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,
("("s "
Unit
Attenuator
Edward A. Lawrence, WASSWD/6
218 Haloid
Ridgecrest, Calif. 93555
Since the topic of "S meters" is a popular
one among radio amateurs, a lot of time is
spent describing these devices, usually along
the lines of how generous or "Scotch" the
meters are at the QTII of the parties in the
QSO. After a few such QSO's, I decided to
build an attenuatar, calibrated in "S" units.
My aim was to attain an accuracy of I db or
better, using 5% ~w resistors and simple
construction so it would be easy to duplicate.
What I wound up with is very similar to
the attenuatar described on page 40, January
'67, 73.
As a sidelight, I started out by calculating
both "tee" and " pi" pads, and used "pi"
because all values of resistance are close to
standard values, but (especially for high
attenuation pads) the values for "tee" pacts
can get quite small; and expensive.
I figured the values required from the
tables in the Allied's "Electronics Data
Handbook", page 8, 5th edition. (Allied
Radio, 75c, full of good info.)
Since HS" units are supposed to be 6 db, I
figured data for steps of 1,2 ,4 and 8 times
that amount, or 6,12,24 and 48 db. With
these steps, any number from 0 to 15 "5"
units of attenuation could be selected. However,
8 "S" units proved to be too much for
one step, as shown by the lowered attenuation
at 30 MHz, due to the inherent shunt
capacitance of the resistor used in the series
leg, plus the stray capacitance of the switch.
So I removed the 8 "S" unit step and
installed another 4 "5" unit step. This allows
selected steps of attenuation from 0 to II
"5" units.
Here are the values I calculated, and the
actual values used, based on 51 ohms. The
steps are switched in series, as required for
the desired attenuation.
Front panel showing the switches for the
various steps of attenuation.
Resistance Values for 51 Ohm Attenuator:
"S" units DB
1 6 154 150 38 39
2 12 85 82 96 100
4 24 58 56 4ffi 390
8 48 51.5 51 64006800
After the attenuator was completed, the
attenuation was measured at 3 kHz and at 30
MHz. With the test equipment available it
was possible to measure more accurately at
30 MHz than at 3 kHz. Below is the data
from the tests.
Atten Step Predicted Measured Measured
"S" units DB etten DB at 3 kH z at 30 MHz
1 6 6.2 6.0 6.02
2 12 12.3 12.2 12.16
4 24 23.3 24.2 24.05
8 48 48.5 47.5 39.11
Now if we want to make an educated guess
as to how far up we can expect good results.
say I db error out of 24 db, then we can use
the measured error in the 48 db step to calculate
the capacitance across the series leg,
and from that calculate the frequency where
the I db error will occur. Go through the
math if that is how you get your kicks, or
take my word for it. It comes out to about
2 pF. And this will cause a reduction of I db
at about 220 MHz. And since the resistor is
Rl
ideal actual
R2
ideal actual
14 73 MAGAZINE