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Newb: SWR Question

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Papageoff

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First time posting g but I've lurked for a while. I'm trying to learn and I hope you folks can help me.

I have a Uniden 980ssb tied to a 4' firestick via 18' of cable on my truck. The antenna is attached to an L-bracket using a spring and quick disconnect to the center of my toolbox. I added a ground wire (12ga iirc) from the toolbox down to the frame at a factory ground point.

I went to retune my SWR today and with my external meter the best I could do was ch1- 1.1 and ch40-3+. The internal meter showed similar numbers so I unhooked the external and the numbers changed to ch1- 1.5 and ch40-1.4 with no other adjustments.

So, it seems like the antenna cable length is affecting the SWR which I didn't think it was supposed to do. Is my cable acting as part of the driven element and if so is that a problem and if so how do I fix it?

Thanks.

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prcguy

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In the case of a bed mounted tool box, the lid of the box is probably acting like a floating and insufficient size ground plane for the antenna. Your ground wire is good for a DC ground but not for RF, the antenna wants to have a lot of sheet metal under it and is unaware of the vehicle frame under it. Antennas are somewhat blind and pretty narrow minded.

In this case with an insufficient ground plane, the coax shield can become a radiating part of the antenna and can cause varying problems when you change its length. You might try the same SWR test while someone grabs the coax and slides their hand up and down the coax to see if the SWR changes. If it does you need to increase your ground plane size or decouple the coax from the antenna.

You can decouple the coax using a good 1:1 choke balun or get an FT-240-61 ferrite core (about $12) and wrap 12 turns of your coax through the core near the antenna. The specific 61 mix ferrite with 12 turns will target the 26.5 through 30MHz range with more than 8k ohms of choking impedance and probably close to 40dB of isolation.

First time posting g but I've lurked for a while. I'm trying to learn and I hope you folks can help me.

I have a Uniden 980ssb tied to a 4' firestick via 18' of cable on my truck. The antenna is attached to an L-bracket using a spring and quick disconnect to the center of my toolbox. I added a ground wire (12ga iirc) from the toolbox down to the frame at a factory ground point.

I went to retune my SWR today and with my external meter the best I could do was ch1- 1.1 and ch40-3+. The internal meter showed similar numbers so I unhooked the external and the numbers changed to ch1- 1.5 and ch40-1.4 with no other adjustments.

So, it seems like the antenna cable length is affecting the SWR which I didn't think it was supposed to do. Is my cable acting as part of the driven element and if so is that a problem and if so how do I fix it?

Thanks.

Sent from my SM-G892A using Tapatalk
 
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Papageoff

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In the case of a bed mounted tool box, the lid of the box is probably acting like a floating and insufficient size ground plane for the antenna. Your ground wire is good for a DC ground but not for RF, the antenna wants to have a lot of sheet metal under it and is unaware of the vehicle frame under it. Antennas are somewhat blind and pretty narrow minded.

In this case with an insufficient ground plane, the coax shield can become a radiating part of the antenna and can cause varying problems when you change its length. You might try the same SWR test while someone grabs the coax and slides their hand up and down the coax to see if the SWR changes. If it does you need to increase your ground plane size or decouple the coax from the antenna.

You can decouple the coax using a good 1:1 choke balun or get an FT-240-61 ferrite core (about $12) and wrap 12 turns of your coax through the core near the antenna. The specific 61 mix ferrite with 12 turns will target the 26.5 through 30MHz range with more than 8k ohms of choking impedance and probably close to 40dB of isolation.
Hmm... Not sure what difference if any that it would make but the antenna is on the body of the box facing the tailgate, not the lid. What you described makes sense though. I'll try that test tomorrow if I can get some time... It may be a few days. Thanks for the suggestion.



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K4EET

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Papageoff,

Firstly, Welcome to Radio Reference!

prcguy gave you some good tips to look into to see if you can "tame" your antenna system.

Ideally, when inserting an SWR meter, it would have minimal effect on the antenna loading. But that obviously is not your case due to the coax shield acting as part of the antenna. Your #12 AWG ground wire to the truck's chassis is probably also acting somewhat as a radiator.

You cite the SWR on Channels 1 and 40 but you should also check it throughout the band (if you have not done so already) so you will know what the antenna's response is. If the SWR is 1.5:1 or so on the edges and decreases to 1:1 at Channel 20, then that's perfect!

Personally, I have never trusted the built-in SWR meters on many CB rigs because they are generally small and often poorly calibrated. I know because I used to test them for a friend's shop when they came in for repair. On the Uniden 980 SSB, the SWR meter only has markings for 1.5:1, 2:1 and 3:1. That is good enough to know if you are below 2:1 (as long as the meter is fairly accurate) but not good enough for accurate SWR readings. This is where an external SWR meter comes in handy and those with cross-needles are the easiest to use.

Let us know how you make out on decoupling the coax so that you can use an external SWR meter. And once again, welcome to Radio Reference!

Cheers! Dave
 

Papageoff

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Papageoff,

Firstly, Welcome to Radio Reference!

prcguy gave you some good tips to look into to see if you can "tame" your antenna system.

Ideally, when inserting an SWR meter, it would have minimal effect on the antenna loading. But that obviously is not your case due to the coax shield acting as part of the antenna. Your #12 AWG ground wire to the truck's chassis is probably also acting somewhat as a radiator.

You cite the SWR on Channels 1 and 40 but you should also check it throughout the band (if you have not done so already) so you will know what the antenna's response is. If the SWR is 1.5:1 or so on the edges and decreases to 1:1 at Channel 20, then that's perfect!

Personally, I have never trusted the built-in SWR meters on many CB rigs because they are generally small and often poorly calibrated. I know because I used to test them for a friend's shop when they came in for repair. On the Uniden 980 SSB, the SWR meter only has markings for 1.5:1, 2:1 and 3:1. That is good enough to know if you are below 2:1 (as long as the meter is fairly accurate) but not good enough for accurate SWR readings. This is where an external SWR meter comes in handy and those with cross-needles are the easiest to use.

Let us know how you make out on decoupling the coax so that you can use an external SWR meter. And once again, welcome to Radio Reference!

Cheers! Dave
Thanks for the info on the meter. My 980 has a digital readout of the ratio and was, 1.4-1.5 ish on the edges and 1.0 on 19 without the external meter connected.

When I had the external meter in line, I compared the internal meter's readings and they were as close as they reasonably could be. For that reason I thought I could trust my radio's meter but maybe that's misplaced, idk.



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Papageoff

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So, from what I've read, to make a balun at home I'd need about 4-5' of cable. It's hard to recall as I installed this over almost 2 years ago, but I don't think I have that much slack in the cable. I looked at chokes and thought maybe one of them would work.

https://smile.amazon.com/eBoot-Piec...&qid=1544022569&sr=8-3&keywords=ferrite+choke

Would this be suitable? Do I need one at both ends or just one or the other?

edit: I just found this on EBAY, it looks huge and I have no idea where I'd hide it, but I think it's what I'd need.
https://www.ebay.com/itm/HF-antenna...-CMC-130S-3K/252743228968?hash=item3ad8ab9e28

Does that look right?

PRCGUY: Your suggestion of the bare magnet might work for testing. I'd be worried about how to protect it in the bed long-term, though. How tightly do I wrap the cable around it? Do the loops need to be bound together (for reasons other than neatness) or spread apart for any reason?Also, getting 12 turns might prove difficult depending on the available slack.




Finally, my preference for optimizing things aside... Is this situation going to damage the radio or the truck if I chose to ignore it? I mean, the ebay balun once I buy the terminals and what not is going to be close to $100. The SWRs seem okay using the internal meter and given that it matches the external meter I think it's probably trustworthy.
 
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prcguy

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A single snap on ferrite in your amazon link will do less than nothing at CB frequencies but a string of them about a foot or more long will give some reasonable choking. The MyAntennas choke in your Ebay link is one of the best on the market but you should do some testing to see if you actually need one before spending that much $$. You can also get some scrap coax and wrap that around an FT-240-61 core and install a male and female connector so it can go in line with your existing coax. That would be a very good choke for cheap.

Another type of test you can do is attach about 9ft of wire to the grounded base of the antenna and stretch that out horizontal. Then test with and without your external meter and coax jumper to see if the match on the radio meter stays the same with or without the added coax and meter.

Bottom line is a choke will reduce or eliminate RF on the outside of your coax and will calm things down but your signal strength to others will not change much with or without the choke. If your antenna needs more ground plane under it and you can give it more your signal will get a little better on transmit.

So, from what I've read, to make a balun at home I'd need about 4-5' of cable. It's hard to recall as I installed this over almost 2 years ago, but I don't think I have that much slack in the cable. I looked at chokes and thought maybe one of them would work.

https://smile.amazon.com/eBoot-Piec...&qid=1544022569&sr=8-3&keywords=ferrite+choke

Would this be suitable? Do I need one at both ends or just one or the other?

edit: I just found this on EBAY, it looks huge and I have no idea where I'd hide it, but I think it's what I'd need.
https://www.ebay.com/itm/HF-antenna...-CMC-130S-3K/252743228968?hash=item3ad8ab9e28

Does that look right?

PRCGUY: Your suggestion of the bare magnet might work for testing. I'd be worried about how to protect it in the bed long-term, though. How tightly do I wrap the cable around it? Do the loops need to be bound together (for reasons other than neatness) or spread apart for any reason?Also, getting 12 turns might prove difficult depending on the available slack.




Finally, my preference for optimizing things aside... Is this situation going to damage the radio or the truck if I chose to ignore it? I mean, the ebay balun once I buy the terminals and what not is going to be close to $100. The SWRs seem okay using the internal meter and given that it matches the external meter I think it's probably trustworthy.
 

Papageoff

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A single snap on ferrite in your amazon link will do less than nothing at CB frequencies but a string of them about a foot or more long will give some reasonable choking. The MyAntennas choke in your Ebay link is one of the best on the market but you should do some testing to see if you actually need one before spending that much $$. You can also get some scrap coax and wrap that around an FT-240-61 core and install a male and female connector so it can go in line with your existing coax. That would be a very good choke for cheap.

Another type of test you can do is attach about 9ft of wire to the grounded base of the antenna and stretch that out horizontal. Then test with and without your external meter and coax jumper to see if the match on the radio meter stays the same with or without the added coax and meter.

Bottom line is a choke will reduce or eliminate RF on the outside of your coax and will calm things down but your signal strength to others will not change much with or without the choke. If your antenna needs more ground plane under it and you can give it more your signal will get a little better on transmit.
Hmm... I saw a YT video where the guy said to remove the ground to the frame and replace with a ground to the bed and tie the bed and cab together. While I don't want to drill holes in my truck, I'm scientifically curious if that would make a difference.

Thoughts?

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RFI-EMI-GUY

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The only right solution is a Larsen NMO27 antenna and the required hole drilled into the center of the cab.

Tune it once and forgeddaboudit.

No need for counterpoise tricks or signal robbing ferrite chokes.

Firesticks are 'meh...
 

Papageoff

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The only right solution is a Larsen NMO27 antenna and the required hole drilled into the center of the cab.

Tune it once and forgeddaboudit.

No need for counterpoise tricks or signal robbing ferrite chokes.

Firesticks are 'meh...
Yeah, but I'd like to be able to trade the truck at some point, so... No rooftop holes.

Is there another antenna that you think is superior for my application that wouldn't require rooftop mounting? I'm frequently open to the purchase of additional toys.

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prcguy

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A firestick the same length as the Larson will work about the same and there are a lot more mounting options for a 3/8-24 threaded antenna than an NMO. With that said I would never put a Firestick in the center of a cab unless someone had already put a ball mount there.

Mount a Larson NMO27 in the same toolbox the OP has his Firestick on and you will have the same type of problems. Ferrite chokes don't rob any signal, they just absorb RF currents on the outside of the coax where they don't belong.

I think the match the OP sees on his radio SWR meter is fine and if he is getting reasonable range with the Firestick then call it a day and be happy.

The only right solution is a Larsen NMO27 antenna and the required hole drilled into the center of the cab.

Tune it once and forgeddaboudit.

No need for counterpoise tricks or signal robbing ferrite chokes.

Firesticks are 'meh...
 

Papageoff

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Ferrite chokes don't rob any signal, they just absorb RF currents on the outside of the coax where they don't belong.

I think the match the OP sees on his radio SWR meter is fine and if he is getting reasonable range with the Firestick then call it a day and be happy.

Well, I suppose if it was that easy, then people would do it by default. I have a firering cable so that makes putting anything near the antenna a bit of an ordeal.

I'm going to try the wire suggestion when I can, because that seems the simplest way to confirm that we are on the right track. It just gets dark so early now that it's hard to do stuff after work.

Any thoughts on the bed vs frame ground point, though?

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prcguy

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Unless the wire to the bed frame adds some groundplane right under the antenna, I don't think it will benefit anything. The flat surface of the tool box top right under the antenna has the most influence. The sides of the box will have a very small effect. If the wire went from the base of the antenna down at an angle for the length of the bed like a ground radial, then you would see an improvement. If the top of the tool box was 10 X 10ft and the antenna was in the middle then we wouldn't be having this conversation.

The match you saw on the radio SWR meter with the external meter disconnected sounds very reasonable, especially if it takes a dip in the middle channels. If that's the case then I would use it.

Well, I suppose if it was that easy, then people would do it by default. I have a firering cable so that makes putting anything near the antenna a bit of an ordeal.

I'm going to try the wire suggestion when I can, because that seems the simplest way to confirm that we are on the right track. It just gets dark so early now that it's hard to do stuff after work.

Any thoughts on the bed vs frame ground point, though?

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RFI-EMI-GUY

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A firestick the same length as the Larson will work about the same and there are a lot more mounting options for a 3/8-24 threaded antenna than an NMO. With that said I would never put a Firestick in the center of a cab unless someone had already put a ball mount there.

Mount a Larson NMO27 in the same toolbox the OP has his Firestick on and you will have the same type of problems. Ferrite chokes don't rob any signal, they just absorb RF currents on the outside of the coax where they don't belong.

I think the match the OP sees on his radio SWR meter is fine and if he is getting reasonable range with the Firestick then call it a day and be happy.

The use of ferrites to absorb (convert to heat) the errant current flowing down the coax does indeed rob useful signal that could otherwise be put into a proper counterpoise and radiated as a stronger signal.

It is like having an HF vertical on poor soil when you sop up signal into ferrite. It will tune up, but the signal will be weak.

The Fire stick antenna is a gimmick antenna unless it has an effective groundplane. It is marketed with mounting and feed options that are inferior to a properly installed NMO27. I don't think there is any cost savings whatsoever.

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RFI-EMI-GUY

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Yeah, but I'd like to be able to trade the truck at some point, so... No rooftop holes.

Is there another antenna that you think is superior for my application that wouldn't require rooftop mounting? I'm frequently open to the purchase of additional toys.

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No holes? In a truck? I gather this truck has never hauled anything in the bed?..

Seriously the NMO27 works better and is less fiddly. You do have to trim it to a chart, carefully. And tune it for VSWR. But once your done, you are done. There is a rubber plug you can fill the hole with on trade in, or leave the antenna for the next owner. Ready for a new CB.

By the way, always have the doors shut and tailgate closed like normal when you adjust VSWR. And do it in the open, away from other metal stuff and overhead wires.

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Papageoff

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Interesting. Well like I mentioned, it's not actually on top of the box. So, if we use the word under somewhat liberally, then the bed floor is "under" the antenna. I have a pic attached showing it. ...and yes I'm changing the mount to get rid of that rust.

If there is a way to get the bed of the truck to be the counterpoise then I think I'd be in high cotton. Honestly, I thought that's what I'd done by by the ground wire to the frame. I mean eventually all of the body is tied together electrically. I figured that was good enough.



Unless the wire to the bed frame adds some groundplane right under the antenna, I don't think it will benefit anything. The flat surface of the tool box top right under the antenna has the most influence. The sides of the box will have a very small effect. If the wire went from the base of the antenna down at an angle for the length of the bed like a ground radial, then you would see an improvement. If the top of the tool box was 10 X 10ft and the antenna was in the middle then we wouldn't be having this conversation.

The match you saw on the radio SWR meter with the external meter disconnected sounds very reasonable, especially if it takes a dip in the middle channels. If that's the case then I would use it.

9e5d6b7aa8b84a2c9cf04acb4f0ea671.jpg


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prcguy

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If the antenna system was designed to use the coax shield as a radiating element, then yes ferrite on the coax would rob some power. An end fed antenna using a non resonant length of wire and a 9:1 balun would be an example of an antenna that uses the coax shield as a counterpoise, but you have to choke those near the radio to avoid hot RF on the radio and mic.

A vertical antenna with a single counterpoise wire is actually a dipole and that single counterpoise radiates, and choking off the coax to prevent RF on the coax and coax radiation would be a primary goal and would not rob any useful signal that would radiate as a stronger signal. For a vertical antenna with multiple counterpoise wires or a ground plane, the ground plane or counterpoise does not radiate. In this case a primary goal would be to insure the coax does not have RF flowing on the shield and adding a choke balun would not rob any useful signal that would radiate as a stronger signal.

The counterpoise in both above examples connects to the ground side of the antenna feedpoint and a 1:1 ferrite choke balun would have no effect on RF currents flowing into a counterpoise since the choke balun is downstream from the feedpoint.

In your example of the vertical over poor soil, I'm assuming its a vertical attached to a ground stake with no ground wires or counterpoise and that leaves the coax shield as the only counterpoise. That is a situation you don't want to create and if so choking off the coax will degrade performance because someone made a mistake installing a vertical with no counterpoise, leaving the coax as half of a dipole laying on the ground and radiating all the way back to the radio. I spend at least several days a week playing with choke baluns and fixing other peoples RF problems, so its a very familiar topic for me.

The firestick is simply a short loaded antenna and they do have an optional gimmick coax feed for mounting to truck mirrors with poor ground plane. The no ground mount came out after I left the CB market around 1980 and I have virtually no experience with it. If the OP is using the no ground cable, then that may explain the changes when he inserts a meter and jumper since that cable harness is probably a tuned length. In that case I might try a regular coax run that grounds to the tool box.

Otherwise I've installed hundreds, possibly 500 firesticks and compared them side by side to everything on the market at the time and they work about the same as a similar length Wilson or base loaded A/S or Turner was popular back then, or Motorola, or you name it. I swapped lots of antennas doing dozens of range tests between every antenna available and for its size the Firestick holds its own against anything of the same length.

My first choice would be a Larson or similar roof mounted, but the OP indicated he can't drill a hole, so a Larson is out of the question anyway.


The use of ferrites to absorb (convert to heat) the errant current flowing down the coax does indeed rob useful signal that could otherwise be put into a proper counterpoise and radiated as a stronger signal.

It is like having an HF vertical on poor soil when you sop up signal into ferrite. It will tune up, but the signal will be weak.

The Fire stick antenna is a gimmick antenna unless it has an effective groundplane. It is marketed with mounting and feed options that are inferior to a properly installed NMO27. I don't think there is any cost savings whatsoever.

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RFI-EMI-GUY

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Remove the ground wire and try it that way and see what happens it may be acting like a counterpoise,and also your L mount looks kind of rusty from the photo.....
That rusty old mount might have excessive resistance and not effectively grounding the RF current to the tool box or even to the fire ring connector. Also, the signal is not going to radiate well in the forward direction with it below the roof of the cab.

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