Same idea as as the Wimo HF mini dipole, but this time I “loaded” the R1 dipole insulator for 3/8″ thread with two 15m mobile whips that I scrored at a hamfest for €10 a piece (around $14). These mobile whips have a black coated fiberglass base of around 1,2m lang. Inside the coating there’s a 1mm wire slightly coiling to the end where a small coil of around 10 turns is wound. The ends are steenless steel whips that can extend the antenna up to 2.3m. Tuned for 21.2 MHz, the length is around 2.1m.
On my 5m aluminium mast, there’s a nice wide dip in SWR and the RXZ measurement shows an Rx of 0 so there not only a 50 Ohm impedance, but there’s resonance as well. The coils bring down the impedance a bit so at resonance, the impedance is very close to 50 Ohms. MMANA calculates an efficiency of -.73 dBd so that’s not too bad. Over the full 450 kHz bandwidth of the 15m band, the SWR is better than 1:1.5, excellent.
So… does work? I haven’t tried it yet, but it looks OK. I never realised this is a pretty nice /P setup for 15m. It even has some directivity on 5m AGL (6 dB sides compared to front and back). I’m probably going to combine these 15m whips with a pair of 10m whips using Wimo’s “dual band center part”:
Well… next holiday maybe, we’ll see.