[TowerTalk] Force 12 B-1/B-1S Baluns

[Steve Hunt]

Yes, the choke reactance may add to the coax braid reactance producing 
an increase in the net CM impedance; in fact with certain lengths of 
coax the braid impedance is high enough that you don't need a choke! But 
now think about much more complicated it would get with something like a 
triband beam where the single choke is operating with three different 
electrical lengths of coax. The solid engineering solution is resistive 
choke impedance and sufficient of it for the application.

Air-cored coax-wound chokes perform exactly like any high-Q 
parallel-tuned circuit: very high impedance over a very narrow 
bandwidth; reactive at all frequencies other than right at resonance; 
tuning very susceptible to proximity effects. How many times have you 
seen a coiled-coax choke taped to a boom - think about that in terms of 
a high-Q parallel tuned circuit!

73,
Steve G3TXQ





On 14/05/2013 18:57, Jim Thomson wrote:
> ##  OK, I get it.  And if the coax braid  Z, and is reactive..but is XL instead of XC, the
> net CM path Z  could also increase ?
>
> ## I did the maths, if you double the power, the diss will double....so diss is proportional to power,
> this is good to know.   Then I suppose you have to factor in the weight of the ferrite used.
>
> ##  I played around   with some of jim browns designs with large diam loops, but they are tough to use in some
> applications..esp with 40m yagis...and 80m rotary dipoles.   With the 40m yagi, u have the boom to deal with.
> The 80m dipole,  u have the mast to deal with.   U gotta get those coax loops well away from metal or the stray C
> from coax loops to metal creates more problems.  Close wound loops on  4-8 type x 31/43  2.4” OD torroid
> cores  seems to solve most of the stray C issues.   Then the entire mess goes inside a gasketed nema box.
>
> ## 213 u  or  393 teflon coax..silver soldered, seems to work pretty good so far.
>
> ##  The  1/4  wave  or  3/4  wave coax...  minus 2%  also works good...but that’s a monoband device.
>
> later........ Jim   VE7RF
>
>
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