Topband: Where to place a preamp? Switching Beverages?

[Rick Karlquist]

Guy Olinger K2AV wrote:
> I am getting comments from people who have installed FCP plus isolation
> transformer 160 TX antennas, how their new TX antenna is quieter than
> their
> K9AY or pennant, maybe a third or half of respondents (!!!) with some hint
> or outright statement of this.  But I think, rather than the TX antenna
> being all that good, it's really the RX antenna's common mode isolation
> really being that grotesquely BAD.

This advice is spot on and something I have advocated for years.
Transformer coupling is the only way to go for feedline isolation.
Unfortunately, many hams want to use a common mode choke
because they are comfortable with that concept.  The DX engineering
active antenna makes this mistake, and as a result has poor
feedline isolation which made it unusable in my application.

Now the argument for common mode chokes and against transformers
is that it it easy to send DC through the former.  The correct
solution is to send the RF through a transformer and send the
DC through a small inductor that is self resonant at the frequency
of interest.  This permits the preamp to be transformer
isolated from the feedline.

Note that this technical problem is independent of whether you
send DC over the coax or over a separate wire.  The separate
wire still requires the self resonant inductor to isolate it
from the antenna side of the transformer.  If you are operating
on more than one band, get a resonant inductor for each band
and put them in series.  They will interact, but not at
frequencies in the ham bands.

Finally, my experience is that a vertical is the worst possible
receive antenna, even if transformer isolated (yep, tried that).
If your receive antenna is not as good as a vertical, it has
serious problems.

Rick N6RK