Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
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 |
#2
|
|||
|
|||
"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 |
#3
|
|||
|
|||
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 |
#4
|
|||
|
|||
"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 |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Low Pass Filter ?? | Antenna | |||
Digital cable filter spam | Antenna |