"Tom Holden" wrote in message
.. .
"Dale Parfitt" wrote in message
news:2O42g.1$HC3.0@trnddc07...

wrote in message
oups.com...
If the antenna is amplified, you can play all sorts of games down
stream. If not, I would just, as Spike Lee says, "do the right thing."

Since the length of the open end of the T is very short, it probably
won't matter much. A common trick is to make a notch filter by
inserting a T then putting an unterminated coax on the T that is a
quarter wave length (I think) of the frequency you want to notch.

Note that the notches made with a single stub are rather broad and even
more important is the fact that it will also notch 3X, 5X, 7X etc the
fundamental.
So, if one places a 1/4 wave open stub at 1.5 MHz to reduce an AM
station, notches will also occur at 4.5MHz, 7.5MHz, 10.5MHz, etc. i.e. a
real mess.

Dale W4OP
A more suitable application for the stub filter for HF reception is to
notch out a strong local VHF-FM broadcast or low-band VHF TV broadcast
station that is breaking through as an image in the HF band. Such images
can be a problem on double or triple conversion radios where the 1st IF is
in the VHF region, e.g. 45 MHz. Channel 2 visual carrier at 55.25 MHz
would be imaged near 5 MHz and 98.1 MHz FM would be imaged near 26.5 MHz.
Cutting the 1/4 wave stub to the transmitting frequency of the offending
image will reduce or eliminate the image. The harmonic notches will have
no adverse effect on HF signals and the stub length will be manageably
short. Of course, a conventional 30 MHz low-pass filter will be effective
at suppressing images across a broader range of frequencies.

Tom
There is another problem to be considered particularly if the antenna in
question is to be used for TX. Consider a 1/4 wave stub cut from 50 Ohm
coax for CH 2 video as above.
The stub with foam coax is approx. 43" long. 50 Ohm cable nominally has
29pF/foot of capacity. The stub will look like a shunt 100pF capacitor well
below its notching frequency. At say, 20M this represents 120 Ohms of
reactance- low enough to ruin the VSWR of a previously well matched antenna.
Pardon if I made a math error- done in my head.
An LPF, as suggested, is a much better solution.
Dale W4OP