how to prevent distroying multiple radios from RF power due to antennas close

fireboat61

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Hello to all, I am gathering information on understanding how to prevent from blowing up the receiver end of scanners and radios due to antennas being close. When I set up my first mobile shack up, I blow out the front end of my scanner because of transmitting 50 watts from a mobile which antenna was to close to the other antenna.

1. is there anything sold that can prevent this?
2. what is distance at a minimum should antennas be apart.
3. Is there an acceptable wattage that will maximize performance but prevent that damage ? ( 5w,10w,25W ect)

I drive a ford f250 4 door.

I am new to understanding theory in regards to distance needed or RF energy,so please be gentle, LOL, seriously though if some of these questions have the simplest answer i am learning.
 

mmckenna

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I am new to understanding theory in regards to distance needed or RF energy,so please be gentle, LOL, seriously though if some of these questions have the simplest answer i am learning.

There's a lot to this. It's not as easy as having a rule about what will work and what won't. Just too many variables.

Best approach I've heard is to set up your transceiver on one antenna, and hook up a watt meter with a dummy load on the other antennas. Transmit and see how much power is getting coupled to the other antennas. 1/4 watt seems to be safe for most reputable radios. Obviously less is better.
There's two issues you need to be concerned about:
1. Physical damage. I've run across this on some radios when there wasn't enough spacing between antennas. It can damage the front end and make the radio deaf.
2. Desensitization. This won't damage the radio, but will cause it to not receive while another radio is transmitting.

Avoiding physical damage is key. If you are using the radios in a public safety type application where it's important to hear all radio traffic (fire ground….), avoiding desensitization would be really important. You'll need more spacing to reduce desense.

Hello to all, I am gathering information on understanding how to prevent from blowing up
1. is there anything sold that can prevent this?
Sufficient antenna spacing. Filtering. Physically disconnecting feed lines when other radios are transmitting.
-Sufficient antenna spacing will vary from radio to radio, antenna to antenna, coaxial cable type, coupling between antennas, etc.
-Filtering can be used to filter out RF from out of band. This is going to be an issue on scanners, if you are running multiple radios on the same bands, or multiband radios.
-Physical disconnect. CHP did, or used to do this, they had a box that would couple the scanner into the low band antenna and let it receive through that antenna. When the low band radio was transmitting, the scanner antenna connection would be disconnected. Sort of like an old T/R switch with one port terminated.

2. what is distance at a minimum should antennas be apart.
There's no set rule, as it's going to depend on the following (plus probably a few I'm forgetting:
-Power level of transmitter. What works for a 50 watt radio may not be sufficient for a 100 watt radio.
-Coupling between antennas. If one antenna is on the vehicle roof, and the other is down on the fender, there will be less coupling compared to them both being on the roof.
-Filtering inside the radio. A VHF only radio is going to have front end filters that are going to reduce the amount of out of band RF that gets to the receiver. In this case, a nearby UHF transceiver may have little impact on the VHF Radio.
-Feed line length. More feedline, more loss, less RF getting into the receiver.
-Antenna gain. More gain, more power getting in/out of radios.

-If life safety reasons dictate you be able to hear all traffic on all radios, then you need to consider not only physical damage, but also desense.


3. Is there an acceptable wattage that will maximize performance but prevent that damage ? ( 5w,10w,25W ect)
No. More spacing + less power is going to help. But there's not one power level that will fix this. It all depends on how much power is coupled into the antenna, feedline and to the receiver.

I drive a ford f250 4 door.
So, you have some real estate to work with. It takes some planning, but you need to consider the following:
Antennas need ground plane under them to work to their full potential. Figure out how much ground plane you need for each one (1/4 wave length in all directions under the antenna is ideal), and start figuring out your antenna placement.
Low band/CB is never going to have a perfect ground plane on a road legal vehicle.
Since feed line losses go up as it gets longer, and as it goes up in frequency, you can play around with your antenna placement, keep the higher frequency stuff on shorter coax runs, and the lower frequency stuff on longer runs.
If you are using cheap hobby radios and/or scanners, figure that they will have little or no filtering on the front ends, so they'll need more isolation.

If you are really pressed for space, and are running single band radios, you could try some band pass filters.

Testing all this out before drilling holes would be a good idea. A good use for a magnetic mount antenna is to use it for testing temporary installs. Use an accurate watt meter that will read down into the sub-1 watt range and a dummy load to see how much energy is getting coupled into the other antennas.
 

KevinC

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Best approach I've heard is to set up your transceiver on one antenna, and hook up a watt meter with a dummy load on the other antennas. Transmit and see how much power is getting coupled to the other antennas. 1/4 watt seems to be safe for most reputable radios. Obviously less is better.
This is what I do. Most of my test gear will handle less than +30, so I use +25 as my safe limit...which is pretty darn close to a 1/4 watt.
 

prcguy

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Put this limiter in front of your scanners and it will protect them very well. This will handle up to 10 watts coming down the coax and limit that that to less than 100 miliwatts to the receiver. It also has very low insertion loss.


I just bought one, better hurry before I buy the rest!
 

Ubbe

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Get that limiter that prcguy recommend for each scanner as 10W from a scanner antenna are probably the most anyone will encounter. There's also another external box that I've seen that sells for $50 that are probably just the same two diodes that most scanners have as protection in their front end but at a higher current rating at 1 ampere which is 50 watt or +47dBm. It will limit the voltage to 1,5volt which is 50mW or +17dBm into 50 ohm. Scannermaster sells Stridsbergs limiter at a $120 price that also has the same type of performance as two protection diodes.

MiniCircuit power limiter

Stridsberg limiter

/Ubbe
 

9Track

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Put this limiter in front of your scanners and it will protect them very well. This will handle up to 10 watts coming down the coax and limit that that to less than 100 miliwatts to the receiver. It also has very low insertion loss.


I just bought one, better hurry before I buy the rest!
Does something like this get installed on the antenna, or at the radio?

As in:

Antenna>RF Limiter>Coax>Radio

or

Antenna>Coax>RF Limiter>Radio

Thanks.
 

9Track

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Also, this would require an adapter, too, right? N-connector>SMA or PL-239/259>SMA?
 

WB9YBM

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Hello to all, I am gathering information on understanding how to prevent from blowing up the receiver end of scanners and radios due to antennas being close. When I set up my first mobile shack up, I blow out the front end of my scanner because of transmitting 50 watts from a mobile which antenna was to close to the other antenna.
I've seen a lot of god answers already posted--agree with most--but too wordy. Short version: if in doubt, disconnect the scanner antenna while transmitting with the other radios. That's it!
 

jonwienke

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Definitely do the coupling test with the watt meter and dummy load. You could possibly perform a similar test with a VNA, with the out port connected to the TX antenna and the in port connected to the RX antenna. But you still should use a watt meter to know the actual input power to run through the dB calculation.

1/4 watt seems to be safe for Uniden scanners (at least it hasn't fried mine), but since there doesn't seem to be a published specification for maximum safe input level, so I don't recommend pushing that level constantly.
 
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