Rigs
selector and automatic antenna switch for your shack
Designed
by sv3ora
When I started collecting vintage rigs, I
ended up in a line of rigs on my bench, that were sitting there,
disconnected from any mains cables or the antenna. I wanted these rigs
to be ready to fire at any time I wanted to, without having to
connect/disconnect cables all the time. I also wanted to be able to
compare different rigs performances at the flip of a switch, which is
the only way this can be done on the HF quick fading conditions. For
power cables, the solution was to leave them connected in the mains
plugs all the time. My
rigs that have an internal PSU, have mechanical switches, so they are
isolated from the mains when they are switched off. The rigs that are
powered by an external PSU, depend on the external PSU main switch for
isolation (in case they haven't mechanichal switches on them), which in
my case is mechanical and switches off the mains power, when the PSU is
switched off.
However, for the RF cables, this was a different story. Having only one
antenna and multiple rigs, means that you have to connect each rig to
the antenna every time you want to operate each rig. This is not only
borring and time consuming (you have to reach the back of the
transceivers to connect/disconnect the conenctors), but eventually
causes the connectors of the coaxial cable and the rigs to wear out. I
decided to make things better and make an RF rig selector for my rigs.
This RF rig selector has been described in this
link.
The current antenna I use, is fine
for transmitting, but in the noisy neighbourhood that I live, it picks
up a lot of noise. I have tried many solutions, without significant
effect in the noise level. This is why I decided to use a separate
antenna for receiving, from that used for transmitting. This antenna
will be some kind of loop probably, so as to be immune to noise or
insensitive to the direction of the noise. It will be placed in a
different location than the transmitting antenna, a location which will
be less noisy. Unfortunatelly,
the space I have for the TX antenna lies in a very noisy location in my
property. So a separate RX antenna, in another physical location is a
must. This means that a separate coaxial for the RX antenna must be
used. Thankfully, the RX coaxial can be very small in diameter, passing
easily through the sides of the windows, without extra holes.
To satisfy all of my requirements, I
developed the circuit shown above. The circuit is able to switch a
common
antenna to four different rigs. Why four? Because this was the capacity
of my switch and the number of connectors I had available. If you have
a greater capacity switch and more connectors, expand the circuit to
your needs.
The circuit of the shack switch,
allows
for 4 separate rigs to be selected, and two antennas, one for RX and
one for TX. TX/RX antenna selection is being done automatically (split
antenna operation) and controlled by the PTT of any of the rigs
connected. This feature can be bypassed by the switch, so that the TX
antenna can be used for both RX and TX. The same switch allows also RX
operation with passive RX antennas of active ones. When in the active
RX antenna position, power is passed to the remote RX preamplifier
through the RX coaxial cable, using a bias-T circuit. The values of the
bias-T circuit have been chosen very large, so as active RX antennas
that operate at LF and lower could still be used. The RF relay defaults
in the TX antenna, so that if there is a power failure, or if the
circuit is not supplied with power, you can still receive (and
transmit) with the TX antenna. The other way around, would be fatal for
both the transceiver and the RX antenna (If you transmitted
accidentally into it).
The PTT circuits are based on my transceivers. Unfortunatelly, there is
no "standard" for the PTT circuits, each rig has it's own way, so the
PTT circuits must be thought for each of them. I followed an "inhibit"
approach for the PTTs. That is, all the PTT switches are connected in
series and DC is passed through them. If any of the rigs transmits, the
PTT switch is opened and the circuit switches to the TX antenna. For
the rigs that do not have an internal relay but output DC on TX
instead, an additional small relay is used (for greater isolation and
losless switching). The only drawback of this "inhibit" topology is
that the PTTs of all the rigs must be connected to the circuit
simultaneously. If you want to exclude a rig of course, you may short
circuit it's PTT connector in the circuit. The PTT circuits as I said,
are non-standard, so you might want to change the circuit to your
needs, but anyway you got the idea.
Notice the connections in the circuit. One section of the RF switch (on
the left) is
used for the positive wire (central conductor of the coaxial) and
another for the negative (braid of the coaxial). Why is that? This is
because I canted to add a special feature to the switch. That is, the
ability to disconnect the antenna from any rig when the rigs are not
used. Previously, I used to disconnect the antenna coaxial from the
transceiver when I was away, so as to protect the transceiver from
antenna static discharges and possibly destroy it's front end circuits.
Now, with a single flip of the switch, I am able to do so. Because I
wanted the switch to operate on different types of antennas (balanced
or not) I decided to short circuit both poles of the antenna at this
position, to equalize their charges.
But equalizing their charges was not enough. I had to find a way to let
these charges go to the ground, so that the antenna is discharged.
Directly grounding the short circuit, did not seem a good thing to do,
because the whole TX wire antenna on the roof would be grounded.
Whether
this is a good idea to avoid lightings or not, I do not know. So I
decided to keep the short circuited antenna floading and instantly
discharge it only when adequate static charge is built upon it. For
this purpose, I used a neon tube, permanently connected to the switch
NC (not-connected) position. When the switch is in the non-connected
position, the tube
lights up and discharges the antenna (both poles) if an appropriate
amount of static
charges has been built upon it. When the switch is in any of the
selected rigs connections, the tube is disconnected, preventing it from
lighting up when you transmit into the antenna. Note that this
configuration, requires that the output
(antennas) coaxial connectors must be isolated from the metal chassis
of
the RF switch!
Isolation of the output antenna
connectors has been done with a PVC sheet and isolated screw rings.
Also
note the usage of BNC connectors on TX and SMA on RX. I used BNC
connectors for various
reasons. They are excellent connectors with quick lock/unlock features.
You do not need to screw them (and wear them out) and once fit in place
they are not unscrewed. Once fitted in place, they allow for rotating
the connection without unscrewing the cable or bending it. They can
handle 100W easily. Despite all these features, they are much smaller
in size and lighter. Their reduced size fits easily to reduced diameter
cables like the RG-58 and similar. In an RF switch where there are lots
of cables connected, this does make a difference. They are also very
common and very cheap. There are even types that do not require
soldering at all to fit a coaxial to them. I use BNC connectors even at
my antenna side, as they have been proven to be quite waterproof. The
types of BNC connectors I choose are not silver plated. Despite silver
plated connectors are better, in the long term they are corroded by
humidity and become much worst than the nickel plated connectors. The
sonnectors I used are nickel plated with gold plated central
conductors.
I have found these types to be much more durable over the years,
despite being cheaper. The same goes for the RX SMA connector, but I
used an SMA connector there so as to accomodate thinner coaxial cables
for RX.
The BNC sonnectors used, are the
square flange types. I
used this type of connectors because when they are fitted onto the
chassis, they cannot be unscrewed, unlike the single-hole types. For
the RX though, I used an SMA connector because it is even smaller and
it can accomodate smaller diameter cables. The
coaxial cable used for the internal switch connections on TX, is the
RG-223.
This cable is silver-plated (both the central conductor and the braid),
it has double braid for increased shielding, it is of the same diameter
as the RG-58 and it has a bit lower loss. The cable loss is negligible
though for such small pieces of cable. The same type of cable has been
used for the internal switch-relay connections as well as for the
connections
of the selector to the rigs. Appropriate lengths of RG-223 cables were
cut and fitted with BNC connectors at one side and the appropriate rig
connectors at their other side. For the RX antenna, you may use the
thinner diameter cable you can find. I used a small piece of very thin
coaxial (taken out of the WiFi card of an old laptop) and passed this
piece through the side of the windows of the shack and through the
mosquito net of the windows. No extra holes are required that way! For
the rest of the RX cable, you can use whatever cable diameter you want,
but I tried to use the smallest diameter I could find, so that the
cable is as much phantom as possible.
All the coaxial rig cables are grounded at the connectors side. I
used a piece of coaxial braid and fitted it to the connectors screws.
Then I soldered the braids of the coaxials onto this piece. Notice the
black ring screw isolators at the antenna connector, to isolate it from
the
chassis. Speaking about the chassis, do not use a plastic chassis for
the RF switch, use only a metal one! The picture below, as well as all
the next pictures, show the RF cables arrangement, but note that the
circuit in these pictures is not complete yet.
The coaxial cables are soldered onto the
switch contacts. Where a ground connection is required, a piece of
braid accomplishes this. Do not use thin wires, the device has to allow
for at least 100W of HF RF power to pass wthrough it. I have tested the
switch with 200W of power and there were no problems at all. The neon
tube
directly connects to the appropriate switch contact and to the chassis.
The most important part of an RF switch
is of course the switch itself. For 100W of HF RF power, I would
suggest you to use a porcelain switch. I had a 5-positions 4-sections
small porcelain switch, which I used. I connected two sections at each
side in parallel (adjacent pins connected together). That is, two
sections in parallel for the positive wire and two sections in parallel
for the braid. I did that for various reasons. First, by using two
contacts for each connection instead of one, you increase the power
handling capability of the switch. Then, you ensure a sure-contact
throughout the years. Any corrosion or wearing on the switch contacts
would cause contact problems eventually. By using two contacts for each
connection instead of one, you double the probability for a good
contact. After all, I had a switch with more sections, so why not make
a
good use of them?
The completed selector is shown above.
The relay was been taken out of an old CB radio. Use the best quality
relay you can afford, as this will be switched quite often and it must
handle at least 100W of RF power.
The results from the RF switch operation
are quite satisfying. The overal construction is kept small and low
profile. The switch makes a good contact despite being small. The
automatic discharger seems to work well. On receive, there is some RF
leakage, as I expected, in the near by cables, which is noticed in the
higher HF bands or in very strong signals. The very sensitive receivers
we use, are able to detect that. This RF leakage occurs even when the
switch is in the NC position, where the antenna is disconnected and
floating. So, to be honest I have
not figured out if the leakage is from the switch or from the external
cables in the shack. On TX, there is of course severe leakage from the
transmitting coaxial to the rest of the ports. This IS expected. There
is leakage even without using any selector at all, in the nearby
receivers, when a transmitter operates at such high powers. There is
nothing you can do about it really, unless your receiver has a mute
capability, which I did not bother to take care of.
The TX/RX switching is taken care automatically and this is very useful
and relaxing for the operator as he does not have to worry about
anything. The active or passive RX antenna selector and the feature to
disable the auxilary RX antenna are really useful and you can do many
antenna and rigs comparisons on-the-fly with it, by the flip of a
switch. Depended on the noise level and the sensitivity you want to
achieve, the switch will provide you the most optimal RX conditions
instantly!
The most important thing though, is that the goal of this project was
achieved. I am able to switch the antenna to whatever rig I want at the
flip of a switch. And before I go away, at the flip of a switch I can
isolate and automatically discharge the antenna when needed. This is so
much more convenient than having to connect and disconnect cables all
the time. I
can also now use a sepatate antenna for RX,
which greatly improves reception in my case. This antenna is
automatically switched by any rig I have and I do not have to worry
about anything. I can also do comparisons between different antennas on
RX, which is crucial in deciding which antenna is better for receiving.
All these features make this little simple to build circuit, so useful
and an integral part of the shack.
UPDATE:
I have designed a simple device for the shack, that allows me to
connect many old and new rigs together and use different transmit and
receive antennas. The device has the next features:
1. Accepts up to 4 transceivers in ports TRX1, 2, 3, 4.
2. Accepts any number of receivers in the RX port. The receivers are
tapped to a common bus of the RX port, with BNC-T connectors, pretty
much like the old 10Base2 computer networks.
3. Accepts 2 antennas, one of them is used for tranceive and the other
only for receive. There is a choice to use passive RX antenna or
Active, where the power for its remote preamplifier is injected to the
coaxial with appropriate filtering.
4. Automatic TX/RX antenna switching option on PTT.
5. Accepts 3 different types of PTT signals for the transceivers. DC-in on PTT, grounded port on PTT, open contact on PTT.
6. Exports 2 different types of mute signals for the receivers. Ground to mute and open contact to mute.
7. Allows for selection of automatic muting on PTT or monitoring of the
receivers when the transceivers transmit. Receivers input ports are
protected in either mute or monitor position, when the transceivers transmit.
8. Simultaneous receiving in all receivers and one of the selected
transceiver ports. Option to isolate receivers from the transceivers
ports on RX and connect the receivers to a discharge protection tube
instead.
9. Direct connection to the shack PSU. On switching off the PSU,
receivers are disabled and protected, default antenna is set to TRX.
10. Operation without power is allowed, with basic capabilities only.
----------------
UPDATE 2-Jan-2023
The
schematic below, includes some more features, like a control line to
bypass the tuner and automatic switching between the TRX ports and the
RX port, whereas at the same time disconnecting one of these ports
from the antenna. This provides maximum isolation and the previous
problems of the different front ends of the machines interacting
together, are now solved. The auxiliary RX antenna connector was used
for this circuit, as I am running out of space in this small enclosure,
and also because I have never used this feature up to now. Maybe on a
next bestion, I will rebuild this feature again if needed.
-----------------
UPDATE 11-May-2024
The
schematic below, is an even more updated version. In this one, the
external tuner is always bypassed (using the PTT out line), both for the AUX RX port and the ports 1-4,
when they are in receive state. When ports 1-4 are in transmit state,
the tuner is re-inserted into the circuit. This is the most desirable
operation, since a tuner setting that gives the lowest SWR, does not
always mean an RX peak. I have also redrawn the schematic properly.
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