Hello Mike,
Sunday, October 18, 2015
> 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
> ==========
Major typo in my original post, the matching transformer was best
tapped at, or one coil down from, the *LOADING* coil end, not the
earthy send, sorry.
I will assimilate and work on the good suggestions, divorce through
the display of pyrotechnics be damned ;) Thank you Mike! I have yet
another question, but will start a fresh thread.
--
Best regards,
Chris mailto:[email protected]
|
