Topband: RE: ground screens

[Eric Scace K3NA]

Hi Randy --

   NEC4 radial studies suggest the following for radial ground screens:

a) You probably want more radials with your very poor ground characteristics.  I can send you some charts separately (these will
probably appear in the next edition of ON4UN's book).  For a given total amount of wire, there are some configurations to be aware
of:

   18 radials of 1/8 wavelength is about equal in performance to 9 radials of 1/4 wavelength.  If you use less than 2 1/4
wavelengths total radial wire, that wire quantity would best be deployed as 1/8 wave radials.  The difference can be as much as 1 dB
if you choose either a shorter or longer radial length.

   28 radials of 1/4 wavelength is about equal in performance to 11 radials of 3/8 wavelength.  Between 2 1/4 wavelengths and 7
wavelengths of total radial wire, the 1/4 wavelength radial size yields best results.  The difference is less, about a half dB max
compared to shorter or longer radials.

   37 radials of 3/8 wavelength is about equal in performance to 28 radials of 1/2 wavelength.  Between 7 wavelengths and 14
wavelengths of total radial wire, 3/8 wave radials are about optimal.  The difference is small.

   Total wire laid down is important.  For very poor ground, going from 1 wave total wire to 2 waves total wire buys you 1/2 dB
increase in signal in all directions.  [You didn't say how long your 80m vertical radials were, but I'm going to guess they are 1/4
wave; that means you have just 1 wave of wire under each vertical on your 4-square.]  Going from 2 waves to 4 waves total wire
yields 0.7 dB further increase.  [You have 4 waves of radials on your 160m tower.]  Double the wire again for another 0.7 dB.
Double again to 16 wavelengths total wire, in 1/2 wave radials, will add another 0.8 dB.  (If you stick with 1/4 wave radials, your
improvement is far less -- just 0.3 dB for an increase from 8 waves [32 radials] to 16 waves [64 radials].)  If you can manage to go
to 32 wavelengths total wire, in 1/2 wave radial sizes (64 radials), you gain another 0.8 dB.  At 64 waves of wire (128 x 1/2 wave
radials) you gain another 0.5 dB.  Total signal increase from 16 x 1/16 wave to 128 x 1/2 wave on very poor ground is +4 dB.

b) You can lay the 80m and 160m radials over each other with no impairments.

c) There is no benefit (and possibly some harm) in tying the radials to bus wire located mid-way between vertical elements.  Ground
screen wires that are not radial-aligned appear to carry less current and contribute much less to the overall pattern.  Truncating
the ground screen at a bus wire reduces system effectiveness somewhat.

   -- Eric K3NA

-----Original Message-----
From: yccc-bounces at yccc.org [mailto:yccc-bounces at yccc.org]On Behalf Of
Randy Thompson, K5ZD
Sent: 2004 February 8 19:24
To: YCCC Reflector
Subject: [YCCC] More antenna questions


Hello gang,

I got a lot of great advice from the reflector when I asked about 160
antennas back in December.  Have implemented some of the suggestions and the
antenna is definitely working better, but I still believe there is room for
improvement.

I have 90' high Rohn 25G tower with a 6-ele 10m monobander at the top and
two 4-ele 10m monobanders at 30 and 60 feet.  Have removed anything coming
off the tower except the coax and rotator cable that are run inside the
tower and exit at ground level (I think this did more to make the antenna
work than anything).  Also have an 80m 4 square hanging by ropes from this
tower.

The tower is on a granite hill, so ground conductivity can be considered
very poor.  (It's New England, that's just how it is.)

Question 1 - Radials

I have 16 1/4-wave radials that are laid out on top of the ground that go
out in all directions from the base of the tower.

Each of the 4 square elements also has 8 radials that are laid out on the
ground from the base of each element.

Does it matter how the radials are mixed?  I.e., does it matter if some of
the 160 radials go above the 80m 4 square radials and vice versa?

Question 2 - More radials

I see in the ON4UN book that the 4 square radials should be hooked together
at the intersection points between antennas.  Right now, mine are not.  Each
radial is independent of all others.  Is it worth tieing them together?

If so, how much should I tie in the 160 radials as well?

Thanks!

Randy Thompson, K5ZD
11 Hollis Street, Uxbridge, MA 01569 USA
E-mail k5zd at contesting.com
Web: http://www.k5zd.com

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