BC355N Desense

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billyfromhill

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I have a BC355N installed in my vehicle. It is connected to a Larsen NMO/150/400/800 antenna on a NMO mount that's installed in my roof. I also have a Kenwood TK-790 radio using a 1/4 wave VHF antenna on an NMO mount that's installed about 24" in front of the scanner antenna. When I transmit with my TK-790, the scanner desenses and stops receiving until I stop transmitting unless the signal the scanner is receiving is extremely strong, in which case the signal gets all staticky. The scanner does this on both the low and high power settings (45W and 5W respectively) of my mobile. The coax for both radios runs together under the flooring of the passenger seat and up the B pillar to the roof.

I don't know why the scanner should be desensing, even at 5 watts. I thought 24" would be far enough apart as to not cause any issues. The scanner also receives very poorly on the 800 MHz band, to the point that another BC355N that I have in my apartment with back-of-set antenna receives said 800 MHz frequency better than my mobile setup. Could the scanner have a blown front end or could there be an issue with my antenna? Both radios were in my old vehicle but the antennas were even farther apart then they are now.
 

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kayn1n32008

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I have a BC355N installed in my vehicle. It is connected to a Larsen NMO/150/400/800 antenna on a NMO mount that's installed in my roof. I also have a Kenwood TK-790 radio using a 1/4 wave VHF antenna on an NMO mount that's installed about 24" in front of the scanner antenna. When I transmit with my TK-790, the scanner desenses and stops receiving until I stop transmitting unless the signal the scanner is receiving is extremely strong,..

I don't know why the scanner should be desensing, even at 5 watts. I thought 24" would be far enough apart as to not cause any issues.
The scanner has very little filtering to reject adjacent frequencies. Even if the antennas were at opposite ends of your vehicle, your scanner would likely still have desense, even transmitting a 5 watts.
 

k0aa

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I have a BC355N installed in my vehicle. It is connected to a Larsen NMO/150/400/800 antenna on a NMO mount that's installed in my roof. I also have a Kenwood TK-790 radio using a 1/4 wave VHF antenna on an NMO mount that's installed about 24" in front of the scanner antenna. When I transmit with my TK-790, the scanner desenses and stops receiving until I stop transmitting unless the signal the scanner is receiving is extremely strong, in which case the signal gets all staticky. The scanner does this on both the low and high power settings (45W and 5W respectively) of my mobile. The coax for both radios runs together under the flooring of the passenger seat and up the B pillar to the roof.

I don't know why the scanner should be desensing, even at 5 watts. I thought 24" would be far enough apart as to not cause any issues. The scanner also receives very poorly on the 800 MHz band, to the point that another BC355N that I have in my apartment with back-of-set antenna receives said 800 MHz frequency better than my mobile setup. Could the scanner have a blown front end or could there be an issue with my antenna? Both radios were in my old vehicle but the antennas were even farther apart then they are now.
Your Kenwood would likely desense the scanner even if it and the scanner were connected to antennas located on the front and rear bumpers.

Most - but not all - scanners I've used have not handled 5 watts from a nearby handheld VHF or UHF Fire Department radio. By nearby, I'm saying within 8-10' from the scanner or its external antenna.

The scanners are designed to be semi-affordable which means they will only be semi-selective in rejecting the radio energy from nearby transmitters.

As for whether or not the radio energy from your Kenwood could permanently disable components in your scanner's case, that's a possibility. I don't think anyone can state with certainty without troubleshooting your scanner and determining why it lost 800 MHz sensitivity.

I have one more bit of info to share with you. Before I retired in 2013, my FD work vehicle was a 2008 Ford Escape. It was equipped with a VHF Kenwood TK-790 and a Uniden BC-355C scanner. The scanner antenna was mounted near the right rear corner of the hood. The Kenwood's antenna was mounted on the roof over the rear seat. The scanner had no problems receiving signals, but the Kenwood's radio energy desensitized the scanner so much that the scanner blanked out and didn't need an audio mute circuit when I transmitted on the Kenwood.

The scanner didn't blank out when a 5W portable radio was nearby. In those instances, the volume was turned down to mute the strange sounds coming from the scanner's speaker.

Best wishes for a successful resolution of your radio issues.

Sent from my HTC One M9 using Tapatalk
 

jonwienke

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Could the scanner have a blown front end or could there be an issue with my antenna?
Yes, and yes.

The first thing to check is how much RF from your Kenwood is feeding back into the scanner. Get a RF power meter and 50Ω dummy load. Connect the dummy load to the output of the meter, and the scanner antenna lead to the input. Key up the Kenwood on each of your commonly-used frequencies at max power, and see how many watts of RF are hitting the scanner. This will vary depending on both antenna tunings and the exact frequency being transmitted. If the meter reads more than 1/2-watt, you may be risking damage to the scanner front end. Damage would likely be noticeable at highest frequencies first if the protection diodes are merely damaged, rather than totally fried.

Try swapping scanners, but do not TX with the Kenwood. Does the mobile still get crappy 800MHz reception compared to the desktop? If so, it's probably an antenna issue. But if the mobile scanner still has issues on the desktop, then a scanner problem is more likely.

Regarding temporary desense, A 5W TX 10 feet away is several orders of magnitude stronger than a normal signal, and most receivers are going to struggle handling an off-frequency signal that is 100,000x greater (or more!) than the signal you're trying to receive. With any radio, you're going to have issues if another radio is transmitting nearby.
 

billyfromhill

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OK I'll try switching out the scanners and see if that makes a difference. Would a higher-end scanner be more resistant to desense and a stronger front end? I've been thinking about picking up a BCD996P2 because my area recently went to a P25 phase II system but has everything patched to VHF. I wouldn't want to blow out an expensive scanner if it's not going to be more resistant,
 

jonwienke

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Would a higher-end scanner be more resistant to desense and a stronger front end?
Yes. A more expensive receiver will handle strong-signal interference better. It will probably resist damage from extremely high signal input better, but that isn't necessarily a design priority, and may not be the case.

Measure the amount of RF feedback hitting the scanner before you do ANYTHING else. If it's less than 1/10-watt on all your TX frequencies, you can rule out receiver damage from RF coupling between your TX antenna and your scanner antenna. My 436HP routinely gets hit with 1/2-watt of RF during TX and hasn't suffered any damage, but in the absence of a manufacturer spec or other data, I'd be cautious about assuming that any scanner can handle that power level safely. If it's over 1 watt, you definitely need to do something about your antenna setup, or you're going to have issues with receiver damage.

Another test to try is to get a splitter or T fitting so you can run both scanners off the exact same antenna input. That will give you a better apples-to-apples comparison of receiver performance. Both scanners should pick up the same traffic if programmed identically. If one scanner misses traffic the other receives, then you know that scanner is underperforming.
 

billyfromhill

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Anybody have a recommended power meter? Is something like the Diamond SX40C any good?
 

Ubbe

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If the scanner receives worse in the car, with that great groundplane, then something is absolutly wrong.
First check the ground shield of the coax and antenna mount. Measure the resistance between the shield of the coax connector and vehicle ground, should not be more than 2 ohm.

Seconds, to reduce the coupling between antennas you usually tilt them in opposite direction, one antenna 25 degree to left and the other to the right. But that might not be possible with the mounts you are using.

But as the reception are so bad to start with it needs to be adressed first as it indicate some sort of problem.

/Ubbe
 

billyfromhill

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The scanner antenna has good continuity to vehicle ground and the resistance to ground was 0.4 ohm. My VHF antenna had the exact same results.

Unfortunately I can't tilt the antennas with my mounts. See photo in OP.

Anybody have a power meter they recommend? I'm also going to try to swap my scanners and see what happens but I want to know if it's receiving too much power when I'm transmitting.
 
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