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[Towertalk] NVIS antenna--reflector addition

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Subject: [Towertalk] NVIS antenna--reflector addition
From: (David Jordan)
Date: Wed, 05 Jun 2002 16:07:08 -0400
Don't make it to complicated...

When you are making your antenna comparisons you've got to de-tune the
antenna that isn't not in use or it will act as a parasitic element and
you won't have a clear indication of either antenna's performance. The
same goes for comparisons of a Vertical to an is supposed to
favor low angle radiation, but only if it is installed properly with an
adequate radials system, etc. If the vertical doesn't have a good radial
system it will also have a high angle component and it also has to be
detuned during testing.  Don't believe me...hook a watt meter to the
vertical while testing on the dipole.  Terminate the vertical to a dummy
load and measure how many watts of power coming down the coax from the
vertical, HAHA.

NVIS on 40 work well between 16-30ft above the earth.  If you place a
reflector under a dipole element configured as NVIS make the reflector
5% longer.

I use four half waves in phase in the NVIS configuration feed with open
wire line.  The dipole elements are 140ft long each and the two dipoles
are spaced 70ft apart.  The feed points are about 16ft above the ground
with the ends of the elements about 25ft above the ground.  Within a
0-400 mile range the NVIS out performs an extended ZEPP at 60ft by
5-10db typcially. At  500 -1,000+ miles out the ZEPP is about 5-10db
stronger than the NVIS.

The NVIS does appear to significantly attentuate the foreign broadcast
station QRM when compared to the ZEPP at 60ft.

For local rag chew, local net operations, short skip, working mobile
stations, etc. the NVIS is a great antenna.


p.s. when you use full wave elements if you plan to install a refector
you will want to break the reflector element in the center, just like
the driven element but make the NVIS reflector elements 5% longer.

Richard Karlquist wrote:

> I have been experimenting with NVIS on
> 40 meters.  Last weekend I compared an
> inverted vee with 60 foot apex, 90 degree
> apex angle to an inverted vee with 30 foot
> apex, 120 degree apex angle.  They were
> indistinguishable except for stations within
> about 100 miles, in which case the lower
> one was a few dB better.  I also compared
> them to a ground mounted vertical.  Although
> the vertical was about the same as the vee's
> for DX, it totally sucked within 1000 miles.
> Many stations that were S9 on the vee's
> were inaudible or nearly so on the vertical.
> I was expecting some difference, but am
> amazed it was so drastic.
> I'd like to try the ground wire test, but the
> question is what do you use?  A wire laying
> on the ground, cut for 1/2 wave in free space?
> Or recut for correct resonance while laying
> on the ground?  Or elevate the wire 6 feet and
> tune for resonance?  Or do you need a whole
> grid of wires?  The ARRL antenna handbook has
> a graph showing that a perfect ground is worth
> 2 dB or so vs an average one at typical NVIS
> angles.
> I could put a half wave wire under the antenna
> with a relay in the middle of it to "deactivate"
> it so I could A/B the system with and without
> the wire.  Does this sound reasonable?  On
> the inverted vee's, there are relays that
> disconnect the unused vee from the coax
> so that it turned into two floating 33 foot
> wires.  Hopefully, this makes it invisible to
> the vee under test, allowing me to use a single
> support and feedline without mutual coupling
> errors.
> BTW, I also tried the vee's on 15 meters (they
> loaded up beautifully on the 3rd harmonic).
> The 60 foot high vee totally trounced the 30 foot
> high one, at least during the weird band
> conditions on Sunday.
> Rick Karlquist    N6RK
> > -----Original Message-----
> > From:
> > []On Behalf Of Chris BONDE
> > Sent: Wednesday, June 05, 2002 11:03 AM
> > To: Al Williams
> > Cc:
> > Subject: Re: [Towertalk] NVIS antenna--reflector addition
> >
> >
> >  From what I have read, the use of the reflector makes the
> > ground part much
> > better.  There are a few site that explain this.  Search on
> > NVIS.  Does
> > EZNEC use a perfect ground?  What will your ground be?
> >
> > I used, long time ago, an end fed antenna.  I found that it
> > worked better
> > just after my wife watered the garden.  So I put a wire,
> > acctually, the
> > rest of the aluminum clothes line wire, under the end feed
> > under the gound
> > and tied the one end to the grounding rod.  Worked better
> > than after water
> > the garden and lasted longer, even on rainy days.
> >
> > Just some thoughts.
> >
> > Chris opr VE7HCB
> >
> >
> >
> > At 10:33 AM 2002-06-05 -0700, Al Williams wrote:
> > >Our club is planning to have an NVIS antenna for 80m
> > >and 40 meters during Field Day.  However:
> > >
> > >The World Radio Books publisher's "Near Vertical Incidence
> > >Skywave Communication" book purchased from ARRL
> > >contains two articles (by the same author) showing an
> > >NVIS wire reflector added to the driven NVIS wire element.
> > >
> > >Because the ground itself is a reflector, and because
> > >EZNEC confirms that adding the reflector is superfluos,
> > >we do not think a reflector will help.
> > >
> > >Agree?
> > >
> > >k7puc
> > >
> > >
> > >_______________________________________________
> > >Towertalk mailing list
> > >
> > >
> >
> >
> > _______________________________________________
> > Towertalk mailing list
> >
> >
> >
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