[TowerTalk] FW: buried radials problem

[KM5VI]

Just a thought...

Could the corrosion on the surface of the steel radial wires be presenting a
high RF impedance (relative to the ground rod path) when dry due to RF skin
effect?  Perhaps either the moisture changes the electrical characteristics
of the surface corrosion or it allows better coupling of the radials to the
ground (still meaning ground losses)?   It always seems to be more
challenging to achieve a decent quality ground plane in dry soils compared
to wet ones...

For what it's worth, corrosion rates on the order of .010" to .020" per year
are not unheard of for mild steel buried directly in certain types of
soils....   which would pretty well destroy a 14ga or 12ga steel wire in
just a few years.

Also - if your ground rod is copper then it could be making your buried
steel wires act like sacrificial anodes since steel has a lower galvanic
potential than copper.  This would tend to accelerate the corrosion rate of
the steel wire.

73
KM5VI




-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces at contesting.com] On Behalf Of
Eduardo Araujo
Sent: Tuesday, March 05, 2013 10:36 AM
To: towertalk at contesting.com
Subject: [TowerTalk] buried radials problem

Dear people, I experience a situation that confused me a little bit.

I have a vertical with 60 iron fence wire buried radials 1/4 wave long plus
a ground rod. I installed it 2 years ago lying on the ground and now they
are buried.

Prior to the contest I decided to check the buried radials of my vertical. I
have bundles ranging from 4 to 7 radials connected to a square ring around
the base of the tower. I provided 20W input and I measured the current
entering each of the bundles which is a procedure I have done many times.

To my big surprise there was little to no current entering most of the
bundles except one which also had a ground rod besides the radials.

This bundle took aprox 80% of the total current. Then I decided to measure
the current entering the ground road and it was aprox 100% of the total for
that bundle.

This situation was the opposite I always had measured before where the
ground rod practically had insignificant current.

2 months had passed without any rain so I tried to spread some water around
a radius of 4 meters but situation didn´t change dramatically but yes an
small increase in all radials current

That night I thought a lot of possibilities but the strongest one was that
for some reason most of the radials had dissapeared or at least have been
reduced to short ones.

Next day we have 27mm of rain. Another surprise, everything back to normal.
I had current in all bundles and almost nothing into the ground rod.

I expected that having 60 radials it should not matter if it rains or not. I
expected to have a relative good shield of the ground 

Question 1: Any guess what could be going on ?

Question 2: Is it a normal situation? Does anyone experience the same?

Question 3: Could it be possible that radials could have been eaten by the
soil and I am having very short ones? and that made them so much soil
conductivity dependent? 

Question 4: if the soil is so dry why the ground rod collect so much current
from the same soil? 

As I have no explanation so far, I am planning to add another 60 radials and
for a reason of cooper cost I am planning to use what I think should be
similar to what I read here many times as WD1. It is for exterior telephony
drop consisting of a pair of 0.8 mm of stiff wire which looks like iron
covered by cooper and both separated by 3mm of a very heavy plastic. I
measured 13 ohms of DC resistance in 60 mts of wire.

My plan is to split the 2 wires and use them as radials.

Question 5: It is a good or a bad idea to use this kind of wire instead of
fence wire?. Cooper will be the last option of course

Many thanks in advance for your opinions.... Eddy, LU2DKT

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