Bryan,
Having a tapped coil is one way to feeding/matching to 50 ohms, and my
experience (for 181 kHz) is that around 2 or 3 turns is about right. I now
prefer feeding the coax inner to the cold end of the coil and earthing the
braid to the "earth mat". When tuned the resistance presented varies with
ground loss, but can be matched down or up with either an autotransformer or
L match.
My input about "Q times" is based on the feeder connected to the cold end,
and no tapping. However, a tapped coil is only a variation for feeding
power to it, and the discussion about current "going sideways" via stray
capacitance is still a fact of life.
If you do measurements, use very low power, otherwise you will fry the
probe.
73, Bob
----- Original Message -----
From: "captbrian" <[email protected]>
To: <[email protected]>
Sent: Thursday, March 03, 2005 10:14 PM
Subject: LF: Re: Re: Re: Current "lost" in loading coil
Thanks Vernall but..
<<"it will be some Q times the voltage applied at the "cold end". ">>
Where would you measure this? ..... there isnt any "voltage applied at
the
cold end". [It is plugged into the finest earth that cunning hams can
devise.] The voltage at the "colder " end depends where you tap (if that
is
how you feed )? Or am I wrong ?
If fed with a coupling coil I confess I am lost .
Bryan 50 50 N / 00 16W
----- Original Message -----
From: Vernall <[email protected]>
To: <[email protected]>
Sent: Wednesday, March 02, 2005 8:58 PM
Subject: LF: Re: Re: Current "lost" in loading coil
> Hi all,
>
> The discussion has brought out some interesting points on what goes on
in
a
> practical loading coil. I would like to investigate (measure) some
vector
> relationships of current and voltage on a practical setup, but in the
> meantime I am occupied with preparing for an LF DX weekend at Quartz
Hill,
> so it will be a while till I can find time to do quantitative testing.
I'm
> interested to find out if maximum current in to the "cold end" of the
> loading coil corresponds with maximum radiation (far field, not near
field)
> from the system (system being not only the wire connected to the "hot
end"
> of the loading coil).
>
> One can expect a big difference in relationships between tuned and
untuned.
> When tuned (the usual situation for LF transmitting) one can expect an
> approximate linear voltage rise (similar volts per turn) and at the top
"hot
> end" it will be some Q times the voltage applied at the "cold end". If
> there is similar stray capacitance per turn, then the current "lost" has
an
> approximate linear distribution up the coil, as "lost current" is
> proportional to driving voltage, which increases along the coil.
>
> There will also be a phase shift due to physical length of wire in the
> inductor, with 360 degrees corresponding to a wavelength (even for
applying
> DC, there is a finite time from applying excitation at one end till
current
> comes out the other end, it is the "propagation delay" that forms the
phase
> shift for steady state sinewave excitation). A coil winding length of
100
> metres of wire is about 4.5% of a wavelength, or some 16 degrees for 136
> kHz. The stray capacitance will of course modify the ideal situation of
the
> 16 degrees being proportional to length along the winding, especially
around
> the upper part of a resonant coil where the voltage is highest.
>
> 73, Bob ZL2CA
>
>
>
>
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