, not going to be an issue.

Keep in mind that if the scanner are tuned to a frequency that opens a filter that are in the same frequency range as the transmitter, the signalpower will go directly to the first amplifier transistor. If tuned to another frequency range or if scanner are turned off it will attenuate at least 30dB, 1000 times.

The power level decreases exponentially with the distance. 1 foot between transmit and scanner antenna will for sure give dangerously high levels but 3 feet should be safe at moderate transmit power levels. All scanners should have protection diods at the antenna input to stop any dangerous levels from being exposed to sensitive electronics. I have seen that Icom pcr1000 that earlier where popular with radio amateurs, which often have their high power transmit antennas in the same mast as their receive antennas, sometimes had their protection diods blown, together with some front end semiconductors. Normal scanner use shouldn't be a problem if it isn't a static discharge and the scanner lacks a DC path to ground with a bleeding resistor or coil or if a close by lightning strike creates a huge static voltage spike.

/Ubbe

Be carefull and don't mount your scanner and phone close together. A few years ago I burnt the receive out of a 396t because I had them mounted beside each other in the two center vents of my dash.

Steve
kb8rce

steelcty said:

Be carefull and don't mount your scanner and phone close together. A few years ago I burnt the receive out of a 396t because I had them mounted beside each other in the two center vents of my dash.

Steve
kb8rce

Click to expand...

Did it make your canner go completely deaf?

Yes, had to send it to Uniden for repair.

racing1 said:

Did it make your canner go completely deaf?

Click to expand...

Was it under warranty? Also was the weather stations also deaf?

No had to pay the flat rate labor fee, No weather,

There are filters made of discrete components in a scanner for each frequency band but they all end up at a common amplifier transistor. If that blows all bands stops working.

In US you have both 2-way radio and cellular sharing the same 800MHz band and when a scanner are tuned to the 700-900Mhz it opens the door for cellular phones transmission if they are operating in the low band. In cities cellular often use the 2GHz band. Out on the country side a cell phone probably use 800-900MHz most of the time and sometimes at full power, which is something like 2watt, no risk of permanent damage at a one foot distance between cell phone and scanner.

/Ubbe

I work as a dispatcher and had my radio go down. Had to use an handheld radio and didn't even thing about my scanner a few feet away. It didn't do any harm to my scanner but did flicker the monitors.