[TowerTalk] NVIS Placement & Beam Separation

[Al Williams]

I used several NVIS antennas about 8' high long before learning about the
name--some 50 years ago.  They were half-wave folded dipoles made out of
300ohm
twin-lead and fed with same.  Rig was a Meissner sig shifter with 7 watts
output on 20 meters.  Perhaps my most notable qso note was one who said I
must be
running a kw because the signal was so strong. Was he ever surprised!

Back to this posting--I just spent about 30 minutes with EZNEC modeling the
subject NVIS (i.e. 150' each side at 8') at 3510khz.  Naturally the major
lobe was
straight up, but I was surprised to find considerable variance in strength
with
changing the length of the antenna.  A change from +- 150' to +-250 resulted
in a 20db difference in signal level.  I used a good ground for the
model--not perfect.

Also NVIS articles suggest optimum heights between .1 and .2 wavelength with
the gain dropping off pretty sharply below .1 wavelength.

73 k7puc
-----Original Message-----
From: David Colburn <whcc@freewwweb.com>
To: towertalk@contesting.com <towertalk@contesting.com>
Date: Thursday, June 29, 2000 3:26 PM
Subject: [TowerTalk] NVIS Placement & Beam Separation


>
>NVIS QUESTION:
>
>I am about to run a 300 foot NVIS Doublet (fed with 450 open line).
>
>The NVIS would run at 8 feet (above grade) atop wooden fence posts.
>The utility lines are about 30 feet up.
>
>~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  (Electric 150ft long, 30ft high)
>...........................................................................
.
>......  (CATV 150ft long, 30ft high)
>---------------------------  (Phone 75ft long, 30ft high)
>
>
>
>````````````````````````````````````````````````````````````````````
>(Doublet 300ft long, 8 ft high)
>
>Should I have any concerns about running parallel with the power and phone
>lines at this spacing?
>
>(Note: The CATV line is unused and sags some at points, the electric power
>cable
>sags much less.)
>
>********************************************************************
>
>BEAM QUESTION
>
>If I have four tri-banders and one trapped dipole on my Rohn tower
>(all fixed, not rotated) what sort of spacing can I get away with?
>(Note:  I have reasons for avoiding a rotor on this tower.)
>
>Here is the planned setup:
>
>82 feet pointing East (Force 12  C3S or C3SS)
>73 feet pointing West (Force 12  C3S or C3SS)
>70 feet for N & S coverage (Cushcraft C4, 40, 20, 15, 10 trapped dipole)
>60 feet pointing SE (Mosley Classic)
>50 feet pointing SW (Generic tribander)
>
>(Note:  I think I have read here that 9 feet of separation is OK.  I just
>don't know if the beams are not pointed in the same direction that makes
>a difference.  Also don't know if squeezing the C4 in there is OK ...
>thought so if it is parallel with the tri-bander booms.)
>
>I do not believe that I will exceed the Rohn 45 specs.  It will be guyed
>with Phillystran at approximately 35 and 70 feet.  I will use a heavy-duty
>20 foot mast that extends 12 feet out the top of the tower.  That mast
>will be topped with a 2/440 vertical whose base will be at 82 feet.
>
>Sound OK to everyone?
>
>(Note:  The QTH is in Florida, just North of Tampa)
>
>- Thanks! & 73, DavidC  K1YP in Hudson, FL
>
>
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>


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