A Radio forum. RadioBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » RadioBanter forum » rec.radio.amateur » Antenna
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Ground rods.at HF



 
 
Thread Tools Display Modes
  #1  
Old September 4th 05, 01:56 PM
Reg Edwards
Guest
 
Posts: n/a
Default Ground rods.at HF

I post the following as a copy of a reply I made to the uk.amateur
radio newsgroup.

=====================================

I am considering the using some army surplus aluminium alloy poles

as
part of the rf ground for an inverted L antenna. They are thick

walled
4ft tubes able to withstand stand being driven into the ground, and

are
lying around waiting to be used for something.
The issues which come to mind are corrosion through contact with the
earth and electrolytic reaction where a copper cable is attatched

with
steel bolt and eylet connector.
Has anyone on here tried this?

John
m1jta

====================================
John,
I have used 3 or 4 feet alumininium alloy tubes as earth rods, on and
off, for many years. I have found them just the same as any other or
metals - ie., no bloody good.

Electrically, at HF, a 3 or 4-feet earth rod is no better than a
single, thin, radial, shallow-buried, horizontal wire of the same
length which is much easier to install.

The resistance to ground has nothing whatsoever to do with the
conductivity of the metal or surface corrosion products. It has
everything to do with the resistivity of the soil in which the rod is
embedded. ie., the soil in the immediate vicinity of the rod.

Resistance to ground depends almost entirely on rod length and is only
very slightly dependent on rod diameter.

Only old wives believe resistance is related to surface area of the
rod or electrode and dig great holes in their back gardens to bury
unwanted, scrap, hot-water cylinders. They would do better by
approaching their local scrap metal merchant.

A 3 or 4 feet rod in typical garden soil has a resistance to ground of
the order of 100 to 200 ohms and by itself is useless.

To locate one rod immediately next to another does next to nothing
regarding ground connection resistance. A pair of rods must be spaced
apart by at least twice their length before the resulting ground
resistance approaches half the resistance of one rod.

A collection of a number of rods must be spaced apart by many times
greater than their depth of burial to obtain the full benefit of all
being connected in parallel.

Consequently, the connecting wires from rods to a focal point
themselves constitute a good system of shallow-buried radials and the
short rods at the ends of the radial wires can be dispensed with.
Exactly the same result can be achieved just by extending the radials
by another, insignificant 3 feet. The rods are both wasted effort and
wasted copper (or aluminium).
----
Reg, G4FGQ


  #2  
Old September 5th 05, 04:24 AM
Richard Harrison
Guest
 
Posts: n/a
Default

Reg, G4FGQ wrote:
"The rods are both wasted effort and wasted copper (or aluminum)."

Likely so.

Radials are placed to capture displacement current to and from the
vertical radiator, to prevent its travel at a high densitY in the earth
where it would cause high loss.

B, L, & E found that more radials were better than longer radials. More
radials put the displacement current capture closer to the vertical
radiator where its density is higher. Radials need extend outward only
as far as there is any current or until a point of diminishing returns
is reached. At great distance from the vertical radiator, the earth`s
crust, which may be affected, has a great cross section, so current
density is low and so are losses. There is much less displacement
current to deal with near the ends of radials. Displacement current is
low near the ends of the radials and the earth out there has a large
cross section and a low resistance. Due to skin effect, the closer to
the surface, the higher the current. This is especially true at high
frequencies.

  #3  
Old September 5th 05, 04:40 AM
Richard Harrison
Guest
 
Posts: n/a
Default

I sent too soon.

The problem with ground rods at high radio frequencies is that depth of
earth renetration may be low as compared with length of the ground rod.
Contact that counts is that which connects with the current path.

There is a special problem with the aluminum material itself of the
ground rods mentioned by the questioner. Aluminum, zinc, and magnesium
are galvanic anodes used for cathodic protection of less active metals.

A copper ground rod is likely to last forever in the soil. An aluminum
ground rod is likely to soon be sacrificed in its attempt to protect
other connected metals in the same galvanic soup.

Best regards, Richard Harrison, KB5WZI

  #5  
Old September 5th 05, 05:42 PM
hasan schiers
Guest
 
Posts: n/a
Default

The best answer I've seen, based on current research is:

N=(SQRT(2*PI*L))/A

N equals the square root of the quantity 2*PI*L divided by A, whe

N = optimum number of radials for high efficiency
L = amount of wire available in meters
A = distance between wire tips at the far end (a measure of radial density)

A = 1.3 for 95% or greater efficiency (approximation) read article for more
precise values.
A = 2.6 for 85% efficiency (approximation)

Example 1:

You have 500 meters of wire available. How many and how long for the two
values of A?

N = (SQR(2*pi*500))/1.3 = 43 radials. Length = 500/43 = 11.6 meters

N= (SQR(2*pie*500))/2.6 = 22 radials. Length = 500/22 = 18 meters

Example 2: (you have space constraints and the max radial length available
is 15 meters)

How many radials are required and how much wire is required?

The circumference of a circle with a radius of 15 meters is 2*PI*15 or 94.2
meters. With the tips of the radials seperated by 1.3 meters we have
94.2/1.3 = 72 radials. If we go for slightly more loss, we have 94.2/2.6 or
36 radials, 15 meters long.

Now you can plug in your own limitations for radial length, and get a feel
for how many of them you will need for 95% efficiency (A=1.3 meters) and 85%
(A=2.6 meters).

This info was presented in some ARRL publication, as I recall, and is also
presented in the 4th Edition of "Low-Band DXing" by John Devodere.

....hasan, N0AN

"Walter Maxwell" wrote in message
...
On Sun, 4 Sep 2005 22:24:24 -0500, (Richard
Harrison) wrote:

Reg, G4FGQ wrote:
"The rods are both wasted effort and wasted copper (or aluminum)."

Likely so.

Radials are placed to capture displacement current to and from the
vertical radiator, to prevent its travel at a high densitY in the earth
where it would cause high loss.

B, L, & E found that more radials were better than longer radials. More
radials put the displacement current capture closer to the vertical
radiator where its density is higher. Radials need extend outward only
as far as there is any current or until a point of diminishing returns
is reached. At great distance from the vertical radiator, the earth`s
crust, which may be affected, has a great cross section, so current
density is low and so are losses. There is much less displacement
current to deal with near the ends of radials. Displacement current is
low near the ends of the radials and the earth out there has a large
cross section and a low resistance. Due to skin effect, the closer to
the surface, the higher the current. This is especially true at high
frequencies.


Hi Richard,

You've just presented the best abstract of BL&E I've seen ever seen.
It should be must reading for anyone who asks questions concerning the
purpose of radials, how many, and how long.

Walt, W2DU



  #6  
Old September 5th 05, 09:31 PM
Richard Harrison
Guest
 
Posts: n/a
Default

I appreciate Walt, W2DU`s kind words regarding my posting about ground
rods and radials.

I did not recommend any particular number or length of radials. RCA`s
Brown, Lewis, and Epstein examined how many and how long ground radials
should be in the 1930`s. The FCC accepted their experimental work,
conducted at 3 MHz, and used it to set ground system standards for
broadcasting in the medium wave band. These standards are still
applicable. B.L.&E. did good work.

Another RCA alumnus, E.A. Laport, abstracted some of B.L.&E.`s work in
"Radio Antenna Engineering". Figs. 2.17, 2.18, and 2.19 on pages 119 and
120 show field strength as a function of the number of radials. All the
Figs. show near perfection with 113 radials, so it seems the FCC rounded
up to 120 radials and made it the rule. It`s worked well, giving us
good broadcast reception when the earth is dry and sandy or wet and
swampy. Laport`s figures show performance with 1/2 and 1/4 the ideal
number of radials.

As Walter flattered me, I`ll reciprocate. Get hold of the April 1973
issue of QST. Look on page 35. Walter is pictured there. He is a real
good looking fellow!

Best regards, Richard Harrison, KB5WZI

  #8  
Old September 5th 05, 10:27 PM
Walter Maxwell
Guest
 
Posts: n/a
Default

On Mon, 5 Sep 2005 15:31:18 -0500, (Richard
Harrison) wrote:

I appreciate Walt, W2DU`s kind words regarding my posting about ground
rods and radials.

I did not recommend any particular number or length of radials. RCA`s
Brown, Lewis, and Epstein examined how many and how long ground radials
should be in the 1930`s. The FCC accepted their experimental work,
conducted at 3 MHz, and used it to set ground system standards for
broadcasting in the medium wave band. These standards are still
applicable. B.L.&E. did good work.

Another RCA alumnus, E.A. Laport, abstracted some of B.L.&E.`s work in
"Radio Antenna Engineering". Figs. 2.17, 2.18, and 2.19 on pages 119 and
120 show field strength as a function of the number of radials. All the
Figs. show near perfection with 113 radials, so it seems the FCC rounded
up to 120 radials and made it the rule. It`s worked well, giving us
good broadcast reception when the earth is dry and sandy or wet and
swampy. Laport`s figures show performance with 1/2 and 1/4 the ideal
number of radials.

As Walter flattered me, I`ll reciprocate. Get hold of the April 1973
issue of QST. Look on page 35. Walter is pictured there. He is a real
good looking fellow!

Best regards, Richard Harrison, KB5WZI


Thank you, Richard, for considering me for the MAS (mutual admiration
society). You are too kind. The issue of QST you referenced contains
the first installment of my series of articles, "Another Look At
Reflections," which form the first seven chapters of 'Reflections 1
and 2'. Incidentally, the 3rd edition is in the publication stage. I
will be posting the release date on my web page at w2du.com.

Back on the subject of radials, unless the FCC has changed the
requirements since I was involved, the requirement is for 90 radials.
However, most of the AM BC stations I'm familiar with use 120, even
tho not required.

Walt, W2DU
  #9  
Old September 6th 05, 02:32 AM
Walter Maxwell
Guest
 
Posts: n/a
Default

On Mon, 05 Sep 2005 14:25:09 -0700, Dan Richardson wrote:

On Mon, 5 Sep 2005 15:31:18 -0500, (Richard
Harrison) wrote:
[snip]
As Walter flattered me, I`ll reciprocate. Get hold of the April 1973
issue of QST. Look on page 35. Walter is pictured there. He is a real
good looking fellow!

[snip]

If you don't have that QST handy you can see Walter he

http://users.adelphia.net/~k6mhe/BLE_de_W2DU.html

73,
Danny, K6MHE


Well, Danny, ya had to go and do it din't ya? Now people who see my
face in post offices will know how to trace me through my mug shot you
just posted. I thought I'd gotten away with it. Somebody hire you to
post it? CSI? Law & Order? At least after I'm sent away to Attica
everybody, including you, will be able to find me.

Ya wanna know sumptin? I don't even have any remorse!

Walt
  #10  
Old September 6th 05, 03:22 AM
Walter Maxwell
Guest
 
Posts: n/a
Default

On Mon, 05 Sep 2005 14:25:09 -0700, Dan Richardson wrote:

On Mon, 5 Sep 2005 15:31:18 -0500, (Richard
Harrison) wrote:
[snip]
As Walter flattered me, I`ll reciprocate. Get hold of the April 1973
issue of QST. Look on page 35. Walter is pictured there. He is a real
good looking fellow!

[snip]

If you don't have that QST handy you can see Walter he

http://users.adelphia.net/~k6mhe/BLE_de_W2DU.html

73,
Danny, K6MHE

Danny, I was just perusing the data in the post re the above url and
found a typo. In the ground radial data in the line showing 30
radials, the data for 0.4/wl indicating 158 mv/meter should read 185
mv/meter.

I've searched through my files for the one I sent to you containing
this data, but I can't find it. I'm assuming you simply copied my
data, so it's probably my error, which I'd like to fix in the
original. I guess all I can do is ask you to place a correction on
that incorrect piece of data to avoid giving the impression that the
remaining data may be suspect.

Walt, W2DU

 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Inverted ground plane antenna: compared with normal GP and low dipole. Serge Stroobandt, ON4BAA Antenna 8 February 24th 11 10:22 PM
How to measure soil constants at HF Reg Edwards Antenna 104 June 25th 05 10:46 PM
Why a Short Lightning Ground? [email protected] Antenna 13 March 5th 05 04:09 PM
The Apollo Hoax FAQ darla General 0 July 22nd 04 12:14 PM
QST Article: An Easy to Build, Dual-Band Collinear Antenna Serge Stroobandt, ON4BAA Antenna 12 October 16th 03 07:44 PM


All times are GMT +1. The time now is 04:06 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright 2004-2014 RadioBanter.
The comments are property of their posters.