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[TowerTalk] Baluns and SWR

To: <towertalk@contesting.com>
Subject: [TowerTalk] Baluns and SWR
From: K7GCO@aol.com (K7GCO@aol.com)
Date: Thu, 24 May 2001 05:33:29 EDT
<< In a message dated 5/17/01 9:39:16 AM Pacific Daylight Time, 
tmartin@chartermi.net writes:
  <<  I have read so many posts about baluns and SWR that I am thoroughly
 confused!  I am going to be erecting a 96 foot tower with a 5 element
 Telex 20 meter monobander and a 2 element S-402 by Mosley and a 3
 element Telex 10 meter monobander.  I want to avoid the ferrite core
 balun and use a coax balun.  I can get no consensus about how many
 windings, how many inches for the form, etc.  The manuals, antenna
 books, and TowerTalk members suggest 12 turns, 6 turns, 8 turns, 4-6
 turns, all on either a 4 inch form, or a 6 inch form, or air wound and
 taped. The list of suggested turns and methods of windings goes on and
 on, not to mention what type of coax to use: 213, 58, 8U, 8X, teflon,
 etc. The Mosley instructions don't mention a balun at all which I didn't
 use at a previous QTH.  I didn't notice any problems with front to back,
 gain, or TVI/RFI without a balun on the 402.
 
 So, what's a fellow to do???
 
 Also, I agree that trying to get the perfect match 1:1 SWR is an
 obsession with a lot of hams.  Of course, the use of solid state rigs
 has fostered this mania.  My 830 will get out fine with a 5-1 SWR.
 However, it is a bit disconcerting to see the reflected meter pegging
 the post on voice peaks when using the amp.  73 to all.  Tom
   >>
 
 Tom:  The so called "coax choke" does very little in the high current 
section near the feedpoint.  You really need a better feed system instead of 
another "Band Aid."  There isn't enough capacity between the too few turns to 
create resonance and an effective Hi-Z choke contrary to what many have been 
led to believe.  If you check the RF Spill Over beyond the coax coil choke 
with a "Palomar RF Meter," there is still a lot of RF below the choke in this 
area contrary to what you have been lead to believe.  
 
The area in the coax where the coax coil choke really works is in the 
Hi-Voltage Area 1/4 wave lower but that is too far down.  This is right at 
the Hi-Voltage area where the capacity is higher between the turns and an 
Effective Hi-Z Choke is formed with the right number of turns.  When you have 
the right number of turns you can run your fingers down the coil and the SWR 
is upset progressively less and less to near the bottom where there is "no 
affect" on the last 2 turns.  I call it the "Truman Choke-- the RF stops 
here."
 
 I use it in making 1/2 wave center fed verticals.  The 1/2 wave L 
configuration works great.  The coax shield is at the bottom 1/4 wave up to 
the coax choke. 
 
 All these "Band Aids" on TT on beams, rotators and towers usually don't 
solve the major problems.  The major problem is a feed system that has RF 
Spill Over.  So far I seem to be other only one warning everyone of this 
"Evasive Problem" that is so prevalent with coax.  Most Mfgs don't even know 
how prevent it and in some cases--couldn't care less.  A simple test is to 
place your hand on the coax shield near the feedpoint and see if the SWR 
changes on the MFJ SWR Analyzer.  If it does you have a bad feed system.  The 
gamma is the worst of all unless it's unbalance is compensated for and there 
is a way to do it.  Shift the DE to one side and use the "K7GCO Hand Balance 
Test."
 
 On uncompensated gamma match systems you can also place your hand on the 
center of the DE and the SWR will change.  When feed systems are "properly 
balanced" you can touch the center of the DE or the coax shield and there is 
no change in SWR on the MFJ SWR Analyzer.  I can also touch the ground of the 
MFJ back at the center of the DE wiht a length of coax and there is no change 
of SWR "when things are balanced".  The Palomar RF Meter on the coax reads 
"Zero." 

Would you believe that when you get the DE and feed system properly balanced, 
the center of the DE where "Zero Voltage" now exists perhaps for the first 
time, there is "Zero Current."  It is so cold there a "RF Frost" is created.  
If you put your hand there for over 5 seconds it will be frozen there.  You 
will have to unbalance the DE to get some RF current there again to thaw out 
your hand to remove it.  
                 Now that's an "Absolutely true story--I just made it up."
 When I make the balancing adjustments I use just a 1/2 wave of coax to the 
MFJ.  I also find less SWR change as the beam is rotated.  On some beams and 
towers I've had 1:1 SWR for 360 degrees of rotation which is not often seen 
in particular with a 75M inverted vee 10' below used as guy wires.  A lot of 
so-called surrounding objects affect is not the object unless real close but 
the RF Spill Over from bad feed systems on to it that varies as the beam is 
rotated.  In other words the beams feed system is often the problem--often 
not the nearby object.  Try a good feed system and you will see this.
 
 The 200 ohm T match like Telrex used was a good feed system and a factor of 
their good performance.  Telrex lengthened the DE to compensate for the T 
Match reactance.  I like balanced 100 ohm coax which is ideal for quads.  I 
have lengthened Yagi DE's and used a series Xc's to obtain 100 ohms or to 
even 50 ohms and 1/4 wave 75 ohm balanced coaxial stubs to get the 100 ohms 
for the feedline.  A 25 ohm beam Z can be matched to 100 ohms with a 50 ohm 
balanced coaxial 1/4 stub.  A 36 ohm beam Z can be inverted to 100 ohms with 
a balanced 60 ohm coaxial feedline (there is a way to make them)--No RF Spill 
Over.   Baluns or ferrite cores or anything else needed just as you desire.  
I've used FD DE's of 100 ohms also for the 100 ohm balanced coax--No RF Spill 
Over.  
 
 I have a 10M 3 element beam on a .35 WL boom tuned for absolute max gain of 
10.2 dBi.  It has 8 dB F/B, a 6 ohm feedpoint Z and narrow bandwidth.  6 ohms 
is too low to step up so I use another technique to get 50 ohms and it's 
patentable.  No RF Still Over either.  I now have 3 ways to broadband it 
again.  I have always liked great F/B but 3 years ago the band was dead on 
10M and if there was someone on the front of the beam there was never anyone 
on the back.  I needed max gain and not F/B.  I wanted to see just how this 
design would work since no mfg has ever marketed this design and I certainly 
found out.  It turns out that this beam has been a "Real Killer" on 10M and 
with the band open, poor F/B hasn't been a problem.  The poor F/B keeps the 
band open on the rear also.  It often has "Infinite F/B." I will build this 
on 12,15,17,20&40M also.  I will also tune 3 element quads this way also.  
W7WJP had one and it "Really Talked."  It had a 25 ohm feedpoint Z matched 
with a 36 ohm 1/4 WL stub.
 
 With link coupling in the final tank circuit the 100 ohm balanced coax 
connects nicely.  Or use a Johnson Match Box.  There is another simple way to 
convert 100 ohm balanced coax into a single 50 ohm coax without a tuner in 
the shack.
 
 I've mention the need for "Zero RF Spill Over" many times, the advantages 
balanced 100 ohm coax and other better ways to get max RF into the antenna.  
Yet all the "Band Aids" for poor design dominate on TT.  This is the year 
2001 and bad techniques still predominate with the mfgs and hams.  The open 
wire line from the 20's still works absolutely great into a T Match and beam. 
 Open wire line still works great into LF dipoles using 1/4&1/2 wave 
multiples when you know how to use tuners.  Some do not and state so 
regularly without realizing it..  
 
 There is a saying "Progress comes one (Opinionated Old Timers) death at a 
time."  Many mfgs thrive on "John Cheap Skate Retiree Hams Budget".   I've 
made Special Arrangements in Hell for Special Ovens for them.  Others like 
M2, Antenna Mart and Raibeam are doing a great job in giving better designs.  
I've been talking to them and who knows--there may be some more better ideas 
showing up--with No RF Spill Over.  If you can get to Sea Side, Or Ham Fest 
June 1,2&3 you can learn some new techniques presented at the antenna talks 
for beams and a great Mobile Antenna idea that makes Mobiles "Speak Up."   I 
just got a FT-100D.  

I have just installed 2 Raibeam 2 element beams vertically polarized on a 10' 
boom and 10' higher the 6M Raibeam 5 element "Butt Kicker" on a 22' boom 
horizontally polarized 10' higher.  I can instantly select either 2 element 
individually or both tegether, or the 5 element on top or both--In, 90 or 180 
out of phase.  

I had a similar set up on 10M around 1960 and I owned 28.503 MHz.  I never 
had to move.  I used 2 of the old Workshop Beams from the late 40's of 5.5 dB 
gain each on a .29 WL boom detuned to 50 ohms vertically polarized and one 
20' above horizontally polarized.  I still have a great 4X5 negative picture 
of that.  What really surprised me was that the 2 vertically polarized 10M 
beams of only 8.5 dB gain 35' high over Seattle's low ground 
conductivity--did so well.  

Would you beleive the angle of radiation was so low it kicks up a dust--"RF 
Dust That Is."  I have a lake in the back yard and it kicks up an "RF Mist 
Fog Cloud" in the shape of the reflected RF beam pattern.  You have to wear 
"Special RF 3D Glasses" to see it.  It's been there all this time but no one 
ever used the "RF 3D Glasses".  When the ducks on the lake or boats sail into 
it, they dissappear like in the Bermuda Triangle.  
                     That's an absolutely true story also--I just made it up.
One Ham meter reader heard me on his Hearing Aid 3 blocks on away on 10M and 
honed in on me. That was "HAI".  I will recreate that configuration on 10M 
and other bands again as I just have on 6M.   Now if 6M doesn't open this 
year I'll really be "RF Pissed".

There is a spray for beams that prevents frost and ice from forming I'm told. 
 It's  Maine product I will test it in SD this coming winter where Ice 
loading can occur. An ice laod is bad enough and if the wind comes up--you 
are in trouble.
K7GCO

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