[Antennas] RG-178 balun turns count

[Brian Cake]

    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
>
>
> On 07/01/2010 10:09 AM, Brian Cake wrote:
>>      That rule of thumb does not apply to off-center-fed antennas, such 
>> as
>> the C pole, where the common mode potential at the feedpoint can be high.
>>
>>      73
>>
>>      Brian KF2YN
>> ----- Original Message -----
>> From: "Roy"<royanjoy at ncn.net>
>> To:<Antennas at mailman.qth.net>
>> Sent: Thursday, July 01, 2010 12:47 PM
>> Subject: Re: [Antennas] RG-178 balun turns count
>>
>>
>>
>>> About:
>>>
>>> "I have a 2" diameter coil form. How do I calculate/test for the number
>>> of turns required to be an effective choke at 14MHz?"
>>>
>>>
>>> The old rule is to aim for five times the reactance in the coil than the 
>>> Z
>>> of the load....5 X 50 = 250 ohms.
>>>
>>> 73,    Roy     K6XK
>>>
>>>
>>>
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