EMRFD Message Archive 229
Message Date From Subject 229 2006-12-09 14:20:21 Paul Wilton R2 pro on 2m Having recently been given a copy of EMRFD as a birthday present, I am
taken with the idea of build a transceiver for 2m (and maybe 6m later
on) based around the R2 pro plus LM2 and T2.
If I build it, it will be
a) A replacement for my FT726 for 2m SSB. Provided I can swing it
+/-100 kHZ around 144.300, the tuning range should be fine
b) Something to experiment with
My question is what performance I can expect on 2m? Is it likely to be
better than my FT726 in terms of sensitivity, selectivity and close in
intermod performance? If not, I'll probably go the usual route and
build a transverter to go with my K2. I've had a look around on the web
for performance figures for typical R2 systems but have found none on 2m.
230 2006-12-11 14:13:53 Rick Re: R2 pro on 2m Hi Paul,
Since I probably have as much experience as anyone with R2 type
things on 2m I'll throw my 2 cents in.
First--pondering the question "should I design and build a system
based on the R2pro or just keep using my FT-726" is a bit like
asking "should I start taking violin lessons or just go to a
concert?" The R2pro approach is a lot of work, and if I my evil
alter-ego professor side designed it properly, you will need to learn
an awful lot to get it up and running well. Those are the questions
I ponder all the time, so you are not alone. In fact, the first
question I always ask before taking on a new project is not, "how
well will it work?" but "how much will I learn?"
I gather from your comment about just going the usual route and
building a transverter for the K2 that you have built transverters
before. If not--I highly recommend exactly that route to get on 2m.
From all reports, the K2 is an excellent box, ideally suited for use
as a tunable IF radio. I designed and built transverters for all the
bands up through 10 GHz for a more than ten years before I finally
decided to replace the commercial transceivers I used as tunable IFs
with something I designed specifically for the purpose. The
transverters were a great education, and a K2 with a well-designed 2m
transverter would be hard to beat.
Now, on to 2m. Nic Hamilton published some pioneering work on 2m
direct conversion receivers that I highly recommend tracking down if
you want to do a serious 2m direct conversion system. I have built a
number of them, but never quite took the shielding seriously enough.
LO leakage, either through poor shielding of the LO system or
radiation out the antenna port, will kill you on 2m. Effects that
are annoying in low-cost direct conversion systems at HF are
debilitating at 2 meters for a number of reasons that take up most of
chapter 8 in EMRFD. I won't go into them here, because this is the
EMRFD web site and I know you already have the book.
In particular, the LM2 preamp reverse isolation isn't quite good
enough. For a really high performance system I'd scrap the LM2 board
and build the circuitry on pages 12.24 and 12.35 ugly style, and then
seal it all up in a die-cast box with SMA connectors. Then for
receive, I'd design a two-stage common-gate U310 amplifier, with lots
of reverse isolation. My goal would be to get the LO leakage out the
antenna port to be less than about -80 dBm.
(That said, there are a large number of LM2-miniR2-T2 systems out
there working well, and many people find them perfect for small
portable applications and serious weak signal work.)
If you have a T2 board already, by all means use it. I have a number
of those on 2m. If not, use the new microT2 board described in
December QST, and available (soon) from Kanga. You will have to do a
bit of design work to put it on 2m, but not much--mostly just
frequency scaling the few band-specific components. The buffer
amplifier will need bypass and coupling capacitors appropriate to 144
MHz, and a 2N5179 instead of the 2N3904. The input to the buffer amp
can come from your VXO or some other VFO system.
If you take shielding of the individual stages very seriously, with
lots of diecast boxes and feedthrough capacitors (see pp. 12.33 to
12.48 for examples), and build a nice ugly constructed VXO system,
you will end up with a great radio. It won't have memories, agc or a
number of the other features that many people consider essential. It
won't have any spurs or synthesizer noise either, and if it drifts,
you will understand it well enough to fix the problem....but it will
take a lot of time to design and build.
There is another way. My original need for the whole R2 etc. series
of projects was not for direct conversion radios, but to serve as the
tunable IF for transverters. That approach avoids all the quirks of
direct conversion systems, including the need for battleship
shielding. All of my 2m R2, miniR2 and R2pro systems are used as
tunable IFs for the higher bands--that's why none of them are quite
well shielded enough to use directly
231 2006-12-12 13:59:39 Glen Torr Re: R2 pro on 2m
Hi Rick and All,
>So instead of using a K2 with a transverter, you could pick an HF
>band (I like 7 MHz) and build a good R2pro-microT2 system
>there, and then use a transverter to get to any other band.
Would a single conversion transverter using a 137 MHz oscillator be a viable approach to develop a 2 meter transceiver from 7 MHz using the microR2 and micro T2?
232 2006-12-12 14:40:40 Rick Re: R2 pro on 2m Yes, a 2m transmitter-receiver with a 137 MHz LO and 7 MHz IF with a
microR2 and microT2 would be fine. You want to be careful to have
enough selectivity to prevent radiating either the image (just below
130 MHz) or the 137 MHz LO out the antenna port.
A diode ring mixer and any of the Toko style 2 and 3 section helical
resonators would work well. I also like to use cavity front-ends
salvaged from old mobile radios, but they may be unavailable or
unfamiliar. Downeast Microwave in Frenchtown NJ has helical filters
for the ham bands at lower cost than some of the parts catalogs.
I have experimented with a number of different configurations on 2m,
and used the microR2 on the cover of October QST for listening to 2m
with an old Ameco nuvistor converter for several months.
There are many possibilities. For 2m I might be inclined to do a
good ugly constructed VFO, temperature compensated using the W7ZOI
technique, sealed in a tin can, and split the output to both the
microR2 and microT2 boards so I could have normal transceive
234 2006-12-13 06:17:11 wilts123 Re: R2 pro on 2m Rick
Thanks for your very detailed reply. It certainly gives me something
to think about and some helpful pointers.
If I go the transverter route, then the "obvious" solution is to buy
the Elecraft 2 and 6m transverters since these will be well integrated
with my K2. However, rather than just "assembling" something, I
fancied more of a challenge, hence the original questions about going
the R2 route.
In the end, I will just have to make an engineering trade-off between
how much time I put in and how much I learn against what the final
results could be.....