[TowerTalk] TH7 SWR trouble + TH6 to TH "2.5" conversion

[K7GCO@aol.com]

In a message dated 03.05.00 02:31:03 Pacific Daylight Time, 
markku.a.oksanen@nokia.com writes:

<< 
 > Having finally invested in a Bird 43 I am a bit mystified with my TH7.  It
 > is at 145 feet or so and should be properly put together however the 10
 > meter SWR is 2 @28.000 and 1.4@28.6  at the bottom end of the cable (160
 > feet of RG-123). Equally 15 meters is around 2 at the lower end.  20 is
 > fine.  The antenna is in the "DX" settings.  Now the issue:  I have a
 > K1TTT 6 turn coax balun up there with about 2.5 inch pigtails to those
 > funny curved aluminum things that should go to the manufacturers balun-
 > any guesses what the correct length should have been to make this work or
 > is there an other problem here?  How sensitive is the TH7 feeding harness
 > for the way the wiring, position of and so on is?  I would rather ask
 > before going up there on a wild goose chase...
 > 
 > Second- has anybody turned a TH6 to two element beam such that you use the
 > driver and the "reflector cell" on the back of he beam?  Equally well I
 > could do the director end.  Dimensions?  Simulations?  Thanks
 > 
 > Markku OH2RA markku.a.oksanen@nokia.com

     In regard to the high SWR problem I'd change the balun first--if you can 
reach it.  Just having a 6 turn coaxial balun in the high current area on the 
feedline--IS NOT AN EFFECTIVE BALUN in particular without some donut toroids. 
 Who ever started the wrap a few turns of coax for a balun in the Hi-Current 
area should be hung with it.  Why is it that hams will add a few turns of 
wire at a verticals base to lower the resonant frequency like from 3.8 to 3.6 
MHz and not get a choke affect they expect in a coaxial cable coil before a 
balanced feed point??  If this coil formed a "RF Choke" the vertical wouldn't 
resonate at 3.6 MHz.  Why has this total nonsense been started and totally 
ignored?  Coiled coax baluns have to be in the Hi-voltage area 1/4 WL lower 
and of enough turns (far more than 6 turns) and Xc between the turns to form 
a resonant tank circuit to develop the Hi-Z needed to choke off RF--at that 
frequency.  Now the 1/4 WL coax shield before the balun still radiates so 
this is the wrong choke no matter how used.  However, there is just one 
application for this type of choke where it really shines. 

The donut toroids only work in the high current area as they are RF 
resistors.  You need a bunch of them also and they get heavy.  They don't 
work in the Hi-voltage area--there is no current.  Toroids are a lossy balun. 
 

It's better to have a beam with a matching system that doesn't have any of 
the "Dreaded RF Spill Over" that needs cleaning up.  (Old K7GCO Axiom)

The dumbest design I've ever seen is when a Mfg has a balanced feedpoint and 
risks having the buyer just connect the coax to it without a balun.  Or leave 
it up to the buyer to spend more money and somehow come up with a balun that 
doesn't have RF Spill Over or increases losses with higher SWR on the band 
edges.  Why not assure the success of a product by having a matching system 
with no RF Spill over?  Some actually do.  Connect the Palomar RF Current 
Meter to the feedline of several beams or dipoles and get a RF Spill Over 
Lesson in good and bad matching system design.  Open wire feedlines have so 
many advantages when properly used.  

If a different balun doesn't cure the problem change the coax and cut to 
multiples of 91' 2" for .66 VF (or 136.7' or 227.9' also) as these are Magic 
1/2 WL Multiple Lengths.  Some bridges are actually sensitive to certain coax 
lengths.  RF spill over on the shield gets into the SWR circuitry and screws 
them up.  I haven't found that with the Bird but have with others.  If that 
doesn't work take the beam down and do a complete clean up of all the joints, 
coat with Anti Seize with aluminum particles like RaiBeam uses--not zinc.  
(quad 1 joint/element soldered joints--only need solder).  Better yet put up 
a multiband Raibeam or a 2 element quad with individual driven elements like 
Antenna Marts at say 50' and at 145' and own the world.  The Hex Beams have 
been doing very well also.  It's reflector doesn't even have a soldered joint 
  Let me know what you find.  k7gco

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