[600MRG] Radiation Resistance Question from XSV

[William E. Isakson]

I used both a mid loading coil without top load wires and in a different
setup used a top loading coil at the point where the top load wires
attached.   I used no base load coil in either case.  The ERP goes up for
the same input when you move the base coil to the mid load (and yes the
coil is different) but the ERP goes up because the current in the lower
section of the antenna goes up--in fact, effectively the lower section of
the antenna IS your antenna because the part above the coil contributes
almost nothing in comparison.   My 38 foot (approx) vertical that was mid
loaded worked about the same as the 18 foot top loaded antenna, by the way.
  By not using a base load you get the same current in the lower part of
the antenna (where the current is high) as you would get with that same
amount of lower portion of a quarter wave high vertical.  The part of any
of the antennas above the coil gets the same amount of current as would the
same amount of the top portion of a quarter wave vertical.  It is the
current that contributes to your ERP.  Remember, at the top of a quarter
wave vertical (or any other length at the top) the current is zero.  At the
bottom it is maximum. If it has a coil at the bottom, that coil uses up all
of the effective 1/4 wave antenna that would have been under the portion of
antenna above it.  The only reason that you would want to put any coil at
the bottom is for the convenience of making adjustments to the coil.
 I^2xRrad, yes?  That is radiating.  The current is higher.   If the
antenna is the same length in the case where there is a base load compared
to the case where there is a mid load or a top load, the Rrad should
effectively be the same.  It is the current that changes.
Bill

--------


Bill Isakson     AC6QV
Roseburg, Oregon USA
bill.isakson at gmail.com

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On Sat, Mar 1, 2014 at 4:29 PM, Ralph Hartwell <w5jgv at w5jgv.com> wrote:

> > If I were to add a loading coil, say about half way up the vertical
> section
> > of my top loaded 630m antenna, would the Rr remain the same, even tho'
> > the base loading coil would need less inductance?
>
> The Rr would increase since you have elevated the loading coil, but you
> will
> require more inductance for the elevated coil than you will need for the
> base loaded system. That means the resistance loss of the elevated coil
> will
> increase, thereby eating up more of your expensive RF power going to the
> antenna. Of course, if you have a big amplifier and don't mind the
> increased
> electric cost, you just throw more watts at the antenna.
>
> Pat, W5THT, WD2XSH/6, used a large elevated coil at the top of his vertical
> where the loading wires are connected, and that seemed to work quite well
> for him. I opted for a honking big high-Q base loading coil on my antenna
> since I cannot easily break my vertical antenna into sections.
>
> EZNec is your friend!
>
> Best DX & 73,
>
> Ralph  W5JGV - WD2XSH/7
>
>
>
>
>
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