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  #1 (permalink)  
Old 05-23-2018, 1:32 AM
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Default Impedence matching to cb radio

Hello, I'm thinking of installing some co-phased RG-59 A/U into my cb radio in my big rig.

Since RG-59 is 75 ohms or 72ohms and the SO-239 connector on the radio is 50 ohm impedence, I was wondering what the best way might be to match the antenna and coax impedence to the input of the cb radio.

Maybe a transformer or trasmach, or antenna tuner? Or could I have the coax a certain length so that it matches the 50 ohm impedence where it connects to the radio?

Also, do any companies make 75 ohm antennas?

Hopefully someone can help. Thanks.
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Old 05-23-2018, 7:56 AM
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The typical co-phasing harness for two CB antennas is a crude 2-way power divider with roughly 50 ohms impedance at the radio connector and at both antenna connectors. This is because the length of the 75 ohm cable used is very specific and critical to maintain a 50 ohm match throughout the system.

If you look up a "Wilkinson power divider", its very similar but without the 100 ohm resistor across the two outputs. The co-phasing harness is close enough and final matching is done by tuning the antennas.

If you buy a premade harness just install it and go on with life. If you want to make one I hope you have an antenna analyzer or something to help you get the final cable lengths correct.

Quote:
Originally Posted by Driverj30t9 View Post
Hello, I'm thinking of installing some co-phased RG-59 A/U into my cb radio in my big rig.

Since RG-59 is 75 ohms or 72ohms and the SO-239 connector on the radio is 50 ohm impedence, I was wondering what the best way might be to match the antenna and coax impedence to the input of the cb radio.

Maybe a transformer or trasmach, or antenna tuner? Or could I have the coax a certain length so that it matches the 50 ohm impedence where it connects to the radio?

Also, do any companies make 75 ohm antennas?

Hopefully someone can help. Thanks.
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Old 05-24-2018, 1:00 AM
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Interesting, I thought every part of the coax was 75 ohms or 72. That will make things easier to setup. Still have to purchase an mfj antenna analyzer, gotta come up with the funds.

I'm going to just buy a pre-made harness, less to deal with right now. Maybe make one in the future though. Still have to figure out how to route the coax to the inside of the cab which is enough trouble.

Never even heard of a wilkinson power divder until now, that needs some looking into.

Thanks for the help.
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Old 05-29-2018, 11:10 AM
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Every point along the coax is the same impedance.

Radio is very different than your electric service. RF feedline has some unique properties when cut to a specific length versus frequency. For most applications shorter is better because more cable equals more loss. But, if the cable is just the right length you can think of it in terms of degrees long instead of a ruler length. When the cable is exactly 0 (or 360 depending on how you think of it) or 180 degrees long, a half-wavelength, its impedance appears as invisible to the radio and antenna. The impedance at the antenna is mirrored at the radio. At a quarter-wavelength and odd multiples the cable appears as an impedance matching transformer. Phase shifts can be used in a variety of ways. These lengths are dependent on the velocity factor of the cable. If you don't know what it is you'll not be able to calculate the length.

Impedance mismatch for the connector at 27MHz isn't very significant. The common UHF connector, PL259/SO239 itself does not have a constant impedance and is actually a poor choice for UHF (its name is old).

The nominal impedance of a ground plane antenna with the plane at 90 to the vertical is very near 25 Ohms. (the Dipole is near 75 and a ground plane with angled radials gets us near 50) On a vehicle all bets are off. A base loaded antenna is a whole different thing.
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Old 05-29-2018, 12:09 PM
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To add to this good post, when the radio and antenna or load is 50 ohms non reactive, the length of cable is irrelevant and changing lengths will do nothing except add more loss with more coax. Its when one end of the circuit, the radio or antenna end is not 50 ohms, then the length of the coax can become an impedance transformer.

Two 50 ohm antennas fed with 50 ohm coax each and connected together at some point will result in a 25 ohm load to the radio, or a 2:1 VSWR. Using critical lengths of 75 ohm coax from each antenna to a common point can transform the impedance back to or near 50 ohms at the radio.

Quote:
Originally Posted by AC9BX View Post
Every point along the coax is the same impedance.

Radio is very different than your electric service. RF feedline has some unique properties when cut to a specific length versus frequency. For most applications shorter is better because more cable equals more loss. But, if the cable is just the right length you can think of it in terms of degrees long instead of a ruler length. When the cable is exactly 0 (or 360 depending on how you think of it) or 180 degrees long, a half-wavelength, its impedance appears as invisible to the radio and antenna. The impedance at the antenna is mirrored at the radio. At a quarter-wavelength and odd multiples the cable appears as an impedance matching transformer. Phase shifts can be used in a variety of ways. These lengths are dependent on the velocity factor of the cable. If you don't know what it is you'll not be able to calculate the length.

Impedance mismatch for the connector at 27MHz isn't very significant. The common UHF connector, PL259/SO239 itself does not have a constant impedance and is actually a poor choice for UHF (its name is old).

The nominal impedance of a ground plane antenna with the plane at 90 to the vertical is very near 25 Ohms. (the Dipole is near 75 and a ground plane with angled radials gets us near 50) On a vehicle all bets are off. A base loaded antenna is a whole different thing.
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Old 05-30-2018, 5:28 AM
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Some good info there, thanks. I definitely need to do some more reading about feedlines so I have a better understanding. Want to start installing my setup in my semi.

I'm thinking maybe some duel or quad band antennas and an Icom IC-718 or a Yaesu FT-818 plus a cobra 29 lx on a switch with a whistler TRX-1 and a WS1040 on a diplexer.
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Old 06-03-2018, 1:16 PM
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The first thing to know is a truck tractor is about the worst vehicle to install a radio in. Don't expect anything to work ideally. A mirror mount is a horrible ground plane. Fiberglass roof? You guessed it. 53 feet of metal behind you? Guess what that does to your radiation pattern.
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Old 06-04-2018, 12:21 PM
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Hmm, thought it might be better then a car or truck because you can get the antennas up a lot higher, but I see what your saying about the radiation pattern. Some of the incoming/outgoing signals must get blocked or absorbed, reflected, and reduced in strength by the big metal box.

Not expecting near perfection with the setup but hopefully it works decently. Never put up any antennas anyways so I might not know what I could be missing.

I suppose the truck makes a horrible counter poise because it's not flat and directly underneath the antennas.

Not sure about the roof or what most of the truck is made of. It all looks like fiberglass to me but I read that it's tin steel. Gonna try climbing up to the roof and sticking a mag mount on there, see what happens. That would be the antenna for my scanners, if that doesn't work I'm thinking of mounting an antenna to my hood mirror. Dual antennas on mirrors would be for transceivers.

Need to find a mount I can attach to a metal rod (part of the hood mirror). Then I have to find a long grounding strap or use ground wire then figure out if I'm going to hook it up to the negative battery or the frame. Some other stuff too. Part of the fun of it.
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Old 06-04-2018, 1:28 PM
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A whip antenna is looking for lots of flat sheet metal right under it and a ground wire to the frame or battery is not a ground plane. Its a DC ground but the antenna will probably not know its there.

What I have had good luck with on mirror mounts, especially one mounted to a fiberglass door or cab is have a second tuned antenna connected to the ground side of the mount and pointing straight down. This makes the mirror mounted whip into a tuned center fed dipole and will increase your signal strength noticeably.

The two whips do not have to be identical like the main one can be a 4ft and the grounded downward facing one can be a two foot or whatever. The main thing is they both have to be a resonant CB whip and tuning can be a little tricky on these.

I would first check the main whip on the mirror using a length of 50 ohm coax and an RF choke near the mount to see where the antenna resonates and what the approximate impedance is. Then add the downward facing whip and check resonant freq and impedance again. This should give you (or me at least) an idea of which whip to tune first. If the whips are resonant in the CB band but the impedance is low you might trim a little off the lower facing antenna. If the resonant frequency is low then tune the upper antenna first a little then the bottom one a little to see what happens. If there is only going to be one mirror mount antenna then tune for the best match on ch 20 and your done.

For a dual mirror setup all of the above should give you a good starting point and I would also go for a good match on the lower channels at first which will leave the whips a little long. Then do the same on the second mirror, then connect the co-phasing harness and see if the antennas need further tweaking. If you get a similar match that is peaked on the lower channels then all is good in co-phasing land and you can then trim the antennas the same amount each to get the best match on ch 20 or where ever you hang out.

Quote:
Originally Posted by Driverj30t9 View Post
Hmm, thought it might be better then a car or truck because you can get the antennas up a lot higher, but I see what your saying about the radiation pattern. Some of the incoming/outgoing signals must get blocked or absorbed, reflected, and reduced in strength by the big metal box.

Not expecting near perfection with the setup but hopefully it works decently. Never put up any antennas anyways so I might not know what I could be missing.

I suppose the truck makes a horrible counter poise because it's not flat and directly underneath the antennas.

Not sure about the roof or what most of the truck is made of. It all looks like fiberglass to me but I read that it's tin steel. Gonna try climbing up to the roof and sticking a mag mount on there, see what happens. That would be the antenna for my scanners, if that doesn't work I'm thinking of mounting an antenna to my hood mirror. Dual antennas on mirrors would be for transceivers.

Need to find a mount I can attach to a metal rod (part of the hood mirror). Then I have to find a long grounding strap or use ground wire then figure out if I'm going to hook it up to the negative battery or the frame. Some other stuff too. Part of the fun of it.
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Old 06-05-2018, 10:25 AM
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A whip antenna is looking for lots of flat sheet metal right under it and a ground wire to the frame or battery is not a ground plane. Its a DC ground but the antenna will probably not know its there.

There goes that idea then.

I always thought of whip antennas as 102 inch metal antennas that are thin and flexible, but apparently they can be fiberglass and really long or short. Those fiberglass antennas are quite rigid though.

What type of a mount would I use for a vertical mirror mount dipole? I have like a six inch bird perch mount installed right now so the lower ground side antenna wouldn't be very far away from the truck body, that can't be good. Maybe there is another mount I could put on the mirrors to get the antennas further away from the truck body.

Is the radio choke for reducing EMI, and parasitic capacitance since the lower antennas on the ground side, and proximity effect losses and something to do with the asymmetrical coax and symmetrical dipole, or is that just a balun? Need to read up more on this stuff.

What parts of the mount are grounded? I know with a bird perch mount the top part is below the stud nut and washer. The whole mounts grounded then except the stud nut?

I"ll give those tuning methods a try if I put up the antennas.

I would love to have the dual antennas resonant on 11, 10 and 2 meters. It would be neat having dipole antennas when everyone else uses monopoles.
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Old 06-05-2018, 11:12 AM
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Here is a commercial mount that will take one whip pointing up connected to the hot side of the coax and one pointing down connected to the ground side: https://www.ebay.com/itm/MFJ-347-HAM...kAAOSwJEla46VF

When I mention a CB whip I am referring to a vertical monopole working against a ground plane of some kind. A vertical monopole can be a full 1/4 wavelength stainless whip, or 1/4 wave or 5/8 wave of resonant wire wrapped around a shortened fiberglass rod or a SS whip with a loading coil in the middle, etc.

Here is something to think about. If you take a typical SS mirror mount on a fiberglass cab and stick a CB whip on it, the mirror frame ground plane presented to the antenna is completely inadequate. In this and many cases the coax running back to the radio becomes the other half of the antenna and it can radiate as much as the whip. Since the coax winds through the truck cab mostly horizontal and is usually wrapped up into a mess, whatever it radiates doesn't do much good for communications and it can pick up noise that will travel on the outside of the braid up to the antenna to be received by the radio.

If you turn the mirror mounted whip into a dipole with a second resonant whip pointing downward, its a now a complete antenna hanging in space and if tuned right will radiate all the power you put into it instead of lighting up the coax as half your antenna. Placing an RF choke at the feedpoint of the antenna will help insure the coax doesn't become a radiating part of the antenna. The second whip pointing down will also contribute to the radiation and improve the strength of your transmit and receive signal.

Quote:
Originally Posted by Driverj30t9 View Post
A whip antenna is looking for lots of flat sheet metal right under it and a ground wire to the frame or battery is not a ground plane. Its a DC ground but the antenna will probably not know its there.

There goes that idea then.

I always thought of whip antennas as 102 inch metal antennas that are thin and flexible, but apparently they can be fiberglass and really long or short. Those fiberglass antennas are quite rigid though.

What type of a mount would I use for a vertical mirror mount dipole? I have like a six inch bird perch mount installed right now so the lower ground side antenna wouldn't be very far away from the truck body, that can't be good. Maybe there is another mount I could put on the mirrors to get the antennas further away from the truck body.

Is the radio choke for reducing EMI, and parasitic capacitance since the lower antennas on the ground side, and proximity effect losses and something to do with the asymmetrical coax and symmetrical dipole, or is that just a balun? Need to read up more on this stuff.

What parts of the mount are grounded? I know with a bird perch mount the top part is below the stud nut and washer. The whole mounts grounded then except the stud nut?

I"ll give those tuning methods a try if I put up the antennas.

I would love to have the dual antennas resonant on 11, 10 and 2 meters. It would be neat having dipole antennas when everyone else uses monopoles.
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Old 06-06-2018, 8:11 AM
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Okay, I see. Funny that the coax can radiate as much as the antenna even though it's designed not too.

I think I'm going to install some workman 102" whip antennas. I don't know about the downward facing antennas yet.

Is rusting on the mount, coax and antenna much of a concern? Maybe use some coax seal or plasti-dip on some stuff?

I'm also concerned about the coax receiving interference because I plan on running it down the mirrors to under the hood and along the bottom of the truck to the sleeper where it will come up through a hole and routed to my radios from there.
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Old 06-06-2018, 11:13 AM
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The coax can radiate due to RF currents flowing on the outside of the braid, not from the inside. What kind of truck is this going in? 9ft whips on most big rigs would be a good 16ft or more in height and would hit stuff overhead and possibly get you a ticket.

Mirror mounts come in aluminum, chrome plated steel or stainless steel. I would use a steel mount and rust is not a big problem in my experience. You do want to seal up the coax connections to keep water from getting into the connector or wicking up the coax braid.

If the truck is diesel then there is no spark plug noise to radiate into the antennas or coax, so running the coax in or near the engine compartment should not be a problem there. If its gas then you risk more noise if you run coax near the engine.

Quote:
Originally Posted by Driverj30t9 View Post
Okay, I see. Funny that the coax can radiate as much as the antenna even though it's designed not too.

I think I'm going to install some workman 102" whip antennas. I don't know about the downward facing antennas yet.

Is rusting on the mount, coax and antenna much of a concern? Maybe use some coax seal or plasti-dip on some stuff?

I'm also concerned about the coax receiving interference because I plan on running it down the mirrors to under the hood and along the bottom of the truck to the sleeper where it will come up through a hole and routed to my radios from there.
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Old 06-06-2018, 3:15 PM
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A fiberglass antenna is actually made of metal encased in fiberglass. Many large dish antennas are (and some still are) made of metal mesh in a fiberglass frame. Whips have a very thin wire inside. They typically don't perform as well as a solid metal whip and won't handle much power. Some prefer them because they're more rigid and might work better in adverse weather. A helical wound or some other not-straight wire inside fiberglass is common and a good way to do it, may or may not be a good performer. Some truck cabs are all steel, some have lots of fiberglass or whatever. An antenna on top of a metal cab with perhaps a tiny hole drilled in a non-metal air deflector if there's one up there to permit the wire up through would make a decent installation. Magnetic mounts are okay but not as good as fixed. Also, at highway speed x2 as you pass other traffic in the opposite direction will yank off less strong magnets. That 100MPH wind is no whimp.

It's too bad, especially for trucking, the USofA didn't choose UHF for citizens band way back when. (Some countries did, although it was a rough start as 27MHz was already popular and it's hardly standardized around the world) It would have certainly been better for the most common use then (minus the late 70s craze) and now.
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Old 06-08-2018, 3:53 AM
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Yeah I think the whip is gonna be to long. Have to find a smaller center loaded. Don't know if top loaded or center loaded is better. Nope, not gas, it's diesel, that's a plus.The alternator and battery wires and other wires must not have to much of an effect then? Chromes nice to have for semis.

So if I install dipole antennas I imagine it would still be beneficial to mount the mount to the truck body as well if it is indeed metal? Or would that cause a bad radiation pattern?

If I have an antenna on the top of the cab and it's metal stringing the coax through the air deflector might help support it a bit, otherwise dangle it down the back of the truck I guess then zip tie it underneath the truck to where it will come in through the sleeper.

Magnetic mounts my only option I guess because I'm probably not going to drill a hole on top of the company truck for an NMO mount.

Doesn't UHF get less range then HF though? What would the benefits be? Might be nice if they didn't have all that noise that you get on am if it was fm. Most truckers probably could care less though. Lot of the new school drivers don't even use cb anymore which I think is not smart.

Some truckers use VHF I think? Not sure. Forgot what it's called, think they use it in canada a lot more. Another good reason to have a ham radio.
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Old 06-08-2018, 4:43 AM
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Quote:
Originally Posted by Driverj30t9 View Post
Doesn't UHF get less range then HF though?
At the power levels of legal use, and intended use, not much more. Those that are using SSB, and beam antennas, will get a lot further, but that is not the typical user. Since a short distance is being covered, UHF works fine, and uses smaller antennas.

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Old 06-08-2018, 9:11 AM
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Ahhh, that's right, the wattage. UHF must get out quite well if someones running it at the maximum legal operating power.
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Old 06-10-2018, 9:05 AM
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For a working trucker in the US who wants to use the radio for work, the only radio game in town is the CB, because that's what is actually used on the road and at the loading docks, etc.

It's true that many truckers don't use the CB nowadays, at least not until there's a traffic hold-up, and then they fire it up to get the drift on what the problem is, what lane is open, what alternative route can I get off on unless already stuck in the queue, etc.
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Old 06-10-2018, 11:38 AM
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Quote:
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It's true that many truckers don't use the CB nowadays, at least not until there's a traffic hold-up, and then they fire it up to get the drift on what the problem is, what lane is open, what alternative route can I get off on unless already stuck in the queue, etc.
If they had the radio on to start with, they might not have gotten into the jam.

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Old 06-11-2018, 9:30 AM
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Yeah, don't drill holes if it's not your truck. That is one reason why vice-grips on the mirror are so common.

There's a lot of detail but as for line-of-sight signals which is what CB is supposed to be, UHF, VHF, and HF are all about the same as far as signal distance. My point is that a UHF installation would be easier and certainly offer more predictable performance. It's a moot point though, too late now. But we wouldn't have had all these people trying to run 1000 Watts, over-modulating, whistling, repeating "audio, audio, audio". More than several miles while out on the highway isn't very useful. CB as it was intended is a fine service. In UHF it may well have remained that way. But perhaps not. I'm merely hypothesizing.

Some figure with cell phones, live traffic maps on the phone, and that sort of thing the CB isn't as useful as before. I'd still want one if it were me. It would be tough to beat getting local directions that way. There's other licensed and unlicensed services out there.
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