Jim:
Search of ARRL website for QST using N7BV claims no article written by him
exists.
Can you be a bit more specific.
Would love to see article.
Thanks,
Ed McCann
AG6CX
*******
Sent from my iPhone
> On Apr 27, 2017, at 1:04 PM, Jim Brown <jim@audiosystemsgroup.com> wrote:
>
> An effective common mode choke is a low-Q parallel resonant circuit, with the
> resonance placed near the operating frequency(ies). Typical circuit Q is
> around 0.5. Power dissipated in the choke due to common mode current is I
> squared R (or E squared divided by R), where I and E are the common mode
> voltage and current. In an antenna system that is reasonably close to
> balance, taking the feedline into account, common mode voltage and current at
> the choke is moderate, and a choke with common mode Z of at least 5,000 ohms
> can handle a fair amount of power. That means 500-600 W with high duty cycle
> and 1-1.5kW with low duty cycle.
>
> If, however, the antenna system is badly unbalanced, as ANY OCF antenna is,
> the common mode voltage and current at the choke are MUCH higher, so that
> choke that handles 500-600 W at high duty cycle may fry with 100W at a low
> duty cycle! And, as N7BV noted in a QST article several years ago, if high
> SWR and feedline length combine to place a current peak at the choke, the
> DIFFFERENTIAL dissipation due to I squared R inside the coax can fry the
> choke.
>
> Repeating my advice -- all-band antennas fed with open wire line are
> YESTERDAY'S antennas. They transmit just fine, but they cannot be effectively
> choked to kill RX noise and to prevent feedline current in the shack. The
> noise problem is new within the past 20 years, where, by 2017, the average
> home, including our own and those of neighbors, EACH typically contains 20-30
> noise sources, each of them connected by wires that act as antennas to
> radiate their noise to our antennas. Any antenna that cannot be choked is a
> poor choice if you live within a few city blocks of your nearest neighbors.
>
> 73, Jim K9YC
>
>> On Thu,4/27/2017 10:47 AM, Guy Olinger wrote:
>> Having disassembled a couple of the Carolina Windom devices that W0UCE
>> burned up running 1500W CW to them, I can attest to their weakness.
>> The windings were RG8X wound on what appeared to be adequate ferrite
>> cores. The RG8X had melted the inner dielectric and allowed the center
>> conductor to short to the shield. There was some case to be made that
>> the ferrite rod had heated, but the melt did not seem to begin at the
>> jacket next to the ferrite rod.
>
>
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