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Coax length on mag mount question

Unit 75 said:
Mag mounts do not properly "couple" the shield of your coax to the vehicle body. The vehicle body is the OTHER HALF of your antenna and only "minimal capacitive coupling" to the body occurs with your mag mount. Your coax shield needs to be solidly connected to your vehicles body. Shortening your coax threw your antenna balance out. You have a common mode current problem and they are using nearby conductive objects to attempt to "balance" the current flow on your antenna so BOTH sides are equal (Kirchholff's current law) that is WHY your experiencing RF burn and OTHER erratic behaviour with other equipment in your truck. You need to increase the "counterpoise area" so you have a BALANCE. Getting one of those ground plane Wilson kits that screw on between the coil and mag base will also help you out, improve balance and reduce RFI.
-73's from the Bam brother.
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Definitely interesting. I have ran some hard mount setups over the years with very short coax, obviously no issue with the braid connected directly to the vehicle. I really had had no idea that changing this one would matter. The length is part of the equation with a mount like this, as I understand you and Road Squawker.

Thanks for the response. Hope you had a good Thanksgiving down there in Sweet Home Alabama.
 
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dledinger said:
I cut it at the connector end. I agree on connections that pop up at higher power, I have seen that also. I tend to think I ruled that out when I added a jumper in and everything went back to normal.
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But it didn't. When you added in the jumper you changed the overall length of the coax and instead merely re-tuned what is being used for a RF ground. You would find that if you removed all the coax and changed it to a length of an electrical quarter wave or three quarter wave it would appear to be working as normal too.
 
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jessejamesdallas said:
If your getting rf burn from the mic...just run a ground strap off one of the radios mounting bracket screws to a good grounding point (like the firewall)...problem solved...

your welcome.
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But the problem isn't solved. You still have RF on the outside of the coax braid, you still have common mode RFI and as a result you still have more noise on receive than you should have and you still have RF spewing out everywhere when you TX instead of out of the antenna.
 
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Unit 75 said:
You need to increase the "counterpoise area" so you have a BALANCE. Getting one of those ground plane Wilson kits that screw on between the coil and mag base will also help you out, improve balance and reduce RFI.
Click to expand...

No they won't. They're too short to be of any use.
 
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Son was by last night having issues with his feed line into the headliner. I had installed it about 15 years ago. He was having periodic high SWR.

We pulled the headliner down and found that the dielectric around the center conductor had shrunken back 3” or so, leaving the center conductor routed through a couple inches of bare copper braid.

Had never seen anything like that. Cable is marked Belden RG213, and if I remember correctly was salvaged from somewhere else at the time I ran it.
 
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M0GVZ said:
But it didn't. When you added in the jumper you changed the overall length of the coax and instead merely re-tuned....
Click to expand...

We were talking about a potential error with the PL259 installation, and I simply meant that it went back to behaving as before with no obvious symptoms.
 
M0GVZ said:
No they won't. They're too short to be of any use
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That's kind of like saying the length of coax he cut off to begin with was too short to present the problem that wasn't there before he cut it. Those radials may appear to be short but it will help balance the currents. It certainly worked for a friend of mine with the exact same problem or I wouldn't have shared that information.
-73's
 
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Unit 75 said:
That's kind of like saying the length of coax he cut off to begin with was too short to present the problem that wasn't there before he cut it. Those radials may appear to be short but it will help balance the currents. It certainly worked for a friend of mine with the exact same problem or I wouldn't have shared that information.
-73's
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That’s the kind of discussion I was looking for out of this post.

I kind of like when folks disagree...that’s the point of friction where I can learn something ;)
 
Shadetree Mechanic said:
I wonder if it would help to put a choke at the antenna?
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Yes and no. I have choked mobile antennas in the past, and it has solved CMC issues. The thing you have to consider is with a magnet mount antenna is that if you put the choke at the antenna's feed point, several feet of the coax shielding will be close enough to the antenna that it will still pick up the signal and still have common mode currents. If a choke is needed, it really does work better if it is on the other side of the vehicle's body, hence I prefer to use them with permanent mounts. The thing about magnet mounts is, with this type of antenna, you need the choke as close to the feed point as possible, so if you put a choke on such an antenna installation you will also have a hunk of mass outside of the vehicle where it can catch wind, so you need to do something to said choke so it doesn't damage your vehicle. I suppose it is possible to locate the antenna where the coax goes straight into the vehicle, but that is not normally the case with magnet mount antennas.

So how do I determine if a choke is needed? I simply add a six foot length of coax, and if SWR changes noticeably a choke is needed, especially if the SWR goes up.

When it comes to the feed point and the "balance" that someone brought up, don't worry to much about this. The chances of any vehicle providing anywhere near the impedance needed for the so called "other half" of the antenna to be in balance with the vertical element is astronomically small. Because of this, it is a good thing that balance is not what determines how much CMC you have flowing on your coax. That is simply determined by the impedance the shielding of the coax is presenting compared to the impedance presented by the so called "other half" of the antenna system. Its much the same as calculating current for a DC parallel resistor circuit, all that is happening is the outside shield of the coax is providing a low enough impedance compared to the car that some antenna currents are flowing on it. Putting an RF Choke at the feed point is like adding an additional resistor to that path that is the coax, causing fewer currents to flow on it.


The DB
 
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Unit 75 said:
Getting one of those ground plane Wilson kits that screw on between the coil and mag base will also help you out, improve balance and reduce RFI.
Click to expand...

Unit 75 said:
Those radials may appear to be short but it will help balance the currents.
Click to expand...

75, I've heard Wilson and plenty of other folks claim these short radials, like I think you're describing here, will solve problems with mobile setups and improve the signal noticeably.

Do you have a link to this product, and if so, could you post it? I looked but I don't see the product anymore.

I think the only thing this hub with radials at the bottom of the radiator will only provide some inductance. I want to try this idea out on an Eznec mobile antenna model and see what modeling predicts with this item.
 
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Sorry, Marconi. I do not have a link. The threading is pretty standard so it should fit your application. It would be interesting to post a before and after results when you complete your modeling. I do know that my friend, who was using a Wilson 5000 mag mount, had to chop a small section of feed line that was badly crimped near the PL259 end. As a result, he began having some RF issues inside the cab of his Dodge Ram pick up, but only when he was running his amplifier. He started experiencing some RF burns from his handheld microphone. I remember laughing and told him his lips looked a bit bigger. He never had those problems until he chopped his coax. He told me that "the burn" with the amp on disappeared after he installed that screw on ground plane kit. I thought that was kind of cool but said that he really needs to use a solid mount and get rid of that mag mount because it wasn't providing adequate bonding for his antenna. Well, he did. He drilled a hole in his roof. He still has that GP kit on that antenna even after he changed the base out...lol but hey, no more RF issues inside his cab. He was the guy that came to mind when dledinger made his post.
-73's
 

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