A 2 (or 3) Band Portable HF Vertical Aerial for HF - UPDATE

I recently made a two-band portable HF antenna from on old CB antenna (see https://www.myadventuresinengineering.com/2020/08/a-2-or-3-band-portable-hf-vertical.html?m=1). It is an end-fed half wave vertical on the 10m band and a quarter wave on 20m. End fed half wave antennas fed with broadband transformers should be able to operate at twice the design frequency as a full wave antenna. Therefore I thought I might be able to tune the antenna as a full wave on the 6m band, depending on the matching network. Note that a full wave dipole has a different radiation pattern than a half wave, and would be expected to produce higher angles of radiation, i.e less useful for DX.

With my new vector network analyser (see https://www.myadventuresinengineering.com/2020/08/nanovna-v2-vector-network-analyser.html?m=1) I set about measuring the VSWR over an extended frequency range. With the antenna initially adjusted to be a half wave at 28MHz, I was hoping for a good match at twice this frequency, i.e. 56MHz. I could then increase the length to resonate at 50MHz. Alas, it was not to be. 

Fig.1 - Measuring the VSWR. The NanoVNA is solar powered using my Solarmonkey.
Fig.1 - Measuring the VSWR. The NanoVNA is solar powered using my Solarmonkey.

The measured VSWR at 28MHz was 1.2:1 as expected, but the VSWR at 56MHz was peaking at 8.4:1 (Fig.1). Adjusting the length did not help. I suspect that the original impedance matching network is not broadband. Note that I cannot easily get access to this due to corroded fixings. Maybe one day I will break it open and replace the original matching network with a broadband transformer such as in:



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