[TowerTalk] TH7 SWR trouble ... Coax balun question

[K7GCO@aol.com]

In a message dated 04.05.00 06:53:13 Pacific Daylight Time, 
markku.a.oksanen@nokia.com writes:<< 
 Hi, 
 I think I may have left something out:  You may remember that the TH6
 instructions call for a 12-turn balun located at the junction of the boom
 and the mast, couple of feet from the feedpoint with the coax to the
 feedpoint taped to the boom.  OK, I did not do this for the TH7, however I
 do have about 8 inches of taped RG213 on the boom between the 6-turn, single
 layer coil. I am beginning to think that this may be the reason, the taped
 part may contribute a small capacitance to one side of the feedpoint.  Any
 other opinions?  
 
 How did others use the coax balun with a TH7?  How is it attached?  Length
 of pigtail wires to the antenna feedpoint?  Length of coax between pigtails
 and balun coil.
 
 Markku
 OH2RA
  >>
Yes, lift this hot shield off the boom.  6 turns does very little as a choke 
in the Hi current area.  As I pointed out before these chokes need an 
excessive number of turns to be effective in a Hi-current area.  They work 
great at a high voltage point 1/4 WL lower with enough turns so that the 
inductance and Xc between the turns forms a resonant tank circuit creating a 
Hi-Z choke.  Of all the other baluns out there I don't know which is the best 
as I don't use them.  Buy the Palomar RF Current meter and measure the spill 
over from some of these baluns and that's a sure way to find the best one 
with the least of the "Dreaded RF Spill Over".  I have feed systems that 
don't need them even with coax feed and I don't use a gamma.  Is there a 
hairpin match on that beam?  There is a way to feed a 3 band hair pin match 
with coax without a balun and not have RF Spill Over that was in QST about 30 
years ago. The coax shield was run parallel to one side of the hair pin with 
the shield connected to the hairpin center tap on the boom and at the top 
feed point.  The center lead of the coax jumps across to the other feedpoint. 
 Another length of coax is connected to the other side of the hair pin just 
to balance it up physically and electrically.  It worked like a charm and is 
then almost equivalent to the hair pin match that was in the WWII radio books.
 
 Never buy a beam with a balanced feedpoint that you have to spend more money 
and time to come up with a way to properly feed it.  When properly fed there 
are beam designs that have great F/B not destroyed by RF Spill Over and don't 
pick up a lot of vertically polarized noise.  Few hams have ever had the joy 
and experience of using one.  Too many compromises 
 
 When you finally resolve your feed problem I'd consider getting a multi-band 
2 element quad with individual fed DE's like Antenna Marts or a Multiband 
RaiBeam and install it at about 35-50'.  The Hex Beam has been doing well 
also and it's the smallest.  The % of time you use this lower beam and it's 
dominant performance will amaze you compared to the higher ones.  The higher 
ones will open the band sooner and hold it open later.  When you have a lower 
one to compare it to, you will be surprised how often the lower one is 
used--with the proper feed systems and joints that don't get lossy.  One beam 
can't do it all and you have to have 2 or 3 at different heights to really 
observe all this.  Let me know how lifting the coax off the boom works.  
K7GCO.       >>

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