[TowerTalk] 1/4 & 1/2 wave Verticals and Radials

Jim Reid kh7m@hsa-kauai.net
Sat, 26 Feb 2000 10:27:24 -1000

It has been said:

> Also, ANY 1/4 wave vertical _absolutely requires_ a
> ground plane to work against!!!  1/2 wave and 5/8ths 
> wave verticals are the only exception to the ground 
> plane requirement, and even they can benefit
> from the decoupling characteristics of a good GP.  

First,  about radials and 1/2 wave vertical antennas:

Here on Kauai,  out at WWVH,  they do use Rohn,  
1/2 wave vertical arrays.

They found an extensive radial system was required to
get their field strength back up,  out in the far Western
Pacific areas!  The problem occurs because the bottom
end of the 1/2 wave vertical is a "high voltage" point;
and,  yes,   the current,  and hence the radiation is highest
up at the feed point 1/4 wave up the tower.  However,
the high voltage point near the ground is a source of
intense E field lines which penetrate the surface of the

At WWVH,  1/2 wave vertical,  center insulated,  
Rohn tower arrays are used on 5, 10 and 15 mHz.  These
towers have been up since 1970-71,  and now need
to be replaced -- Rohn will not quote them again!!
Evidently they no longer believe the big tower center 
insulator is a good idea,  hi.

You should see the 2.5 mHz tower!!  A full half wave plus
tall!!  But at 2.5 mHz,  it is a single 1/2 wave tall tower,  not
an array as  for 5, 10 and 15.

Anyway,  without a high conductivity radial system on
the ground surface below the lower end of the verticals,
a LOT of current is induced into the ground,  which flows
out as the voltage on the bottom end of the 1/2 wave
vertical rises,  then flows back toward the tower
as the voltage reverses polarity on the other half
RF cycle.  Even near the beach,  there was a lot of
field strength loss to the WWVH signals until the
big radial system was installed.

These radials below the 1/2 wave tall antennas  do not 
need to be resonant,  either.  They just need to
be long enough to give the current a low loss
path to flow in as the lower end of the full half
wave vertical oscillates  in voltage polarity.
It would have been nice to put the original
Rohn tower arrays out in the ocean;  then maybe
none of all these radials would have been needed,  hi.

Second,  your post discusses the benefit of using
several "on the ground"  radials beneath 1/4 wave

You commented about using the 1/4 wave atop a
fence,  or elevated several feet from the ground.
Per the experience of some,  if the 1/4 wave vertical is
raised,  not ground mounted,  but say raised  up about
1/4 wave,  then 8 elevated radials will essentially
equal the boost in transmit efficiency about as much
as 60 to 90 ground mounted ones,  or will recover about  4 to
5 dB of what would otherwise be lost to ground loss, "heat",
warming the worms,  etc.  Four elevated radials saves
maybe half the ground loss,  2 or 3 dB which would otherwise
be available to the worms.  `120 or more radials,  on the
ground,  will supposedly recover "all"  the xmit power
which might be lost in the ground,  said to approach 6 dB
saving of total power.  Or,  said another way,  a ground
mounted vertical without radials,  will deposit about 75% 
of the available power from the rig into the ground,  for 
the purpose of diathermy for the worms! (More than
half the rig power to ground loss, as the vertical antenna
radiation resistance,  in ohms, will be less than  the 
ohmic loss of the coupled ground resistance.)

You must decide how many radials you wish to 
install,  versus the saving of transmitter output
power from ground loss and conversion to more
radiated power.  Running 100 watts to an elevated
vertical with four elevated,  resonant radials should
result in say 30 to 50 watts of total radiated power,  or
your signal would be down "only" 3 or 4dB (some say
that would be down about 1/2 S unit)  at the DX operators
rcvr across the world somewhere.  Add more radials,
up to 40 or so,  and your signal strength would increase
out there,  maybe 1 to 2 dB;  go on up to 90 plus
radials,  and for sure your signal would be 2 and maybe
even 3 dB stronger yet, or,  if lucky,  back up to
the full 100 watts radiated which left the transmitter
system.  BTW,  all of this info,  with only slightly different
power savings estimates is given in the ARRL 
Antenna Book,  in the two editions I have,  not the
very most recent,  radial number and power savings
are given in Table 3 within the chapter titled, "The
Effects of the Earth."  The data presented by the ARRL
was first presented  by John Stanley, K4ERO, in the
December 1976 issue of QST,  if you have the issues
or CD covering '76.  And most of that data was taken
from "Radio Broadcast Ground Systems", and "Ground
Systems as a Factor in Antenna Efficiency", Proc. IRE,
June 1937, Brown,  Lewis,  and Epstein.

For a 1/4 wave elevated vertical,  you also want
the ends of the radials way up above ground for
the very same reason as above, that is,  there will
be very high RF E field potential at the far ends
of the radials.  And, of course, the elevated radials 
must also be resonant as they will greatly impact the antenna 
impedance/vswr,  and you want them symmetrically placed 
below the vertical so that  the RF field radiated from the 
radials will evenly cancel  in the "far field" thus maintaining
the desired low radiation angle from the vertical.

IMO,  it is best to ground mount the vertical,  and not to
bother trying to resonate the "on-the-ground" radials.  And
use at least 16 plus radials so that at least 50% of your
rigs output power is effectively radiated,  with the
results as described above.  It is not easy to get an
elevated vertical/radial system properly set up,  and high
enough to achieve high radiation efficiency.  I think it
is easier to accomplish with the ground mounted vertical.
And,  if you can,  install 60 or more radials,  such that
no more than 1 or 2 dB or so of your power is used to
warm up the creepy crawlers and wigglys!  And make
your radials as long as possible,  at least 1/4  wave,
and as others have written,  they do not all have to be 
the same length, nor any particular length,  beyond at 
least 1/4 wave, nor,  for that matter real straight!
And a solid sheet of conductor directly below the vertical
element to which the radials would attach is also an
excellent idea,  as was written.

Of course,  your thoughts about the 1/2 wave vertical
without a ground radial system are true,  and work fine,  
if the lower end of this vertical is also elevated 1/4 wave,  
or more up.  Then,  most of the E field lines from the bottom
of the 1/2 wave element will then return to the upper
half of the radiator,  rather than terminating down on
the ground where current flow would occur.  Otherwise,
to get all the efficiency of radiation possible,  be prepared
to put in a good radial field.

Thought you might find interesting the 1/2 wave vertical 
installation at WWVH which does use ground mounted 
radials,  and why they were installed.

73,  Jim,  KH7M

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