[TowerTalk] Conclusion, G5RV vs. Dipole

[n4kg@juno.com]

Brian,

I looked up your ARRL reference but did not see any 
quantitative data.  Generally speaking, the closer
the transmission line is to the antenna wire, the
greater the coupling.  At 90 degrees, there is NO
coupling.  At 80 degrees, yes, there will be some
coupling, but not much. Remember also that the
further the feedline is from the antenna wire, the
less the coupling.  The PRIMARY effect of unwanted
coupling is to fill in NULLS which may be on the
order of 10 to 40 dB down from the main lobe(s).
I doubt that the main lobes are affected much by
incidental coupling unless the feedline runs VERY
close to the antenna wire.

73,  Tom  N4KG


On Mon, 16 Oct 2000 alsopb <alsopb@gloryroad.net> writes:
> 
> Look in the 17th addition of the antenna handbook.  The section on
> antenna currents on transmission lines.
> 
> It shows that unless the feedline is at right angles to the 
> radiators
> and remains equidistant from the two halves, equal same direction
> currents are induced in the feedline.  In other words, the feedline
> radiates.  This happens regardless of what feedline one uses.  Coax
> guys can use ferrites to quash this.  Ladder line guys can't.  A 
> good
> portion of this radiation will be vertically polarized.
> 
> I believe due to the installation difficulties of ladder line, that
> obtaining near to the perfect conditions is quite difficult.  I
> suspect that indeed, most of the installations do have
> feedline radiation.  The questions are: How much and is it 
> important? 
> Is the criteria RF in the shack or rf burns or just what.  I wish
> somebody would model this stuff once and for all to obtain
> quantitative answers.
> 
> 73 de Brian/K3KO
> 
> 
> n4kg@juno.com wrote:
> > 
> > Unless I am mistaken, there is a HUGE misunderstanding
> > of how the G5RV antenna works.
> > 
> > The ONLY way it could act as a Top Loaded Vertical is for
> > the currents in each side of the balanced transmission line
> > to be EQUAL and flowing in the SAME direction.  If that were
> > the case, they could be tied together or replaced by a single
> > wire and attached to the two sides of the horizontal wires at
> > the top.  I do NOT believe that is the case.
> > 
> > IF the feed is indeed as a split balanced dipole of various
> > electrical lengths, then the ladderline is acting as a balanced
> > transmission line with EQUAL but OPPOSITE currents in
> > each side and it does NOT radiate.
> > 
> > As a balanced dipole, it acts as:
> > 
> > 3/8 wave (short ) dipole on 80M
> > 3/4 wave (long) dipole on 40M
> > 2 Half Wave dipoles in phase on 30M (Voltage Node feed)
> > 3 Half Wave Long Wire on 20M (current Node at feed)
> > (almost) 2 Full wave LW's in phase on 17M  (near voltage node)
> > 9/4 wave LW on 15M (neither current nor voltage node at feed)
> > 5/2 wave LW on 12M (current node at feed)
> > Two 2WL LW in phase on 10M (voltage node at feed)
> > 
> > BTW, as originally designed, the ladderline was meant as
> > an impedance transformer that provided a reasonable SWR
> > on SOME bands (but NOT ALL) without the need for a tuner.
> > 
> > de  Tom  N4KG
> > 
> > On Mon, 16 Oct 2000  David Jordan <wa3gin@erols.com> writes:
> > > Right on Tom...
> > >
> > > A dipole at 1/8 wave above ground is a great NVIS antenna.  Two
> > > dipoles at 1/8 wave above ground spaced .67 wavelength feed with 
> open
> > wire
> > > line
> > > and feed in phase from a center drop or open wire to a tuner is 
> even
> > > better.  Most of the radiation goes straight up and it's omni
> > > directional!
> > >
> > > The G5RV is a top loaded vertical on the lowest freq w/ the twin
> > > lead radiating as a vertical does...a real compromise if one is 
> looking
> > 
> > > for short skip high angle radiation.
> > >
> > > Have Fun,
> > > dave
> > > WA3GIN
> > >
 


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