Transmitting near scanner antenna

[Ubbe]

Powering off WILL NOT protect from excessive RF, because the diodes and transistor that fry are UPSTREAM of the band filters.

If you see a solution that has a transistor upstream of the bandpass filters then the designer have made a huge mistake. I have never seen a receiver or a schematic with that design due to the risks we are mentioning here.

/Ubbe

 

[kevinparrish]

I've been using the Stridsberg Engineering "RF Power Limiter" p/n LIM-01 for several years in conjunction with mission critical receivers.

Bias-Tee

 

[KR3LC]

The original poster was talking about 1000 W transmitter, not 50 or 100W. He didn't mention what frequency the transmitter is operating at and I would think that plays a role too.

Anyway I have for years operated with a vhf/uhf dual band ham antenna and a scanner antenna on my car separated by the width of a typical car only. Running 50 W I've never damaged any scanner. I've ever had.

However, I did have a Uniden HomePatrol repaired once with a damaged front end that I assumed was due to a proximity issue in another vehicle. In this case the ham antenna was mounted on the front left fender near the windshield and the scanner was used within about 2 feet.

 

[doctorbubba010]

Great thread gents,
Getting ready for installation I have the holes drilled. Was going to run a Larson 27S in one NMO connection With a cobra 75 cb with possibly another Scanner antenna fairly close (as seen in picture) would a 4watt output disrupt a scanner?

 

[jonwienke]

Great thread gents,
Getting ready for installation I have the holes drilled. Was going to run a Larson 27S in one NMO connection With a cobra 75 cb with possibly another Scanner antenna fairly close (as seen in picture) would a 4watt output disrupt a scanner?

Probably not, at least not going to damage anything. 4 watts isn't likely to hurt anything, and coupling across bands is poor, so very little is likely to hit the scanner if it's got a VHF/UHF/800 antenna.

 

[doctorbubba010]

I’m planning on doing just that

Larson 27S with a Larson VHF/UHF/800

when would you suggest It becomes to much sauce for the scanner. If I were in the future get a stronger CB set up (yes I’m tracking FCC and the 4 watts is the law)

 

[WB9YBM]

Explain to me why it never appears to be a problem with anybody but a few hobbyists.

Could it be because the hobbyists don't have as much cash (and/or maybe the expertise of an organization/business) to throw at the situation?

 

[doctorbubba010]

Absolutely right. Or don’t care to ask. Just throw the flag in.

 

[W4WMH]

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:

CALIFORNIA HIGHWAY PATROL MOBILE RADIO EQUIPMENT 2001

mmckenna
I've seen these pages in the past, and am interested in a box as such for my POV to allow a scanner to use the low band antenna. Does anyone on RR have specs to build one or least know what components would be required? I use a TK690H / 890H combo radio, so it has the DB-25 that could be programmed to trip the relay. I just don't know what components would be required, and assume you can't just use a 12Vdc auto relay for RF. Any information would be greatly appeciated.

 

[mmckenna]

mmckenna
I've seen these pages in the past, and am interested in a box as such for my POV to allow a scanner to use the low band antenna. Does anyone on RR have specs to build one or least know what components would be required? I use a TK690H / 890H combo radio, so it has the DB-25 that could be programmed to trip the relay. I just don't know what components would be required, and assume you can't just use a 12Vdc auto relay for RF. Any information would be greatly appeciated.

You can find coaxial relays online. I have one sitting at my desk at work, however it's a 28volt coil.

You could pull TX sense signal output off Pin 11 of the 25 pin connector to trigger the relay, however I do not know what the specs are on that output. You could make an RF sensed switch, probably plans for that on the net somehwere.

 

[ladn]

When I was a field photojournalist, my company car had a 90W VHF Mitrek into a 3dB Antenna Specialists antenna mounted on the trunk deck. On either side was a scanner antenna (models varied, as did scanners). Never fried any scanners. Naturally, every time I keyed the Mitrek, I would have no scanner reception.

 

[TailGator911]

My scanners, both fixed and portable, are only about 3ft from my Icom 746Pro and I transmit @ 45w on 2m local nets 4-5 times a week like clockwork. Never had a problem. But, then again, I survived covid in March with diabetes and heart issues. I'm just lucky like that. Dumb luck and the kindness of strangers, I suppose...

 

[cmdrwill]

I guess some were really lucky. Blown out receivers ...

 

[jaspence]

Why is 1000 watts needed? 10 or 20 watts on 10 meters (right next to 11 meter CB in frequency) can easily give me contacts a 1000 miles or more away.

 

[LD723]

I've always wondered if you can have a setup with a transceiver and instead of a scanner, an SDR with filters and such to prevent it from being fried if possible lol. I'm not going to try it but I've always wondered if its possible lol. My local pd has Uniden BCD996P2's in their vehicles and I've always wondered if it also has ever gotten overloaded in the front end. I'm not sure how many watts they run their mobiles I would estimate around 25

 

[mmckenna]

I've always wondered if you can have a setup with a transceiver and instead of a scanner, an SDR with filters and such to prevent it from being fried if possible lol. I'm not going to try it but I've always wondered if its possible lol.

Sure, notch filter would do that, or even something broader. However, it would block out reception on that segment of the band, making the scanner useless there.

My local pd has Uniden BCD996P2's in their vehicles and I've always wondered if it also has ever gotten overloaded in the front end. I'm not sure how many watts they run their mobiles I would estimate around 25

Overload is one thing, damage is another. Overload will wipe out the receiving, but not necessarily damage anything. In my wife's car, the VHF whip in on the roof connected to a 45 watt VHF Motorola. When transmitting, the FM radio will lose reception on all but the strongest FM stations. No damage to the radio. The AM/FM antenna is down on the front right fender. VHF antenna is dead center on the roof.

 

[prcguy]

10 or 20 watts in the CB band doesn't do much these days. I'm not advocating using 1,000 watts, but comparing 10m on a busy day with a noise floor of maybe S1 or S2 to a noise floor of S9++ on CB is not a fair comparison.

Why is 1000 watts needed? 10 or 20 watts on 10 meters (right next to 11 meter CB in frequency) can easily give me contacts a 1000 miles or more away.

 

[K9DWB]

Why is 1000 watts needed? 10 or 20 watts on 10 meters (right next to 11 meter CB in frequency) can easily give me contacts a 1000 miles or more away.

Not exactly an apple to apple comparison I'd think. 10 meter Amateur vs 11 meter CB represents two vastly different worlds and I can't see how there's any way to say they're similar other than they're both radio bands, users talk into a mic on a transceiver and hear sounds on the speaker. Beyond that not much anything else is remotely comparable.

I know CB is 4 watt legally and I am not advocating running illegally. Historical statement: I used to run with the other truckers with a CB of some variety when I drove for various regional companies. At one point, I had a radio setup that somehow transmitted at more watts than some HF base radios output and yet I still got walked on all the time. I am not advocating it, just saying what was done in the past. On CB, 4 watts will give you lot lizards or solar powered trucks with their drivers sitting at the T/A or Flying J truck stops. There's 250 contacts 1/4 mile away on 11 meter CB at one parking lot. Sounds a lot different than 10 Meter to me too.

I'm glad you can make a QSO 1,000 miles away with 10-20 watts. I just won't be buying tickets to your apple to orange comparison.

 

[Firekite]

Explain to me why it never appears to be a problem with anybody but a few hobbyists.

Because the pros account for it in their design and equipment. And for a fleet if they swap out a scanner body to send the “bad” one back for warranty service it’s a minor inconvenience.