[Premium-Rx] High Local RF Power with Premium Receivers

[Gary Geissinger]

Guys,

Between the 27 MHz high power transmitter in the house right behind mine, a
number of strong pager transmitters in my area (along with a local MW
transmitter), and Field Day every June I have a few things that I do to help
with strong local interference.  I hope some of this is of help.

1.  A band reject filter was built for 27 MHz that helped a great deal, but
did not completely take out the local interference.

2.  Station grounding was improved using 1 inch wide (and 1/4 inch thick)
braid between all the radio gear in the shack.  The braid was soldered to
the copper cold water pipe.  This actually made a dramatic difference in
reducing interference getting to the receiver by paths other than the main
RF antenna input feed.  

3.  A surplus passive bandpass filter for HF was installed (bought off of
EBAY).  It knocks out the medium wave band and anything over 30 MHz.  This
ended the medium wave and pager interference.

4.  Finally, a Harris RF-551A preselector was procured (EBAY) and a custom
interface built so that it tracks the receiver tuning.  While the front end
of the preselector is not as good as my receiver's, the preselector can
tolerate 100W DIRECTLY INTO THE FRONT END and is very narrow in HF terms.
For field day our club ran a test of the RF-551A preselector.  With a
receiver (Icom IC-756, just an okay HF receiver in my estimation) at 7 MHz
and a transmitter at 14 MHz (at 1500 Watts) there was no noticeable
interference with the antenna spacing at 20 feet. When we switched the
receiver to 10 MHz, the interference was noticeable, but weak stations could
still be worked.  Without the preselector, reception was impossible at both
7 MHz and 10 MHz with the transmitter power reduced to 100 Watts. Based on
this experiment, there are now 4 of us in our club who use the RF-551A
preselectors for field day.

4a.  We repeated this test using an Elecraft K2 as the receiver (modified
with the signal limiting diodes to improve strong signal handling).  This
receiver has one of the best front ends available in ham gear.  Results with
the RF-551A were the same (no surprise here).  The K2 receiving on 7 MHz
with the transmitter at 100 Watts at 14 MHz heard interference, but medium
power signals could still be heard and worked.  This is impressive given the
20 foot antenna spacing.  Clearly the better front end makes a big
difference; however, the RF-551A still solved the problem best. 

5.  As an experiment, an MFJ noise/signal canceller was procured and tried
with a second antenna.  On some interference sources the improvement was
impressive.  In other cases; however, the broadband nature of this unit
actually made the problems worse.

I hope some of this helps.  Electro-magnetic compatibility can be a tough
problem.  

Gary WA0SPM
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