#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
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
FAQ on WWW: http://www.contesting.com/towertalkfaq.html
Administrative requests: towertalk-REQUEST@contesting.com