Here is a very useful accessory for the station. It allows to switch three transceivers to three antennas with the possibility of inserting a SWR-meter for a permanent measurement whatever the selections made.

I have only one antenna in the current configuration but the three inputs are busy and it allows me to connect the dummy load permanently on one of the outputs.

The realization of an antenna switch is simple when it comes to HF. Low insertion losses and minimal influence on impedance can be expected while using common materials. This is the case here, but if you want to build a switch for VHF or UHF, you will necessarily choose a project based on coaxial relays.

Supplies

The enclosure is a basic model closed by a screwed lid, a Hammond 1550C (115 x 90 x 51 mm). This will be fine for this project.

The idea is to distribute in the bottom the eight SO239, to connect the two switches with rigid wire of good section and then cover it by the cover.

The switches are three-position models found at tube-town.de and supporting 6A at 125VAC, well made with good contacts.

The SO239, of Chinese origin obviously, are not of a super quality rated finishes but they will do the trick. So I prefer to not tel where I bought them in France.

As for the wire used, it's 2.5mm2 rigid which I removed the sheath.

The knobs are of the type "chicken head" bought at the same place as the switches.

In all, enclosure, knobs, switches, SO239 x8, the project costs less than 35 €.

The schematic diagram

The schematic diagram is very simple. Each common goes out on a SO239 to allow to insert a measuring device, here a CN410M DAIWA, which will provide a SWR/Power indication whatever the chosen transceiver or the antenna used.

The connections are made using rigid copper wires and the ground of the SO239 are connected by their attachment to the aluminum housing.

Construction

A multi-diameter drill is handy for drilling the eight holes for SO239, but a drill press would be even better. Indeed, even by precisely marking the locations with a punch, no hole is really aligned. Fortunately this part is at the back of the case and not in front!

The drilling plan of the lid shows two small holes, each 9mm from the center of the main hole and 45 °.

Each switch has indeed a lug to prevent it pivots when tightening. You can see the technical information about it by clicking on the thumbnail of the switch - paragraph "Supplies" above- or here.

So that the switches could be held in place at the right height and perfectly centered, I attached them to two skewers attached together, perfect to simulate the height of the lid until the wires are soldered.

The first wires to be prepared are those concerning the commons, they must be soldered first on the SO239 because they are then covered by the two switches. The other wires are very accessible and can in turn be prepared and welded in any order.

On the wiring diagram, only the outline of the switches is drawn in order to see the connection of the common SO239s. The left switch will see its first position named 3, the second, 2 and the third, 1. With the knob pointing to the left, position N°1 will be at the top. The second switch, meanwhile, will be used in the normal order, its position N°1 naturally at the top since its button is oriented to the right.

This will result in a backward left-to-right numbering for each SO239 row.

The antennas are connected to the bottom row and the transceivers to the top. The left common will be connected to the TX connector of the meter and the one on the right will go to the ANT output of the meter.

The front panel

Having already experimented with other projects, I applied the same method. It consists in creating a design of the facade in 150 DPI with the serigraphy, print it on photo paper and then laminate the result before cleaning the holes using a punch.

This gives a semi-rigid facade which, once cut to the dimensions of the housing, is held in place by both the nuts of switches and self-adhesive double-sided pads for the corners.

Measurement

Here is a measure of the impact that the switch can have on the SWR because of its insertion. A MFJ259B is connected to one of the inputs and the dummy load on one of the outputs. The CN410M is connected between the commons, all with short PL/PL RG58CU leads.

But in practice with the 100w of the FT450D, the 28 MHz SWR is at most less than 1.2 on the CN410M and is measured 1.0 on the transceiver. The set has therefore very little influence and does not affect the performance of the station.

In use

The knobs have a good grip and switch easily. On the other hand, the stability of the set is not good, even if I equipped the case with rubber feets . We have to imagine the weight of the connected coaxials that tend to tip the case backwards - I had stalled one of them to avoid this - plus the fact of being obliged to hold it at each action on one of the selectors so that it does not slip backwards.

I then got a kind of velcro called "Dual Lock 3M SJ3560", the one with two identical faces planted with pimples. Two portions of 50mm by 25mm and the turn was played. The switch is now firmly inked on the shelf and it is not the action on the selectors that will move it.

Conclusion

A useful accessory that easily fits on the shelf of the station because incoming and outgoing cables are connected to the back of the housing and not on the sides as is often the case on commercial switches.

It has quickly become indispensable and moreover, it is always a pleasure to use a "homebrew" material ;-)

Its only defect is perhaps to have only three inputs or not to have an earthing of antenna cables during a storm. That said, my station using a remote tuning box powered by the coaxial, it would be a very bad idea to risk bypass the power supplied by the control box. As for thunderstorms, I always preferred to undo the coaxial and unplug the sockets, it's safer.