Channel Jumper wrote:
KRLD - operates with a effective radiated power of 50,000 watts.
Probably when it was built, there were no houses where the transmitter
is located. As urban sprawl continues, and property values increases,
being in prime real estate becomes an issue.

Simple blocking circuit: a 365 pf variable capacitor and a 240
microhenry ferrite core inductor or coil, connected in series
and connected between the two conductors of the feedline, all
placed in an aluminum box. Readily available RCA or "F" connectors
can be used to connect to the feedline.

Sometimes I see people use two of these, say in the same
box with an aluminum baffle between the two parts, loosely
coupled with, perhaps, a reasonably small capacitor, in
the feed line (or a third, parellel wired, tuned circuit)
between them. Nice little hacker's project, and something
that probably tunes to a good null, but once you're trying
to put three tuned circuits in a homebuilt trap you may
be better off buying a commercial product.

There are some problems that cannot be solved by
high-pass filters or nulling circuits. These are
harmonics, or, less likely but troublesome when
present, heterodynes. Both can be caused by bad
metal-to-metal contacts in the environment -
gutters and downspouts, fencing, even the buried
ground system below the tower. Don't have any
experience tracking these things down, but there's
plenty of information available on reducing RFI -
radio frequency interference. Start with the
Radio Amateur's Handbook and other publications
of the American Radio Relay League.

Calculations:

You might consider an ERP of 50K watts versus, say,
a ham radio at 100 watts. Usually a ham radio at a
distance of one city block pefectly acceptable while
the 50KW AM band signal is well known to cause
problems.

But let's say we want to reduce down to 50 watts.
That's 1000:1 in power. It's is common to just
count the number of decimal places in ratios of
this sort - amplification or attenuation - so that's
10^3 - 3 decimal places. Communications engineers
call one power of 10 a Bel, so it's three Bels,
or, more commonly, 30 deciBels (dB).

So what you want is something that reduces the
unwanted RF signal by at least 30 dB. Thats' doable.
I suspect you could find a high-pass filter with
a low-frequency cutoff of 2 Mhz and an attenuation
of 30 dB, for a 50 or 75 ohm feedline, without much
of a problem.

George Cornelius

George Cornelius wrote:
Channel Jumper wrote:
KRLD - operates with a effective radiated power of 50,000 watts.

[...]

Simple blocking circuit: a 365 pf variable capacitor and a 240
microhenry ferrite core inductor or coil

Rethinking...

I chose the standard tuned circuit components used in
the input stage of an AM radio over the years, but looking
closer I would guess that for a low impedence feed line
you would get better results - a sharper null - using a
smaller inductor and a larger capacitor.

So let's say we divide the inductance L of the
coil by 10.

For 1080 Khz, you need 1/sqrt(L C) = 2 pi f,
or C = 1/( 4 pi^2 f^2 L). This means you'll
need to tune C to about 900 pf for a 24 uh
coil. If you use a 365 pf variable C and add
750 pf or so in parallel to it, then, changing
to using this smaller coil, you should be able
to tune the null plus or minus a few hundred
Khz around the station frequency.

George Cornelius