Saturday, January 23, 2021

Simple end fed tuner to fit in a cheap project box

In the supermarket the other day I spotted these little boxes, three for $3. I thought they'd be good little project boxes for something so, that was the trigger for this.

I've been watching videos from Charlie Morris, ZL2CTM, recently - he's really doing a wonderful job. One of his projects is an End Fed Half Wave Antenna tuner with SWR meter

My portable radios have built-in SWR meters and I figure it's better to tune for a minimum with the meter in the radio than one in the tuner - that also made the project simpler.

I didn't have the T68-6 toroid, just a smaller T20-6, in the junk box. I grabbed a 12 position switch from Jaycar and a 270pF polyvaricon (variable capacitor). Here's the very simple circuit.

I think there might be an elegant way to mount the toroid on the back of the 12 way switch with the windings looping around each of the terminals but I'm not quite there with this initial build.

Here's a close up.

In the back yard there is an end fed wire that rises up to the top of a 6m squid pole and then angles over to a pole on the roof of the garage. I guess it's about 12m of wire. A counterpoise runs along the ground under the back deck.

The box worked well for a project like this, it was easy to drill but strong enough to hold things like the switch without flexing.

The tuner worked well on 20m with a deep dip in SWR but a little more inductance (more turns) is needed for 40m.


As mentioned, I'm working on a neater wiring system for the switched toroid. This is looking better:


Unknown said...

Hi Peter,
With an end-fed having a high impedance , and hence a high voltage , what level of RF power can you run with a polyvarcon varicap ?
I have a FT817 (5W max) which possibly should be OK ?

BTW , I really enjoy your website.

73/72 Frank . EI7KS in IO63XC

Peter Marks said...

Hi Frank,

Yes, the voltages can be a problem for polyvaricons. I've had them melt when running WSPR. I think 5W is about the limit for continuous carriers but 10W peak on SSB can work.