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[TowerTalk] Re: Force12

To: <>
Subject: [TowerTalk] Re: Force12
From: (
Date: Wed, 10 Oct 2001 19:07:32 EDT

      In a message dated 10/3/01 2:38:44 PM Pacific Daylight Time, writes:<< 
        Force 12 spec depends upon 100 feet of coax:
        >  Sounds like their BW  quote depends on there being enough 
       > loss in the  feedline to achieve it.  
              Some years ago,  Dave Leeson,  now W6NL posted the
       explanation here on Tower Talk.  The length of line creates
       a double tuned transformer which per theory does,  in fact,
       increase the "system" bandwidth.  It is NOT added loss,  it
       is a form of matching transformer.  I do not recall the details,
       but suppose they can be found in the archives.  These
       posts were at least 5 years ago,  perhaps even further back.
       Dave is a PhD in EE,  and has written many articles/books
       in the tech literature;  so presume he knows about what he
       suggests to be true!
       And,  yes,  I would guess that the "transformer" of a 100 foot
       coax would only be optimum for a particular band area;
       probably on 15 meters if I remember anywhere near correct,  hi.
       73,  Jim  KH7M
      Jim: Since 100' seems to be a standard or average length I have this 
suggestion.  Use the "K7GCO Magic Length" of 91' 2" or multiples which is a 
1/2 wave or multiple real close on all bands based on a fundamental base 
frequency of 3.562 MHz for .66 VF (1.781 KHz for           182' 4").  Antenna 
R measurement can be made in the shack very accurately on all bands with the 
simple Antenna Scopes (RF Whetstone Bridge) or the fancier Z measuring stuff 
except 30&17M (minor error) for starters.  The fancier measuring equipment 
also measures reactance also but at resonance it's Zero.  Adjust antenna DE 
lengths so it is in the middle of the band and/or the GD'ed frequency of the 
feedline.  It gives very accurate R readings if there is no RF Spill Over and 
no PHD degree needed.  Factory Specs check out most of the time I've found 
with 1/2 wave or multiple coax.  
   Since all my coaxes are this length or multiples, I can play phasing games 
with any coax that happens to be connected to antennas I want to phase.  I've 
never had all these SWR measurement problems everyone seems to have since the 
40's although the longer the coax the lower the SWR.  Some SWR actually 
improves the match for a higher power input at the end of the feedline for 
the 50 ohm output rigs--the desired goal.  Pi network finals match a fair 
range of Z's and did a good job in the last Century.  
  If the SWR at the end of the coax was the lowest on a certain frequency for 
a beam for example, that's what it was with the bridge at the feedpoint also 
or by GD'ing.  It never failed so I quit checking it at the beam.  Maximum 
transfer of power from the rig to the input of the feedline was determined by 
the resistive load like 50 ohms it saw there and not at the antenna even if 
on another frequency just off a bit.  There seems to be something forgiving 
and certainly repetitive about this length into "clean matching systems."  
Factory specs checked out every time.  A little SWR isn't all that bad as it 
lowers the SWR and can enhance more RF into the input of the feedline.
   One time back at Hy Gain in about 1960 a tribander was tuned up on the 
test tower using very good coax and transferred to Andy Andros's tower to 
test.  A length of Columbia brand new Pollyfoam coax was used--the first run. 
 The same SWR curves with the same length of coax were not obtained.  This 
was a very embarrassing
      Columbia pulled a huge boo boo.  They just changed the solid dielectric 
to Pollyfoam.  It seems the Columbia Engineers didn't know the basic Zo 
formula for coax.  The shield OD was the same as was the center conductor OD. 
 The formula contains the factor of 1/sq root of the dielectric constant "e" 
which went from 2.3 for solid to 1.6 for pollyfoam (.66 to .79 VF) and in 
order to keep the Zo 50 ohms, the center conductor diameter had to be 
increased from about .07" to .1" if I remember right from 41 years ago.  If 
anyone can look up this article on coax design that was in QST about 40 years 
ago I'd like to review it again as mine is packed.  When I first looked at a 
cross section of this Columbia Pollyfoam coax and since I knew the formula, I 
could see without measurement the shield and center were the same and the Zo 
trouble was obvious.  With a quick calculation in my head I predicted it was 
higher.  If "e" in the denominator decreases (less dielectric) the Log of 
another factor in the formula denominator has to increase for the Zo of 50 to 
stay the same for the same OD.  The only thing that could change was the 
center conductor and it had to increase about .03" and--it didn't.  So the Zo 
had to go up and it screwed up the SWR readings.  That is why the center 
conductor of RG 8 Pollyfoam is about .1" diameter to create 50 ohms Zo.  
  The larger diameter center wire was too large for the ID of the older PL 
259 center pins and the ID had to be enlarged and was enlarged from then on.  
In a typical Type N Center Pin, 3 strands of the center conductor wire of 
Pollyfoam has to be removed so the rest would slip inside the Type N center 
pin.  I had an article in CQ about 35 years ago about all this in detail 
showing cross sections of various Zo's of coax, the center conductor size, 
diameters and other differences.   
  With physical measurements it came out about 60 ohms as did tests with 
various resistive loads.  Another length of regular coax solved the SWR 
change problem.  I still have some of the Columbia 60 ohm coax and it makes 
great 1/4 wave stubs for matching 50 to 75 ohms.  If you ever find some 
Columbia RG 8 Pollyfoam, it might be some of the first run 60 ohm 
coax--cherish it.  I even have some 56 ohm coax from WWII. 
  In case you didn't know RG8 Pollyfoam coax only has up to .2 dB/100' less 
loss at 30 MHz over solid .66 VF dielectric.  As I recall the solid has17% of 
the loss is in the shield, 3% is dielectric loss and 80% is center conductor 
loss.  The dielectric % loss of solid dielectric starts to increase a bit 
faster over 30 MHz over Pollyfoam and the larger center conductor reduces its 
loss.  Since the center conductor has the highest % loss by about 4 times at 
30 MHz, increasing the center conductor was of fair benefit for a small 
diameter change.  Pollyfoam's advantage starts to be of value above 30 MHz at 
a faster rate with increasing frequency. 
  Another problem of Pollyfoam is this.  The dielectric constant is 
unfortunately not constant and can vary the resonant length of it as much as 
3' over 100' and alter phasing relationships.  In most single coax 
applications the variance of VF is not important.  I used equal lengths of it 
on a beam I have and I could immediately see something wasn't right.  I have 
a way of changing polarization's of a beam in the shack (you would never 
guess how--6 different polarization's) at the end of 2 equal physical length 
feedlines 91' 2" long GD'ed at 28.5 MHz.  I was not getting the specific 
polarization's with 2 equal physical length Pollyfoam coaxes I got with solid 
dielectric coax.  One was about 36 degrees too long.  From my tests I guessed 
less than 45 degrees.  I had to shorten one Pollyfoam coax 3' (36 degree) to 
GD at the same frequency as the other and did so for all matching 2 coaxes 
cut from the same 1000' roll.  One pair came fairly close.  Remember that if 
the VF changes--so does the Zo.  
  2 equal lengths of solid .66 VF dielectric will GD so close I stopped GDing 
the second one.  It's a good idea to still GD each as I found a piece of bad 
solid dielectric coax one time.  This would have caused a lot of hard to 
detect trouble had I not GD'ed it.  I GD'ed only one length of the 2 
Pollyfoam coaxes and cut the other one the same length.  This is one reason 
why the GD'er and knowing how to use it is so important for the competitive 
  Would you believe they mix Baking Soda into the dielectric to get it to 
foam?  It's hard to control the mixture and that's why it varies.  Just 
kidding.  When they make Pollyfoam they blow nitrogen into it and they have 
not been able to fully control the variables, therefore the VF varies.  For 
144 MHz and above I think it's great coax if the VF is not critical.  You can 
GD it if you know how.  
  Variances in coax and even SWR bridges can make a well designed beam look 
bad.  Water in the coax can screw it up also.  So the beam Mfgs aren't always 
to blame.  Open wire line never makes an antenna look bad, has nothing to 
saturate and nothing to create an unbalance.
 This all points out many problems with coax few are aware of.  None of these 
exist with open wire line.  Any variance of wire spacing alters the Zo very 
little.  Regardless of any Zo change, if the tuner can match the Z at the end 
of the feedline, 100% of the power transfer from the xmitter to the input of 
the feedline less the very small losses in a tuner properly designed and 
tuned.  The same is true matching the Z at the input to a coax cable.  
However, a lot more of it gets to the antenna with open wire line for years 
and years and years through all kinds of hazards.  The Z at any frequency of 
the DE is of little concern with open wire line where with coax it's a major 
concern.  Solder lugs are a lot cheaper that coax connectors by about 20 dB 
(new concept of measurement) as is the feedlines comparative cost.  There is 
virtually no loss in a soldering lug connection ever if properly soldered and 
coated with silicone grease.  Or in many cases, joints can be soldered for 
even less loss and greater longevity.  Feedline life is many dB longer for 
open wire line and can actually outlive you (I have one that will) with no 
increase in loss.  You can make open wire line where you can't make coax.  If 
damaged open wire line is easily repaired with a soldering iron where coax 
isn't.  Perhaps a more serious look ought to be made of open wire line.  
There are no Reflectors where the problems of open wire line properly used 
are discused, there are for coax problems on a regular basis on and on and 
on.  It's not very user friendly at times.

I may actually make some very low loss 50 ohm or higher coax for VHF/UHF with 
3M adheasive backed (or contact cement) copper foil covering 2" PVC OD pipe 
and about a .8" center conductor foil covered PVC pipe held with slotted 
disks (so condensation could drain) for moon bounce antennas.  It could 
tolerate very high SWR's if needed also.  It's also easy to splice 
mechanically and electrically.  About a .3" center conductor would give about 
a 100 ohm coax for certain applications.  With 6" OD and a .25" center 
conductor about 180 ohms can be created.  2 series connected would give a 
balanced 360 ohm balanced coax for "certain applications" for VHF/UHF.  K7GCO 

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