Step attenuator for sale

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From: Mike Czuhajewski (Mike.Czuhajewski@bbs.abs.net)
Date: Sun Feb 04 1996 - 00:24:10 EST


Step attenuators are useful items for QRPers, both in
homebrewing (testing filters, receiver sensitivity, etc) and
operating (milliwatting). This one is 75 ohms, not 50, but
there's a way around that at slight penalty. It's a Kay
Elemetrics model 442D, 1 to 101 dB in 1 dB steps, with
individually selected sections of 20, 20, 20, 20, 10, 5, 3,
2 and 1 dB. The maximum power rating is one watt (but if
using it for milliwatting, I'd keep the power well below
that to be safe). I didn't get a chance to look in the Kay
catalog to see the rated frequency, but on a network
analyzer it looks pretty good to 1 GHz.

W6TOY bought this at the Odenton swapfest, at a table near
the Maryland Milliwatts. He paid $25 for it, and didn't
realize it was 75 ohms until it was too late. He could have
returned it but I told him I'd either get a buyer or take it
myself. (I have an Imelda Marcos relationship with step
attenuators.) It's yours for the same price, plus shipping,
but I'm a value-added retailer--this one comes with a
printout of the network analyzer evaluation. I slapped it
on one of the HP 3753D network analyzers at work and did a
sweep of 1 MHz to 1 GHz on each of the 9 sections. It gives
a good, graphic display of the attenuation across the range,
and also digital readout at 4 marker frequencies of 30, 144,
500+ and 999 MHz.

What use is a 75 ohm attenuator if all you work with is 50
ohms? All you need to do is slap a 75/50 ohm conversion
device on each end and you're all set. The technical name
for them is "minimum loss pad", and they're an L resistor
network. They add 5.7 dB each of loss, or a total of 11.4
if you use them on both ends. That's not necessarily a
drawback; you know what the added attenuation is and take it
into account when you use it. There are quite a few times
when you use an attenuator when you don't need to go under
10 dB, so it's only a slight drawback. Instead of a 1 to
101 dB attenuator, they make it an 11.4 to 112.4 dB unit.
(You could use wideband toroidal transformers instead, which
would have less loss, but those would be somewhat sensitive
to frequency when you use them across a large span.)

At no extra cost, since I got several for nothing, I'm
throwing in a pair of old Texscan ZM-57 minimum loss pads.
These look like little BNC attenuators, with male and female
on either end. They were of the wrong sex when I got them--
the female ends were 75 ohms, male were 50; they were more
suitable for using a 50 ohm attenuator in a 75 ohm system,
so I opened them up, swapped the center contacts end for
end, and put them back together. They aren't as pretty as
they were when new since I had to surgically remove the
labels, but they don't look terrible...and the price is
right.

Mechanical details--the 442D is 8.5 inches long, 10 when you
include the female BNCs at each end, and 1.25 high by 2
wide. It is rather hefty, probably weighing a pound or so.
When the minimum loss pads are attached to the ends, the
overall length is 13 inches. The color is white, indicating
that it's relatively new, since the older models were gray.
The case has a few scuffs and dings but in pretty good
shape. One of the switch handles is slightly bent but works
OK. One of the switch mounting nuts was missing, so I
replaced it with what I could find at home; doesn't match
but does the job. This unit even has a pedigree of sorts--
it still has a USAF TMDE label on it. That's Test,
Measurement and Diagnostic Equipment, once known as PMEL,
Precision Measurement Equipment Lab; that's the USAF shop
which tests, aligns and repairs all test equipment on a
base. (When you're in a technical USAF career field like I
was before I retired, you get to know the function quite
well since everything you do depends on them.)

Kay is still very much in business, by the way.

If interested in the attenuator, please contact me and we'll
work something out. W6TOY just wants his $25 back and
doesn't care where it comes from. This is your chance to
have a nice step attenuator and take me off the hook--I
already have ten commercially made step attenuators, to say
nothing of the ones I built myself, so I hardly need another
one!

73 and Queue Our Pea DE WA8MCQ wa8mcq@bbs.abs.net

--
Mike Czuhajewski, user of the UniBoard System @ abs.net
E-Mail: Mike.Czuhajewski@bbs.abs.net
The WB3FFV Amateur Radio BBS - Located in Baltimore, Maryland USA
Supporting the Amateur Radio Hobby, and TCP/IP InterNetworking


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