[TowerTalk] TowerTalk Digest, Vol 172, Issue 38
jim at audiosystemsgroup.com
Thu Apr 27 16:04:00 EDT 2017
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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