[Antennas] RG-178 balun turns count

Fri Jul 2 14:38:15 EDT 2010

Brain,

I'm not going to belabor this any more. Except to say this.  Did you 
actually read Chapter 6 of Jim Brown's paper - including the examples he 
gave showing the measurement he took?

Going QRT of the subject.

73,
Danny, K6MHE

On 07/02/2010 10:44 AM, Brian Cake wrote:
> Danny,
>
>    I disagree, for the reasons stated in my last posting. And my 
> example does apply to Ian's chokes. Yes, I am aware that the power 
> loss in a ferrite balun due to the differential current is purely due 
> to the cable loss. But the power loss due to the common mode potential 
> at the feedpoint driving current into the common mode resistance of 
> the balun can be very high indeed, as I noted in my last posting. If 
> you have EZNEC, just run a model, and it will substantiate what I say. 
> If not, build a 20m C pole using a GM3SEK balun, find a clear 
> frequency and run it key down for a while at 100W input, then check 
> its temperature...but be careful, it will be hot!!!
>
>    When I built my first C pole for 20m, I used a choke balun having 
> an unidentified core material from my come-in-handy box. I measured 
> the balun inductance, and at low frequencies it was just fine, but 
> when I used the balun on the C pole it got stinking hot! That was when 
> I was forced to calculate the effects of the common mode resistance of 
> the balun, and led to the designs in my C pole article.
>
>    In short, I contend that the W2DU type of sleeve balun will not 
> work well with a C pole or any other type of OCF dipole, unless great 
> care is taken to make sure the common mode losses in the ferrite 
> material are low, and that the common mode inductance is sufficiently 
> high. For this purpose, 43 material is a poor choice above 2 MHz or 
> so,  61 material is good to 15 MHz or so, and 67 material is good to 
> 100 MHz or so.
> a
>    73
> a
>    Brian KF2YN
>
> ----- Original Message ----- From: "Danny" <k6mhe at k6mhe.com>
> Cc: <antennas at mailman.qth.net>
> Sent: Friday, July 02, 2010 1:12 PM
> Subject: Re: [Antennas] RG-178 balun turns count
>
>
>> 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.
>> a
>> 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]
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