rf - Free and Open Source Software

sumed. A signal generator of sufficient outpu
t power can also be used. If one uses a
method such that connection of the transmitter
to the receiver for reference level setting
is no t necessary, a transmitter of fixed
po wer output of any level can be used.
Summary
When commercial laboratories make gain
measurements using so me of the methods described
they take elaborate precautions to avoid
effects that will alter true gain readings.
However, even with simple equipment-even
the regular sta tion transmitter and receiver in
many cases- meaningful results can be obtained.
Even if one does not measure the gain of
any antennas, the material in this article
should give a better insight to many amateurs
as to how the gain figure for an ant enna
is determined. Particularly, it should clarify
how antenna gain is always related to some
reference. Thus, unless one knows the reference,
one can easily read good-sounding
but not really useful gain figures for some antennas.
Finally, it should be appreciat ed that gain
is only a numeric and not the only meaningful
cha racte ristic of an antenna, although too
to gain figures. Ot her factors such as the
vertical and horizontal radiation pattern
forms, front-to-buck ratio, impedance, bandwidth,
etc. are just as im portant and, indeed ,
in some applications more significant for best
communication than gain. ...W2EEY/l
Albu.. " nt,lns thre, 12'"
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A Different TR Switch
The TR Switch described in May 1963 73
:IIaga:inc on pages 12 and 14 has undergo ne
a metamorphosis or change for the better.
Whi le in some areas of the country the
grounded grid configuration wilt fun ction
well, it behooves the amateur in a metropolita
n area, especially where there are several
marine, coastal, point to point commercial
sta tio ns operating, to use a different circuit.
The rf chokes, especially the one in the cat h­
ode of the grounded grid tubes, have a self
resona nt frequency and 10 and behold, commercial
stations can be heard in the background.
Weakly, but still there. No amount
of decoupling will eliminate them. Different
values of chokes can be used, but then a sacrifice
in gain on the amateur frequencies results.
The most satisfactory circuit ry tried to
date uses a cascode rf stage lightly cou pled
to the transmission line, and a cathode follower
output to match the receiver input impedance.
A tube rectifier is used as an rf rectifier
to provide de voltage bias on transmit times.
There is no time constant- it is instantaneous,
for CW, in order for fast break-in. When using
SSB, the bias holds long enough between syllables
so that the receiver stays blocked as
long as you keep talking. When receiving,
the rectifier has no effect, unless a kw station
next door fires up. In that case, he will create
enough bias in the TR switch to prevent overload
of the set. It will lower the gain of your
receiver, but without this effect, you would
have another kind of trouble. You would
probably go to the other end of the band or
change bands. It will be apparent to the experienced
constructor that this unit can be
used as a tuneable preselector merely by substituting
the I mh L2 rf choke with suitable
tuned circuitry. M. C. Smith, W6GMC
MAY 1969 45