[TowerTalk] Narrow Band Filters

[Michael Tope]

----- Original Message -----
From: "Jim Lux" <jimlux at earthlink.net>
>
> ----- Original Message -----
> From: "Richard (Rick) Karlquist (N6RK)" <richard at karlquist.com>

> Lots of complexity for probably not much gain (other than tech gee whiz).
> Realistically, you're looking at thousands of dollars in hardware
> (especially if you're running QRO).  Consider that a single computer
> controlled LC to handle hundreds of watts runs in the kilobuck range, and
> you're probably going to need 4 of them (two radios, 1 Tx trap, 1 Rx trap
> for each).  For FD, you've got 1000 ft of separation potentially
available,
> so, investing in coax might be a better deal.

If you use the MFJ style noise cancellors, you could
do it fairly cheap. Standard stubs or lumped element
bandpass filters will take care of crossband interference.
The noise cancellors would be used to null the
broadband noise from the co-band/adjacent-mode
station. When you are dealing with a stable local
source, the noise cancellors like the MFJ unit are very
easy to adjust. If fundamental overload from the
co-band/cross-mode station is a problem (unless you
match the group delay in both RF paths, the cancellor
won't produce a broadband null), you could use a
narrow tunable filter on receive (something like the old
Drake R-4C preselector). Again very easy to adjust (just
peak the receiver noise).

One the other hand, long pieces of coax are even simpler.
We do our field day on a mountain top, so we could
conceivable put separate receive antennas down the
hill (towards the northeast of course) to improve isolation.
The co-band/adjacent-mode stations could share a single
receive antenna (and coax) located at maximum distance
from the TX antennas (a small tribander with a power
divider for instance).

73 de Mike, W4EF................................