Wednesday, May 01, 2024

Simple End Fed Half Wave antenna for 40, 20, 10m using Jaycar toroid

Just for fun, using parts already in hand, I've built a portable half wave end fed antenna that is resonant within each of the 40, 20 and 10m bands. It's built on a mains extension cord holder purchased a while back from Bunnings for not much money. The matching electronics is built in to the holder.

The Unun (or autotransformer) is 2 turns on the transmitter side and 13 turns in total. I don't know why many builders use the funky swap over half way through the winding - perhaps it's to keep the output away from the input? I used a simple approach.

The toroid core was purchased from Jaycar catalog number LO1238, branded "Duratech". There is a 150pF high voltage capacitor across the antenna connector.

It makes quite a convenient way to carry the antenna wire. The long wire was cut to 20m but needed to be shortened a few times to get it to resonate in the 40m band. About 1m of counterpoise wire is dropped on the ground.

It's a bit low on 40m but I didn't want to shorten further so that the other resonant points would be in higher bands. Here's 20m:

10m looks OK too:

A small antenna tuner will be able to match this to a transmitter on each of these bands without too much trouble.

Above is the plot with ham bands shown in grey.

A simple project with easy to find parts. I haven't seen these cable holders recently but hopefully there are similar low cost options.


Richard, VK3TXD, has interesting designs at Oz-Tenna and a useful introduction document here.

VK6YSF tests an Unun with the LO1238 toroid.

Some discussion on the EEVBlog.

HFKits describes a similar end fed matcher.

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