For MURS/GMRS: what type of radio would work best for the job?

[03msc]

You would need to go into a professional radio apprenticeship program. It goes something like this:
First year, unlearn CB lingo and coax myths and memorize ITU phonetic alphabet. That takes nearly the entire year.
Second year, don't talk, just listen to commercial radios.
Third year is spent installing commercial radios in vehicles and buildings under strict supervision of an actual radio professional. Mostly old and dirty cars will be selected for the apprentice with used coax and power cables.
Forth year, memorize all current commercial radios and their specs then progress onto learning various commercial radio programming software.
Fifth year, become proficient at using a radio service monitor and learn all pertinent FCC radio specs. Progress into tuning duplexers and isolators. Purchase, program and use a current Luxury radio like a APX or current Harris. At some point you might graduate.

In the end you might find the non professional way of life was more fun.

Hmm...so I'm wayyy farther along than I thought

 

[smittie]

You would need to go into a professional radio apprenticeship program. It goes something like this:
First year, unlearn CB lingo and coax myths and memorize ITU phonetic alphabet. That takes nearly the entire year.
Second year, don't talk, just listen to commercial radios.
Third year is spent installing commercial radios in vehicles and buildings under strict supervision of an actual radio professional. Mostly old and dirty cars will be selected for the apprentice with used coax and power cables.
Forth year, memorize all current commercial radios and their specs then progress onto learning various commercial radio programming software.
Fifth year, become proficient at using a radio service monitor and learn all pertinent FCC radio specs. Progress into tuning duplexers and isolators. Purchase, program and use a current Luxury radio like a APX or current Harris. At some point you might graduate.

In the end you might find the non professional way of life was more fun.

I took the military class. Cost me a lot more than five years.

Smittie

 

[KK4JUG]

Unless your business is made up of family members, I don't think GMRS will be legal.

 

[prcguy]

@prcguy, I want to learn (or unlearn) coax myths. What's that about?

Respectfully,
Smittie

Too much to go into here, consider the apprenticeship program. Or Google "what is correct coax length for my CB antenna?" That will give you some good examples of what not to do. And if you pass the apprenticeship program and someone later asks "hey good buddy, whut's yur SWRs?" You can announce "I currently have 22dB return loss at resonance, thanks for asking!"

 

[smittie]

Too much to go into here, consider the apprenticeship program. Or Google "what is correct coax length for my CB antenna?" That will give you some good examples of what not to do. And if you pass the apprenticeship program and someone later asks "hey good buddy, whut's yur SWRs?" You can announce "I currently have 22dB return loss at resonance, thanks for asking!"

I wrestled with the coax length question on my current CB radio install. Still not convinced that it always needs to be 18 feet but also still do not know enough to figure out what it does need to be.

Respectfully,
Smittie

 

[mmckenna]

I wrestled with the coax length question on my current CB radio install. Still not convinced that it always needs to be 18 feet but also still do not know enough to figure out what it does need to be.

Respectfully,
Smittie

That's what he's saying.
The 18 foot myth is just that, a myth. But CB'ers love to perpetuate that because they don't understand it and want to make sure no one else understands it either.

The correct length for your coax is however long it needs to be to get from the bottom of the antenna to the back of the radio, and a few inches of slack. Anyone that tells you it needs to be a specific length for CB or any other radio services is not understanding how this works.

 

[prcguy]

That's what he's saying.
The 18 foot myth is just that, a myth. But CB'ers love to perpetuate that because they don't understand it and want to make sure no one else understands it either.

The correct length for your coax is however long it needs to be to get from the bottom of the antenna to the back of the radio, and a few inches of slack. Anyone that tells you it needs to be a specific length for CB or any other radio services is not understanding how this works.

Or, the antenna system is screwed up and in some cases the coax length is critical because the coax becomes a radiating part of the antenna system and changing the length will change the antenna system tuning. In that case you fix the actual problem and avoid tuning the coax. Or some things like using twin mirror mount antennas with a coax phasing harness, the harness length is critical because its a frequency specific power divider.

 

[K6GBW]

Yes yes yes! Coaxes should be custom length for the application. And please please learn to crimp/solder PL259's. You can really tell alot about a person by the connectors!

 

[smittie]

Alright. I believe you guys. Here's my experience.
A Cobra 25 radio mounted in a 2021 Jeep Gladiator. I have read a number of articles calling out the CB myth of 18 feet. So, I decided to try cutting coax to length. RG8X coax, soldered PL259 (tested using a multi-leter), factory installed NMO. Browning BR-140 antenna. Best I could get was 2.1 on channel 1 and 3+ on channel 40. I read the cut sheet that came with the antenna. It gave cut lengths for frequencies but it specified 17 of RG58A/U coax. I blamed the coax, decided the CBers must be right, ordered 17 of RG58A/U with NMO and an FME connector.
Tested that by connecting the NMO to the mount on the Jeep and the coax to the radio via an SWR meter.
I was still not able to get better than 2 on channel 1 and 3+ on channel 40 using the BR-140. I switched to a Laird C27 and was able to get Channel 1 to 1.6 and 40 to about 1.9. At this point I installed the coax in the vehicle and tried the SWR again. Everything is above 3 with with channel 1 higher than channel 40.
This is where I am at now. I plan to replace the Cobra 25 for several reasons but one is the hope that I have better luck tuning it (yes, it occurs to me that the problem is probably the operator rather than the equipment).
I mentioned that I learned most of what I know about RF in the military. Everything I worked with or set up was a complete kit from the manufacturer. Never had to tune antennas.
So, what am I not doing right? What do I need to do different? How do I do it right? How do I know when it is as good as it is going to get?

Gratefully,
Smittie

 

[K6GBW]

The radio won't make any difference as to the tuning of the antenna. In looking at your set up, I'm betting the biggest issue is where it's mounted. Running a radiator so close to the windshield frame is going to throw the SWR off regardless of the type of antenna. People that mount CB antennas too close to the cab of their pick up trucks have the same problem. For a Jeep, I've had the best luck mounting the antenna on the rear tire carrier. The reason is the top of the Jeep is fiberglass and therefore RF transparent. Usually something like a Francis or Firestick in the 3-4 foot range works well. The antenna you're using is an electical quarter wave and needs a substantial ground plane to work right. Its best mounted on a large metal roof or trunk lid. On your Jeep it's just going to frustrate you.

Keep in mind the wave length for CB radio is 11 meters. That works out to 36 feet! That's a big wave to be transmitting off of a tiny antenna on a car with no ground plane around it. PRCGuy and MMcKenna might have some other ideas. But if this where my project, I'd move that antenna.

 

[smittie]

@K6GBW, thank you.
No tire carrier on my Gladiator. Just a tailgate. However, I am thinking about moving the antenna to the front bumper. 3/8" steel. I'll play the RUgged Radio mount out a little more but the front bumper is my Plan B.

Appreciate your input.

 

[K6GBW]

Actually, the front bumper would probably work pretty well. Another side benefit would be that you can see it so there's less chance of tearing it off when you're off road.

 

[smittie]

@K6GBW ...and it gives me an indicator of exactly where the front of the truck is, which can be very handy offroad and on.

 

[mmckenna]

One thing the CB guys forget to include is the velocity of propagation with the different types of coax. So "18 feet" of physical length isn't 1/2 wavelength on all types of cable.

As for the SWR issues, that is likely due to your mounting location. The ground plane will impact the tuning of the antenna. Cut charts will get you pretty close with a good ground plane, but if you don't have one, cut charts are a mere suggestion.

An antenna analyzer that will show you a graph of where the antenna is resonate will help you solve a lot of issues. NanoVNA's are a good solution, but you're looking at something in the $75 range (when I bought mine) and the learning curve can be a bit steep. Makes it much easier to see the SWR, though.

 

[KK4JUG]

Did we stray a bit from the original topic?

 

[smittie]

Did we stray a bit from the original topic?

My bad. Sorry.

 

[03msc]

My bad. Sorry.

Feel free to start your own thread in the install questions subforum though! As you can see, people are glad to help.