On Thu, 07 May 2009 11:18:14 -0700, Kevin Normoyle wrote:
>Can we agree that the real problem is that QRO Baluns require venting,
>rather than airtight enclosures?
YES! The key issue is dissipation. BUT: A robust coaxial choke (that
is, very high resistive impedance at the operating frequency) will
dissipate very little power because it reduces common mode current to a
very small value.
Dissipation becomes an issue when the core sees enough differential
field from a high power transmitter. That only happens when it's a 2-
wire winding. There is also the issue of saturation, which in turn can
lead to distortion.
Rob said:
>It's probably not a big deal if an antenna is slightly unbalanced and
>there is some resulting common mode current on parallel wire line,
In the transmit mode, common mode current is an issue to the extent that
it puts more RF in your shack (or your neighbor's living room stereo).
In the RX mode, it's an issue to the extent that the feedline picks up
local noise and couples it to the antenna.
For example, let's say you have an antenna up in the air 70 ft, and the
feedline runs down between your house and your neighbor's. There was
only 15 ft between my house in Chicago and my neighbor's house. So -- at
6dB per doubling of distance, a noise source in my neighbor's house
could be 12-15dB closer to the feedline than to the antenna.
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