Hi Gil,

Here's how I look at EFHW tuners. Two things have to be right:

1)The capacitor and transformer's inductance have to resonate at your desired frequency. Nothing will work right if that L-C circuit isn't tuned for your operating frequency.

2) The turns ratio of the transformer must be right to match the impedance of the resonant 1/2 wave wire to 50ohms.

Actually, there's a 3rd: the antenna wire also has to be resonant at the desired frequency.

Here's where it gets tricky for you. If you add or subtract turns you change both the turns ratio and the inductance, so the resonant frequency changes as well as the turns ratio.

I've never done an 80M EFHW but I've designed a lot of 10M to 40M ones and

~~4.7K~~ 2.5K is about right for their antenna impedance. VK5ZVS's design has a turns ratio of only 4.8:1 which will only transform 50ohms to 1.18Kohms

. Maybe an 80M wire is so close to the ground that its impedance is lower than a shorter wavelength antenna, but 1.18K suprises me.

Here's what I would do:

- With a 1.2K resistor and the variable cap, tune your box to 80M. You should get about 1:1 with that resistor.

- Rig your 80M wire antenna to your box and use your box and the dipper to trim the wire length for a dip. You won't get a 1:1 SWR, but now you'll have a resonant wire and a resonant box. The only thing that will be not correct is the turns ratio and from the SWR you'll know what turns ratio you need.

- Assume you need to go up in turns ratio and calculate what you need to get to 1:1. Re-wind the transformer accordingly.

- Rewinding screwed-up the box's resonant frequency so with a resistor load, re-tune the variable cap so your box is resonant at 80M. This time use a resistor = 50ohms * (square of turns ratio).

- Test your box with the wire antenna. It should be about right. Now the only trick is to figure out what your variable cap is set to so you can get a high voltage cap.

Good luck and keep us posted,

Bob