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]