RadioBanter - View Single Post - Problem of nearby transmitter breakthrough

September 20th 05, 05:22 PM

wrote:
On Mon, 19 Sep 2005 23:35:12 -0400, "Bob Chilcoat"
wrote:

The receiver is already in a (steel) waterproof box. Tomorrow I will try
moving the receiver as far away as I can. If that doesn't fix the problem,
I'll try the stub antenna notch filter solution. I have a BNC Tee. Can
anyone point me to the 1/4 wave length formula for 120.6 MHz? Does 0.591
meters (23 5/16") sound right?

Wavelength is 300/F in meters. One quarter wave is 75/f in meters.
However your using coax cable and the speed of light (nominally 300 in
the examples cited) is slower depending on the coax used. for foam
types it's about .8 (varies by brand) and for solid types usually
around .66 (still varies some).

So for 120.6 a 1/4 wave section is 75/120.6=.62189M (24.4838").
A section of RG58 (solid dialetric) would be .66 that length or .4104M
(16.159").

To give you an idea of how critical that length is the 1/4Wave of the
same coax for 123mhz is .4024M (15.84").

So even small variations in VF (velocity factor for the coax) or
gutting error can make for big mistuning.

years ago, many cable pay-tv signals were scrambled simply with an
interfering carrier inserted at 2.25 MHz above the video carrier. The
'decoder' was simply a notch trap that removed the carrier.
It was quite easy to remove this 'scrambling carrier' with a stub of
coax, twinlead or even zip cord. Once you know the approximate length
its a matter of just trimming away in 1/4" intervals. I forget if the
qurater-wave stub remained open and the half-wave stub was shorted, or
vice versa, but you get the drift...it only takes a few minutes to try.
That said, its easy to do sitting on the floor behind the tv with a
steady signal but if you are trying to trap an intermittent signal in
the field it might be rather maddening without a test oscillator and a
means of measuring the signal. Then its placement into the 'system' of
iffy impedance may cause it to shift slightly.

IF you could find an old cable TV "Channel A" or "Channel 14" positive
trap it would be a cinch to retune it. I don't think any of the
commercial vendors would sell them as one-off, though...they only cost
about $6 new. Maybe somebody has one in a junquebox?

-Bill- I'm surprised my junkbox didn't yield one