Topband: QRM Canceling Experience

[Bill Tippett]

        I've been experimenting with an MFJ-1025 and two identical
Beverages, staggered ~1/4 wl and separated ~1/8 wl.  These spacings
(1/8 to 1/4 for both dimensions) are recommended in the DXE NCC-1
manual.  Listening in the ARRL 160 is a perfect test since there are
many strong signals interspersed with weak DX signals.  I'm able to
reduce most interfering signals by about 20-25 dB, but not much beyond
that.  I'm just curious what others have observed if anyone has tried
this with Beverages?  I'm guessing that it's difficult to achieve
better rejection because of varying phase with real signals.  Using
stable test signals, the 1025 is capable of ~55 dB nulls with very
careful adjustment.

        The modified 1025 seems to work fairly well under these severe
contest conditions.  I've heard a few very weak IMD products but
nothing significant even with both channels at maximum gain.  I plan
more experimenting tonight to see if reducing gain will even eliminate
the weak IMD.  I've done all W8JI mods (excluding removing the BCB
filter) listed on his webpage.  I also added a 2:1 impedance
transformer to both inputs since both of mine were ~160 ohms on
Topband, even though other bands were OK.

        If anyone is using an NCC-1  **with Beverages**  (i.e. not
verticals), I'm especially interested in how much rejection you can
achieve on real signals?

                                                                 73,
Bill  W4ZV

P.S.  It's really amazing to hear some big guns CQ-ing away with
hordes of DX calling them...typical ARRL 160!