Scanner disconnect on RF sense

[leightonstarwalt]

Hey guys,

I'm working on getting a mobile setup installed and had a question regarding my scanner.

I'll have three radios and my scanner.. 4 antennas total. Scanner has a dedicated antenna.

My question is, is there anything on the market that can disconnect the antenna from my scanner if I key up on my other radios? I've seen t/r switches, but they seem to be more for operating a receiver and transmitter on the same antenna.

I'd like for the scanner antenna to be disconnected if I transmit on any of the other radios.

I would prefer to be able to buy something pre-made, as I lack the tools to build much of anything from scratch. I'm aiming to have everything installed before I go on a road trip from California to Illinois on the 2nd week of September.

 

[RFI-EMI-GUY]

The Icom ICR-9000 has such a circuit built in to the HF antenna port. The manual can be downloaded on the internet.

You could use a TR switch with an RF sense. Not sure if that combo is commercially available.

The other method is a solid state RF limiter. Those are commercially available, but pricey. You could build one, with diodes, but it would be a potential for IMD distortion.



Sent from my SM-T350 using Tapatalk

 

[Ubbe]

What scanner is it? It probably already have protection circuits and you wouldn't have to worry about the scanner being damaged.
You do not care about the two other radios that has to take the RF power as well?

/Ubbe

 

[leightonstarwalt]

What scanner is it? It probably already have protection circuits and you wouldn't have to worry about the scanner being damaged.
You do not care about the two other radios that has to take the RF power as well?

/Ubbe

I's a Uniden 996P2.

I'm not really concerned about the other radios:

Motorola XPR4500 - UHF DMR
Yaesu FT-8800r - Dual Band FM 2/70

Third radio is TBD, but likely will be another Dual band for D-Star.

 

[jonwienke]

If each radio has its own antenna, there is a simple test to determine if you need to worry about isolation. Connect an RF power meter to the scanner antenna, key up each of the other radios in turn on whatever frequencies you use at max power, and observe the power reading you get feeding into the scanner antenna. I get about 500mW feeding into my scanner when I key up on VHF and that hasn't hurt anything, but I wouldn't want to hit it with anything more. On UHF, the needle hardly even budges.

 

[leightonstarwalt]

If each radio has its own antenna, there is a simple test to determine if you need to worry about isolation. Connect an RF power meter to the scanner antenna, key up each of the other radios in turn on whatever frequencies you use at max power, and observe the power reading you get feeding into the scanner antenna. I get about 500mW feeding into my scanner when I key up on VHF and that hasn't hurt anything, but I wouldn't want to hit it with anything more. On UHF, the needle hardly even budges.

You know, that's actually not a bad idea.

Just ordered a wattmeter.. I'll need to check SWR on all the antennas before I put them into service anyhow.

 

[jonwienke]

Just remember that the coupling between any given pair of antennas is frequency-dependent, so you need to test frequencies from each band you intend to transmit on with each radio.

 

[cmdrwill]

We get PD, Fire and other radios in the shop with smoked receiver frontends. So yes a real concern/problem.

" 500 miliwatts " may be too much.

 

[majoco]

A co-axial relay in each antenna lead to disconnect the radio powered by the 'press-to-talk' line in your only transmitter.

 

[jonwienke]

" 500 miliwatts " may be too much.

My 436 has survived that level of RF feedback numerous times with no damage or detectable loss of sensitivity. But that doesn't mean every scanner will survive that. It would be nice if manufacturers published a maximum safe input signal level spec, so that one could be confident knowing a safe signal threshold.

 

[jonwienke]

A co-axial relay in each antenna lead to disconnect the radio powered by the 'press-to-talk' line in your only transmitter.

Not practical if transmitting with multiple radios.

 

[majoco]

Not practical if transmitting with multiple radios.

Same principle applies - any PTT could kill all the inputs to the other receivers - any transceiver has Tx/Rx changeover relay - even energising more than one transmitter at a time - been there - done that.

 

[jonwienke]

But as the number of radios increases, the number of PTT interconnections needed to ensure every radio RF mutes every other radio on transmit increases at the rate of N * (N-1), N being the number of radios. The OPs setup would only require 9, because the scanner doesn't transmit, but that is still a ridiculous rat's nest of extra wiring for something that may not even be an issue, depending on test results.

 

[jonwienke]

And that also doesn't address the fact that most radios don't have an easily accessible PTT signal connection that could be used to drive multiple isolation relays.

 

[jim202]

Years ago, I had a GE MSTR II on VHF, that I smoked the front end FET on. Replaced it and it happened again. As it was the only radio in my truck at the time, I was scratching my head. Then the light came on. I had to drive right by a 150 foot fully steerable dish antenna that was working at 440 MHz. It was used for ionospheric sounding and measuring the layers of the atmosphere.

This antenna transmitter ran a bunch of power and in the near field, I would guess that there was still a bunch of power coming into my antenna. The road was within about 100 feet of the antenna.

Not wanting to continually need to be replacing the front end FET on the radio, I has several discussions with some other radio engineers. The consensus seemed to be that placing a couple of hot carrier diodes back to back going from the input of the FET to ground should resolve the problem. So I used a couple of 50 Ohm resistors to isolate the diodes from the input and the FET.

I put a 50 Ohm resistor from the gate of the FET to the diodes. A second resistor from the diodes to the input that use to go to the FET. Had to cut the circuit trace on the board to make this work.

After the mod, never lost the front end FET again. Didn't see any signal loss or intermod problems.

By the way, for those that might ask, I don't know what the exact power output of the radar transmitter was, but it was high. Used a whole, very large room of the building for the power supply and transmitter. Antenna was fed by some very large wave guide that a small person could crawl through. A very interesting place on top of a hill with several other research sites there.

 

[RFI-EMI-GUY]

Antenna specialists made a custom relay for CHP so that a scanner could share the two way radio antenna. They have since discontinued the practice because they have a separate UHF antenna for scanners.

CALIFORNIA HIGHWAY PATROL MOBILE RADIO EQUIPMENT 2001

http://www.wb6nvh.com/CHP/Rlybx01.jpg

 

[Ubbe]

I can't remember seeing any scanner schematic that didn't had protection diod on the receiver front end.
FET in the old days where prone to fail by static electricity. It only took a thunderstrike miles away to blow those FETs. It's much different today.

A parabol dish are extreamly directional and don't send out any dangerous RF levels outside of it's loob.
100 feet at 440MHz would attenuate 55dB. If 0.5W are concidered as a safe level, the transmitter needs to output 158kW in your direction. If two antennas on a car roof are cut to resonates on the same frequency and are 20 inches apart, it would take a 50W transmitter to produce 0.5W from the other antenna.

/Ubbe

 

[jonwienke]

The degree of coupling is frequency dependent. On lower frequencies, the near field is larger and you'll get more coupling even if the antennas are resonant at both the low and high frequencies (e.g. a typical dual-band UHF/VHF antenna). In my truck, TX @ 25W feeds 500mW to the scanner. On UHF, I can barely see the needle move at the same TX power.

Antennas are about 36 inches apart.

Theory will give you a general idea what to expect, but there is no substitute for testing.

 

[prcguy]

There are calculations for establishing the far field where propagation is stable and repeatable, but there is a seat of the pants calculation to get an idea of very close spaced antennas. Levels in the near field are very complex and difficult to predict but you can estimate roughly 22.5dB loss in the first wavelength from the antenna, then add maybe 6dB to that to get a rough worst case level.

The actual level could be much lower or a little higher and can change radically with just a few inches of change but at least you can see if the estimated levels are going to be dangerous to another radio with a close mounted antenna.
prcguy

The degree of coupling is frequency dependent. On lower frequencies, the near field is larger and you'll get more coupling even if the antennas are resonant at both the low and high frequencies (e.g. a typical dual-band UHF/VHF antenna). In my truck, TX @ 25W feeds 500mW to the scanner. On UHF, I can barely see the needle move at the same TX power.

Antennas are about 36 inches apart.

Theory will give you a general idea what to expect, but there is no substitute for testing.

 

[leightonstarwalt]

Holy moly. I didn't realize there were so many replies! Looks like I need to check back here more than just once a day.

The wattmeter should be here Saturday. That'll give me some time to get everything wired and install my Havis console that arrived today.

I'll be sure to post results of testing; although I'll only have one radio to test with for now; the Moto XPR4500.. I won't get the 8800 until I arrive in IL.