Message-ID: <002601bf006e$a43c96e0$2e97b38f@w8k3f0>
From: "Dick Rollema" <d.w.rollema@gironet.nl>
To: "LF-Group" <rsgb_lf_group@blacksheep.org>
Cc: "Koos Fockens, PA0KDF" <kfockens@worldonline.nl>, 
 "Ger van Went, PA0GER" <van.went@hetnet.nl>
Subject: Fw: LF: LF-antenna with top load
Date: Thu, 16 Sep 1999 20:08:47 +0200
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To All,

As a answer to a question by Mike, G3XDV,  I have sent the following e-mail
to the reflector:


>Mike, G3XDV wrote:
>
>
>> 1. No top load
>> Efficiency: 10.8%
>
>> 2. One horizontal top load wire of 5 m
>  Efficiency: 12.5%
>>
>
>> 4. One horizontal top loading wire of 10 m
>Efficiency: 11.9%
>
>> 6. One horizontal top loading wire of 20 m
>> Efficiency: 10.0%
>
>
>Dick,
>
>There's something wrong here isn't there?. You have a reducing
>efficiency for increasing top load length. Surely that can't be right.


I replied:


>Yes, I agree it looks strange. But it results from the way  efficiency is
>calculated: radiation resistance divided by the sum of radiation and loss
>resistance. When the loss resistance increases more than the radiation
>resistance efficiency goes down.  In the real world the actual efficiency
>will certaily get better as the small increase of wire loss is almost
>swamped  by the total resistance of the system: earth loss + coil loss +
>wire loss.
>What really matters is the increase of Rs.
>
>It is better to ignore the efficiency figures; I used them only to
calculate
>Rs from the real part in
>Z = R + jX


After sending the e-mail I realised I should have explained why I mentioned
efficiency at all if it was better  ignored.

The reason is that the computer program does not produce  radiation
resistance directly. What comes out of the machine is Z = R + jX for an
antenna over perfect ground, plus the efficiency. The only loss involved is
then due to the resistance of the antenna wires, so R = wire resistance +
radiation resistance.

>From this follows:  Efficiency = {Radiation resistance/(wire resistance +
radiation resistance)} x 100%.

So I needed the efficiency to calculate the radiation resistance.

Peter, DF3LP,  in his e-mail suggested to set the wire resistance to zero so
directly obtaining Rs. But my program AO does not allow that; one has to
insert the material the wires are made of.

73, Dick, PA0SE