"Richard Clark" wrote in message
news
On Fri, 10 Sep 2004 03:50:44 GMT, "A-Tech"
wrote:

It seemed to me that a reasonable attempt would be to use a piece of
300ohm
flat-lead and short
it at an appropriate distance from the screw-terminals of the antenna.
None
of my attempts have
yielded any observable improvements. It may be that the filter must be
"deeper" (higher Q?) than
what my attempts provide.

Any ideas?

Hi Bryce,

Length can be very critical as you can imagine. Trying to find the
sweet spot, over and over, can become taxing if not simply fruitless
(undoubtedly your experience to this point).

A simple method is to connect the twin lead in the usual manner.
Measure out a little more than 1.5M to work with (this will be too
long, but it is better than being too short). Take a razor blade (old
style used for scraping paint) and working from the far end cut
through the insulation of both leads at once to short them out.
Repeat at 1/16" intervals (or 1mm intervals if we are sticking with
metric) and note results. You don't want to cut through completely,
just enough to nick the copper and make a good short with the blade.

Keep in mind that while you are handling the line doing this, you are
part of the tuned circuit (and possibly detuned circuit too). It will
give you a quick ball-park and reduce the cut-and-try.

73's
Richard Clark, KB7QHC

Unfortunately, you will find that the -3dB BW of a single stub is in the
neighborhood of 5% or 5MHz at your frequency of interest. The
discrimination between your desired and undsired channels will be
negligible.
We manufacture a line of small multiple cavity symmetrical and asymmetrical
notch filters- getting under 0.5% BW with small topology filters is nearly
impossible at VHF.

Dale W4OP

A company in FL - well, they might have been blown away by now - called
something like Pizio Technologies makes VHF bandpass filters from "crystal"
elements. Such filters are capable of rejecting adjacent channels in the
150 MHz range and are needed in some cities if you have to put a receiver
downtown.
As Dale points out, it takes a serious filter indeed to perform what you
wish such a filter to perform. On the other hand, I have deliberately tuned
some higher end FM broadcast receivers to a weak adjacent station in the
presence of a fairly strong station to find a bit better than mediocre
performance. Some receivers have much better filtering than other
receivers.
So, as another summery: rotate antenna for best SNR; get a receiver
with better filtering; and consider a special-made crystal filter if you
still have a problem. 73 Mac N8TT

--
J. Mc Laughlin; Michigan U.S.A.
Home:
"Dale Parfitt" wrote in message
news:_X90d.8722$Va5.8116@trnddc01...


Unfortunately, you will find that the -3dB BW of a single stub is in the
neighborhood of 5% or 5MHz at your frequency of interest. The
discrimination between your desired and undsired channels will be
negligible.
We manufacture a line of small multiple cavity symmetrical and
asymmetrical
notch filters- getting under 0.5% BW with small topology filters is nearly
impossible at VHF.

Dale W4OP

"J. Mc Laughlin" wrote in message
...
A company in FL - well, they might have been blown away by now - called
something like Pizio Technologies makes VHF bandpass filters from
"crystal"
elements. Such filters are capable of rejecting adjacent channels in the
150 MHz range and are needed in some cities if you have to put a receiver
downtown.
As Dale points out, it takes a serious filter indeed to perform what
you
wish such a filter to perform. On the other hand, I have deliberately
tuned
some higher end FM broadcast receivers to a weak adjacent station in the
presence of a fairly strong station to find a bit better than mediocre
performance. Some receivers have much better filtering than other
receivers.
So, as another summery: rotate antenna for best SNR; get a receiver
with better filtering; and consider a special-made crystal filter if you
still have a problem. 73 Mac N8TT

PTI is definitely still in business and are very capable of building such a
filter- unfortunately in small quantities they are cost prohibitive.

Dale W4OP