[TowerTalk] Arrow had cheapest toroids

[Joe Subich, W4TV]

 >> I'm going to try the bifilar winding with 14 gauge thhn (solid).
 >> Seems like the cheapest solution with some so-239's and a plastic
 >> box.

Be careful with the bifilar parallel wire chokes.  The Zo of the
"transmission line" in the choke is higher than 50 Ohms (generally
in the 70 Ohm range).  If the length of the transmission line is
more than a very small part of a wavelength it can cause a
significant change in SWR .

The coaxial based current baluns are much better in that respect
as they represent a much smaller "bump" and upset the overall
system far less than a bifilar balun.

73,

    ... Joe, W4TV

On 5/24/2010 3:46 PM, Jim Brown wrote:
> On Mon, 24 May 2010 12:21:42 -0700, knormoyle at surfnetusa.com wrote:
>
>> Arrow Electronics  had prices that came close to the 250 piece prices jim quoted,
>> for my small quantities. (www.arrow.com , just type part # into search)
>
> Wow -- thats great news!
>
>> I'm going to try the bifilar winding with 14 gauge thhn (solid). Seems like the
>> cheapest solution with some so-239's and a plastic box. Jim gave that the thumbs
>> up with power, I think. Any comments on that? I might double stack the cores with
>> the bifilar turn count Jim gave. I suppose glass taping the cores is a good idea?
>> or unnecessary?
>
> Do NOT double-stack -- that will change the tuning. Note also that my measurements
> are for #31, not #43. If you want to wind some with #43, Id be happy to measure
> them for you (Kevin is my neighbor a few ridges over) and can add them to my
> published data. I dont have any spare #43 cores laying around.
>
>> my antennas are well matched (<2:1), so I shouldn't be seeing a breakdown voltage
>> problem with the thhn? (with 1500W). I'm assuming the thhn pair is close spaced to
>> get close to 100 ohms.
>
> I have NOT done extensive testing for long term power handling for bifilar chokes,
> but I saw VERY little heating when I stuck it in series with my Titan at 1.5kW for a
> few minutes of keydown. You should be fine. The primary failure mode is due to
> excessive common mode, which in turn is due to serious imbalance.
>
> 73,
>
> Jim K9YC
>
>
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