On Fri, 28 Feb 2014, Stuart Longland VK4MSL wrote:

Hi all,

Well, I've been doing some further thinking on this idea of a homebrew
set. Gareth pointed me to the "minima" homebrew set:

http://www.phonestack.com/farhan/minima.html

I must say, that thing is truly inspiring. It got me thinking though.

I'd like to reach up into the VHF; 2m if possible. Now I bought some
Si570 ICs the other day, and they're quite capable of reaching 160MHz.
So 144MHz is no problem. Taking the 3rd harmonic should get me to 70cm.

Deciding on a suitable IF is the next challenge. The higher I make it,
the easier it becomes to make a filter that will reject images. I
considered a bog-standard 10.7MHz IF at first, but then considered what
would happen at 70cm where the band goes from 430-450MHz.

At one point, there was a wave of FM communication receivers that used
21.4MHz (twice 10.7MHz) IFs, so those should be reasonably available. Of
course, I've seen some odd filters in the HF range used in such receivers
too.

When early clunky cellphones were obsolete, I was able to buy some at
garage sales. This was the mid-nineties. They all had crystal filters in
the frequencies above 30MHz, some around 45MHz but also some around 70MHz
if I remember. The good thing was they all converted a second time to 455
or 450KHz, so there was a conversion crystal there too.

If you have to convert to some other frequency for actual selectivity,
then the you'd need to get a crystal to do that conversion, which suddenly
makes those filters expensive.

The reality is there were lots of crystal filters out there in the HF
range. Probably more under 10MHz than above, but they were all over the
place, on odd frequencies. If you need selectivity there, then you have
to hope to find one that is for SSB, or go the ladder filter route. I've
seen SSB filters at really odd frequencies, so in commercial equipment
there isn't a standard.

WIth all the recent portable shortwave receivers on the commercial market,
there may be some now that are broken, offered up at a low cost, but
having an IF filter around 45MHz, and then a conversion to 455KHz or
450KHz, the latter filters not perfect for SSB but in some receivers there
is a narrower one than the AM filter.

Michael