Inverted-L: will it do for Marconi Day?

With much warmer, if still rather variable weather in the air now, attention is turning to peparations for International Marconi Day on April 22nd.

I don't have a van or a property from which to operate at the site of Marconi's Carnarvon VLF station, so equipment rationalisation is critical - not least because I have to carry it part-way up a north Wales mountainside!

Marconi's trans-oceanic VLF inverted-L, as it was between 1914~1919 (callsign: 'MUU')

 

I thought I might try an inverted-L antenna; this is very much in the spirit of the old trans-oceanic station, as it was arranged as just such an antenna. For a few years, Marconi and his engineers claimed the inverted-L was a 'directional antenna'. My recent PDF-based, 197-page, richly illustrated book about the station looks in detail at how this claim came to be believed. You can buy a copy for £9.99, wherever you happen to be, here. You also get a free Google Earth-based plot of features identified during fieldwork in 2022.

Anyhow, up went the 9m pole in a field free of cows, and a ~21.3m wire run up and away from it as a sloping inverted-L. The end of the wire ended up a couple of metres above ground. The wind was up at about 40 knots, so the matching traces I obtained tend to be somewhat noisy on occasion.

The 9:1 unun feeding the wire was hooked up to a very simple QRP tuner; if this will handle things, anything will. I crocodile-clipped a ~5m radial wire and tested things both with and without it.

So, here are the results. At 40m, the wire is too close to a half wave, and tuning is consequently too sharp, unless one is only using digital spot-fequencies. It will need shortening or lengthening in practice; I'll have to experiment some other day. The best match frequency can be shifted either way. Green with counterpoise, orange without:

 Next, 30m. Pink with counterpoise, orange without:

Now the bread-and-butter band of 20m; green with counterpoise, red without. Some minor re-matching would be necessary in changing from digital to the higher reaches of the SSB portion and vice versa; it is a wide band, after all:

Next, 17m, where the counterpoise has essentially no effect and the matching near-perfect across this very narrow band:

 

15m next, where the counterpoise becomes much more useful. Red with, purple without counterpoise. Despite being a wide band, the matching is such that re-matching with frequency changes is unlikely to be necessary:

At 12m, a very narrow band, things are very good, the counterpoise (green) making a very small difference to without (red):

Finally, at 10m, a rather expansive band. The 5m counterpoise didn't allow effective matching, so I uncoupled it, leaving only around 75cm attached. This only had the effect of shifting the frequency downwards - though this would be a very useful feature for rapid re-matching, without fiddling with the matchbox, if shifting from the SSB portion of the band to digital:

And for reference, here's the full HF sweep without the matching box attached. Dips at 5.1, 11.1, 18.1, 32.6 and 39MHz.  The 60m band should be very usable, but I didn't test it on this occasion: