[amsat-bb] Re: Arrow and ELK Comparative Antenna Tests

[Clare Fowler]

It is commendable to see some direct antennas comparison measurements being 
made.
Computer modeling is one thing. Realizing it in hardware is quite another.

The biggest problem in making field measurements is to reduce and try and 
eliminate reflections
from the ground, adjacent structures and overhead wires and your antenna 
mount.

The pattern of the antennas under test can be very significant.
Two antennas can have the same direct forward gain. But one can have
a narrow beam width main lobe but with relatively large side lobes,
while the other has a broader main lobe beam width and low side lobes.

While they both can give the same forward gain results the second antenna
is obviously the better antenna.

To test for ground reflections raise and lower your antenna and also move it 
back and
forward and see if your signal strength varies.

On a different day and different location repeat the measurements.
Your results may well leave you wondering why you made the
statements you so confidently did.

To reduce ground reflections raise the antennas as far as possible above 
ground.
Use a high gain directional antenna for the signal source so that little of 
the radiated
power is directed at reflective surfaces.
The greater the spacing between the rx and tx antennas the more likelihood 
of
a reflection but the spacing needs to be such that a small change in spacing
has little effect on the results. As a rule of thumb spacing about 10 
wavelengths.

In actual satellite operating a difference in forward gain of a db or less 
is hard
to detect however the effects of the pattern can be significant in pointing 
a hand
held antenna at a moving target.

Having made hundreds of comparison measurements on 13cm helix antennas
I know how much time and effort it takes.  However I think it is very 
worthwhile
to actually make real antenna comparison measurements to confirm the 
theoretical.

Clare VE3NPC



----- Original Message ----- 
From: "John Kopala" <jkopala at gmail.com>
To: <amsat-bb at amsat.org>
Sent: Sunday, April 24, 2011 11:49 PM
Subject: [amsat-bb] Arrow and ELK Comparative Antenna Tests


> On Saturday, April 23, Pat Stoddard (WD9EWK) and I did some antenna 
> testing in an attempt to answer
> the questions about which is the best antenna for portable satellite 
> operation.  We only tested the
> antennas in receive mode to determine their relative gain.  Time 
> constraints prevented us from
> performing additional testing to determine if the transmitted output was 
> consistent with the receive
> gain of the antennas.  For the time being we will assume (and we all know 
> the dangers of doing so)
> that the transmit performance closely matches the receive performance.
>
> The antennas tested were an Arrow (3 x 7 elements), an ELK (4 elements), a 
> PortaFox configured for
> 145/435 operation (4 elements), and a Home Brew 4 by 9 element "arrow" 
> antenna.  The standard Arrow
> antenna was the only antenna equipped with duplexer, but not the basic 
> duplexer which is installed
> in the handle.  We did not measure the insertion loss of the duplexer on 
> the Arrow antenna, but this
> was obviously not a significant factor in the overall performance.  A 
> duplexer could still be
> required depending upon the antenna chosen and the radio(s) to be used.
>
> Using the Arrow antenna as the reference antenna and 145.300 MHz as our 
> test frequency, our
> measurements indicated that the Arrow and the ELK antennas had identical 
> gain.  The PortaFox antenna
> showed 2db less gain than the Arrow and the ELK.  The Home Brew 4/9 
> element crossed yagi showed 2db
> more gain than the Arrow and the ELK.
>
> On 435.300 MHz, the Arrow antenna had 2db more gain than the ELK and 8db 
> more gain than the
> PortaFox.  The Home Brew 4/9 element had 3db more gain than the Arrow.
I