6m antenna project
ka2zev at yahoo.com
Mon Dec 3 08:21:01 CST 2007
I'm still working on finding antenna bases for the AS coil / whips that Frank has.
After the Saturday Taco's meeting I got an urge to try my hand at something.
Radio Shack of all vendors sells a base load magnetic mount CB antenna that uses the AS style whip and base. This is their 29.99 unit. I will bring it into tacos next Saturday if anybody wants to see it.
In an old copy of the 'Handbook' there was an article on how to tune similar antennas into ham stuff. I think the article was for the 220 band.
Anyhow, I got to wonder how hard it would be to make this work at 52MHz?
On the plus side, if I had killed the coil/whip, I still had an AS style antenna base to work with. So the project would not be a total loss.
I used my MFJ VSWR analyzer to do this.
For those of you with the VNA tool, that might yield a more precise result.
Couple of words about the antenna first.
It's a loaded coil with a fiberglass whip.
It has about four inches of adjustment range.
A word about plastic products.
You know how some plastics feel like quality products, and some simply don't.
The plastic this was molded out of is in the second category.
Strong but brittle feelings, almost too shiny to be real.
The whole antenna radiated the feeling of 'cheap junk'.
Just for the record.
The antenna coil body seemed to be glued to the internal parts.
At the time I didn't know how to remove this plastic tube from the coil assembly.
Ended up using a cut off wheel on the dremmel tool.
You would expect a metal part connecting the coil winding to the antenna whip.
No so here. It's a chrome plate plastic that makes the connection.
I did say cheap.
This thing is going to be loaded with 'bi metallic' issues after a few years of service.
The copper wire is soldered to a brass pin, pushed into the plastic, making contact with the chrome plating. It works, but for how long.
From the day this antenna was assembled it starting to deteriorate due to oxygen.
Talk about a throw away product.
After some dust and noise, I was able to cut a channel into the plastic, to the point where a screw driver can be pushed into it, twisted and cracked the tube off the coil assembly.
Then the question of how to tune for a new frequency.
I counted the windings.
Five on the (lower) and 22 (Upper).
With the stinger bottomed, it gave me a vswr curve in the 28 MHz range.
So if I over cut the windings, I can compensate by raising the whip.
I cut the upper coil.
So two turns at a time I removed windings.
Then replotted the resonance.
Each time it moved a few MHZ up the dial.
After a bunch of wire was removed it actually tuned up on 52MHz.
Not too bad.
I wish I knew more about how these things worked.
I think the magic number was about 28 inches (in 4 inch segments) off the top, and one turn off the bottom. I saved the cuttings and the tune notes for Saturday.
Taking a turn off the bottom was a wild guess.
I did it because the Z Gage was more than a little high.
It proved out to be a right guess, but the how and why is not clean in the mind.
Call it inspiration, not based on logic or thought.
Anyhow, I'm going to firm up the solder joints and make it presentable.
After the next taco's meeting I may spray it in plastic, and cover with heat shrink tubing.
I want the brain trust to look it over and decide if it's sane or not.
If I need to do a second one, I will boil the coil first to see if that frees up the glue the OEM used.
"You are, what you do, when it counts"
"Gravity, the quickest way down"
Mayor John Almafi
"You ever drop an egg, and on the floor you see it break?
You go and get a mop so you can clean up your mistake.
But did you ever stop to ponder why we know it's true?
If you drop a broken egg you will not get an egg that's new?"
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