Hi TopBanders,
I recently posted a request for opinions about radials. I was pleased to
get a lot of private mails - Thanks to all! I also got requests from
other guys looking for info asking if I had gotten any responses since ALL
responses where directly to me and none were posted on the reflector. :-)
This info has helped me to correctly install the radial grid for my new 3 el
in-line vertical parasitic array. Here's hoping it will help others too.
This is condensed mainly from opinions of well respected guys on this
reflector, and a little of my own after some reflection... It's probably
basic info to many here, but to me and perhaps some others, it's new and
useful!
And, maybe it will also inspire further debate on some of the points
presented here.
73,
Tom, K1JJ
CT
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Condensed Opinions and Experiences with Vertical Radial Ground Systems:
1) Ground radials have very little effect on pattern formation. By laying
them out longer or concentrated in favored directions, this will NOT bias
directivity or efficiency in any particular direction.
2) Radials are used to reduce ground losses only. Picture a vertical as one
half of a dipole. RF current normally flows back and forth thru each dipole
half.
Now, like the dipole, the vertical element is one half, and other half is
the Earth soil. RF current does not flow efficiently through average soil.
The copper radials are laid upon the Earth as a "shield" or path to return
(collect) the RF current back to the base. This happens 1.8 million times a
second on 1.8 mhz.
Another way to picture it: The vertical has a large group of long resistors
all around attached to its base extending out. The resistors represent the
Earth soil. Your job is to short these resistors out with wire to produce a
low resistance path back to the base.
3) Very low current flows through each individual ground radial wire since
the total current is divided up amongst many. So, thin copper wire is OK.
4) The best place for ground radials is on the surface, not buried. I find
it very easy to staple the wire down to the grass and let it grow around the
wire... the grass will push it down. Mowing is easy. Best done in the early
Spring or after a short cut mowing job. Take solid #6 copper wire and cut it
into 5" pieces. Bend them in half into 'U' shaped staples and stomp into the
ground holding down the radial every 10' or so.
5) If you use more than one vertical element, tie the two groups of element
ground radials together and terminate them using a common bus half way
between the elements. This saves on wire, reduces parallel resonances, and
gets ALL radials shared for each working element.
6) Some say the height of the vertical is the maximum radial length needed.
But most say 1/4 wave radials are an important minimum no matter what the
height.
7) 50-60 radials appears to be the amount required to get into the "flat"
part of the efficiency curve for fair soil - diminishing returns. 120
radials is nice, but appears to be overkill.
8) A lamda of about .03 is the maximum distance between radials for
"reasonable" efficiency. This is the maximum "gap" between each radial
before a reasonable efficiency is lost. I believe this means that Earth
currents between the ground radials become a meaningful part of the loss if
this number is exceeded.. By using 60 radials you will easily be within
this parameter when using 1/4 wave long radials.
Once the radials are less than about .025 to .05 wavelength apart at the
farthest point (the tips), adding more wire is a wasted effort. This is
13'-26' apart on 160M.
9) An additional, concentrated short ground screen is a waste. If 60 radials
or more are used, the first 40' or so of the radial field is very dense
already - an additional screen will have no worthwhile effect. I was told
this screen is rarely used at BC stations - and if used, it's usually a
copper flashing material that is used more to keep from tripping over
radials and perhaps to protect equipment placed at the base from lightning.
10) The successful use of only a few elevated radials are way over blown.
Four elevated radials on HF have very little shielding effect and are down
as much as 3-5 db over a full-blown 120 ground radial system. Four will work
for a VHF ground plane that is essentially in free space, but for HF, the
Earth is a major part of the equation and RF currents must be collected
around the vertical in the near field and returned to the base in an
efficient manner. A few elevated radials cannot do this efficiently simply
due to the wide open missing gaps. The current wisdom is that at least 30
elevated radials are needed to do the job of 120 ground radials. Maybe
more. Elevated radials 1/8 wave high may be an exception to this.
11) Unless 1/4 wave parasitic vertical elements (reflectors and directors)
have their own good ground radial systems they will degrade the expected
gain. Even a few ohms in the base will drop db quickly. F-B will also be
degraded since the element will have less current due to resistive ground
losses to form the proper pattern. They should have as good a radial system
as the main driven element.
12) A symmetrical radial pattern around the base is important! It's just a
matter of collecting RF currents away from the lossy Earth soil. Even a
directional vertical array has energy off to the sides in the near field, as
very little pattern formation is completed close in. All energy in all
directions needs to be collected. The main purpose of the radials is to
reduce losses. IE, the elements radiate in all directions in the near field
, so you need a screen in all directions to intercept current and return it
to the base.
13) Once the ground loses are minimized through a heavy concentration of
radials, it is up to God and the Earth miles away as to how good your low
angle efficiency will be. If your far field ground is poor, a chunk of the
lowest angle lobe will be bitten out, just like Pac Man. Search "Brewster
angle" for more information on this.
14) I've found that #14 insulated PVC SOLID copper wire is great for
radials. It costs $12.77/500' at Home Depot -good deal. Easy to splice into
when connecting interlaced radials to form a grid.
15) When connecting interlaced radials together, bind with wire, solder,
then tape real well and then coat with RTV or some good weatherproofing
compound. The solder will fall apart into a white powder if exposed for long
in the soil without protection. If sealed off, the sealed insulated wire
will keep the copper inside bright and shiny for years...sealed against
water and soil contaminants.
16) When using a single vertical with radials, interlacing or connecting
the radials together with a single wire from radial to radial is a waste of
copper.
17) Some feel that adding another wire to help narrow the gap or "fill in"
by forming a 'Y' is useful when the radials are very long instead of
increasing the number.. This applies to the radial areas far out from the
base where the radials become spaced far apart.
18) Radials on 160M should be AT LEAST 100' long. If they are too short,
increasing the number and density will not cure this problem. It's a matter
of covering all areas. Once a certain density (as described earlier) is
achieved , adding more radials the same length is a wasted effort.
19) When using a steel tower, three copper wires connected together at the
top and run down the tower legs to form a "cage", and connected to the base
will insure that tower joint problems will not be a factor.
I suppose the cross sectional area of the steel tower makes the resistance
low enough so that the copper wire does not help in this area too?
73,
Tom, K1JJ
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