[TowerTalk] [spiderbeam] spider beam gain measurements

corneliuspaul at gmx.net corneliuspaul at gmx.net
Mon Aug 11 14:06:43 EDT 2003


...a few days ago I spent 3 days out in the fields doing some antenna
measurements with reference to a dipole at the same height and 
thought I would let you know the results.

Initially I only wanted to do a comparison between 2 spiderbeams, one of
them using a different feed method suggested by a user.
These measurements were done first, finding out that there is no difference
between the 2 feed systems.

Then, because I had never been on such an ideal test terrain before (see
description below), I decided to stay longer and also run some more tests
of the spiderbeam against a reference dipole at the same height (10m),

+++++ description of the test setup +++++

There is apicture of the the test setup on my website, have a look at the
topmost picture in the gallery on www.qsl.net/df4sa/spiderbeam.htm
I found a very nice field which is open to all directions, only a few trees
here and there but they dont matter at all. The terrain is incredibly,
totally flat in all directions for at least 30 kilometers (typical north
German terrain characteristic), making it very well suitable for such test

For the first test,
I set up 2 identical spider beams, each on a 10m aluminum pole as you can
see from the picture. 
The antennas were spaced 60m apart from each other, I had the station in
the middle (where the car is) and could switch between the 2 antennas
The feed lines to each antenna were identical, the coax cable was brand new.

For the second test,
I took down all the directors and reflectors of one spiderbeam and kept
only the driven element (multiband-dipole).

+++++ test results +++++

I made lots of contacts with stations in DL, EU and DX, never telling the
QSO partners which antenna is which one, only switching between "antenna 1"
and "antenna 2" several times. Both antennas were always rotated to radiate
exactly in the direction of the QSO partner.

The results on 10m and 15m were:
10% of the people said - spider beam is better by 3 S-units 
60% of the people said - spider beam is better by 2 S-units 
20% of the people said - spider beam is better by 1 S-unit 
10% of the people said - no difference

The results on 20m were:
30% of the people said - spider beam is better by 2 S-units 
50% of the people said - spider beam is better by 1 S-unit 
20% of the people said - no difference

What do I conclude from this test? 

The differences between the results on 15/10m and 20m are mainly because of
the installation height. 
The higher the antenna is installed (in terms of wavelength) the more
visible becomes the gain of the beam vs. the dipole. This is a well know
fact and can also be viewed easily in simulation.

The gain difference between the spiderbeam and a dipole is at least one
S-unit, most of the times 2 S-units. 
Theoretically, one S-unit is 6dB. Most S-Meters are not very accurate, but
90% of the people always saw a signal increase with the spider beam, no one
saw it the other way round.
The theoretical gain of a 3element yagi like the spiderbeam over a dipole
is around 6dB. 
So the gain measurements made on that weekend match very well with the
calculated values.

So all in all I conclude (once again...) that the antenna works exactly as
specified... :-)
There is no magic involved either, it just shows the typical behaviour of a
3ele yagi vs. a dipole, installed rotatable at the same height... 
(BTW it also shows again that a rotatable dipole is not such a bad
antenna...) :-)


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