Re: [amsat-bb] GRRRRRRRRRRR!

[Jon Ogden <na9d@xxxxxxxxxxxxxx>]

on 3/1/01 6:47 PM, John / NS1Z at ns1z@arrl.net wrote:

> But not likely to the extent one would experience by using the opposite
> polarity, say LHCP to RHCP? I can envision the received (I think that is
> what you meant to say) signal not being 'perfectly' circular in either pole,
> as seen by my linear antenna. However, I cannot imagine that lack of perfect
> circularity ever approaching a true linear polarity. This is where I should
> think the greatest signal attenuation would occur. I know opposite polarity
> isolation can be as great as 20 dB for linear (Vertical to Horizontal)
> antennas. Anyone have info on LHCP to RHCP isolation?

Theoretically, the isolation between LHCP and RHCP is infinite if I remember
correctly, in real practice, you'll never get that good.  You are right in
that you'll rarely get an elliptical polarization approaching linear, but it
could happen.  I think it happens with AO-10 as you get wildly shifting
changes due to its pointing angle and tumble.

> Why
> do they use it on International satellites and Linear on Domestic?

Because satellite ops are better than everyone?  :-)

Seriously, I think because CP is less prone to some of the problems that
linear can have.  I think that spin modulation of the satellite with linear
antennas would cause a gradual shift from vertical to horizontal to vertical
to horizontal is the satellite rotates.  Secondly, I think there are times
when you might actually get some Faraday rotation of the signal as it goes
through the atmosphere (it happens with HF signals for sure and I would
think it could happen at VHF and UHF depending on the atmospheric condx).

I am sure there are other reasons as well.  Keep in mind that with your
linear antennas you are receiving a CP signal not trying to receive a
particular polarization of a linear antenna on the bird.  That would be much
harder.

> I also noticed I didn't
> experience the "whoop whoop" of the signal due to polarity rotation that
> some operators seem to experience. This is, I imagine that's what causes
> them to speak in those halting dialogs. 8-0=

I don't know about the whoop whoop you speak of but the reason why people
speak haltingly is that they listen to themselves on the delay.  In normal
speak we hear ourselves speak instantaneously.  With headphones on and on
the bird, we hear our selves maybe 1/4 to 1/2 second later. Hence the
L O N G  D R A W N out words.  I tend to take the headphones off one ear so
I can hear myself!

> 
> Anyway, it works for me and I have found no compelling reason to spend more
> for the circular. Again, based on my experiences. Your mileage may vary - I
> get 42mpg with my Saturn SC2...........

To each their own.  If you are happy with it, there is no reason to change.
Certainly a good preamp will also make up the difference.

73,

Jon
NA9D


-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

"A life lived in fear is a life half lived."

----
Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA.
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org