(long) RE: [TowerTalk] One more ground radial question

[Jim Lux]

At 04:49 PM 12/17/2003 +0000, kb9cry at comcast.net wrote:
>  I would agree 5 radials just won't cut it and your theory of impedance 
> changing during rain is probably right on.  Also your planned number of 
> radials of 12 - 24 is also low.  The best bet is 60 1/4 wavelength 
> radials but any number of shorter lengths will work.  My experience, with 
> good loamy and clay soils, is 45 is the minimum required.  I'd plan to 
> install around that many if you can; the 5 you have now is not 
> enough.  Commercial stations usually use up to 120 radials but extensive 
> tests by amateurs have determined that 60 is optimum.  Check this list's 
> archives for past discussions.  Phil  KB9CRY

While 160m is fairly close to the broadcast band antennas that George 
"60-120 radials" Brown wrote his paper about, there's nothing special about 
them being 1/4 wavelength long.  Also, don't forget that all that analysis 
and testing about radials was done in B.C. (before computers) days with 
slide rule and CRC tables by your side. It was tedious enough that you're 
not going to try lots of different schemes for grounding, and you'll tend 
to pick approaches to evaluate that have nice symmetry so the analysis is 
simpler.  Even the more recent articles in QST, for instance, have 
essentially echoed Brown's early analytical approach.  Someone should get a 
copy of NEC4.1 and do some REAL modeling of this. (There is a paper in last 
year's ACES conference that used NEC4 to analyze the performance of ground 
rods.  J. Patrick Donohoe, "Modeling guidelines for the computation of 
electrode grounding resistance using NEC", Proc. of 19th Ann. Rev. of Prog 
in Appl. Comp. EM, Mar 24-28, 2003, Monterey, CA., pp844-849 )

There are basically three situations you might be in, radial wise:

1) Where the radials ARE the ground plane, and have to carry all the 
current to form the image of the vertical.

This would be the case with dense radial networks above ground, of any 
length, including grids, sheets, etc.  In this case, the groundplane (or 
counterpoise) isn't a resonant thing, and you choose the size according to 
the electric field from the vertical, so that "most" of the field is 
covered.  A ballpark of making the radius of the counterpoise equal to the 
height is a nice compromise (it just happens that it winds up being 1/4 
wavelength, then, when the vertical is 1/4 wavelength long...).  If you 
have a substantial "top hat" on your vertical, then the counterpoise should 
be bigger.  There are a number of nice programs out there that handle this 
kind of calculation graphically, especially for rotationally symmetric 
arrangements.  In the ideal case, you're approximating a continuous 
conductive sheet, so no current appears on the "bottom" side, and the 
actual ground is irrelevant (at least, in the near field)

2) Where the radials are designed to make a good connection to the "real 
earth" and they are either laying on the surface or buried.

Here, you need to agonize about things like whether the current is flowing 
in the radial or the earth, and what the optimum arrangement of wires is to 
reduce the overall impedance, given that a) soil is not a nice uniform 
resistor, it has a significant dielectric constant; and b) the soil 
conditions change (radically) with moisture content; and c) the current 
isn't flowing in a nice idealized sheet anyway.  This is the classic case 
subjected to analysis by Brown and others (including recent articles in 
QST).  By the way, a more optimized arrangement of conductors would be to 
have branching radials, so that the "density" of radial wires is roughly 
constant (i.e. more radials, the farther out you get, so the distance 
between radials is roughly constant).  CLose to the antenna, you don't need 
as much copper. You can also analyze (to death) the current distribution 
from the antenna induced in the ground, etc. The construction hassles of 
such an approach tend to discourage it, though, when you can just go out 
and plow in 120 straight radials of whatever length.

3) Where the "stuff at the bottom" is just another radiating element of the 
antenna.

A 2m "groundplane" with drooping radials, or a discone, would be good 
examples. In this case, the length and position of the radials is very 
critical, since they carry significant current.  Anything with 2,3, or 4 
radials fits in this category, and they are basically some form of bent 
dipole or biconical, and can be considered as such.  Anything with loading 
coils or capacitors on the radials would also fit in this category. So 
would using something like the MFJ artificial ground, which is just an 
antenna tuner with a different label.


What happens when ground conditions change:
In case 1: Nothing... you've created a synthetic ground plane. (Which is 
why the broadcasters do this..they don't want their field strength varying)

In case 2: As ground conductivity increases, the resistive losses decrease, 
so the SWR will tend to get worse (less R to swamp out the X from the 
antenna) and the bandwidth will narrow.  However, the efficiency of the 
antenna will improve (less loss R, so more power goes into radiation.  This 
is the basic tradeoff between Q and loss/efficiency.

In case 3: Lap of the gods... You're probably inducing significant current 
in the ground, so increasing ground conductivity will probably increase the 
current, which might dissipate more power in the ground, but it could 
actually get better.  Consider the ends of the spectrum: a bone dry desert- 
no current flows, very little loss; a salt water swamp - the surface is 
almost a perfect reflector, so most of the RF is reflected and not 
absorbed.  It's in the middle where losses are significant. Only modeling 
or measurements would tell for sure.



The upshot is that if your SWR is getting worse when it rains, you have a 
reasonably good #2 situation, because your "connection" to the soil is good 
enough that the soil conductivity affects it.  What you might want to do is 
tune/adjust the antenna for resonance when it's raining (because that's 
when the loss resistance is lowest and the system has highest Q and highest 
efficiency).  Then, when it dries out, it can only get better.  Adding more 
radials will reduce the effect of changing soil conditions, reaching the 
limit of case #1, where you've made an artificial ground.



> > I have to admit that I am in aan enviable position...whatever the number of
> > radials I need that is what I install.  The more the Merrier.  in my major
> > radial field below the vertical I had 120 radials it worked like a 
> charm and
> > the cows didn;t seem to mind one bit.  Seriously,  my view is that you are
> > under-radialized (a word?).
> >
> > -----Original Message-----
> > From: towertalk-bounces at contesting.com
> > [mailto:towertalk-bounces at contesting.com]On Behalf Of Jerry Connelly
> > Sent: Wednesday, December 17, 2003 9:18 AM
> > To: TowerTalk at contesting.com
> > Subject: [TowerTalk] One more ground radial question
> >
> >
> > Hello
> > I do not want to flog the proverbial dead horse but I am
> > trying to understand something here.  My inverted L for 160m
> > seems to work fairly well but the tuning is far from o.k.  Right now
> > I have 5 on-ground radials.  Three are 1/4 wave long and two sre 30'
> > shorties. I intend to end up with 12 to 24 but weather hasn't been usable
> > yet.
>
> > I got the length pruned to lowest swr (about 1.6) but everytime it rains or
> > snows
> > it goes up to 3 or 4.  I realize a tuner will handle it but I like tuning
> > for freq. of
> > interest and only using a tuner when away from that freq. range.  My 
> yard is
> > average to poor soil.  It has lots of shale and a small amount of granite.
> > Is the moisture improving the soil's conductivity and lowering the
> > impedence??
> > Does this mean I need more (possibly many more) ground radials??  I wanted
> > to bury
> > them several inches down....so would that help even the swr curves some??
> > Forgive my ignorance but ground radials are new to me and I don't want to
> > copperplate
> > the backyard only to find out something else is really the culprit.
> > Thanks
> > JerryC
> > KC8TES
> >