[TowerTalk] 6M coax balun

[Jim Thomson]

Date: Tue, 31 Aug 2010 18:37:35 -0400
From: "Roger (K8RI)" <K8RI-on-TowerTalk at tm.net>
Subject: Re: [TowerTalk] 6M coax balun



On 8/31/2010 5:10 PM, Hector Garcia XE2K wrote:
> Tony
>
> if you want to read more about the baluns for the 6m band and the antennas  go
> to this web page
>
> http://www.g0ksc.co.uk/creatingabalun.html
>
> I just build the Hybrid for the LFA's and was very easy.
Part looks good, but the "air core" 5 turns on the paint can is unlikely 
to be very effective.

73

Roger (K8RI)

###   14 x  of the  large  type 43 beads..  [ 1" OD x .5"  ID  x 1.125"  long]    will give you 2083 ohms  on 6m.  If you use RG-393 coax,
you can then stuff  5 kw CCS all day long into it.   If you use large  type 31 beads,  you will get  just over 2 k ohms, resistive, with 10 x large
beads... and  just over 4 k ohm, resistive, with 20 x large beads. [3 k with 15 x beads] 

##  If you opt for a few turns of coax in a solenoid air wound config..... or coax wound on type 31  2.4"  OD cores.... make damn sure you keep the 
outside of the coax well  away from the boom... as in at least the OD of the coax coil.     IE:  3"-4" OD coil.... keep it 3-5" away from the boom...
or the stray C  from the coax to boom  will screw up the balun really fast.   On these 80-40  baluns, made from coax wrapped onto type 31 cores,
depending on how tight the turns are wrapped onto the core..... it's not easy to get the coax  turns away  from the boom  [ or mast if a 80m rotary dipole is used] . 

##  This is where a large size nema box comes in handy.... then the cores and coax can be suspended in mid air, dead center in the nema box...and away from metal. 
The string of large beads on RG-213/393  coax has an advantage too....easier to install..and zip windload.  You also don't have a stray C to boom problem.  You can't blow
up the large type 43 beads..on any band..with YC-156 power levels.    The large type 31 beads will also work.. but the choking Z has to be a lot higher, since the type 31
material  is mostly resistive.  I squared x Resistive ohms  = real heat     I squared  x  reactive ohms = reactive power =  'wattless power' .    It amounts to a 2 x edged sword. 
Modern thinking dictates a predominantly resistive component, but to pull that off, the Z has to be real high... like 2-3+ K  ohms.    A type 31 choke balun that's only 1 k ohm,
and predominantly resistive, will go off like a bomb.  A  2 k ohm choke, also made from type 31 material, will  dissipate exactly  1/2  the power of a 1 k ohm/type 31  choke. 

I will be doing some experiments this fall/winter, where type 43 beads are used at the feedpoint... and type 31 beads are used 1' downstream. 

Later... Jim   VE7RF