Single antenna setup for both transceiver and scanner

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Firekite

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I’m planning my mobile setup for my truck. I’ll most likely end up with one mobile VHF/UHF transceiver, and I have a Uniden BCD436HP scanner. I’d like to look at using a single dual-band antenna to service both devices.

I know I can install two separate NMO mounts with two separate antennas, and as long as the RF coupling between the antennas is reasonably low at transmit frequencies at full transmit power, the scanner would probably be ok and neither significantly desense over time nor for that matter burn out the front end.

Before I decided to make the leap to getting my ticket and therefore having to take transmit into account, I’d considered primarily the Larson NMO150/450/758 as a great scanner antenna for my needs. Now I’m also considering the NMO2/70B instead even though it’s twice as long and lacks a spring, just for the extra transmit gain.

What I’d really like to do is simply go with a single antenna (not sure which yet) on an NMO mount centered on my roof with a single LMR200 feed line to a single point where it can then be shared by both the mobile radio and the scanner. But I can’t imagine I can use a simple splitter without murdering the scanner. Am I overthinking this? Do the modern x36/SDS Uniden scanners have reliable RF sense overload protection of some sort built in? If not, I assume I need a transmit-receive switch to protect the scanner. Right?

It seems like most “affordable” transmit-receive switches these days are targeted toward SDR users to protect their receiver in a base shack. I’m not sure why, other than maybe it’s just the most common use case for such a component these days. Is there something magic about SDR I’m not aware of that would make such a switch specific to SDR? Or since a scanner is simply a receiver, would something like the MFJ-1708B be an appropriate choice for this sort of mobile application?

I know I could get a second antenna and NMO mount setup for about the same price, but I’m trying to keep things simple and minimize the number of antennas I have to get out and remove when entering my garage, car washes, etc, and I can’t imagine scanner reception would be any better on a separate antenna. And I don’t really need (or want) my scanner to be receiving and blaring as I transmit anyway.

Am I on the right track? Is there a perfect solution I just haven’t found yet? Insights and experiences would be welcome, as this is my first rodeo making these kinds of plans and decisions.
 

mmckenna

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It seems like most “affordable” transmit-receive switches these days are targeted toward SDR users to protect their receiver in a base shack. I’m not sure why, other than maybe it’s just the most common use case for such a component these days. Is there something magic about SDR I’m not aware of that would make such a switch specific to SDR? Or since a scanner is simply a receiver, would something like the MFJ-1708B be an appropriate choice for this sort of mobile application?

A simple splitter would allow the 50+ watts RF power to be dumped into the scanner. That would cause all the expensive parts inside to release their magic smoke. You don't want that unless you are looking for a quick way to destroy your scanner.

It appears that the MFJ-1708B will do what you want. It will disconnect the scanner from the antenna/transmitter when you key up, thus preserving the magic smoke inside your scanner. Looks like their are some jumper settings you'll need to address so both radios can receive at the same time.

But, be aware, the joke is MFJ is an acronym for Mighty Fine Junk. Not high quality stuff, and I'd probably get a little uneasy about putting a $500+ scanner in the hands of MFJ. But, it's your equipment, your money, your truck, do what you want.
The fact that MFJ calls it "Good" isolation between ports. "Good" isn't as good as "excellent". Since some scanners can generate birdies, and some transceivers can generate local oscillator noise, I'd be curious how well this would work, but not curious enough to spend $110 bucks to find out.

I know I could get a second antenna and NMO mount setup for about the same price, but I’m trying to keep things simple and minimize the number of antennas I have to get out and remove when entering my garage, car washes, etc, and I can’t imagine scanner reception would be any better on a separate antenna. And I don’t really need (or want) my scanner to be receiving and blaring as I transmit anyway.

Honestly? This is exactly what you should do. It'll be cheaper in the long run, and if the Mighty Fine Junk fails, you are not going to be out $500+ for a new scanner.
A dedicated 2/70 antenna for the dual band radio is going to work better.
A dedicated scanner antenna will work better for the scanner, especially if you listen to stuff in the 700-800MHz range.

The hassle of removing two antennas instead of one seems trivial compared to the cost and possible drawbacks of a single antenna system.

Am I on the right track? Is there a perfect solution I just haven’t found yet? Insights and experiences would be welcome, as this is my first rodeo making these kinds of plans and decisions.

You are probably one of a thousand people that have come on this site to ask the same question. Yes, it would probably work, but with concerns. There isn't a better solution out there. But, like I said, your money, your radio, your car, you decide. Personally, I'd go with a dedicated scanner antenna and a dedicated transmitter antenna. If I really had to only do one antenna, I'd probably use the AM/FM-good-time radio antenna for the scanner with a splitter so I can still listen to tunes.
 

mmckenna

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Wow, wait a minute. I took a closer look at the schematic and thought about it some more, it does look like the "transmit" port is disconnected when not transmitting, which means your dual band radio will not be connected to the antenna when not transmitting. Looks like it could be modified to do what you want, but I'd still recommend separate antennas for what you are doing. Modifying MFJ sounds like a recipe for headaches.
 

ActionJackson

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If I really had to only do one antenna, I'd probably use the AM/FM-good-time radio antenna for the scanner with a splitter so I can still listen to tunes.

This is exactly what I did back in my CB days, 40+ years ago. Worked like a charm, especially receiving L.A. City Fire on low band.
 

Firekite

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If I really had to only do one antenna, I'd probably use the AM/FM-good-time radio antenna for the scanner with a splitter so I can still listen to tunes.
I’ve heard worse ideas! But yeah I’ve been considering getting two NMO mounts/feedlines and mounting the 2/70B in the center of the roof and the 150/450/758 on the roof just ahead of the third eye brake light on the cab.

But I still think the 2/70B would give me all the receive I could want for the scanner, and I’m still surprised a simple little rugged T-R switch doesn’t exist as a widely available commercial item. Regarding the MJF unit, Dave Casler tested it to show right about 90db in mW of attenuation IIRC. All I need to do is avoid dumping the brunt of TX power directly into the scanner, not isolate it perfectly, though of course I want the transceiver to receive when not transmitting.

Ugh. If no legit T-R switches are available, I’ll probably end up having to run multiple antennas on multiple mounts and route multiple feed lines. It’s a crew cab F250 with no sunroof I plan on keeping a long time, so there should hopefully be enough room on the roof to provide a great ground plane for the transmit antenna while achieving sufficient separation to keep from burning out or desensing the scanner. Fine :)
 

mmckenna

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It’s a crew cab F250 with no sunroof I plan on keeping a long time, so there should hopefully be enough room on the roof to provide a great ground plane for the transmit antenna while achieving sufficient separation to keep from burning out or desensing the scanner. Fine :)

Well, you've got lots of room then. I had a 2011 F150 crew cab with the VHF quarter wave in the center and an NMO on the rear above the 3rd brake light. I used it for my 800MHz radio for a while, then I use it for a CB antenna (NMO-27) for a while. Not a perfect ground plane for the lower frequencies, but it'll work fine for a scanner.
Current personal truck is a 2018 F-350 crew cab with just the VHF whip.
The work truck is a 2017 F-350 regular cab. I have the antennas mounted side by side, one VHF quarter wave and one 800MHz colinear. Centered on the front to rear axis and spaced 18 inches in from the cab edge. Works just fine.
But then again, neither of these trucks go into a garage, so no issues for me.
 

prcguy

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I would install two separate antennas and use an RF limiter on the antenna feeding the scanner. Here is a cheap limiter that will handle up to 2 watts in from a nearby antenna, then clamp it to a lower level that should not hurt a scanner. RF coaxial limiter 1MHZ-1000 MHz 10dBm F/ VHF amplifier, short wave SDR receiver | eBay This one should be ok for antennas with a couple of feet isolation and up to about 50w of transmit power.

You might want to measure what comes off the receive antenna worst case with the highest power you will run on your transmit antenna, then choose an appropriate RF limiter.

I've found much better ones on Ebay surplus that will handle up to 60 watts input continuous and clamp to about 10mw feeding a receiver with super low loss from basically DC to 3GHz. That one cost me about $70 surplus but it was many hundreds of $$ new.
 

majoco

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What transceiver? I'm not up with recent VHf/UHF transceivers, but some HF transceivers have a "Receiver out" socket which you can connect to another receiver - I used this back in the 80's with a Yaesu FT901DM with a Kenwood R2000 on the side - very handy for spotting up and down the band or other bands while still hammering away on the key in a contest. The receiver in the 'transceiver' is obviously protected against the 500W going up the coax, so the accessory one is too.
 

mmckenna

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What transceiver? I'm not up with recent VHf/UHF transceivers, but some HF transceivers have a "Receiver out" socket which you can connect to another receiver

OP said:

I’ll most likely end up with one mobile VHF/UHF transceiver, and I have a Uniden BCD436HP scanner.

I'm not aware of any dual band mobile transceivers that have an RX output. Would be a nice solution, though.
 

vagrant

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I've found much better ones on Ebay surplus that will handle up to 60 watts input continuous and clamp to about 10mw feeding a receiver with super low loss from basically DC to 3GHz. That one cost me about $70 surplus but it was many hundreds of $$ new.
Can you advise on the model number of that particular one or any others that can handle similar wattage?
 

prcguy

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The one I got is an Advanced Control Components model ACLM-4896C46. The Ebay seller had a couple of them but it looks like he sold out. I should have bought several. I think this one handles about 10W continuous as is and for 60W continuous you bolt it to a heatsink.

Can you advise on the model number of that particular one or any others that can handle similar wattage?
 
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