[Lowfer] Why put a coil under the top hat ???

[ALAN MELIA]

Hi Andy I may be wrong about this but all the comments on this topic I have read (I take QEX) have been from people using simulation or quoting and extrapolating HF "lore". I am afraid you cannot beieve any of it, even from Cebik who was a very clever man but had not used LF!!

The models used for simulation do not work for top-loading at LF.

If you put your antenna in the middle of a field with no surrounding "lossy environment" the assertions (and simulations ) might be true. However in a normal amateur location with trees, houses, sheds etc around then the situation becomes different and the simulation model is so inaccurate it doesnt yield anything useful. 

The major gain from a top loading coil is that all of the vertical is at a relatively low voltage so minimum curent is driven through the capacitive coupling to the lossy surroundings. This will in most cases override any "effective height" effect produced by the "optimum" positioning of the loading coil. The capacitive top-hat should of course be above the coil because little current flows in any wire beyond the top-hat (unless it is so long that it "becomes" the top-load.)

Dont worry about coil Q for a top-loaded coil it is almost irrelevant, very little current will flow in it so the losses will be minimal. Instead choose a format that is easy to make and "aeronautically" efficient i.e. keep the windage a low as possible. Mike G3XDV and Peter G3LDO (137kHz exponents from the starting years) wound coils on 2 litre coke bottles (place "open" in the freezer for a while, cap unscrewed, when cold screw the cap on tight. As they warm up they will become very firm but very light coil formers.) The form factor doesnt matter despite what people will tell you......at least in practice, I doubt you will be able to measure the difference.

http://www.alan.melia.btinternet.co.uk/spiraltop.htm

These measurement show the effect of a top loading coil although not the same kind you are using. The efficiency improved a lot as the top-capaciti was increased. I dont recommend spirals they have two much windage but it was an interesting experiment. Note the drop in ground loss as the capacity is increased.

Best Wishes
Alan G3NYK