[Antennas] RG-178 balun turns count

Fri Jul 2 16:37:45 EDT 2010

Danny,
 Chapter 6 of Jim Brown's excellent paper includes only measurements of 
the magnitude of the balun common mode impedance...there is no way to tell 
whether it is all reactive or all resistive or a bit of both.
 In the section titled "The Dissipation Question" he states "the worst 
case puts the full transmitter voltage across the choke under the condition 
of infinite imbalance". This is absolutely not the worst case for antennas 
that are fed off-center, where the common mode voltage can be many times the 
feedpoint voltage. This is the condition that exists in a C pole and any OCF 
antenna.
 These are also my last comments. Nice doing battle with you!
 73
 Brian KF2YN
----- Original Message ----- 
From: "Danny" <k6mhe at k6mhe.com>
Cc: <antennas at mailman.qth.net>
Sent: Friday, July 02, 2010 2:38 PM
Subject: Re: [Antennas] RG-178 balun turns count
> 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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