[amsat-bb] antennas for portable use

Thomas Doyle tomdoyle1948 at gmail.com
Tue Jul 17 05:00:53 PDT 2012


I did a lot of comparisons between my Elk and Arrow II. My results
were exactly the same as those of WD9EWK/VA7EWK but I was never sure
why the Elk performed better. Comparison tests I have been running
between a DIAMOND A430S10R and an Olde Antennas 7CP-70cm have given me
an insight as to why the Elk works so much better.

The tests between the Diamond and the Olde were the most inconclusive
tests I have ever run until suddenly my ancient grey cells managed to
figure it out. The Diamond has a longer boom which gives it more gain.
The Olde has a very short boom but is circularly polarized. I had them
mounted on opposite ends of the cross boom with a switch for quick
comparisons. During part of a pass the Diamond would work  better.
During other parts of the pass the Olde would work better.

The chart at the top of the SV1BSX web page holds the answer.
http://sv1bsx.50webs.com/antenna-pol/polarization.html
When the orientation of the sat antenna lines up with the Diamond it
works better and continues to work better until the sat rotates to the
point where the cross polarization loss cancels the gain advantage the
Diamond has over the Olde at which point the Olde works better...

What has this to do with the Elk - Arrow comparison. Since the 2M and
70cm antennas on the Arrow are mounted at right angles to one another
we have a problem when working linear polarized sats. If the Arrow  is
turned to maximize the downlink signal it is automatically turning the
uplink antenna to the angle that will maximize the uplink cross
polarization loss of up to 30 dB.

I do my testing on the linear sats and listen to my own downlink
signal.  When the Arrow is rotated to maximize the the received signal
my best guess is that it is turned  off the angle of the sat antenna.
This would spread the cross polarization loss between the uplink and
downlink antennas. The exact offset angle would depend on transmit
power, receive sensitivity and a ton of other things.

There is nothing wrong with the Arrow II it just can not work as well
as the Elk on linear sats unless you have a quick wrist and rotate the
antenna between transmit and receive. Even if you have very quick
wrists resulting from a miss spent youth playing foosball there still
is a problem. Trying to figure out the angle to turn the antenna
during transmit is all but impossible. Since the sat is rotating the
angle for transmit is not always 90 degrees from the angle for
receive. All antennas work to some degree and all this stuff is only
important if you are trying to maximize performance.

73 W9KE Tom Doyle


On Mon, Jul 16, 2012 at 3:28 PM, Patrick STODDARD (WD9EWK/VA7EWK)
<amsat-bb at wd9ewk.net> wrote:
> Norm,
>
> I use an Elk log periodic for AO-7 and all my other satellite work. I am
> able to work passes to the predicted LOS time, subject to local
> surroundings.
> I was only able to work down to 4 or 5 degrees elevation with my Arrow
> dual-band Yagi, before I switched to the Elk. You can see this setup
> in action working satellites - including AO-7 mode B - in some of
> my videos at: http://youtube.com/va7ewk
>
> For AO-7 in mode A, the Elk is my 2m antenna, and a Buddipole dipole
> on its tripod and telescoping mast is used for the 10m downlink.
> Since this requires more setup, and the 10m downlink is weaker
> than the 2m downlink, I don't work mode A very much.
>
> 73!
>
>
>
>
> Patrick WD9EWK/VA7EWK
> http://www.wd9ewk.net/
>
>
> On Monday, July 16, 2012, Lizeth Norman wrote:
>
>> Hi all!
>> Just wanted some comments as to what ops are using for AO-7 portable.
>> More importantly, some info on how they perform near loss of signal.
>> Pictures of antenna arrays would be greatly appreciated.
>> Norm n3ykf
>> _______________________________________________
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-- 

Sent from my computer.

tom ...


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