Transmitting near scanner antenna

SquierStrat

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Hello. Was wondering what everyone does to protect their scanner from harm, when it's antenna is right next to an antenna that transmits? My antennas would be about 20 feet apart, and the transmitting antenna transmits at 1000 watts at times. Does simply keeping the scanner turned off keep it safe? If i wanted to keep it turned on, what needs to be done to protect it?
 

Hit_Factor

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Your scanner antenna is much further away from my KW transmitter.

I put a HP rf limiter ( eBay) on my scanner antenna, before it goes into a stridesburg multi coulpler.
 

KC4ASF

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Hello. Was wondering what everyone does to protect their scanner from harm, when it's antenna is right next to an antenna that transmits? My antennas would be about 20 feet apart, and the transmitting antenna transmits at 1000 watts at times. Does simply keeping the scanner turned off keep it safe? If i wanted to keep it turned on, what needs to be done to protect it?
IMHO your scanner will be useless whenever the transmitting is going on. So just turn it off..
 

mmckenna

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Hello. Was wondering what everyone does to protect their scanner from harm, when it's antenna is right next to an antenna that transmits? My antennas would be about 20 feet apart, and the transmitting antenna transmits at 1000 watts at times. Does simply keeping the scanner turned off keep it safe? If i wanted to keep it turned on, what needs to be done to protect it?
Turning the scanner off does absolutely nothing to protect it. Turning the power off only disconnects the power. It does not disconnect the antenna from the receiver. Any high level RF will still get to the soft expensive bits on the inside.

You would need to do one of the following:
1. Separate the antennas. Exactly how much would depend on antenna gain, feed line losses, how much coupling between the antennas, and how much RF your scanner will tolerate before sustaining damage. I don't think any scanner manufacturer specifies how much RF power the receiver will tolerate.
2. Disconnect the antenna from the scanner, either manually, or using a T/R switch.
3. Filters designed to reduce the TX signal below the point the scanner would be damaged.
 

jonwienke

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Get a RF power meter and a dummy load, and attach the dummy load to the output side of the meter.

Connect the input of the meter to the scanner antenna lead, key up your transmitter, and see what kind of power reading you get on the meter. I've hit my Uniden scanners with 250mW without any ill effects, but anything more than that is probably not safe. There are too many variables to accurately guess how much RF is feeding into one antenna from another; measure it, and then you'll know whether you need to worry or not.

You can use the same setup downstream from a limiter to test whether it's working correctly.
 

vagrant

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Not knowing bothered me, so I would disconnect the antenna at the scanners/receivers when I was transmitting. That got old really quick as I have too many feed lines. I also considered spacing at home and on my vehicle when installing, but not knowing still plagued me. I then did what jonwienke advised on everything at the house and on the vehicle. No problem as I stepped up the wattage, everything was okay and my spacing was correct. Regardless, I still installed several HP limiters (5086-7283) at home and in the vehicle, call it insurance.

I believe this was previously discussed, so a search should provide other experiences with some of them using 1000 watts at their QTH.
 

Ubbe

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Turning the scanner off does absolutely nothing to protect it. Turning the power off only disconnects the power. It does not disconnect the antenna from the receiver. Any high level RF will still get to the soft expensive bits on the inside.
Actually, turning the power off will disconnect the receiver from the antenna. There are band filters in a scanner that are selected using switching diodes. When the scanner are powered off all switch diodes are disconnecting all receiver circuits from the antenna. Most scanners have protection diodes at the antenna input that blows at higher input levels but are easy to get at, not covered in resin, and easy to replace if you can solder and are less than $1.

Try to keep the level into a scanner below +10dBm or something like that. Free space loss and Watt to dBm calculators:


/Ubbe
 

jonwienke

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Turning off the scanner doesn't disconnect the diodes, or the first RF preamp transistor or two, which are upstream of the band filters and are what get fried first. Do a search of people who actually fried their scanner with excessive RF, and you'll find it's not just diodes needing replacement.

And taking actual measurements of the RF hitting the scanner is always better than using a calculator fed with guesses and estimates.
 

WB9YBM

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Hello. Was wondering what everyone does to protect their scanner from harm, when it's antenna is right next to an antenna that transmits? caMy antennas would be about 20 feet apart, and the transmitting antenna transmits at 1000 watts at times. Does simply keeping the scanner turned off keep it safe? If i wanted to keep it turned on, what needs to be done to protect it?
I usually play it safe & unplug the scanner--and as additional insurance ground the scanner antenna...
 

captainmax1

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I keep it safe and unplug the antenna from the multicoupler when I'm going to transmit in my house or in my vehicle. I keep the multicoupler within easy reach and takes just a couple seconds to unplug a BNC connection. I fried one years ago with only 5 watts and antenna's were 8 foot apart. Plenty of stories in RR about other people doing the same thing.
 

Ubbe

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Do a search of people who actually fried their scanner with excessive RF, and you'll find it's not just diodes needing replacement.
Yes, correct. That's what's going to happen when you have a scanner powered on. All the switching diodes are conduction and let the RF signal pass all the way to the first transistor. It will get permanent damaged by a relative low power signal and loose some of it's gain and will go into intermod a lot easier. Always have your scanner powered off when not in use or having power transmitters active nearby.
I keep the multicoupler within easy reach and takes just a couple seconds to unplug a BNC connection. I fried one years ago with only 5 watts and antenna's were 8 foot apart. Plenty of stories in RR about other people doing the same thing.
They have absolutly no protection or bandfilters or any switching diodes that could stop the signal and it doesn't matter if a multicoupler are powered off. They have to be disconnected from the antenna to protect them from harmful signals or use a RF protection device between antenna and multicoupler.

If you have antennas at the same level, as on a car roof, you can use the calculator to estimate how much signal the other antenna receives. If they are at different hights, like on a mast pole at home, you have to add the amount of dB isolation it will add. It might not be much isolation as most antennas create side loobs and if you have a discone or dipole that are installed offset at the side of a mast it might be sitting right in that side loob if a vertical transmit antenna are sitting above it.



/Ubbe
 

jonwienke

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Yes, correct. That's what's going to happen when you have a scanner powered on. All the switching diodes are conduction and let the RF signal pass all the way to the first transistor. It will get permanent damaged by a relative low power signal and loose some of it's gain and will go into intermod a lot easier. Always have your scanner powered off when not in use or having power transmitters active nearby.
Powering off WILL NOT protect from excessive RF, because the diodes and transistor that fry are UPSTREAM of the band filters. Meaning they are exposed to the same RF regardless of the state of the band filters.
 

nd5y

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There are thousands of public safety vehicles and command post trailers with scanner antennas mounted a few feet from 50 to 100 W LMR antennas and you never hear of their scanners (or radios) being damaged.
 

jonwienke

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There are thousands of public safety vehicles and command post trailers with scanner antennas mounted a few feet from 50 to 100 W LMR antennas and you never hear of their scanners (or radios) being damaged.
That may be correct for radios (their receivers have to be able to withstand their own TX power), but if you search the forums, you'll find several people who fried the front end of their scanner having it near a TX antenna running 50W or more. IIRC one guy did it twice before realizing what the problem was.
 

mmckenna

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There are thousands of public safety vehicles and command post trailers with scanner antennas mounted a few feet from 50 to 100 W LMR antennas and you never hear of their scanners (or radios) being damaged.
A few feet likely isn't much of an issue.

However, CHP does (or at least did) run a relay box on their scanners to disconnect and ground when transmitting:
 

nd5y

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That may be correct for radios (their receivers have to be able to withstand their own TX power), but if you search the forums, you'll find several people who fried the front end of their scanner having it near a TX antenna running 50W or more. IIRC one guy did it twice before realizing what the problem was.
Explain to me why it never appears to be a problem with anybody but a few hobbyists.
 

mmckenna

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Explain to me why it never appears to be a problem with anybody but a few hobbyists.
"Hobbyists" is your answer.
Many of us know to keep proper antenna separation. Many do not. I've seen some weird installs, especially in parking lots at hamfests.

It's not a one size fits all thing. Individual radio models seem to have different tolerances. Different levels of coupling between antennas, different feed line lengths, different power levels.
 

mmckenna

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That's what I think. Back in the 80's I had a 2 meter Mirage amp in my SUV and roof mounted 2m, 220, 440, scanner antennas all less than about 2 1/2 feet apart. Nothing was ever damaged.
Often that's probably enough.
On the other hand, my dad was running a Yaesu 2 meter radio and a commercial UHF radio with about 18" of separation on top of a truck. The Yaesu didn't take kindly to a 35 watt UHF radio that close and it decided to stop receiving.
 

jonwienke

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Explain to me why it never appears to be a problem with anybody but a few hobbyists.
At least one of the posters was a firefighter with a 100W radio in his vehicle.
 
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