Topband: Question on common mode chokes

[Guy Olinger K2AV]

Those would be transmitted signal strength concerns.

On the receive side, if one ever listens on one's transmit antenna, the
feed coax shield is a potential path for RF noise from the house. Any noise
voltage allowed to go from the coax shield to the radials becomes a
differential voltage versus  the vertical wire which now travels back to
the shack *inside* the coax. Common mode choke on the coax near the antenna
takes care of that. The remote tuner likely works against a common case
ground which directly connects the coax shield to the radials.

73, Guy K2AV

On Wednesday, June 3, 2015, Jim Brown <jim at audiosystemsgroup.com> wrote:

> On Wed,6/3/2015 9:42 AM, James Rodenkirch wrote:
>
>> I seem to recall, while reading up on common mode chokes, where I
>> wouldn't need one if I employ an autotuner at the base of the antenna...is
>> that true/factual?
>>
>
> The function of a choke on the feedline is to prevent it from becoming
> part of the radial system. This matters most when the radial system is
> relatively limited, but is quite important when it is elevated. See N6LF's
> work on this, published as a 2-part piece in QEX several years ago, and
> available on his website. Google his call to find it.
>
> The "executive summary" is that ground losses are least when the current
> is equally divided between many radials, which is affected by their length,
> their number, soil quality, and proximity to the earth. The loss in a
> radial is I squared R, where R is loss coupled from the earth. The more
> radials present, the greater the division of the base current between them,
> thus the smaller the I. And because power is I squared, lost power falls in
> proportion to the number of radials.
>
> When there are only a few radials, current distribution will be strongly
> affected by the nature of the earth under them, which can vary a lot over a
> radial field. In this case, radials that carry greater current will
> dissipate more power, and the total power loss will be greater. The
> significance of the feedline choke is that it prevents the feedline from
> disturbing that balance.
>
> 73, Jim K9YC
>
>
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