Bidirectional switchable 40m wire yagi

I switched on the other evening and heard a very quiet DX caller on 7.142. It was YC0LOU from Indonesia and I could only pick up parts of his call. He called and called and had no takers. In fairness, he was extremely quiet but as the sun was gradually moving around the sky, he finally became audible and it was worth giving him a shout. 400w off my inverted V at 7m height got his attention but I needed a few blasts for him to get my call right. I put him on the cluster and he had a pile up.

Now, the point is, had I had more gain, I’d have not only heard him better, but he’d have heard me quicker too.

So I could add more height to my Inverted V but the difference between 7m and 10m isn’t actually that much at 5 degrees off the horizon – not even a db. Hardly worth writing home about.

40m-wire-yagi-drawingAnyway, this was the QSO that made me sit up and take stock of what I could do. I was seriously considering phased verticals for DX when I thought up the idea of having a switchable wire yagi. Either firing East or firing West.

Like me, you may already have an inverted V dipole up for 40m, all you need to is build another one about a quarterwave in front – or behind your existing dipole but out of a single wire. You don’t need to feed this with coax, it’s a parasitic element, like a 2 element yagi.

In its simplest form, depending on how you tune the parasitic wire element, it will either act as a director or a reflector. If you make it smaller than the dipole, it will act as a director and the RF will start at the dipole and will tend to be skewed towards the smaller element. Make it bigger and it will act like a reflector and the RF will go in the other direction.

I plan on making mine smaller and add a relay to switch in a small coil with the inductance of about 2 microhenries (that around 7 turns across a 2 inch plastic pipe about 2 inches long). This way, the yagi will either fire one way – or the other way.

If you build this about 30 feet off the ground, according to MMANA, (depending on wire used), you should end up with a dipole with legs of around 10m long and build the smaller dipole with legs of around 9.6m long. Switch a coil in at the centre of the smaller reflector / director element and bingo, a dual-fire yagi.

Add another switch to cut the passive element in half and your single inverted V dipole will spring back to life.


Green (Inverted V) Red (with loading coil switched in) Purple (no coil)

Gain is impressive enough. At a 5 degree take-off angle you will achieve just over 4 db of extra gain over the single dipole and at 45 degrees or so, you will improve your 500 mile signal impressively.

If you are thinking that 4 db isn’t much, consider that’s like doubling your power (and a wee bit) and also having a receiver that’s now got twice the signal coming in (with some front-to-back as well).

Build it!