End fed half-wave antennas?

These are multi-band antennas (K24) are end fed via a transformer so the high impedance is reduced to somewhere nearer 50 ohms. I see AliExpress is selling them for around £30 shipped to the UK. 

As a QRP operator, the limited power handling does not bother me. Usually I operate at about 5W. 

With high power some have reported RF feedback, often cured with a small counterpoise or choke on the coax. I am tempted to see how this works. I doubt I could make one for that price.

Banggood is selling similar ones for even less.

Fault finding my Par endfed 102040 antenna

Since taking it down I have realised I can check if the coax was faulty by connecting a 10m dipole (endfed at the shack) and running the wire down the garden. If it is the coax I can replace it and get on 10m, 20m and 40m.  

With this antenna I had 10m spots from all over the world (including Australia and Antarctica) with 500mW WSPR.