power switch for your rigs
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
After a bit of brainstorming, the circuit I ended up is shown below. 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 next image, shows the back of the metal chassis used for the RF switch. Note the isolated ANT connector. Isolation has been done with a PVC sheet and isolated screw rings. Also note the usage of BNC connectors. 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.
Below is a first look at the internals of the RF switch. The BNC sonnectors used, are the square flange types. I used this type of connectors because when they are fitter onto the chassis, they cannot be unscrewed, unlike the single-hole types. The coaxial cable used for the internal switch connections, 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 connections as well as for the connections of the switch 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.
Below is an overal view of the internal construction of the RF switch. 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. 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 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 switch 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.
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