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.

I can easily get a 30MHz oscillator brick, so I figured that'd be a good
starting point.

For HF, I whack the signals through a low-pass filter, tune the VFO
between 30-60MHz. It might get a bit hairy on 10m, but for 15m and below
we should be good. A notch filter right on 30MHz might make 10m workable.

For 6m, a low-pass filter on 54MHz should do it, tune between 80-84MHz.

2m and 70cm is where it gets interesting. I spent a few hours trying to
synthesize a 144-148MHz BPF in Qucs last Saturday. Not as easy as I
envisaged. I'd set it up to produce a high-order Chebychev filter, then
find out that the filter calls for 1pF capacitors (!) and very specific
coils. Cauer filters got closer, but not really the holy grail I'm after.

Then I did some thinking: the Icom IC-22b apparently used 5 helical
filters in its front-end to great effect. They're a smallish rig too,
and looking at a picture of the guts of one, I see 5 metal cans that
don't look terribly huge. They also look fairly straightforward
mechanically.

The question is, how good are these in terms of roll-off? I don't expect
them to put up much of a contest against a crystal filter but I'm not
sure what's realistic.

Regards,
Stuart Longland