Ladder Crystal Filters For Radio Amateurs
by Jack Hardcastle, G3JIR
Introduction
In the following pages you will find a detailed guide to
the design of ladder crystal filters. If this is too
mathematical for you do not worry. It is only included for
those of you who really, really, really wish to know about
such things. For everybody else all you need to do is to
read the help file. Even this will be seen to be necessary
only as a last resort. Just jump to the software and enjoy
putting in some numbers, and read the help file later.
The comprehensive ladder crystal filter design program has
been prepared by Horst Steder DJ6EV. Taking my BASIC
routines which were published in Radcom in May 1983 as a
starting point, he has made a number of important
additions which have greatly increased the program's
facilities and ease of use. These additional routines for
tuning individual crystals and also the graphics which
have been assembled by Delphi into the finished product.
Incidentally, the reason these BASIC routines were so
fragmentary is because the PET and the Apple2 computers in
use in 1983 had very limited memory available. In fact,
the memory available for programs was a mere 4k, or so,
something which is laughably small by today's standards.
Our shared interest in these filters goes back as far as
2001 and after several hundred QSOs on 40 metres over this
period we decided we had enough material to make it
worthwhile trying to pass it on to others, in an easily
usable form. I cannot put a date on the decision to publish,
but it was at least two years prior to its publication date
in the November/December issue of QEX. The development of
this work has been carried out using amateur radio as the
principal means of communication; but we have also made much
use of the Internet, and innumerable e-mail's have passed
between us. For instance, the final draft of the text which
was submitted to QEX was number 9d: we gave-up allocating
numbers after 9. This would have extended even further into
the alphabet if we had not used our regular 40 metre 'skeds'
to discuss some of the trickier points of design. It would
be nice to be able to say that it was all finally 'wrapped
up' using only these means of communication, but we have to
admit to needing to make some last minute telephone calls
so as to get decisions back to ARRL without further delay.
So, what are we going to do next. The brief answer to this
is that we have not yet decided. We continuously discuss
things which have the potential to become a future publication.
However, even radio amateurs who are retired only have a
finite amount of spare time to devote to such projects.
Don't forget, we also like chasing DX, working contests and
having local ragchews on 2 metres. And on top of all that
there is that much neglected list of chores our XYLs line-up
for us. So please take this into account if you need to
contact us. We will try to provide informative answers to any
questions which we have not covered in the Help file.
This will be updated at intervals.
Acknowledgements
Firstly, I wish to thank the committee of the Warrington
Amateur Radio Club for allowing this material to be included
on their web site, and for Dave Tarbatt G7SKR for converting my
files into a form suitable for the 'Web'.
Finally, I would like to acknowledge the copyright owners
of the publications included in this website posting.
1. The Radio Communication articles are the copyright of
the Radio Society of Great Britain and are reproduced with
their kind permission.
2. The Crystal Measurement article from the January/February
issue of QEX is the copyright of ARRL and is reprinted with
their kind permission.
If you are not already a member of one of these Societies
please consider joining. Details can be obtained from their
web-sites listed below.
The Radio Society of Great Britain (RSGB)
The American Radio Relay League (ARRL)