The-Ideal-T-R-Switch.pdf

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the
ideal
t-r
switch
Two methods are
in
general
use for pro-
viding an antenna in an amateur sta-
tion installation. One is to use separate
antennas for transmitting and receiving.
Most amateurs interested i n top receiver
performance rule out this possibility, pre-
ferring instead to have the advantage of
main and directivity of their transmitting
antenna for receiving. The other method is
to use a single antenna for receiving and
transmitting, and switch the antenna be-
tween receiver and transmitter with a re-
lay or electronic transmit-receive (t-r)
switch.
Both methods have advantages and
disadvantages. For the cw operator, a t-r
switch allows full break-in operation,
which is necessary for traffic, net,
DX
and
contest operating. If you've ever used full
break-in, you can appreciate how nice i t
is to be able to monitor what's going on
in the way of competition as you listen
between code characters. But the t-r
switch has one drawback. No t-r switch
i s
as efficient as a direct connection be-
tween antenna and receiver. Many opera-
tors, therefore, compromise full break-in
capability i n favor of the best possible
weak-signal performance and use an an-
tenna changeover relay.
I n this article I've presented the modi-
fication of a commercially available t-r
switch to combine the advantages of elec-
any t-r switch
to provide
instant break-in and
best signal reception
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april 1969
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tronic switching with those of an antenna
changeover relay. The technique can be
used with any commercial or homebrew
t-r switch.
When
I
was faced with this choice,
I
wanted to have my cake and eat it, as
usual, so
I
decided to combine the an-
tenna relay with the t-r switch for the
ideal combination of antenna change-
over. The resulting improved t-r switch
provides the capability of full break-in
plus allowing direct connection of the an-
tenna to the receiver.
the same, with only the mechanical con-
siderations being different.
the change-over relay
The antenna changeover relay
I
used
was a Potter and Brumfield KTllA, for 115
Vac operation. This dpdt relay has per-
formed perfectly since its installation. One
pole is used to switch the unbalanced
coaxial feedline, and the other controls
the transmitter. This provides a safety fea-
ture, because the transmitter can't be
turned on until the relay removes the re-
fig. 1. Modifications
to
the Johnson t-r switch are
shown by the heavy lines.
Relay K11 is Potter
&
Brumfield
KT11A
(115
Vac).
VIA
I/P
~BLT
VIR
112 6 8 ~ 7
the t-r switch
The switch
I
used for this application
was the Johnson model
250-39
t-r switch.
It is compact, reliable, and inexpensive and
performs well. Furthermore, exactly the
right amount of vacant space is available
in the case to accommodate the antenna
changeover relay, and just the right
amount of space i s on the front panel for
the required new connectors.
1
refer to this particular t-r switch i n the
following discussion, but any t-r switch,
either commercial or homebrew, can be
used. The electrical connections will be
ceiver input from the antenna feedline.
The transmit switch operates the relay,
one pole of which turns on the transmit-
ter
B
plus.
construction
The changes to the Johnson t-r switch to
add the relay are shown in
fig.
1,
with
the modifications
indicated by heavy
lines. T S l l
i s
a bakelite terminal strip, and
1 1
and 112 are RCA phono connectors. These
1
carry control circuits, and any suitable
substitute can be used. An spst switch
connected to the terminals of T S l l is the
62
5
april
1969
transmit switch. 111 provides the ground-to-
transmit function to activate the trans-
mitter.
If the transmit control for your trans-
mitter requires this line to be off ground,
then another terminal strip or an un-
grounded connector can be used to carry
this control line. 112 ~ r o v i d e sa ground on
the receive function, which may be used
to mute the receiver for phone operation.
connections to the station
To connect the t-r switch into your sta-
tion, make the three coax connections (to
receiver, transmitter, and antenna) as in-
dicated on the t-r switch front panel. Con-
nect the station transmit control switch to
the terminals of TS11. Connect j l l and 112 as
described for transmitter and receiver con-
trol. It's that simple; you're now ready to
go.
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operation
With the station-control switch in the
standby position, the antenna is relay-
connected to the receiver, and the trans-
mitter is switched to standby by the same
relay. This provides best weak-signal re-
ception. When the station-control switch is
in the transmit position, the receiver
i s
disconnected from the antenna and con-
nected to the t-r switch output. At the
same time, the transmitter is relayed on,
ready for full break-in operation.
The photo shows the inside of the t-r
switch with the added relay and connec-
tors. It makes a very neat package. If you
prefer not to modify your t-r switch, an
external antenna changeover relay can be
wired to provide the same switching ac-
tion.
A
Dow-Key model DK60-2C can be
used, with the relay's auxiliary contacts
wired for transmitter control in the man-
ner described.
The cost of the relay and connectors for
modifying the t-r switch comes to about
$4.25. The complete t-r switch described
here can be duplicated as shown for a
little less than $35. It's a low price to pay
for the versatility and good performance
obtained.
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ham radio
----------
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-p
I
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april 1969
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