[amsat-bb] Re: Arrow and ELK Comparative Antenna Tests
jkopala at gmail.com
Mon Apr 25 10:43:54 PDT 2011
Mark, The testing method for the antennas was fairly basic. The antenna range was my front yard, the
only easy location free of obstructions, other than a public park. Our signal source was an
IFR-1200S station monitor feeding an aftermarket dual band rubber duck antenna vertically polarized
and about 3 feet above the ground. The antennas, vertically polarized for the band being tested,
were supported one at a time at about 44 inches above ground by resting their boom on the top edges
of a large cardboard box positioned about 93 feet from the station monitor across the front yard.
The receiver for the tests was an IC-910. The signal level was set on the station monitor to produce
a mid scale, S9 reading on the receiver S meter. An attenuator was used at the receiver input to
reduce the signal so that the addition on 1 db attenuation would cause a single step reduction in
the S meter bar length. We used the Arrow antenna to establish this reference level against which
each antenna could be measured. Each antenna was then tested to determine how much attenuation was
required to match the signal to the reference level. With the PortaFox, the signal strength was
below the reference level on 435 MHz, so we had to create a new reference level for that antenna. We
then used the Arrow antenna and measured how much additional attenuation was required to get down to
the signal level of the PortaFox antenna. The difference between the amounts of attenuation required
to match the reference level indicate the relative performance of the antennas. John Kopala N7JK
At 08:34 AM 4/25/2011 -0400, Mark Hammond wrote:
Thanks John and Patrick. Interesting results and a very nice study. Question---what was your testing
method? I can't glean that from the information below. Thanks! Mark N8MH At 08:49 PM 4/24/2011
-0700, John Kopala wrote:
> >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.
> >Although the ELK antenna shows slightly less (2db) gain on 435.300 MHz, it does have one potential
> >advantage over the Arrow antenna. With the ELK, transmit and receive are in the same plane. With
> >any satellites that have linear polarized antennas, such as AO-27, SO-50, the ISS and maybe SO-67,
> >a crossed yagis can maximize the performance on one band while minimizing it on the other. That
> >does not mean the Arrow won't work, as has been demonstrated by the thousands of satellite QSO's
> >that are made on a regular basis using Arrow antennas. It just means that when you rotate the
> >antenna to maximize the downlink signal, you may be significantly impacting you uplink signal
> >strength in marginal situations.
> >Even though a satellite may have a circular polarized uplink and downlink, don't assume that the
> >orientation of your station antenna as horizontal, vertical, or something in between won't have a
> >significant effect on your signal strength. My experience operating portable with my home brew
> >antenna has convinced me that my horizontally polarized Qagi should either be remounted vertically
> >polarized or replaced with a circular polarized antenna. That is another ongoing project.
> >In summary, unless you plan to build your own, the performance of the Arrow crossed yagi antenna and
> >the ELK log periodic antennas are very comparable and should provide lots of solid satellite contacts.
> >The overall results reflect the adage that bigger is better, but along with more gain, you also get
> >a more bulky antenna that is harder to handle. The home brew crossed yagi I built for portable
> >operation is tripod mounted, can be rotated on its axis, and disassembles for storage in a roll up
> >case. But because of its size, it would be very tiring to attempt to use it hand held for an
> >extended period of time. Pictures of the home brew crossed yagi antenna are on my QRZ page.
> >John Kopala
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