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Coax length on mag mount question

2NC995

2NC995

DAN
Feb 23, 2008
2,219
4,005
273
Coastal NC
So...I shortened the coax on a Wilson mag mount, taking off 7’ or so. Checked SWR and everything is great.

Then a few hours later I go for a drive and upon TX I get an RF burn between my hand and face. Ok...turn amp off, check SWR and all is good. Hopped out looked over antenna, then try to TX once more...burned again, but I also notice amp is oscillating (drawing 40 amps SSB when I’m keyed but not talking) and radio display is going bonkers.

So I cruise back home and put in a jumper in extending the feed line to 15/16 feet. Everything back to normal again.

Obvious solution is putting in a hard mount and going on about my business, but I’d like to have somewhat of an understanding what is happening here.

I can only theorize that I have feedline radiation due to the mounting arrangement, and that it’s more of an issue with the shorter coax?
 

dledinger said:
So...I shortened the coax on a Wilson mag mount, taking off 7’ or so. Checked SWR and everything is great.

Then a few hours later I go for a drive and upon TX I get an RF burn between my hand and face. Ok...turn amp off, check SWR and all is good. Hopped out looked over antenna, then try to TX once more...burned again, but I also notice amp is oscillating (drawing 40 amps SSB when I’m keyed but not talking) and radio display is going bonkers.

So I cruise back home and put in a jumper in extending the feed line to 15/16 feet. Everything back to normal again.

Obvious solution is putting in a hard mount and going on about my business, but I’d like to have somewhat of an understanding what is happening here.

I can only theorize that I have feedline radiation due to the mounting arrangement, and that it’s more of an issue with the shorter coax?
Click to expand...

What kind of amplifier are you using? Is it installed correctly?
Obviously, the amplifier doesn't see a load that it likes and that it can work with. How does the radio by itself work? I'd put in a new run of good coax and tuck the extra away, not coiled up and/or kinked, where it won't trip passengers and just go from there. Nothing inherently wrong with a magmount if it has enough metal to function as a ground plane.
 
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dledinger said:
I can only theorize that I have feedline radiation due to the mounting arrangement, and that it’s more of an issue with the shorter coax?
Click to expand...

Yup. I bet if you tried it with the original length without the amp you'd not have the issue. What amp are you running that draws 40A? I'm guessing over 100W which would be the limit of what I'd consider running on a magmount.
 
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Beetle said:
What kind of amplifier are you using? Is it installed correctly?
Obviously, the amplifier doesn't see a load that it likes and that it can work with. How does the radio by itself work? I'd put in a new run of good coax and tuck the extra away, not coiled up and/or kinked, where it won't trip passengers and just go from there. Nothing inherently wrong with a magmount if it has enough metal to function as a ground plane.
Click to expand...

A DX500. Set up correctly as far as I know. Have been running it for a while with the longer coax and no issues. The only check I did with radio alone was SWR, and it was fine.

For a “fix” I’m planning to just leave the jumper in until I install a hard mount. No sense in running a new feed line to the magnet at this point.
 
M0GVZ said:
Yup. I bet if you tried it with the original length without the amp you'd not have the issue. What amp are you running that draws 40A? I'm guessing over 100W which would be the limit of what I'd consider running on a magmount.
Click to expand...

The original length worked fine with or without the amplifier.
 
Slowmover said:
Durn it. It’s 18-feet needed. How many times we gotta tell you guys?

The square of the velocity of light cubed times three. A New Mexico Minute. How much more simply can that be stated?
Click to expand...

Slow,

That's theroy....I have run coax with the math and using enough to get from radio/ linear to antenna. To each his own. Use good quality coax. I find the coax that comes with a mag mount to be less than stellar, especially if running power.
 
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dledinger said:
Hmm. Is the theory not that the magnet couples the braid to the vehicle body via capacitance?
Click to expand...

If you have a large enough magnet, this is accurate. But if the body of the vehicle (or whatever part of the vehicle the antenna is mounted) is inadequate to be a ground plane, you can still have currents on the braid, even if you have a better mount than a magnet mount.


The DB
 
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dledinger said:
A DX500. Set up correctly as far as I know. Have been running it for a while with the longer coax and no issues. The only check I did with radio alone was SWR, and it was fine.

For a “fix” I’m planning to just leave the jumper in until I install a hard mount. No sense in running a new feed line to the magnet at this point.
Click to expand...


There is a few good vids about installing fresh coax in Wilsons , I would cut the film in the center of the magnet . Crack loose the 9/16 nut drill out the coax hole and run 9 feet of lmr 400 . I have pushed 600+ into a mag 5000 like this with zero problems . I loved zero oxygen mini-8 coax but the 400 really helps clean things up and get the fire down the wire !! $15 on ebay tops and that's with a connector lol

If it talks , keep talkin ;)

73's 4-2-zero New Jersey Brian
 
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Redbeard U812 said:
Slow,

That's theroy....I have run coax with the math and using enough to get from radio/ linear to antenna. To each his own. Use good quality coax. I find the coax that comes with a mag mount to be less than stellar, especially if running power.
Click to expand...


Im sure I will get some boo's here but I cut my coax to needed length for any particular install ;) never get out a calculator or a tape-measure . Never been a big problem so (and I am certainly no expert ) I can only assume all the coax math and rules are for optimal performance , so Im sure I have some loss but its never been an issue for me . (PLEASE DONT BAN ME I KNOW THERE IS SCIENCE AND MATH BEHIND COAX LENGTH LOL )

Just one mud ducks opinion o_O 73's Brian
 
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dledinger said:
Hmm. Is the theory not that the magnet couples the braid to the vehicle body via capacitance?
Click to expand...

if that (the coupling) was effective, then why did you get an RF burn from the mic?

All mag mounts use the shield as part of the antenna. that shield length is calculated into the design.

Turning the amp on did not create a new problem, it was already there after you cut the coax but you didn't notice it at the lower power level.
 
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