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Re: Seeking Mode JA System Opinions



Hi Eric,
Here are a few thoughts on mode JA antennas...

It is not necessary to have "big" antennas to work mode JA.
I have made many mobile contacts on FO-20, AO-27, and UP-14
using nothing more than a vertical on my car fed by an FT-100.

However, if you want reliable, horizon to horizon communications,
it is a different story. Remember that most of the operating time for
these satellites will be under 30 degrees elevation and your best
DX will be in the 5-10 degree range. You don't want to optimize for
overhead passes (unless you are at the poles!)

I wrote a paper for one of the 1998 AMSAT Space Symposium in which
I did a theoretical analysis of using an "eggbeater" style antenna
for the 70 cm downlink that was optimized for low elevation angles.
It is a myth that eggbeaters give you circular polarization as this
is only true off the top of the antennas (i.e. at very high elevation
angles.) Eggbeaters are basically horizontally polarized turnstile
antennas at lower elevation angles.

The bottom line is that this is an adaquate antenna for reliable
communications in analog modes if you mount a 1dB NF preamp right
at the antenna and you get it up high above any clutter (i.e. 40 feet
and higher.) You should be able to receive the satellite beacon on
FO-20/29 from horizon to horizon with this antenna. I used a homebrew
version on the Fuji satellites for several years and it worked as
expected. Note that this antenna is not sufficient for 9600 digital
satellites unless you are at the poles.

For the 2 meter uplink, an eggbeater is really not a very good antenna.
It's radiation pattern is not ideal for an uplink, there is too much
power going straight up and not enough at lower angles. There are also
holes in the pattern which will make you fade out until the satellite
moves out of the "hole." You can overcome this by using a 100-200 Watt
amplifier but you really have to ride the power control because the
peaks in the antenna pattern at certain angles will provide way too
much uplink power while the minimums will require full power just to
barely hear yourself.

Verticals are slightly worse and genrally have a hole in the radiation
pattern somewhere in the 15-30 degree range depending on the specific
installation. This is the worst place for a hole as this is where you
make most you your contacts.

The best omni antenna is a Lindenblad. Several years ago I wrote an
article for the AMSAT Journal that described a 2-element, phased-array
of Lindenblads. This antenna was designed to provide a radiation pattern
optimized for a 2 meter LEO uplink and worked very well. I believe this is
the best you can do with an omni-directional antenna. The down side
is that you cannot buy one and they are a little complicated to build.
They are also quite a bit larger than an "eggbeater."

The best antenna is a yagi for either up or down. If you don't want to
use an elevation rotator, I would tilt it at 15 degrees rather than 30
for best DX.

73,
Tony AA2TX

---
At 09:54 AM 9/1/01 -0700, Eric June wrote:
>Hello all,
>
>I am an old HF guy who is just starting out with the satellites.  I have 
>made a dozen or so contacts on RS-12 using equipment lying around the 
>shack, and now I would like to take the next step and become QRV on the 
>Mode JA LEO birds (FO-20/29, AO-27, UO-14).  My main constraint is some 
>vague CC&R language that doesn't specifically prohibit ham antennas, but I 
>would like to keep as low of a profile as possible.  I am therefore not 
>shooting for an AO-40 or AO-10 capable station at this time.

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