Content-Transfer-Encoding: 8bit
Message-ID: <3.0.1.16.20020515091223.2df7ee54@pb623250.kuleuven.be>
Date: Wed, 15 May 2002 09:12:23
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: RE: LF: Ground loss
In-reply-to: <Pine.LNX.3.95.1020515111709.237A-100000@fitec.omskcity.com >
References: <3.0.1.16.20020514131621.08dfed86@pb623250.kuleuven.be>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii; format=flowed
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

>When I derive the formula I assumed wires capacitance 5 pF/meter
>If wire is VERY short (0.2 meters) it's not true. For such a short
>wire there is about 20 pF/meter. If do some corrections resistance for
>ground rode of 0.2 meter is about 3000 Ohm, not 11700 Ohm.

>Are You sure real resistance is few hundred Ohm WHEN THERE IS NO LONG
>WIRE to connect TX to such a short ground rod? Does You do such
>experiment?
Hello Alex,

I believe there is a misunderstanding, the antenna wire (Lant) remains long
(several 10's meter), only the counterpoison (Lcp) is reduced to 0.2m
(small ground rod). This creates a larger Lant/Lcp ratio in your formula,
resulting in a large R.

Few years ago I did some experiments on antenna capacitance / loss
resistance depending on the radial system. At the moment I had the radials
on the ground (not burried), so I could easily change the radial
configuration. At the base of the antenna I had a 30cm ground stake.

I noticed that :
1. the antenna capacitance was rather independent from the radial system
(180pF with only 30cm ground stake / 200pF with full radial system of in
total 80m of wire.
2. the loss resistance varied from 180 Ohm with only groundstake to 80 Ohm
with the full radial system. When gradually adding radials I noticed that
the first radials made the big change (120 Ohm with 20m radials, 90 Ohm
with 40m radials, 80 Ohm with 80m radials).

The test antenna at that moment was a 7m high 'umbrella like' antenna (in
total 30m wire) kept away from any obstacles as far as possible.

Later test showed that a 14m high antenna close to trees had a larger loss
resistance of about 110 Ohm (winter) to 140 Ohm (summer) but dispite that
it produced a much stronger signal.

73, Rik  ON7YD