View Single Post
  #3   Report Post  
Old October 24th 06, 02:15 PM posted to rec.radio.amateur.antenna
Denny Denny is offline
external usenet poster
 
First recorded activity by RadioBanter: Jul 2006
Posts: 326
Default Minimizing Common Mode Currents by Simple Grounding?

Jeez RIchard, did we quaff a few too many pints of viper venom last
night? The guy asked for advice and got fanged...


Ignore Richard, Brian, his acerbic disembowling was aimed at the
purveyors of the antennas, not you...
However, having said that Richard is right as usual.. Forget those
guys...
Get some wire - Romex or THHN from Home Depot will work nicely... Or
any other source of #18 to #14 wire you can come up with...

Cut it to a half wave via formula:
983/F(mc) for 1 wave, then divide by 2 - exact length doesn't matter
because the tank circuit
will tune the antenna, just about no matter what..

Grab the nearest source of thin plastic bottle you can find that is
roughly 2" - 3" diameter...
- that bottle of designer water your sweetie carries around will work
just fine, just don't fess up when she asks if you have seen it...
And if you leave the water inside, you can tell the guys on the local
repeater, "Can you hear me now?" oops, wrong commercial
you can tell them that you are running so much power you had to water
cool the coils! -

Wind about 6 or 7 turns of bare wire on the bottle, spaced about a
wire diameter apart...
Get a variable cap, about 60 to 100 uuF will do nicely... Wire this
across the coil as a parallel tuned circuit...
Hook the braid of your coax to the bottom of the tank... The coax
center will have an alligator clip on it... The feed end of the antenna
goes to the top of the tank circuit obviously...
Tap the coil 1.5 to 2 turns up from the bottom with the alligator clip,
run low power, swing the cap and look for a dip (that is a 'dip' on the
SWR meter, not your fishing buddies)
Didn't dip did it? Probably because your tank didn't tune exactly to
the band you are on... Add or subtract turns to the tank coil until you
do see meter movement that says you are getting close... Then adjust
your tap point for that magic 1:1...

"One to one, one too one, we don't have no fun until it's one to
one"...

************************************************** ***************************************
A short break for Bruno to to give me my medicine and adjust my
restraints...
Now I feel better!
************************************************** ******************************************


Now the counterpoise... A ground rod will work after a fashion - so
will strychnine, but that is another story...
Better is radials, as many as you can put down - length does not
matter! Drop a wire from the bottom of your tank to the radials
center..
Better yet best! is an elevated counterpoise, half wave long, 2 feet
or higher above ground, the bottom of the tank/coax is tapped to the
center of the counterpoise... Ground rod is unnecessary, unless you
want to put an RF choke from the bottom of the tank to ground just to
bleed static electricity... Your call... The ground rod does nothing
otherwise, and may actually harm your antenna pattern if connected
directly to the tank...
(I know, I know, that's not what they taught you in Sunday School...
It's a cold, cruel world out there.)

Now the common mode choke... Well, you can order a premade choke from
WX0B (great folks, solid products, but definitely not cheap)
Or you can order a 0.5" X 4" iron powder rod from the usual suspects,
then get Formvar magnet wire and wind 6 bifilar turns on it... Then
solder this whole gaschmazz between the tank and the coax... It will
work nicely...
Or, you can stop reading all the Hennie Pennies who run around clucking
that the sky is falling down, wind 6 turns of your coax on a plastic
pail in about 90 seconds, and go for a beer because you are done - and
you have the best choke that money can buy...

BTW, you only have to put up one antenna for 20 because with a bit of
experimenting and a jumper or two, you can find the magic coil taps for
15 meters, also...

OK, Bruno says I will take a nap NOW, before I cause any more problems
on here....

denny / k8do