..."splatter" from a local Am station, particularly around 5
MHz, from a nearby location.

I probably used the wrong term here.

Well, maybe, and maybe not.

As I said,
If it's a broadcast station harmonic being *truly* transmitted
and *accurately* received, up at 5 MHz, it would be somewhat
unusual.

That's been my experience in a quarter-century working around AM transmitters in
the United States, but as soon as I posted it, I read Glenn Hauser's latest DXLD,
in which he remarked that he'd picked up the TENTH harmonic of a Mexican MW
station up in the SW band! Glenn's no fool, and I'd trust his observation; the
answer is that, possibly, the standard of maintenance at that station is
abysmally poor, and the signal being radiated is SO terrible and full of illegal
garbage that this situation -- possible in that country with lax regulations,
unlikely here in the US -- could happen under some strange conditions.

Problem is that as a listener, *you don't know* at this point, since even
expensive communications receivers aren't calibrated like FCC type-accepted
field strength meters, used to measure antenna system performance. The radio
station would likely have one, if it's directional (or would need to borrow one
from time to time), and could quickly verify if there is a REAL spur out at 5
MHz, coming from their transmitter. Before I'd complain to them, though, I'd
test the problem on at least a small number of radios, and with varying antenna
lengths and couplings.

I do get the station at exactly three times,
but I also receive the station at other spots
not harmonically related.

Sounds like the *same* problem I have, four miles from a 50kW station, five or
six miles from some 5kW and 1kW stations: which is why I used a SW highpass
filter that attenuates below 2 MHz, and a notch filter for the 50kW station's
frequency. When these stations drop their power at night and either change from
ND (non-directional), to D (directional) with patterns not necessarily aiming at
ME, I can turn one or both of these filters off...but still hear some *very*
weak 2nd and 3rd harmonics: legal "wisps" of signals.

The odd thing is I used to live 15 miles from the station and
would receive the station on [an] old tube communication receiver
Collins) with only a five foot antenna!

Depending on the station's pattern, if D, and your position within a lobe or
null, this seems normal to me. In the thirties, before compact loop antennas
became commonplace, *local* station radio reception was often obtained with just
a small piece of hookup wire, even as short as you describe. A tube radio can
be made to be extremely sensitive: a Collins isn't exactly chopped chicken
liver!

Unless the station is abysmally weak -- and if it were, it wouldn't be causing
you problems! -- I'll bet an unamplified crystal set at your location (or five
miles away) could pick up the station just fine, with 5 feet of wire as an
antenna.

If it is normal to get cross modulation only 15 miles from [an]
AM radio transmitter, that would imply nobody near a major
urban area (Chicago, Los Angelos,New York, etc) could use
a shortwave radio below 7 MHZ. Is that true?

If you have a radio that is prone to internally-generated cross modulation
effects, then this is true. If you have too much signal from your antenna,
making almost ANY radio overload in one of its RF or IF stages, this could be
true. If you take pains to get a well-designed radio, and to use an antenna
that provides signals you WANT to receive, but not too much out-of-band RF that
you DO NOT want to receive, then you won't have the problem -- IF the cross mod
otherwise is generated in the radio, using a completely broadband antenna!

If the spurious signals are caused by intermods that happen when two or more
transmitting facilities are close together, with complex interactions in their
antenna systems, matching networks, and output stages (*particularly* if one or
more of the rigs is a solid-state transmitter!) you may have all kinds of
spurious signals flying around; your poor old radio will just faithfully pick
them up!

There happens to be a notorious AM radio transmission site in a certain specific
city where I used to work, where four stations intermodulate with each other.
The FCC and all other parties involved know about it, but the problems are
practically insoluble without separating all the rigs and towers; filters
installed have not helped too much. A few miles away, the junk is too faint to
detect; but as you drive by, the AM band becomes a hideous mess (SW bands too.)
Unfortunately, this antenna farm is right by the ingress/egress to a major
commuting bridge, and at the nexus of a gigantic highway complex: so tens of
thousands of people drive through the area each day. I guess they get used to
expecting their AM radios to 'crash and burn' for a mile or two; the fellows who
are MOST upset about it seem to be the managers and PD's of the stations!

When I moved to my new location which is approx one mile
from station...

Hoo-boy, do you have a problem! At a mile away, I'd scarcely expect *any* radio
to behave perfectly. So that other respondents may check, could you please post
the station's call letters and frequency? I am sure that readers in your area
may be able to scan the dial and see what their results are. What is the power
of the station, if you know? And its mode of operation: ND or D? If D, are you
in a main signal lobe? This may be determined by checking one of the websites
that give American BC station stats, based on your location. Try:
http://www.radio-locator.com/

I also purchased a new receiver a JRC nrd545. I have the problem
with both receivers. The JRC has a switchable RF antenuator of
20 db.

Good radio; but it's solid-state. Is the RF attenuator designed like many other
similar circuits: just a switchable gain function in a front-end stage, or a
voltage-controlled attenuator, using solid-state junctions? If so, even a 20 dB
attenuation may NOT eliminate internal cross-modulation in a solid state front-
end device. Older transistor radios were murder, with their sensitive and fussy
bipolar transistors. Newer ones with FETs are still difficult to design with
the same signal-handling dynamic range as boat-anchor tube radios. (And it
should be pointed out that ANY solid state device can act like a "mixer" if the
signal input is beyond the linear power-handling range.)

I myself put off buying a new solid state receiver for years due to my own
location, holding on to some tube equipment after reading all the complaints on
such forums as r.r.s. about cross-mod and overload problems. I was pleasantly
surprised that my Icom R75 could be so easily "tamed" with a tiny handful of
parts and a few minutes of work; and my antennas are 175 feet and 350 feet long,
yet in a metropolitan area bristling with AM stations. So, I have no doubt that
a JRC can be made to work reliably with the proper attention to the input. A
passive preselector, such as the well-regarded MFJ unit, could help; or you
could do as I did, and do some experimentation with antenna coupling, highpass,
and notch filters.

Happy with the results obtained with my Icom and the filters, I then tried out a
new Sony 7600; but no matter what I've done, it's a washout when a local HAM
transmits in either the 80 or 40M bands. I *think* I know who he is, and can
see a sloper and some other antennas at a house a block or two from me; I have
absolutely no reason to suspect that he isn't operating legally, so the problem
is really up to me. If you buy a cheap ($500) radio in a plastic case, expect
trouble from such nearby carriers. The Icom, however, has a metal case and good
shielding, so a combination of proper grounding, good antenna coupling, and
filtering makes it work well. I *could* build a grounded cubic Faraday shield,
insert the Sony radio into it, and run all leads to/fro via appropriate
bypassing devices. But how could I tune the radio? (The mind boggles at such a
Rube Goldberg solution.) So I play it when the HAM is not on the air...

I wouldn't burden the poor radio operator to solve this problem; he simply
couldn't. (This is one reason why *I* am not a HAM operator; as an avid
audiophile myself, I have sympathy for neighbors who might enjoy -- say -- audio
recording. Why ruin their hobby, just to blabber on about yours?)

But, remembering the inverse-square law, I am at least four to five miles away
from one of the main problematical stations that can overload my rig; you are
only ONE mile away. You may be getting a pretty hefty dose of RF at this short
distance. (If you were a bit closer, you might see if your electric toaster can
play the station!)

To summarize:

1. Obtain information about the allegedly-offending station: power, location,
antenna mode (ND or D) and if D, the pattern and where YOU are in its footprint;

2. Test your radio(s) by altering the antenna coupling and reducing the input
WELL OUTSIDE of the radio's rear antenna terminal; do not merely rely on using
the *internal* RF attenuator, which may NOT be effective against preventing
input stage crossmod tendencies.

3. Compare your reception problem with other locations, perhaps using a portable
radio. Tune in your "5 MHz splatter" signal with a portable, and drive or walk
around, and go closer and farther from the station. See what happens.

4. If you have any hint that the signals are spurious ones that MAY change
significantly if you alter the antenna coupling, then try a notch filter to
reduce the fundamental transmitting frequency of the station. This might hugely
reduce the input crossmod and clear everything up; but if junk still remains,
outside the filter's bandstop frequency range, you COULD be picking up *real*
signals being transmitted as spurs, or through re-radiation from metal surfaces
near the transmitter. Either way, the station should know about this; they must
fix the former problem, and possibly might be able to solve the latter one if
they care about their "RF community", even if the trouble is being caused, say,
by a bad gutter, a powerline, or a corroded fence well off their property.

AUTEUR (MW station broadcast engineer by profession up to the '90's.)


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