[Lf] Loading coils.]

[Andre' Kesteloot]

Rik Strobbe wrote:

> Hello Bob & Dave,
>
> I just tried to 'reconstruct' Daves experiment on paper.
> I assumed that :
> - the 'bad' /P coil had a Q of 370 (refernce : my loading coil is made of
> 1mm solid Cu wire and has a Q of 350)
> - the 'good' Decca coil has a Q of 600
> - an groundloss of 50 Ohm (average ground)
> - an antenna capacitance of 500pF.
> For 137kHz you will need a coil of 2.7mH (XL = 2.3 kOhm). For the /P coil
> (Q of 370) the loss will be 6.2 Ohm, for the decca coil (Q of 600) the loss
> will be 3.8 Ohm.
> So the total loss will be 56.2 Ohm for the /P coil and 53.8 Ohm for the
> decca coil.
> With 800W this means an antennacurrent of 3.77A for the/P coil and 3.85A
> for the decca coil. This is a difference of only 2% (or less than 0.2dB). A
> difference that hardly will be noticed on the band and with a lot of RF
> current meters you might need a sharp eye to see a significant difference.
>
> 73, Rik  ON7YD
>
> At 10:36 17/07/00 +1200, ZL2CA wrote:
> >Dave wrote:
> >>
> >> Dear all.
> >>
> >> It was a sunny afternoon ( honestly! ) so I thought I would compare a few
> >> loading coils on 136. Theory states that most losses are in the earth
> system
> >> and losses in the coil would have to be pretty big to make a difference.
> >>
> >> I tried three coils:
> >> My usual coil    380mm dia    500mm high    4mm o.d. wire
> >> My /P coil        200mm dia    210mm high    1.25mm o.d. wire
> >> Ex Decca        810mm dia    210mm high    4mm o.d. litz wire
> >>
> >> Guess what?
> >> No difference at all!
> >> At the test power, about 800W, I got exactly the same aerial current in
> each
> >> case.
> >>
> >> So there.....
> >>
> >> 73. Dave G3YXM.
> >
> >This is a fairly convincing result that you have high ground loss, that
> >swamps observation of the differences in coil loss.
> >
> >Coil loss could be assessed by separately checking them, resonated with
> >polystyrene capacitors roughly equal to your top loading capacitance,
> >and observing the 3 dB bandwidth.  The dynamic resistance can then be
> >calculated.
> >
> >However, working on lowering the ground loss(es) would seem to be a more
> >rewarding pursuit.
> >
> >73, Bob ZL2CA
> >
> >
> >
> >