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[TowerTalk] 160m 1/4 wave vertical help needed

Subject: [TowerTalk] 160m 1/4 wave vertical help needed
From: Dennis OConnor <>
Date: Mon, 17 Sep 2007 05:18:27 -0700 (PDT)
List-post: <>
Been there, done that, got the scars to prove it...  My base is 7 feet high 
with 12 sections on top of that, with a 10 foot stinger...
  The MFJ 259 goes nuts when attached to this tower - just picks up too much RF 
from BC stations and overloads the diodes dumping raw current and  swamping the 
pickup section...
  Frank is right on about working at transmit power levels....
  I will assume that your insulator section is OK..
  I will assume your radials are clamped to the tower base section...
  Your top section is a bit short to tune to the bottom of 160...
  If you need to know the resonant frequency (idle curiosity) get a noise 
bridge - the MFJ one works just fine and is an instrument of a thousand uses...
  The best method to tune it (and at the same time match it) is to build an L 
match tuner...
  Grab a piece of cardboard mailing tube 3" in diameter and 6" to 8" long and 
give it two or three coats of clear polyurethane...  An office supply store 
will have mailing tubes... Then using two parallel lengths of #12 bare copper 
wind it full length on the tube and bolt the ends through the tube with brass 
#10 hardware, so it stays tightly wound (one winding), then unwind the spacer 
winding... Bare copper is easy to come by - go to Home Depot and get either 
Romex or solid copper wire THHN... Slit and remove the insulation...
  Make a flexible jumper with an alligator clip on the end for shorting out 
  Get a variable capacitor - I suspect 500 puffs will be needed... Doesn't have 
to be a high voltage unit as it is working at or near 50 ohms... 
  The coil goes in series between the coax center and the bottom of the 
radiator sections..
  The cap goes from one end of the coil to ground/radials - I'm guessing the 
transmitter end of the coil... Could be the other end of the coil, but that 
will be determined by trial and error...
  Take the rig and a battery out to the base of the antenna...  Set the rig for 
roughly 10 watts out into a dummy load, then hook it to the input of the L 
match... With the transmit on, set the cap to roughly half mesh, then grab the 
alligator clip and short out the entire coil (don't clip it, just hold it 
against the winding with your fingers - wear a glove if you are a worry wart) 
and watch the SWR indication on the rigs meter as you wipe the shorting clip 
along the coil watching for a dip in the SWR...  Usually you will get one... If 
you don't, try more or less mesh on the cap and repeat... Once you see the dip, 
clip the  jumper to that turn and vary the cap... By moving the clip a turn at 
a time and swinging the cap you will walk it right down to 1:1...  In your 
final assembly you can remove most of the unused turns on the coil - leave a 
couple of extra turns for later changes in the antenna or radials...
  A side issue is your radial field... Not nearly enough... If you can't make 
them longer it doesn't matter, but I want to see 50 - 60 radials of whatever 
length you can put down... If they are short that is fine... Just get the 
radial count up there... And don't bother measuring radials, it's a waste of 
time and energy... Just walk them out, snip off and use a sod nail to hold it 
down.  Two steps sideways... Nail the free end down and walk back to the 
tower... Repeat...
  denny / k8do

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