Here are a few additional ideas and comments.
Dig up an old tube RX such as the Drake R4 or an older Xcvr such as a
TS820, etc. If the problem persists on them it is time to check outdoors.
If it goes away proceed to step 2.
Ascertain that your current rig uses diodes to switch RX front end
filters. If possible disable the BC Band in ur rig. This can be done by
lifting one end of the switching diodes. This is not easy to do with some
radios. The older radios do not use diodes but old fashioned switch
contacts. If the problem goes away or even diminishes a bit you can chose
to leave it disabled or upgrade to PIN diodes for all band switching
diodes. The PIN mod to my TS940 reduced the crud from S5 to S1 or less
plus made a major improvement in overal IMD performance. The culprit is
that the total BC band energy is causing the diodes to act as a untuned
crystal set. Each diode is probably contributing a little bit.
I got so frustrated many years ago that I disabled the whole input
network on my TS940 and ran the beverage feedline right to the RF amp
input xfmr thru a very sharp bandpass filter. It worked 100% but not too
pratical. The PIN's have made it livable most of the time.
Other potential culprits are the Beverage matching transformer and the
feedline. Those little bitty 1/2" cores that many people use can go non
linear in the presence of loads of RF on the antenna. They may be fine
out in the boonies. IMHO the BBR-7731 core is the worse possible choice.
I use nothing smaller than a FT114-61 core or even a FT140-61 since I
am LOS to AM towers in most directions. If the cores are causing the
problem then all the ferrites, HP filters or preselectors in the world
wont help. I have thought about adding a multi pole HP filter in the
antenna line just before the xfmr but like many other things around here
it never got done. Before ripping everything apart just remove the xfmr
and hook the feedline directly to the antenna and ground with some short
clipleads and run a test. The impedence mismatch is not important in this
step.
Lightning can also damage ferrite cores. Even a storm many miles away can
induce high voltages on the antenna. I got knocked on my butt once from a
discharge that was many miles away. Ferrite can become permanently
damaged, even with no visible evidence, and cause massive linearity
problems. With 6 Beverages here it is not uncommon for me to replace 5-10
vaporized 2W carbon terminating resistors every year. I should probably
replace the xfmr each time too.
I sometimes wonder if we hams cause some of our own problems. How much
voltage is induced on the Beverage from that big amp and vertical some
short distance away?
Another thought just occured to me while typing.... With all of the
blown resistors I would expect the preamp or relay switching matrix to
fry often too. But yet I only have to replace the Mosfet maybe once every
year or less. I have never lost a relay. My initial thought is that the
ground rods and 700' feedline bleed off the voltage before it gets into
the shack. .....Dunno. Any ideas out there?
Feedlines can be the source of frustration too. Disconnect the feedline
from the xfmr and then tune your radio over the BC band.....signals
should be 50+ dB down, see the ON4UN book for other feedline tests. I had
a 700' run of RG6 CATV drop cable gradually deterioate over about 5 years
from moisture migration. It was run on the ground thru dense woods and
became covered with leaves. The moisture caused severe corrosion to the
braid and foil shield. I now use 1/2" CATV hardline. With any cable use
Penetrox or similar on connector threads. Avoid RS or other import
connectors and adaptors. The materials plus the imperfect grounding of
the threaded nut (or whatever its called) deteriorate rapidly outdoors.
Finally, I use a tunable preselector preamp in the shack. It is a
modified Ameco Mosfet unit that now has a gain control since I almost
never use over 10dB of gain....usually 3-5dB. Broadband preamps out at
the antenna can also be another source of headaches.
None of the above ideas is a cure-all or even applicable to many. They
are just a few things to add to some of the already excellent comments on
the reflector.
Oh and yes, tower guywires can generate crud. I have been informed that
some commercial 2 way sites have added the Radiokit A637 Superbeads on
the guywire at the ground end to eliminate IMD problems at UHF and is
mandated in new construction! They determined thru tests that the
turnbuckle, equalizer plate, guy rod interfaces were acting as diode
joints, which is well known in the amateur field, but Im still amazed
at the cure.
Keep ideas flowing.
73.....Carl
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