I had a great time listening to SAQ's centenary VLF transmission over the weekend, despite my battery connections for the amplified (WellGood) loop breaking. I had a spare but forgot about it, instead using my teeth to strip wire and make bare connections to the battery!
If you missed the transmission, you can hear the whole thing here.
My standard receive set-up for VLF is an 18m-long loop of RG-213, slung as neatly as I can manage in tree branches, with vertical sections of the loop about 4m tall and two horizontal sections of about 5m each. The lower horizontal part simply runs along the ground to the amplifier.
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| The standard configuration, pole or tree branch supports. |
This set-up gives a pretty consistent S6-7 signal strength for SAQ and there is no need for any improvement.
But I did come to wonder whether reducing the ground-run of the loop might improve things somewhat. I thought I'd try to lift the centre up to around 6 metres, thus giving, allowing for sag, roughly 14m vertical sections and the rest as a now clear-of-ground, roughly 4m horizontal section running to the amplifier.
I aligned the whole thing on the German time signal DCF-77, at 77.5kHz CW.
The signal from this 'pseudo-delta loop' arrangement was very slightly under S9+20.
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| The tall vertical version under test. |
I then lowered the whole thing to around 4m at the apex, so that it resembled what I would normally use (see first image), but slung from trees. The signal was indistinguishably different from the previous arrangement, despite having significantly shorter vertical runs.
When the loop was allowed to collapse such that it had no vertical runs to speak of, the DCF-77 signal dropped to around S9, or some 20dB below where it was with reasonable vertical runs.
It seems that the lesson from this quick experiment is that so long as there are a couple of metres of vertical run and that there is approximate alignment with the direction of the transmitter, then that's all that is needed for successful VLF operation. The length of the loop, however, remains important; with a standard 3m-circumference loop, SAQ is only weakly detectable whilst it is strongly detectable at S6-7 on the 18m loop.
The benefit of using a pole rather than trees, though it's something else to carry, is that it's somewhat easier to stick the whole thing in the air - especially if you have no handy trees available or, as is often the case here, those that are available have rotten lower branches! The 'pseudo-delta' shape can be formed simply with a couple of ropes and tent pegs tugging at the lower corners.
