Ken Hough's Website
Amateur Radio

Antennas -- Antenna Switches

This section includes information about:
Multi-port antenna relay/switch box for HF, Manually operated coaxial switch box for VHF/UHF,
Frequency splitter filters/diplexers
Eventually most radio amateurs end up with several antennas with corresponding feeder cables. In the shack this can present something of a rats nest of cables that need to be swapped about. This can also be costly in terms of the amount of feeder cabling needed.

In this section, I've tried to present my own solutions to this problem, and the equipment that I now use.

The first point to make is that for HF frequencies, normal rotary switches and relays can be used. These will not introduce excessively high capacitances or inductances into the circuits. SWR values should remain low.

This will not be the case at VHF and UHF frequencies -- believe me, I've tried it! At VHF and above special coaxial switches and relays WILL BE NEEDED.

However, if two or more antennas are to be fed via a single feeder cable, it may be practical to use a splitter filter or diplexer, especially at VHF/UHF (see below)

The next point to consider is whether switches/relays are to be sited close to the antennas, or if they can be better sited in the shack. The former will require remote operation, whereas the latter might be locally/manually operated.

My HF antennas all match well into 50Ω coax feeders which are all less then 15m in length, so I can use RG58 cables without incurring significant signal losses along the cables. Switching between these is done conveniently in the shack.

Cable losses become significant at VHF and above. The physically larger RG213U feeder or similar is needed at these frequencies. I also use the smaller CLF200 cable which although not quite as good as RG213U, is similar in diameter to RG58.

Below, I have described my own solutions to the problem of having multiple antennas and feeders.

Multi-port antenna relay/switch box for HF:
This relay/switch box can be operated either locally via manually operated rotary switches, or remotely via signals applied via a 9 pin "D" type connector. This device uses six standard 12V changeover relays to provide four outputs to antennas and two inputs from Tx/Rx rigs. Only one output and one input can be active at once. All other connections will be shorted to earth. The selected input and output connectors are indicated by LEDs. In the absence of 12V supply, ALL inputs/outputs are shorted to earth.


Points to note:
1. For local operation, 12V DC must be provided to the DC power connector. For remote operation, 12V DC
   should be applied via the "D" type connector to activate the relays as required.
2. Diodes are included at the manually operated switches to prevent the DC power connection from interfering
   with the remote operation connections.
3. This switch box can be used at frequencies up to 50MHz, but IS NOT SUITABLE FOR USE AT VHF/UHF.

The switch was developed in three stages as described below.
Firstly, six 12V changeover relays were connected together as a block of six plus two using lengths of 18SWG copper wire as indicated in red on the diagram on the left. Double pole relays are indicated (because I already had them), but single pole would be OK.

Next, lengths of 18SWG copper wire (shown in green) were soldered across the relays to provide earthing bars. These were left long enough to be bent down to provide earthing connections to the metal case and also to provide support for the relays (see photo above).

Connections shown in blue were made to two rotary switches.

Connections to SO239 sockets for antennas and Rx/Txes were made using short lengths of RG58 coax cable.
LEDs were fitted into the lid of the switch box in positions corresponding to the SO239 sockets. These were arranged as per the circuit on the left.

Connections between the LEDs and the relays were made via a length of ribbon cable.

Note that 1.8kΩ resistors are specified, but in the photo above, two parallel connected 3.9kΩ resistors were used.
At some point, it might be convenient to operate the switch remotely. This facility has been included in the design as follows:

A 9 pin "D" type socket was fitted into the side of the metal box.
A short length of ribbon cable was used to connect between six of the pins on the "D" socket and each of the relays. Of course, one of the pins on the "D" socket was earthed to the metal box.
Forward biased diodes were included in the +12V supply lines to each of the rotary switches. This allowed for operation either via a +12V supply through the rotary switches, or via +12V applied via the "D" socket.

Lastly, a toroidal choke was included in the +12V supply line to the rotary switches. This was to minimise the possibility of RF being passed back down the +12V supply line. No choke was included in the lines to the "D" socket.
This switch box was fixed to a wall and close my rigs. It has continued to work very well. I find it convenient to have three antenna systems (a large mag loop, a 40m/20m loaded dipole, and a 15m/10m/6m fan dipole) permanently connected to the box along with a 50Ω dummy load -- so no need for repeated swapping of PL259 connectors.

I recommend that other constructors do include the LEDs, but not necessarily the option for remote control.

Manually operated coaxial switch box for VHF/UHF:
At VHF and above, coaxial switches must be used, otherwise SWRs will be very high. These devices are suitable for operation at frequencies of 500MHz or more. They are quite expensive and in the case of remotely controlled devices -- VERY expensive!

On the left is shown a three port manually operated coaxial switch that I use to select between a 2m Yagi antenna and a combined 2m J pole and 70cm Yagi (port number 2 is presently unused). The latter two antennas are fed via a single feeder into a 144MHz/433MHz diplexer -- see below.


Frequency splitter filters/diplexers:
Provided that operating frequencies of antennas are well separated, it is possible to operate them via a single feeder cable and a frequency splitting filter/diplexer. This is the case for the 2m and 70cm bands.

Conventional high pass plus low pass filters can be used to do this as described by DF2CK. I did make up a filter of this type, but was unable to obtain low SWR readings at 70cm. I then built a diplexer that relied on the ("magical"?) properties of 1/4 wavelength sections of coax cable.

If a signal is fed into one end of a 1/4 wavelength stub of coax cable whose other end is short circuited, then the impedance seen by the signal will be very high. Conversely, if the 1/4 wavelength stub is left open circuited, the signal will see a short circuit. Add to this the properties of L/C series resonant circuits and we have the basis for a very effective 144/433MHz splitter/diplexer.

I don't have an original reference to this idea, but have seen a working device and have based my design on that device.
On the left hand side of this circuit diagram a 433MHz 1/4 wavelength coax stub is indicated that is effectively short circuited at the outlet end by a series resonant L/C circuit tuned to 433MHz. This arrangement presents a very high impedance for 433MHz signals at the input (ie top connection) of the device, but will freely pass 144MHz signals. Similarly on the right hand side of the diagram, a 144MHz 1/4 wavelength stub is indicated terminated with a series resonant L/C circuit that is tuned to resonate at 144mHz. This side of the circuit will block signals at 144mHz, but will pass 433MHz signals unhindered.

Dimensions for the 1/4 wavelength stubs are given, but depending on the particular coax cables used, some experimentation might be needed, especially for the 433MHz stub.

Note that perhaps somewhat confusingly, a 145mHz(2m) antenna should be connected via the 433mHz(70cm) 1/4 wave stub and the associated series tuned circuit should be set for minimum SWR at 433mHz. Similarly, a 433mHz(70cm) antenna should be connected via the 145mHz(2m) 1/4 wave stub and the associated series tuned circuit should be set for minimum SWR at 145mHz(2m). See below.
Before use, this diplexer MUST be accurately tuned. This isn't difficult. My procedure was as follows:

1. Connect low power transmitter with SWR meter to the input of diplexer (SO239 socket in circuit above).
2. Connect 50Ω dummy loads to output BNC sockets. Dummy loads can be made from two small 100Ω resistors connected in parallel across a BNC plug. Resistor leads should be as short as possible.
3. Adjust the trimmer capacitors for minimum SWR readings on the two operating frequencies.
4. If SWR readings close to 1:1 cannot be obtained then try using slightly different stub lengths.
I built the diplexer described above into an aluminium die cast box as shown on the left.

The device continues to work well, giving very low SWR readings on 2m and 70cm.

I use this diplexer to feed a 2m J pole and a 70cm yagi from a single length of RG213U coax cable.


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13/03/15