A Coax Switch

Cheap slide switch in a standard 54 × 50 × 26 mm box.

THE SWITCH

Fig » shows a proven inexpensive home-made antenna selection switch. If you question the use of a cheap slide switch and SO239 coax sockets, read on. Measurements in a physics lab showed there to be practically no reflection on HF and even on 70 cm the SWR was below 1.3 : 1! That is explained as follows:

l The contacts in the slide switch have larger contact surfaces than many a bought coaxial switch.

l The wiring and switch contacts, between the top and bottom of the metal case, act as the centre conductor of a coax of near 50 Ω.

It is a standard box measuring 54 × 50 × 26 mm (l × w × h) and the wiring between the switch and the coax sockets is done in 2 mm silver plated wire.

POWER HANDLING CAPABILITY

If the switching is done with power off, the switch can stand 800 W. I have used this switch for more than 15 years and even with 1500 W there have been no problems.

 

While building this switch, you might as well add a test point, e.g. to connect a ‘scope’ or frequency counter. The capacitors of 10 and 220 pF make a capacitive voltage divider and the extra loading, 9.6 pF, does not affect performance on HF. In fact, on 10 m it improves the SWR as the extra capacity, in combination with the wiring it makes a filter which favours that frequency.

 

This shows how the switch is used in my station.