Wideband receive & V-UHF transmit antenna separation

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NM9X

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I've been reading on our forum about how far my wideband scanner antenna should be separated from my transmit antenna and it seems like the answer is "as far apart as possible" but not any closer than about 36". Both antennas are mag mount and I measure the max output of my radio at 82 watts on VHF 2m. My transmit antenna is 5/8 wave dual band for UHF/VHF. I have my antennas separated about 38" right now and have not noticed any scanner problems yet. I have a BCD436HP and an old BCT8 and obviously don't want to do any lasting damage to them. Am I looking for trouble with this setup?
 

prcguy

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Its almost impossible to predict what antenna spacing and power level is the maximum your scanner can handle without damage, but you can predict the power level it sees.

Your 82w transmitter is 19.13dBw and at 146MHz, the path loss at 1m using 0dB gain antennas is 15.72dB loss, leaving 3.48dBw or 2.19w into your scanner, not counting coax loss, etc.

Since this is in the near field, calculations can be off quite a bit in either direction and this is just to get an idea of how much power can be coupled between to close spaced antennas. I used 0dB gain antennas in the calculation and if your antennas have any gain, the coupling will go up by that much.

If you double the distance between the antennas, the coupling will go down by 6dB or four times, but at your spacing you are probably still in the near field where RF behavior is difficult to calculate. You could also put a wattmeter and load on your scanners coax and directly measure what power it sees when you transmit. That is an absolute measurement you can count on.

I would not want to subject my scanner to several watts at its input connector, especially when its receiving in the VHF band and its tracking filters would tuned there.
prcguy

I've been reading on our forum about how far my wideband scanner antenna should be separated from my transmit antenna and it seems like the answer is "as far apart as possible" but not any closer than about 36". Both antennas are mag mount and I measure the max output of my radio at 82 watts on VHF 2m. My transmit antenna is 5/8 wave dual band for UHF/VHF. I have my antennas separated about 38" right now and have not noticed any scanner problems yet. I have a BCD436HP and an old BCT8 and obviously don't want to do any lasting damage to them. Am I looking for trouble with this setup?
 

jonwienke

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If your antennas are mag mount, it shouldn't be that difficult to relocate them so they are farther apart.

And most mag mount antennas aren't rated to handle 82W. If you want to run that kind of power, you should probably look at a more permanent mount.
 

prcguy

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I think I'll ask Whistler Wendy if they can put a rating on their scanners for maximum power input before damage. That along with a simple test of connecting a wattmeter and load on your scanner antenna cable would go a long way in answering this question.
prcguy
 

NM9X

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Its almost impossible to predict what antenna spacing and power level is the maximum your scanner can handle without damage, but you can predict the power level it sees.

Your 82w transmitter is 19.13dBw and at 146MHz, the path loss at 1m using 0dB gain antennas is 15.72dB loss, leaving 3.48dBw or 2.19w into your scanner, not counting coax loss, etc.

Since this is in the near field, calculations can be off quite a bit in either direction and this is just to get an idea of how much power can be coupled between to close spaced antennas. I used 0dB gain antennas in the calculation and if your antennas have any gain, the coupling will go up by that much.

If you double the distance between the antennas, the coupling will go down by 6dB or four times, but at your spacing you are probably still in the near field where RF behavior is difficult to calculate. You could also put a wattmeter and load on your scanners coax and directly measure what power it sees when you transmit. That is an absolute measurement you can count on.

I would not want to subject my scanner to several watts at its input connector, especially when its receiving in the VHF band and its tracking filters would tuned there.
prcguy
I didn't get a reading on the wattmeter in line between the scanner and antenna. Would there be a greater reduction in coupling by lowering the scanner antenna so that its top is below the bottom of the transmit antenna? Thanks for the helpful response!
 

prcguy

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Did you have a load on the wattmeter to absorb the energy or were you using the scanner as the load? Can your wattmeter read down to a watt or so?

Height separation or shadowing one antenna from the other is more isolation and should help, but without knowing what point your scanner will be damaged its impossible to say how much isolation you need.
prcguy


I didn't get a reading on the wattmeter in line between the scanner and antenna. Would there be a greater reduction in coupling by lowering the scanner antenna so that its top is below the bottom of the transmit antenna? Thanks for the helpful response!
 

NM9X

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Did you have a load on the wattmeter to absorb the energy or were you using the scanner as the load? Can your wattmeter read down to a watt or so?

Height separation or shadowing one antenna from the other is more isolation and should help, but without knowing what point your scanner will be damaged its impossible to say how much isolation you need.
prcguy
Oh I put the meter in line with the scanner and antenna. My meter measures transmitter output watts, reflected watts and swr. It reads to tenths of a watt. Maybe not the right meter to measure what I'm trying to in this situation.
 

jonwienke

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Connect a dummy load to the meter, not your scanner. You're trying to determine if the coupled RF is safe for the scanner.
 

prcguy

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If you had it connected to the scanner antenna on one side of the meter and the scanner on the other, it can give you an indication of what's going on. But you have to plug the scanner antenna into the transmitter side of the wattmeter and the scanner into the antenna side of the meter, otherwise it will only read the small amount of reflected power back from the scanner.

Since we don't know if the scanner shows up as a 50 ohm impedance on its antenna input at the frequency your measuring, its best to use the wattmeter with a load. In that case the scanner antenna still plugs into the antenna side of the meter and the load goes on the radio side of he meter.
prcguy

Oh I put the meter in line with the scanner and antenna. My meter measures transmitter output watts, reflected watts and swr. It reads to tenths of a watt. Maybe not the right meter to measure what I'm trying to in this situation.
 

NM9X

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If you had it connected to the scanner antenna on one side of the meter and the scanner on the other, it can give you an indication of what's going on. But you have to plug the scanner antenna into the transmitter side of the wattmeter and the scanner into the antenna side of the meter, otherwise it will only read the small amount of reflected power back from the scanner.

Since we don't know if the scanner shows up as a 50 ohm impedance on its antenna input at the frequency your measuring, its best to use the wattmeter with a load. In that case the scanner antenna still plugs into the antenna side of the meter and the load goes on the radio side of he meter.
prcguy
Yes I had plugged the antenna into the tx side of the meter since that would be the source of rf power and the scanner into the antenna side. While the scanner was on when I keyed the mic, I did not have the scanner set to receive the same frequency as my radio was transmitting. The meter did not register a power reading.
 

prcguy

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If you did what you said and your meter will read down to tenths of a watt and no power was measured, then it looks fairly safe to transmit with your high power 2m rig with the scanner going.
prcguy

Yes I had plugged the antenna into the tx side of the meter since that would be the source of rf power and the scanner into the antenna side. While the scanner was on when I keyed the mic, I did not have the scanner set to receive the same frequency as my radio was transmitting. The meter did not register a power reading.
 

NM9X

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If you did what you said and your meter will read down to tenths of a watt and no power was measured, then it looks fairly safe to transmit with your high power 2m rig with the scanner going.
prcguy
I'll experiment with a dummy load and different antenna locations and see if I get results. I generally only use 10W and have been disconnecting the scanner when using the high output. Thanks again prcguy for the ideas and calculations.
 

jonwienke

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I did some testing this morning. I have 3 antennas on my vehicle, a CB antenna mounted in the middle of my truck cab, and glass mount dual-band and scanner antennas on the rear cab window on the left and right sides. The glass-mount antennas are about 36" apart from each other, and about 30" from the CB antenna.

I put a watt meter with a dummy load on the scanner antenna, and tried keying up the radios. With the CB, I got nothing on the meter--no needle movement, even with the radio power turned up to 60 watts. I presume this is because the scanner antenna is not tuned for low band.

Pretty much the same thing with the dual-band radio on UHF--very slight needle movement, but not enough for a meaningful reading.

But on VHF, I got just under 1/2 watt hitting the scanner with the radio TXing 25 watts.I presume this is because the near field is larger with VHF than UHF.
 

jonwienke

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While the scanner was on when I keyed the mic, I did not have the scanner set to receive the same frequency as my radio was transmitting.
The frequency your scanner is set to is irrelevant--the same power level is hitting the scanner RF amp regardless. If you're concerned about the power level hitting your scanner, use a dummy load until you've actually tested.

And test on multiple bands if the TX radio supports it. Power level may be safe on one freq, but not on another.
 

NM9X

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The frequency your scanner is set to is irrelevant--the same power level is hitting the scanner RF amp regardless. If you're concerned about the power level hitting your scanner, use a dummy load until you've actually tested.

And test on multiple bands if the TX radio supports it. Power level may be safe on one freq, but not on another.
I'll pick up a dummy load and test. Thanks for sharing your results.

Do you think a 5/8 wave and a 1/4 wave antenna would couple with the scanner antenna differently assuming same distances, power outputs, etc.?
 

jonwienke

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There are too many variables to speculate. As I noted, it depends on the antenna designs, their relative positions, the size and shape of metal objects in the vicinity, and the exact frequency being transmitted. Get a dummy load and test. The 1/2-watt RF feedback hasn't damaged my 436 scanner despite repeated keyups, but it's honestly more than I expected. I don't plan on increasing VHF TX power without redoing the antenna configuration.
 

Ubbe

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The frequency your scanner is set to is irrelevant--the same power level is hitting the scanner RF amp regardless.
I include an example of a scanner front-end, this one from a BC780. There is protective diods that limits the voltage entering the amplifier and it will be so low that it cannot hurt the scanner, if the diods doesn't blow.

There's bandpass filters before the first active amplifier and makes it frequency dependent to interferencies.

/Ubbe

 

jonwienke

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None of that changes based on the frequency the scanner is tuned to.
 

jonwienke

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In other words, the scanner may be more susceptible to being damaged by frequency X than frequency Y, but the voltage at which damage happens at frequency x will not change based on the frequency the scanner is tuned to.

Specifically, the protection diodes are upstream of the bandpass filter(s), and therefore will blow at a constant input voltage, regardless of what happens to the signal downstream.
 

prcguy

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Its possible that some scanners don't have the same diode protection. If they don't have diode protection and they do have switched or tracking filters, then the frequency the scanner is on would have an impact on potential damage. If you are transmitting with high power on VHF but the scanner is tuned to UHF and there is a band pass filter in the front end, it could withstand a lot more input level compared to the scanner tuned to VHF.
prcguy

In other words, the scanner may be more susceptible to being damaged by frequency X than frequency Y, but the voltage at which damage happens at frequency x will not change based on the frequency the scanner is tuned to.

Specifically, the protection diodes are upstream of the bandpass filter(s), and therefore will blow at a constant input voltage, regardless of what happens to the signal downstream.
 
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