Antenna Switches

Hams are experimenters by nature. It is proven fact that if there is an investment to be made in the station, antenna should be considered first. This is because transmission signal quality in a given direction is well tied to the antenna. As soon as we go for multiple antennas on our roof, need for multiple transmission lines becomes a need. Often this is found to be cumbersome to lay multiple coaxes. Antenna switch is a simple device which can be placed near to antenna system with a single coax and a control line running to it. Although it is simple switching device, poor quality of relays, boards and even the enclosures can yield to losses or even interference. 

Following are the key considerations when homebrewing or buying an antenna switch:

1. Number of antenna ports
2. Insertion loss
3. Port Isolation
4. weather and EMI proofing
5. Mounting mechanism
6. Control device
7. Control line

Number of Antenna Ports:

How many antennas you would like to have switched. If moderate space is available on the roof, it is quite normal to find 2-4 different antennas. A mix of vertical for lower bands, dipole or beam is common. in certain cases upto 6 antennas can also be found. During the operation if that ham finds one antenna better than other in that condition, he can quickly switch between them and continue.

Insertion Loss:

Antenna switch being inline component between the antenna load and the transceiver, due to the fact of additional contacts in connectors, relays etc. there will be some loss  introduced. Our aim should be to reduce the loss as much as possible. You will have to select quality connectors, relays for this and ensure that the loss is not significant.

Port Isolation:

When one of the relay is activated other how is the coupling behavior with other antenna relay ports matter. More the isolation the better. Reduced isolation will result in unwanted signal leakage through that route.

Weather and EMI proofing:

Antenna switch being outdoor device, selection of the material matters a lot to protect inside from rain, sun, dust etc. Preferably galvanized steel or powder coated aluminum casing with rubber gasket inserts gives a long lasting protection.

Mounting mechanism:

How and where are you going to place the switch should be planned in advance. Most hams prefer mounting the switch to the mast and some mount to the wall near to the roof. Following are some arrangements – flat mounting(with anchor bolts/screws) provision for the wall, worm drive clamp mast mounting,  step teeth and U bolt mechanism. All these have their uses and depends on individual situations and preferences.

Control Device:

While antenna switch is remote, control device is always near to operators. It even simpler device with rotary power switching to trigger respective relay in the remote switch. It typically contains a visual indication on switching port as well. Aesthetically it control device should be stable(should not wobble around the table) on the shack bench and gel well with your rest of the equipment.

Control Line:

Care should be taken on Control line as remote end of it terminates in antenna switch and has great degree of RF exposure. Control Line should have EMI suppressors applied near the antenna switch. Also shielded cable should be preferred. Normally there is no much DC current flowing through them (depends on relay resistance and line length). Watch for cable specifications on resistance/100 meter. If you are running significant length of control line, to carry current required to trigger relay may drop in the line causing relays not to trigger.

In summary, even a simple device when considered for optimal performance has several factors which makes it or breaks. Do not compromise on cheaper components. Consider something which will give you years of operation without maintenance.