[Antennas] RG-178 balun turns count

Fri Jul 2 13:12:00 EDT 2010

Brain,
Your example does not apply toe Ian's chokes. Those are coaxial current 
(choke) baluns wound using ferrite cores. Good choke baluns offer little 
opposition to differential current on a transmission line while impeding 
common mode current. The differential current flows only on the inside 
of the coax (between the center conductor and the inside of the shield) 
and is not effected by the ferrite material on the outside at all. 
However, common mode current flows on the outside of the shield and 
therefore is effected by the ferrite core. The only loss the 
differential current encounter is in the 5 to 6 feet of the coax itself 
as there is no additional loss from the ferrite core because the 
differential current is not exposed to the ferrite core. So, in other 
words, any losses dissipated by the core results form the common mode 
current on the transmission only.
I suggest you re-read Chapter 6 (beginning on page 24) of Jim Brown's, 
K9YC, paper for a good explanation of their workings.
In short, both Ian's and Jim's baluns are a variation of Water Maxwell 
W2DU's bead balun and as such use lossy ferrite in their construction.
These baluns will work very well indeed on you C-pole antenna.
73,
Danny, K6MHE
On 07/01/2010 01:31 PM, Brian Cake wrote:
> Unfortunately the two links cited by Danny concentrate on solving 
> EMC problems, and do not concern themselves with the power loss in any 
> choke balun.
> For example, the apparently excellent performance provided by 
> GM3SEK's cascaded broadband choke, 8kOhms or so over approx. 3 to 20 
> MHz, would result in severe power loss in a C pole. This is because 
> the choke impedance is almost all resistive. A C pole on 20m running 
> 100 Watts has a common mode feedpoint potential of about 530 Volts. In 
> a resistive load of 8000 Ohms this produces a loss of 35 Watts, for an 
> efficiency of only 65%.
>> The balun must be designed not only for high impedance, but also 
> for high Q, as explained in my article. If ferrite cores are used, 
> they must be chosen so that they do not saturate and go non-linear on 
> power peaks, otherwise you might get some nasty signal quality reports!
>> 73
>> Brian KF2YN
>>>> From: "Danny" <k6mhe at k6mhe.com>
> To: <antennas at mailman.qth.net>
> Sent: Thursday, July 01, 2010 1:45 PM
> Subject: Re: [Antennas] RG-178 balun turns count
>>>> Yes that is correct and the five times rule is marginal at best - even
>> for fifty-ohm feed point antennas.
>>>> Here are a couple of URLs that have good papers on the subject and the
>> reasons that the choking impedance should several times greater than the
>> five time rule.
>>>> http://www.ifwtech.co.uk/g3sek/in-prac/inpr1005_ext_v1.pdf
>>>> and
>>>> http://audiosystemsgroup.com/RFI-Ham.pdf
>>>> 73,
>> Danny, K6MHE
[snip]