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

To be pedantic and precise, AM splatter consists of sidebands of audio
modulation that interfere with adjacent signals. No AM radio station can
splatter out of the broadcast band to 5 MHz; it's a physical impossibility.
Even an old messy RCA Ampliphase transmitter, badly misadjusted, would not
usually splatter more than a few tens of kHz either side of carrier frequency.
So therefore you would surely mean that you are getting a *spurious signal* that
is somehow related to the station's transmission, presumably a harmonic or spur.


If it's a broadcast station harmonic being *truly* transmitted and *accurately*
received, up at 5 MHz, it would be somewhat unusual. As a broadcast engineer my
experience is that above the weak third harmonic of the carrier signal there is
rarely much being transmitted; if so, the power levels would be exceptionally
low (maybe tiny fractions of a watt at the fifth or sixth harmonic, at worst.) A
significant signal could be transmitted if the station had a real problem: such
as a spurious transmitter oscillation that somehow got 'passed' by their antenna
system (which, by definition, is well off resonance this far from the carrier
frequency, and therefore incredibly inefficient "out there" at 5 MHz...) Or
there could be a bad component in a matching network; even a re-radiation of the
signal from a corroded fence or roof fairly near the transmitter site. And two
close-by radio transmitters can interfere with each other and produce sum-and-
difference "intermod" frequencies.

But my guess is that you probably are experiencing crossmodulation effects in an
overloaded radio front-end. If so, unless you significantly attenuate the main
fundamental carrier frequency signal of the station, you aren't going to
eliminate the noise. A loop antenna surely can help, if you can orient it to
null out the local station. But you might want to try some experimentation
before spending a lot on the Wellbrook you suggest. De-couple your antenna
connection using a variable resistor or by trying a series of different values
of capacitors in the range of a few tens to hundreds of picofarads -- or even a
"gimmick" capacitor: merely break the connection of the wire leading from the
antenna to the radio antenna terminal and twist a few turns of the (insulted)
broken ends around each other, varying the number of turns and therefore the
coupling. If at some point you can reduce the signal this way so that the 5 MHz
interference goes away, you have confirmed that the radio front-end is
overloading.

You should probably also use a shielded coax line and a matching transformer,
with the coax grounded at both the radio and the transformer ends: thus the lead-
in will not itself act as an antenna for the strong local transmitter signal.

If the station is REALLY transmitting a spur up at 5 MHz (or if there is a
*real* signal being propagated in space due to this station 'interacting' with a
fence or another carrier, producing an intermodulation product), then by using a
bandpass filter to reduce the antenna gain only at broadcast band frequencies
you may not solve the problem: the 5 MHz spur (or whatever it is) won't be
affected. But if the spurious signal is YOUR radio overloading and
crossmodulating, the BC band filter (SW band highpass filter) will help, or even
completely fix the problem.

I have this trouble with my hypersensitive solid state communications receiver
and use a simple series shunt filter tuned to a local 50kW station's frequency,
right across the antenna-to-ground terminals of my radio. The filter is a
variable cap in series with a (metal-shielded) ferrite antenna coil, parts I
took from a throw-away broken transistor radio. I tune the variable cap to the
station's frequency: this filter then shunts most of that frequency to ground
(having a measurable effect of reducing the signal by 27 dB, quite a lot!) The
"one pole" filter is broad, and has a discernible effect through most of the BC
band; but well above 2 MHz in the SW bands it has no attenuation. Therefore, I
leave it in all the time, since I don't listen below the 90M band with this
radio. This simple filter, which took five minutes to make and adjust, enabled
me to cut out most of the crossmodulation effects from several local AM stations
that caused a number of discrete intermod products that peppered the tropical
band and the 80M ham band. It is interesting to note, however, that I *still*
receive some very faint second and third harmonics of two stations, "real ones"
being transmitted but within legal FCC attenuation specs: just milliwatts of
signal that my radio easily detects, close to the transmitters.

My tube-type radio picked up less of these signals, but did get SOME of them.
In the case of both radios, reducing antenna coupling did not suddenly cause
them to disappear: because the signals were really there, not merely caused by
front end overload. The tube radio heard less of them, because it is far less
sensitive than the solid-state modern receiver.

However, cheap portable SW radios and even my fairly costly Sony 7600 pick up
not only front-end crossmod products but also "wipe outs" of large portions of
the SW bands when a local SSB'er starts his rag-chews. I have tried almost
everything: filters, extra grounds, reducing antenna coupling; and nothing
really works to make the cheap radios clean and free from strong interference.
I just wait and use them at night when all the BC stations drop their power, and
when the ham is transmitting...I turn them off.

AUTEUR



--
Ce message a ete poste via la plateforme Web club-Internet.fr
This message has been posted by the Web platform club-Internet.fr

http://forums.club-internet.fr/