[UK-CONTEST] Fw: Filters

[Ian White GM3SEK]

Paul Beecham wrote:
>
>----- Original Message -----
>From: "Paul Beecham" <paul at prolectric.co.uk>
>To: "Ron Fraser" <ron at gm0ntl50.freeserve.co.uk>
>Sent: Friday, March 17, 2006 9:36 AM
>Subject: Re: [UK-CONTEST] Filters
>
>
>> Ok Ron, yes stubb filters are easy to make, just be care full to insulate
>> the ends properly as very high voltages can be present and don't get
>> confused between open and shorted ends. Here I,ve used so239 "T"
>> connectors to fit them into the feedlines. The coax  of course to be high
>> quality to suit transmit power and be mindfull of velocity factor as it
>> effects the length. 73  Paul   G6PZ ----- Original Message -----
>> From: "Ron Fraser" <ron at gm0ntl50.freeserve.co.uk>
>> To: "Paul Beecham" <paul at prolectric.co.uk>
>> Sent: Friday, March 17, 2006 9:19 AM
>> Subject: Re: [UK-CONTEST] Filters
>>
>>
>>> Hi Paul,
>>>
>>> Thanks for the speedy response to my posting.
>>> I forgot to mention that we currently use ICE bandpass filters which are
>>> by no means perfect but do the job.
>>> We'll look into the coaxial stub filters that you mentioned and see how
>>> we get on.
>>>
>>> Wow, what a contest setup you have. Obviously a lot of work and expense
>>> has gone into it.
>>>
>>> We operate , in Iota, as low power dxpedtion from Eu092 so we have to try
>>> and maximise our TX (MPs) and wire antennas as much as possible....really
>>> we should move a few hundred miles farther south which would help!
>>>
>>> Thanks again,
>>> Ron GM0NTL

In addition to the antenna siting and screening, you can use in-band 
peak/notch filters like the FM repeater builders do. The filter on the 
CW station has its notch on the high side, while the one for the SSB 
station has its notch below.

You'd be pushing your luck to get more than 20dB of notch per section 
(and multiple sections would probably add up to too much coax) so it 
certainly isn't a magic key to fully independent TX/RX operating inside 
the same band. However, for the IOTA station that has already tried 
everything else, it might just make enough difference to let the mult 
station do a more effective job of listening in-band.

Peak/notch filters don't have to be big copper cavities. For HF, they 
can be made like coax stubs - the only difference is a few extra 
components at one or both ends of the cable. They do not act as harmonic 
filters on other bands, so you'd still need those as well.

A few years ago, G4SWX and I nearly wrote an article about various kinds 
of peak/notch filters made from coax, working up to a design that has 
notches both above and below the 2m band to knock out strong pager 
signals. "Nearly wrote" means "did all the design, wrote about 95% of 
the article, and then both got side-tracked".

I'm contesting this weekend, but maybe next week...



-- 
73 from Ian GM3SEK