[RFI] replies -- QRM from AM BDCST station
tomcox at iquest.net
Fri Jan 2 16:07:10 EST 2004
Good idea. I tried to put all responses together for the list. I didn't
get any requests to keep private replies off the list, but just in case,
preemptive apologies. Also, apologies for any replies I missed below.
I would appreciate a forward of the good ideas you received.
Please let me know your eventual solution.
I have B'cast interference too.
REPLIES BELOW, in no particular order -- all worth reading KT9OM
I.C.E. (Industrial Communications Engineers) of Indianapolis, Indiana makes
a High Pass filter specifically for this purpose. I have one at my home QTH
in the USA and it works well on an AM station at 1310 kc. I beleive the
filter is rated at 300 watts and can be placed at the output of your
Sorry but I don't have the address or phone number of I.C.E. over here but
beleive they have a web site with just a few of their products. You will
have to call them for more info.
John - HS0ZDJ (W2YR)
I live about 3 miles from a 10kw station on 990khz. I also have an IC-746.
>I've got enough RF coupled into my 256ft horizontal loop, used against
>ground on 160, to cause an old fashioned SWR bridge into its field
>port read up scale with noticeable modulation!
>I had broadcast overload on 160 until I started using an external matching
>unit (tee match style MFJ-969). I still get interference at the 2nt
>but the overload is gone and 160 is usable. In this mode I don't use the
>746's internal tuner. This might be the most satisfactory solution. Maybe
>you could borrow a tuning unit for evaluation.
>I built, years ago, a high pass filter into a Radio Shack variable
>attenuator originally made for CATV use. That helped a DX-440 (consumer
>quality SWL receiver) perform well. I had video sweep gear available
>delighted to see a nice knee at 1600khz and 28 db or so rejection. The
>design was right out of the ARRL handbook. It was not designed for
>If you go the homemade filter route, check your junk box for discarded
>cassette recorders. I found a useful adjustable coil from the bias
>You might also consider a pair of bypass relays for transmitting.
I have a remotely operated 160m station at a former broadcast tower (at
least I did have until somebody broke in and trashed it - it will be
operational again next season). There is a high power AM broadcast
station about 2km away on 790kHz, and as I was receiving on a 5/8 wave
vertical it was quite strong... What I did was to make a simple high
pass/notch filter (3rd order elliptic function) using 3 capacitors and
one inductor which puts a 60dB notch onto 790kHz. This is quite enough
to remove any problems even though the rig is only an IC728 which is not
terribly wonderful. Your case is a little harder because of the much
lower separation to 1340kHz. However, a 3rd order filter will still give
you a more than 40dB notch. I am not smart enough to design filters in
my head, but I have been using a really neat Windows program called
Filter Free. The (free) version I have allows you design up to 4th order
filters; you have to pay lots to get the full blooded one. I see from
their web site that their latest free version only allows 2nd order
filters which is useless, so if it would help I could send you the one I
have. There are probably other free programs out there also.
I took the liberty to run a design for 1340kHz through the program
without any great attempt to optimise it and as a symmetrical 'T'
circuit it gives:
2 series 1nF9 capacitors between input and output 8uH25 inductor from
the junction of those capacitors 1nF7 capacitor from the inductor to ground
You can also do it with 3 inductors and one capacitor but I think the T
is easier to build/adjust. All I did was to build it to the design
values, but make the inductor adjustable (I used coil stock) and just
used the receiver to give me the greatest null on the BC signal. I used
transmitting mica capacitors so it will easily handle 100W.
Note that the attenuation over the rest of the BC band is not wonderful
- only about 10dB, but if there's only one station that doesn't matter.
If there is more than one you have the choice of using a higher order
filter or using more than one simple filter in series. The latter is
much easier to adjust.
If you want actually just want a notch then you can use a series
resonant circuit in parallel or a parallel resonant circuit in series
(!). The values for the parallel resonant circuit are 440nH in parallel
with 32nF, and that would give you about a 20dB notch, with little
attenuation beyond +/- 50kHz.
There are lots of other possibilities which you can only really see by
playing with a filter design program.
Hope that helps.
In pleasantly cool North Eastern Ontario
More information about the RFI