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[TowerTalk] Re: Hair Pin Match-Balanced and Unbalance Blunder

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Subject: [TowerTalk] Re: Hair Pin Match-Balanced and Unbalance Blunder
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Date: Thu, 2 Dec 1999 10:42:08 EST
#21  Recently there was a lot of talk about Balun Losses.  There are "built 
in baluns" by nature of the matching system design with no loss for coax.  
HyGain went out of their way to defeat this concept with the Hair Pin Match 
they used.
     Hair-Pin matches were described in WWII Technical Books.  HyGain uses 
hair pin matches but not in the same way of the original design.  It needed a 
1:1 balun on the coax to prevent RF Spill Over on to the shield.  Many were 
not used with the resulting problems RF Spill Over causes.  With WWII Hair 
Pins, the coax entered the center of the hair pin where it was grounded to 
the boom.  The coax ran up inside the tubing to the feedpoint where the 
shield terminated at the opening on one side and the center lead jumped a 
short distance across to the other side of the hair pin.  This is really a 
"Compressed T-Match" and no series Xc is needed as the jump lead is short.  
The balanced 50 ohm feed points are now close to each other.  No balun is 
needed with power limitations and with all the discussion of Baluns and their 
losses, consider the following.
       There was an article about 30 years ago in QST I think it was where a 
HyGain Tri-Band Hair Pin was modified to look like the original WWII Hair 
Pin.  The larger coax cable was paralleled with the smaller diameter hair 
pin.  I would have thought there would have been an adjustment required.  The 
shield connects to one side of the hair pin feed point and is taped to the 
hair pin wire on the left and right side back to the hair pin center ground 
where the shield is grounded (?) or connected there and then on to the rig.  
Seal coax from water entry at the open feedpoint. Carefully solder a 
grounding strap to the shield and seal the coax at this point.  Another coax 
is paralleled and taped to the other wire just to make it the same diameter 
and retain symmetry.  Connect the shield at the center point and at the other 
feedpoint. If the original configuration is maintained, this has just made 
the same hair pin right from the coax cable--the WWII model. If just coax is 
used and after adjusted for 1:1, the orientation must be fixed.  For 
convenience make it from a 1/2 WL of coax with a temporary PL259 on it so 
that a MFJ analyzer can be used near the feedpoint for test and adjustment 
purposes.  A type N female connector can then be installed permanently and 
then tape the connection.  Type N connectors can still leak. 
       The author claimed low SWR on all three bands of a tri-bander--and no 
balun losses.  I never liked the hair pin match HyGain used.  It was often 
1/8" aluminum wire in the high current section which didn't radiate. When I 
first saw it, no one ever thought it would match 3 bands with one 
configuration--except HyGain or the author of the modification article.  In 
Eznec however, DE's less than or longer than a 1/2 WL barely affect the 
pattern.  My concerns were unwarranted.  It did require an insulator in the 
center of the DE.  
        The Clements Match of about 35 years ago in QST is a balanced feed 
system for unbalanced coax and used just one series Xc.  I've used several 
variations of it and is great for mono banders.  The author Clements, claimed 
that sewing machine noise went away on his 10M beam with his match which then 
passed the test.  This indicates a perfect balanced noise bucking feed 
system.  No RF Spill Over on to coax on transmit and no RF Spill Up into the 
feed point of vertically polarized noise or mostly vertically polarized 
signals being picked up on the coax shield.  
      There is a simple trick of a precise balanced adjustment of a T-Match, 
Clements Match and even the Gamma Match which has a lot of RF Spill Over.  
It's more of a Convenience Match.  Using a MFJ Analyzer, make the adjustments 
for 1:1 SWR.  Then put your hand on the center of the DE.  If the SWR 
changes, the adjustments of the T or Clements Match aren't balanced assuming 
both DE halfs are the same length.  Move one tap or the other 1/2", readjust 
the variable Xc for 1:1 and do the hand test again.  When you get the hang of 
it, it goes fast and Presto--no RF Spill Over!  F/B improves and RFI problems 
just go away and not many beams can make that test.  Fortunately your hands 
affect to unbalanced RF at the center of the DE is detected by the MFJ.  The 
voltage is supposed to be Zero at the center contact point on the boom.  
Improper adjustment of a T or Clements can unbalance a balanced DE.  The MFJ 
Analyzer sensitivity makes this adjustment possible.  I used to use the 
Antenna Scope Bridge and GD Meter to do the same thing back in the 60's.  
Before that I shunted light bulbs from the center out each side the same 
distance and checked brightness.  
       The Gamma often has a lot of RF Spill Over.  On 10M years ago, 
ignition noise from cars was bad without resistance wire.  When I used 
balanced systems this was not a problem.  I used a T-Match and balanced 100 
ohm coax right into a balanced link in the tank circuit and balanced inputs 
to receivers.  No noise.  I learned the value of this in the 30's.
       Now let me tell you how bad the Gamma really is.  A high power W6 
running 25 KW (that's just the buffer) had a call from a neighbor telling him 
his mast has sparks coming off it--not the antenna.  He had his wife modulate 
the rig and watched.  The arcs were coming from the coax shield to the larger 
mast--1/4 WL below the boom where the DE was attached and using a Convenience 
Gamma.  I said "attached" as there is no RF ground up there of any value 
which is often a misconception.  What was happening is this.  Although the 
coax shield and the larger diameter mast originated at the same connection 
point, the RF Spill Over voltages on each were much higher 1/4 WL down.  Due 
to the differences in the diameters, there were different also.  This is 
where the RF is at it's Hi-E or Hi-Z point and at 25 KW the Gamma RF Spill 
Over voltage level would really Zap you.  I've lit neon lamps 1/4 WL down 
from the beam using a Gamma running a KW.  He added a T Match and the 
Fireworks went away as well as other RFI.  It took 25 KW to finally show how 
bad the Gamma really is.  It can be balanced by making one DE side longer 
than the other.  
       Would you believe the "RF Spill Over SWR on the tower molded a 
mechanical SWR in the tower that returned straight again with the T-Match." 
That is an absolutely true story--I just made it up!  There was a Jeeves 
Cartoon in QST showing and open wire line with Mechanical Standing Waves on 
it--my idea.  In the 40's I made up some open wire line with stiff wire and 
bent it into a SWR pattern and told those who asked--it had a high SWR on it 
and it reduced RFI, it's also a far more efficient and worth while way to get 
the RF to the antenna.  It's a true lossless line.  Some believed it.  I've 
had a lot of experience with open wire line!
K7GCO Ken Open Wire Glanzer

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