Hi Alan Its getting late and I am not sure I have absorbed all your data
correctly
1 dont forget the down wire has capacitance to ground as well....around an
average of 5pF per m, also you have the parasitic capacitance of the coil to
ground which will depend on its height above ground. The value (6pF/m) for
the capacitance of top wire is a rough usable estimate and can vary
depending on the height of the wire. The downwire value assumes a single
wire not a cone which has a greater capacity.
2 Transformer matching under the coil can be quite a confusing game. You
rreally need to measure the untuned wire to know the parameters, like ground
loss and coil loss can be measured separately. Some stations have suggested
putting a toroid within the field of the main loading coil can give stange
results (but this was with higher power at 136kHz) Most seem to have found
connecting the bottom of the coil to ground and tapping up the coil the
easiest solution (you probably require too many turns to link couple but
that is another option)......probably less variables
3 Nearby wires/aerials will not have much effect UNLESS they are tuned to
500k as well. Then you will get coupling and a sort of GDO suck-out effect,
which looks like increased environmental loss.See the page below though for
136 it shows the effect.
http://www.btinternet.com/~alan.melia/aecouple.htm
I can seriously recomment a simple bridge, it dispels a lot of the confusion
of trying to estimate ground loss from aerial current and power. You can
make changes to the grounding and if the "R" term doesnt change you aint
improved it despite any changes in aerial current. You can then use it to
get your matching right at resonance, by shorting out the variable capacity.
But you do need a drive souce of about 3 volts rms, and a tuned detector
like a receiver because you pick up lots of volts from MW BC stations which
swamp the null. There seems to be resistance to buiding these but those who
have, have sorted their problems out more quickly.
http://www.btinternet.com/~alan.melia/aelossbr.htm
I hope that helps
Cheers de Alan G3NYK
----- Original Message -----
From: Alan Ibbetson <[email protected]>
To: <[email protected]>
Sent: 02 June 2007 22:30
Subject: LF: Unexplained tuning effects on 500K aerial
> 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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