In message <[email protected]>, Alan Ibbetson
<[email protected]> writes
I have moved the loading coil on my inverted-L from near the far end to
the top of the vertical. I need less coil for resonance now, actually
rather less than a simple 6pF/m of horizontal loading wire would
predict. The loading coil is about 1m underneath my 40m yagi and I'm
suspicious that the yagi and the HF array above it are adding extra
capacitive loading. Given that the tower sections are not explicitly
bonded together and the safety earth is not directly joined to my RF
earth, I'm not convinced I have made things better. My next job is to
see if the loss resistance has gone up or down. The SWR has certainly
changed so the loss isn't near 50 ohms any more.
I made up a 3:2 turns ratio transformer using a ferrite loaded trifilar
winding. Putting a 22 ohm resistor across 2 windings and looking into
all 3 gives a good 50 ohm match (return loss 30dB), so the transformer
works OK.
I put this transformer at the base of my inverted-L with the loading
coil at the top of the vertical as described above. The return loss is
22dB, suggesting a probable aerial input resistance of around 25 ohms.
This is exactly half the value I had before, when I had the loading coil
30m out along the horizontal section of the aerial, so my pessimism
appears unfounded. Perhaps Laurie's "footprint" theory explains the
improvement? I'm not sure.
What I am completely puzzled by is the short length of horizontal wire
needed to bring the aerial (or rather its loading coil) to resonance. I
have 15m of vertical. I started with 44.7m of horizontal wire and, at
6pF/m, calculated I needed a 375 uH loading coil. I made the coil and
both calculation using an oft-quoted empirical formula and direct
measurement with a Peak LCR40 agree that the coil is the expected
inductance.
Here is the puzzle. I actually only needed 36.5m of horizontal wire for
resonance at 502.0KHz. That's about 50pF less than predicted. I know
that the HF aerials are only a metre or two away from the coil and will
probably add some capacitance, but 50pF? That seems hard to believe.
If I align the 40m beam parallel to the 500K horizontal wire it shifts
the 500K resonance LF by only 1KHz relative to orienting it at 90
degrees to the wire. The change in horizontal wire length from 44.7 to
36.5m corresponded to a much greater frequency shift - about 35KHz if I
remember correctly.
Can anyone shed light on this for me please?
--
Thanks,
Alan G3XAQ
[email protected]
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