"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