SpamHog wrote:

The Desrt Ratt HR regenerative receiver has a good reputation, and I'd
love to build one, but I am a quite mediocre homebrewer... If I can
get some potential problems out of the way I can better concentrate on
the many others that will surely crop up!

These are my doubts w.r.t. the DR2 as described in the .PDFs on Paul
Harden's page:
http://www.aoc.nrao.edu/~pharden/hobby/DR2.pdf
http://www.aoc.nrao.edu/~pharden/hobby/DR2descr.pdf .
Can anyone comment on any of these points?

1) Tuning Coil
--------------
The PDF mentions inductance, number of turns and wire gage,
but leave us in suspense apropos the coil former:
"T1 is wound directly on the PCB". Uh?
I guess anything 5-6uH will do, but I can't keep from wondering...

Here is a link to a few pictures of an older version of the Desert Ratt.

http://www.njqrp.org/qhbextra/2/2a.html

You have to scroll down a little bit, but this shows that the coil is
wound on a toroid. As long as it has good characteristics in the 3-30 MHz
range, any one can be used. The permeability may vary from one toroid to
another but you should be able experiment a little to get things right.
Maybe someone else could suggest a proper toroid number. The one there
looks like a T50-?? or T38-??. It's hard to tell.

2) Tuning Varicap
-----------------
"R5 sets the current across D2-D3" The reverse resistance on such
rectifiers is huge, much greater than R5 (10kohm). I noticed that Paul
Harden N5AN once offered V/C measurements on diodes for use as varicaps
(http://www.kkn.net/archives/html/QRP.../msg00360.html)
There, one sees a 1N4148, forward biased. From the specs one gleans
that this diode has a sizeable current when forward biased at voltages
within a reasonable tunng range - but we're talking about something
quite different here. What gives?

Yes, forward biased the 1N4148 will have a high current. If you read
further through that thread, someone else described a problem with using
it in that mode. The high current caused excessive heating and drift.
Not really recommended for use.

3) Q2 (oscillator) biasing
--------------------------
The resistor between C6 and C7 is not rated. A base bias on an regen's
oscillator can be rather, well, critical....

C6, C7, and R? are there to filter and stabilize the Base-Emitter
voltage and bypass the Base-Emitter junction from RF. A value of
100K should be sufficient. The value isn't really that critical.

Here is a link to a really old version of the Desert Ratt.

http://duke.usask.ca/~buydens/ham/ratt/index.htm

The coil is different but the biasing structure of Q2 is pretty much
the same. Also note the slightly different part arrangement on the
lm386. Get a spec sheet on the lm386 to see what each part is suppose
to do.

4) IC Amplifier
------------
R12 is indicated as "10W".
What would a reasonable resistance rating be?

This just looks like a misprint. I believe that should be 10 Ohms.
Just go to google and type in lm386 for all the information you can
stand on that chip. I didn't see any configuration that used it
in quite that way, but that doesn't mean it can't be used that wy.

5) Voltage Regulator
--------------------
The text says that the drop across the forward biased LED1
is 1.3-1.8V, while across the Si rectifier D1 is 0.7V.
In series they'd sum up to 2 - 2.5V, but on the circuit
one reads a total drop of 3.0V, i.e. up to 1 V higher.
Charles Kitchin's design fed the oscillator just 2.1V...
Would 2.5V be enough? Or, do LEDs come with a wide variety
of voltage drops?

The voltage drop across the diode D1 will be as specified
for just about any 1N914 you use, however, the voltage across
a LED depends on exactly which one you choose. Here is another
link to a page that describes the kind of voltage drops you
can expect from LEDs.

http://www.theledlight.com/LED101.html

It seems that this value can be any where between 1.7 and
4.6 volts depending on the type you choose. I seem to recall that
even when you get a LED from Radio Shack the specs are printed
on the back. Personally, I would shoot for the 3.0 volts, listed
in the schematic.

TIA to everyone!

I hope this will encourage others to experiment with this circuit.

Filippo (SpamHog)
N1JPR/I2

--
Martin E. Meserve - K7MEM
http://www.k7mem.150m.com