[TowerTalk] Chicken Wire Ground Screen

[Bill Tippett]

K9YC:
 > It has long been well understood that return current is greatest 
near the base of the antenna, so copper near the tower is more 
important than copper at a distance in reducing I squared R losses. 
This is why AM broadcast stations often use a heavy copper screen 
that looks like monster chicken wire (40 ft x 40 ft is a number that 
comes to mind) in the immediate area of the tower, with radials 
terminated to the perimeter of the mesh.

         I believe this might be another broadcast myth (like 
supposedly needing 120 1/4 wl radials) and which is NOT mandated by 
the FCC.  About 10 years ago I decided to make a ground screen for my 
180' Topband vertical using ~16 50' X 4' rolls of welded galvanized 
fence wire (with rectangular openings about 2" x 4"...not the smaller 
gauge woven hexagonal chicken wire).  I couldn't tell any significant 
difference in performance with this laid over my existing radial 
field (about 110 radials of 100-150') although I didn't actually make 
before and after measurements.

         I later read an edition of ON4UN's book which indicated you 
could achieve a nearly perfect near-field ground using 1/4 wl radials 
spaced ~.025 wl apart at the tips.  Doing the math for 160 this 
implies ~13 feet apart at the tips.  No matter how long your radials 
(even if differing lengths), you simply maintain the 13' spacing at 
their tips for an optimum ground.  For 1/4 wl radials, this implies 
60 (radial circumference / .025 wl spacing = 60 radials).  For longer 
radials, it implies more.

         If you accept this logic (which I do because several 
including W8JI, N6LF and others point to ~60 as an optimal number of 
1/4 wl radials), then there's little point in a ground screen near 
the antenna.  Assuming the approximate dimensions of a ~40' square 
ground screen (I'll assume a circular radius of 20'), your 60 radials 
above are only 2' apart (0.004 wl) at 20' out, so the improvement 
from a ground screen should be minimal.  Of course the radial spacing 
becomes even closer as you near the base.

         My galvanized fence wire has long since rusted out and quite 
frankly I now wish I had never put it down because it's now a hazard 
to trip over.  If anyone is serious about ground screens I believe 
you are well advised to use real copper which will last forever (but 
I hate to think what that might cost).  IMHO you are better advised 
to forget any sort of ground screen and spend your money on at least 
60 1/4 wl copper radials which will last forever.

                                         73,  Bill  W4ZV