History of
the Desert
Ratt
This is an attempt to describe the development of a nice HF regenerative radio receiver design that
was reworked by several different people. My comments try to identify
the improvements, or at least the new ideas, not seen in previous
versions (at least, not in this "series").
1) "$10 receiver has microvolt sensitivity"
the original by Charles Kitchin N1TEV, 1994.
- very low regen stage voltage
- very high Zin, fit for a telescopic whip
- RF output buffer between regen stage and diode detector
- smooth resistive regen control, with two effects acting in
opposite directions.
2) Paul Harden NA5N had built this circuit already in 1995
and credited Charles Kitchin in a mailing list posting. He later
published a version called the "Pipsqueak",
on QRPp. Paul sent out a very similar version ascii schematic to
the QRP-L mailing list. It can be seen here
(From: Ira 'Crash' Knorr (mediumwave@hotmail.com), Subject: Re:
shortwave receiver schematics, Newsgroups: alt.radio.pirate,
1998/09/25). Craig LaBarge WB3GCK built a version of the same (nice pics).
3) The "Desert Ratt" (.pdf) by Paul
Harden
NA5N, 1996-1997 .
- high pass filter
- RF preamp/buffer
- voltage doubler detector, probably better Zout match to AF amp
- AF amp stage after the detector
- AF phase splitter feeding the +/- inputs of the LM386
differentially
4) The "Preá" receiver (.doc
files), by Miguel
Angelo
Bartié PY2OOH, who credits the "famous Desert Ratt".
- DC-decoupled pot between tuning coil tap and ground allows for Q
reduction without changes in polarization
- venier tuning, by voltage controlled capacitance in the same place
- connection from tank circuit to RF output buffer at the low-Z
coil tap
- ...probably a better match than the original, or at
least reduces the load on the tank circuit...
- ...yet nobody else seems to have adopted it, pro'lly because the
circuit exegesis was offered in Brazilian...
- notice that the the 3 Si diodes in DC stabilizer are polarized
wrong (these are NOT zeners)
- no AF phase splitter, no RF HPF
5) "Desert Ratt 3" by Paul Harden
NA5N, 1996 (?). This was probably a work in process, which
was published by Arnie Coro CO2KK on the Radio Habana site, and then
quoted and linked
by everybody and their
cats. There seem to be lots of rigs built by various people
called
"Desert Ratt 3" that may be based on this version.
- unfiltered RF preamp/buffer
- straight RF detector again, not voltage doubler
- antenna trimmer, not HPF; nearly
superfluous 120pf fixed cap in series with 3-30pF variable
6) "A Simple Regen Radio for Beginners"
(.pdf), 2000.- Charles Kitchin pointed out to me that he had kept
experimenting for a few months to reduce the parts count on his
original design, and also came up with one that included a LM386 in the
audio. It was eventually published in QST.
- straight RF detector again, not voltage doubler
- whip antenna,
- printed circuit available from FAR Circuits
- simple audio amp with LM386 (Charles points out that the
split-phase version works better)
- no RF preamp/buffer (although Charles claims that it "needs it"),
no RF output buffer, no voltage doubler
detector..
This is clearly a design optimized for beginners, like the 1994
original.
7) "Desert Ratt 2" (.pdf)
by Paul Harden NA5N, last updated in 2002.
With a detailed
description (.pdf). http://www.aoc.nrao.edu/~pharden/hobby/DR2descr.pdf
- the RF HPS returns
- RF preamp fed with very low voltage, like regen stage
- RF preamp to tank circuit coupling is now inductive, with
impedance step up
- tuning by voltage and reverse polarized Si diodes (not even
varicaps)
- stabilizer now uses LED in series with 1 Si diode, saving some
power
- RF detector is voltage doubler again
- band change by switching caps into the tank circuit
- no more AF amp between detector and phase splitter
8) Now what?
I had originally written here that it would have been nice to hear if
N1TEV, NA5N, WB3GCK, and PY2OOH had any new ideas in the
intervening time, and if there are any other
interesting versions that I missed. So
far the feedback is the following:
- N1TEV wrote that his simple bipolar
designs have become his most popular work in spite of their performance
limitations - e.g., his JFET designs work better. I guess
it may also have something to do with the fact that his
“High Performance Regenerative Receiver Design.” articles
(QEX 1998, ARRL 2000, if I got it right) are still not generally
available on the web. I have no idea how much of those articles
overlaps with this FET project of his: "A Shortwave Regenerative Receiver Project"
(1999), which instead seems designed for simplicity as well..
- WB3GCK never played with regens after the
Pipsquesk.
- PY2OOH may have made further mods, but
none is documented so far - let's stay tuned.
As
things stand now, the "Simple Regen
Radio for Beginners" wins for simplicity. If
you
intend to
build a Desert Ratt for performance, I think, you
are better off starting
from the "Desert
Ratt 2",
not from the "Desert Ratt 3" on the RHC site.
Furthermore, I would consider trying out the following: complications:
- RF volume pot ahead of HPF (especially for those of us who live
in
Europe!)
- toroid core in the tank circuit
- feeding the tuning controls, RF-stages
voltage stabilizer & LED from stabilized 6.8V
- fully separate DC-based (RF-choke coupled pot) and RF-based (cap
coupled Q-reduction pot) regeneration controls, just to get a feel of
both
- also (big maybe) use a real, balanced, double junction varicap
with vider capacity range
- tuning controls: main with with logaritmic100k pot connected to
diodes via
100k, fine with linear 100k pot conected via 1M or so
- couple the regen to the RF buffer at a low Z point like PY2OOH did
- more aggressive AF voice bandpass filtering.