Message-ID: <005c01c51edd$f5de00e0$05bc0150@p2300>
From: "captbrian" <captbrian@ukonline.co.uk>
To: rsgb_lf_group@blacksheep.org
References: <6.1.0.6.2.20050301115700.03662eb0@mail.casema.nl> <004d01c51e98$ec121f00$c401a8c0@quaycustomer>
Date: Wed, 2 Mar 2005 04:11:50 -0000
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Subject: LF: Re: Re: Current "lost" in loading coil
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Can it be that for years we have all looked at the aerial current
distribution diagrams in the handbooks  which show  a constant for the
loading coil and a sine-wave portion for the "whip"  and nodded without ever
demanding that it be a sloping line for the coil?

G3GVB
----- Original Message -----
From: Vernall <vernall@xtra.co.nz>
To: <rsgb_lf_group@blacksheep.org>
Sent: Tuesday, March 01, 2005 7:57 PM
Subject: LF: Re: Current "lost" in loading coil


> Dick PA0SE and others,
>
> I believe a good way to analyse why more current goes in the bottom of the
> coil than comes out the top is to resolve the current in to in-phase (I)
and
> quadrature (Q) components.  RF ammeters or other sampling current meters
> display the resultant (R) current (which equals the square root of [I
> squared plus Q squared]).  The I current eventually does some radiating
> (from the whole antenna system) and the Q current feeds the parasitic
> capacitance (and is a low loss even though it can not be eliminated).  The
> actual mechanism is lots of small Q currents distributed up the coil, some
> between turns and some to the environment, so the above simplifies these
> currents to a single equivalent value of Q at the bottom of the coil.
>
> Using 10% as a difference between I and R currents, the Q current
calculates
> as being some 46% of I or some 42% of R.  The values are quite sensitive
to
> the observed ratio of bottom to top resultant currents, and the value of Q
> is quite a significant current.
>
> What I'm not so sure about is how to tune in practice for best far field
> radiation from the antenna, and whether that condition gives a resistive
> input at the bottom end of the coil.  If my above above suggestion is a
> valid way of working, then it would seem that a secondary matching network
> could be needed to cater for the Q current?
>
> 73, Bob ZL2CA
>
>
> > To All from PA0SE
> >
> > Several amateurs have found that the current at the bottom end of the
> > loading coil is higher than at the top (aerial side) of the coil.
> > In my station the difference is of the order of 10%.
> >
> > William, PA0WFO, has a large coil of 8 mH and a 23 m long wire as
aerial.
> > He measures 1.5 A at the bottom of the coil en 0.6 A at the top.
> > My theory is that the "lost current"  flows via the capacitance of the
> coil
> > to its surrounding (even a metal object in free space has  capacitance).
> >
> > The current at the bottom of the bottom of the coil divides between the
> > capacitances of  coil and  aerial.
> >
> > I suggested to William he  measure the capacitance of the coil and of
the
> > aerial. For the coil he found 150 - 200 pF, depending upon the position
of
> > the coil and for the aerial 210 pF.
> > But these values do not explain the large difference in current at
bottom
> > and top of the coil.
> >
> > In a transmitting aerial the current increases going from the end of the
> > radiator towards the coil.
> >
> > Now to my question:  does this increase in current also occur in the
> > winding of the coil?  My feeling is that the current at the beginning
and
> > end of a coil should be the same; apart from the current that flows via
> its
> > capacitance to the surrounding.
> >
> > I also have read that the coil should be considered as an aerial with a
> > length equal to the length of the coil.  But on 2 km
> > that would be an extremely  small aerial, reckoned in wavelength.  So
> > radiation by the coil must be negligible.
> >
> > There are certainly  experts on the reflector who know the answers.  I
> > welcome  their views.
> >
> > 73, Dick, PA0SE
> >
> >
> >
>
>
>
>