Chris,
I have been following your exploits with getting going on LF
tramsmit. You certainly have a very good signal for the relatively
low RF input so there is actually not a lot wrong with what you are
doing.
The pictures (see below) reveal a few interesting things. The RF
voltage on your loading coil is likely to be quite high, so I would
advise you to move your matching transformer 6 inches or more further
away from it (eg to the edge of the dustbin).
I think you are too hung up on the perfect match to/from your coax.
Bear in mind that in comparison to the wavelength it is a =very=
short piece of cable. Note that 100m of coax at 136kHz equates to
about 100mm (4 inches) of coax on 2m. Obviously you need to take care
not to damage the PA, but I have always found that it is better to
tune for maximum smoke (sometimes literally - don't tell your wife!)
rather than agonise over impedance. Running low power, tune the
antenna for maximum RF current in the antenna wire, then alter your
matching, re-adjust for max current and compare. If it is worse, then
alter the matching in the opposite direction and repeat until you
can't get any more current.
If understand your remark:
"The little FT240-43 cored matching transformer is tapped either one
turn up from the earthy end or a slightly better match is found if I
tap *AT* the earthy end, which seems odd to me."
you get a good match with the coax apparently shorted out at the end.
Looking at the picture of your coil, you seem to have several 'one-
turn' loops around your matching/earthing circuitry and again it
would help to move these away from the coil a little to reduce
coupling. This may explain some of your odd effects as your RF
'earth'.may be 'hotter' than you think.
Note that your antenna may well have an impedance fairly near 50-ohm
in any case - it is theoretically much lower but the environmental
losses will add a lot - so you could try (temporarily) using no
matching at all. As others have recommended, I prefer a proper
matching transformer rather than an auto-transformer, even when in my
case the ratio is almost 1:1. I see no difference in Tx or Rx (QRM)
performance between fully isolated or common eartth.
Another problem is your choice of FT240 material for your
transformer. This is not ideal for LF use - most specs do not
recommend it for use below 5MHz. Much more suitable is 3C90 which is
available from Farnell. An inefficient transformer may explain why
the RF prefers to couple to your coil through stray wiring instead.
.
This may also explain why your tuning is so sharp as the matching
transformer is not doing what you expect it to do. If, once the
matching is sorted out, the tuning is still sharp, simply reduce the
turns on the variometer, or use a smaller coil/variometer in series
to fine tune (I use 500mH).
Good luck.
Miike, G3XDV
==========
> You once asked for some photos and info on my HF aerial, which is now
> also my LF aerial :) I remembered to take some today and they are
> zipped up at:
>
> http://www.chriswilson.tv/aerial.zip
>
> They show my matching coil, loading coil, the strapped HF feeder for
> use on LF, and if you look really closely you might be able to see the
> loop wire. A Google Earth map is included showing the placement of the
> horizontal quad loop, it's fed right in the corner by the house. As
> you can see, the feedline is *right* by the house, and also at the
> lowest point of the loop. This is because the house is actually a
> bungalow! So very far from ideal. The 6 foot ground spike is about ten
> feet from the loading coil which normally lives in the green bin.
> That's the only ground at the moment, and again I should hopefully be
> able to improve this somewhat. The little FT240-43 cored matching
> transformer is tapped either one turn up from the earthy end or a
> slightly better match is found if I tap *AT* the earthy end, which
> seems odd to me. It gives this scan at the moment, and it's VERY peaky
> and a pain to set more accurately for resonance, plus it seems to
> wander about a bit with temperature.
>
> http://www.chriswilson.tv/17-10-2015.jpg
>
> Hope it's of interest. 73 from 2E0ILY
>
>
> > Congrats Genosse Chris!
> > Very great!
>
> > I'm very impression about results and you simple all-wave antenna!
>
> > I'm inform on russian forum about your antenna as we are agreed a
> > year ago. http://136.su/index.php/topic,268.0.html
>
> > 73!
> > Roman
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