[TowerTalk] FW: low band antenna without good ground

[Matt]

GM3SEK brings up an interesting point about skin effect.  Good comments also
about soil characteristics having a profound effect on corrosion life.
Drawing on experience with underground piping - it's not just the soil
conductivity but also the chemical makeup of the soil which is location
dependent.

 

Another interesting point was made about steel being an order of magnitude
better conductor than soil, drawing a conclusion as to screening benefits.
Screening the RF absorption of the soil within the near field (of the back
yard) is said to improve efficiency (by making the radiation resistance
proportionally greater compared to the loss resistance).  But unless the
screen itself provides a low impedance RF return path, (copper) radials
would be necessary in addition to the screen ??? (chime in here please)
The far field benefits would be moot because they have to be derived within
the Fresnel zone of the antenna - which for low band would be quite some
distance away from the back yard.

 

With respect to burying zinc coated (galvanized) steel, I wonder if anyone
has tried cathodically protecting a screen system by bonding it electrically
to a buried sacrificial anode?  Since the zinc has an electronegativity of
about -1.1v, I would think that the anode would have to be a magnesium alloy
(-1.6v typical).  Such installations generally have the cathode material
electrically insulated from the soil in order to limit corrosion current
(and give the anode a useable life) - but insulation coating would not be
practically feasible with mesh.  I wonder if the corrosion current could not
be limited by placing a fixed resistance in the conductor between the anode
and the protected screen?   Food for thought.

 

Matt

KM5VI

 

 

 

Jim Lux wrote:

>On 9/25/11 9:41 AM, Kevin Normoyle wrote:

>> 

>>>  From ON4UN's Low-Band DXing regarding ground enhancements using 

>>> mesh

>> screening...   "Steel is a very poor conductor at RF"

>> 

>> 

>> Looking at VK1OD's table here, zinc is pretty good 

>> http://vk1od.net/antenna/conductors/loss.htm

>> 

>> so zinc-plated steel, or Al would be fine,

 

 

The RF conductivity of the steel is irrelevant, because its high magnetic
permeability forces almost all the RF current outward into the skin of zinc
(even if the thickness of the plating is less than the normal skin depth in
solid zinc.)  The steel is then acting only as a mechanical support for the
'tube' of zinc plating which is where almost all of the RF current flows.

 

This means that the RF conductivity of galvanized wire is determined almost
entirely by the diameter of the wire and the thickness of the zinc plating.
Good industrial-quality stock fencing scores well on both of those points,
and then the mesh construction provides multiple, diversified current paths
which further improve the conductivity.

 

 

 

>> it's just the corrosion issues.?

>> 

> 

>I think so.  It sort of depends on a whole cost/benefit analysis... If 

>you're rolling out wires for a weekend or a couple weeks, cheap and 

>light might be the ticket (go aluminum!).  If you're installing a 

>ground field for your 160m full wavelength vertical to reduce losses so 

>you can beat the guys operating in the middle of a salt marsh for the 

>Stew Perry for the next 20 years, then copper might be a better choice.

> 

>ANd the corrosion thing will be highly site specific.  If you're out in 

>the desert (where soil conductivity is really, really low), practically 

>anything will help.  If you're in that salt marsh, galvanized fence 

>wire's probably not a good choice.

 

At this QTH, where I can *taste* the salt on my antennas, the galvanized
wire mesh is lasting just fine. If it gives me another 5 years, it will have
been a very good investment and I'll do exactly the same again.

 

 

 

-- 

 

73 from Ian GM3SEK