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.
References
Richard, VK3TXD, has interesting designs at Oz-Tenna and a useful introduction document here.
VK6YSF tests an Unun with the LO1238 toroid.
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