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Author Topic: Converting a mag mount CB antenna to 6M  (Read 9309 times)
KF4RNK
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Posts: 1




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« on: June 09, 2000, 03:19:56 PM »

Has anyone converted a mag mount CB antenna to 6M?  My antenna is a loaded antenna with about a 3 ft. whip.
Any and all suggestions are welcome.

Thanks

kbhipp@yahoo.com
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KC5MEI
Member

Posts: 11




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« Reply #1 on: June 12, 2000, 06:02:51 PM »

I haven't done a 6m conversion, but I have done a 2m conversion. The process should be pretty much the same.
Brass welding rod comes in a variety of diameters and is much less expensive than brass designer rod. (It's the same stuff but welding rod is about 57 cents per 3 foot length and the designer rod is $1.50 where I live.) Select the diameter which fits the mag base and buy a couple of lengths. They can be soldered or brazed together to give the length of whip you need. Next disassemble the mag mount, most have a coil and a capacitor to bring them to resonance. Toss out, or junk box, the coil and capacitor. Replace the coil with a length of stainless steel or brass all thread, or a cut down ss or brass bolt (in my first conversion I used a regular steel bolt and the electrolysis made for wierd SWR in about a month). In the Radio Shack base that I used, the coil was covered with a plastic tube, and it screwed into the base and the nipple that held the whip. I had to cut this decorative piece of plastic to  tubing to fit the lenght of the cut down bolt I used to replace the coil. The capacitor was hidden in the base of the mag mount. The magnet is a toroid and I had to remove the protective covering on the bottom to get to the capacitor to remove it. Reassemble the mag mount with the new components and your homemade whip. Calculate the length you need for quarter wave resonance at the frequency of use. Measure from the top of the magnet out along the whip to the calculated length, add a quarter inch or so for a fudge factor and cut the brass rod. Now tune as you would any other whip antenna by adjusting the length at the set screw and by trimming the tip a little at a time until you have a 1:1 match. You now have a 1/4 wave antenna that will handle moderate amounts of power.
Good luck and many happy modifications. Every antenna I have ever built has been 1:1 on the first cut due to very careful measuring and a little fudge factor in the length.
73 de KC5MEI
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KA3VVV
Member

Posts: 0




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« Reply #2 on: June 13, 2000, 02:24:22 AM »

I took a 11meter cb ant or firesick ant . In use it on 6meter. What i did took the vin cover off the ant. The wire that was wrap around the ant under the vina cover I cut it down to  54in for 6 meter. Took some heat sink to replace the vina cover. So take your mag mount cut it for 54 in .
start at 56in cut down. Have your swr met in line.
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WB6BYU
Member

Posts: 13239




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« Reply #3 on: June 16, 2000, 07:09:48 PM »

Basically you just need to reduce the inductance
in the loading coil until the antenna resonates on
6m.  The mobile antenna design chapters of the
ARRL Handbook or Antenna Book can help you
calculate the required inductance (though I don't
have either handy.)  A dip meter or SWR Analyzer
will make the process easier.

First, unwind the existing coil (attempting to save
the wire if possible.)  The coil will probably be attached
to the antenna at one end, ground at the other, with
a tap in the middle which goes to the feedline.
For starters, rewind the coil with half the number of
turns (a wild guess).  Keep removing turns until the
antenna is resonant (lowest SWR) at 6m.  Then
adjust the feedline tap point for best SWR.  The
adjustments interact a bit, but the process isn't
difficult.

(I've also converted such antennas to a half-wave
on 2 meters, and they work quite well.  I replaced the
coil with about 5 turns, tapped about 1 or 1 1/2 turns
up from the bottom.  A 20pf trimmer cap across the
coil tuned it to frequency, and the tap adjusted for
best match.  Works great, and doesn't require much
of a groundplane.)

      - Dale WB6BYU
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KF4BOT
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Posts: 34


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« Reply #4 on: July 25, 2000, 05:17:06 PM »

In the early eighties, I was a radio technician with the Florida Department of Transportation. Their base/mobile communications were done on 6m business band. My job was to install, repair and maintain base and mobile radio equipment for the Department. All mobile installations used CB antennas cut to size. Most were full-length whips, but some were the smaller loaded antennas. All you need to do is reduce the electronic length of the CB antenna to about half its size, then do the fine tuning for the lowest SWR. In the case of a mag mount, or other loaded antenna, just cut off about 50 inches off the loading coil and reconnect the remaining coil. If you need to trim any further, take 1/2 inch at a time off the whip until the SWR is right.
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