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Re: [TowerTalk] TowerTalk Digest, Vol 172, Issue 38

To: jim@audiosystemsgroup.com
Subject: Re: [TowerTalk] TowerTalk Digest, Vol 172, Issue 38
From: Edward Mccann via TowerTalk <towertalk@contesting.com>
Reply-to: Edward Mccann <edwmccann@yahoo.com>
Date: Thu, 27 Apr 2017 21:04:28 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
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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> 
> 
> 
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