[Date Prev][Date Next][Thread Prev][Thread Next] - [Date Index][Thread Index][Author Index]

Re: Helixes and VO-52 (Was: Polarization of HAMSAT downlink)



Hi Joe, IW7ECJ

Owing to the large difference in polarizations for LEO satellites novadays
you actually  need switchable RHCP and LHCP antennas both for the
uplink and downlink for maximum performance particularly if you are
interested in digital satellites.

This configuration requires a considerable money investment particularly
in our country with wiring complexity for the antennas wich is not advisable
to beginners.

If you use only one fixed sense of polarization say RHCP than you lose
about 20 dB when the opposite polarization is required and actually
this configuration is not recommended but it was recommended
before beginning from OSCAR-6 about 30 year ago when RHCP was
standard for AMSAT

Owever to start with moderate investment and good efficiency a linear
horizontal polarization is advisable for uplink and downlink because
in any circumstance if RHCP or LHCP is required in that particular
moment you lose only 3 dB with a little more QSB and this is acceptable
considering that manually switching RHCP/LHCP for the uplink and
downlink during a pass searching for the best signal it also make the life
hard to the LEO operator and probably to lose 3 dB is better.

If you use linear horizontal polarization with high gain antennas in
comparison to your actual Dual Band Vertical you will improve your
EIRP in uplink and you will increase  your downlink efficiency as well
because your G/T ratio is greater.

In addition linear horizontal polarization allow you to use the same
antennas at maximum efficiency with tropo traffic due of polarization
match with the others and this is what many VHF/UHF stations do
using high gain linear antennas for satellite but only for contacts during
orbits with low satellite elevation.

If you add to the above linear antennas a rotator for the elevation than
your EIRP is largely greater than using your actual Dual Band Vertical
Collinear so that it will be easier for you to hear your own stronger
downlink as soon you get again the mike and this will semplify your
tuning to manually compensate for Doppler particularly in SSB where
the automatic doppler compensation is difficult to be obtined due to a
lot of many factors all togheter.

In conclusion to be fully operational with the best compromise of money
of investment of technical complexity and operating efficiency I suggest
you to forget your actual Vertical Collinear as well the QFH and put two
high gain linear antennas one for 2 meters and the other one for 70 cm
with any elevation rotator so that having a better uplink/downlik you will
forget the need of having a precise doppler compensation to solve your
actual problems.

At this point you will say why I am using switchable polarizations instead
of linear

My answere is that switchable is better for satellite but you says that a
full blown Sat station with crossed yagis and switchable polarity is out of
the question now so that the above linear arrangement supported by the
above technical discussion is the way to go.

Actually I am operating only FO-29

Best 73" de

i8CVS Domenico


----- Original Message -----
From: "IW7ECJ, Joe" <iw7ecj@libero.it>
To: <AMSAT-BB@amsat.org>
Sent: Saturday, July 02, 2005 10:05 AM
Subject: Helixes and VO-52 (Was: [amsat-bb] Polarization of HAMSAT downlink)


> Ykes!!! Domenico and all experts out there... does this mean that my idea
of
> building a couple of QFH to start with the sats, won't do any good to my
> chances of hear better DL on VO-52??? I mean, of course, without modifying
> the construction of the 2M Helix which is, as I understand, devised for
the
> majority of the Sats already in orbit... :-(
>
> A full blown Sat station with crossed yagis and switchable polarity is out
> of the question now, so what is advisable to do to start with moderate
> investment?
>
> I've made a bunch of QSOs on the VO-52 using my poor Dual Band Vertical
> collinear, but it's clearly not the right way to get on the sats...
>
> Any hint would be very appreciated!
>
> 73 de Joe, IW7ECJ
>
> PS... Dom... I don't hear you anymore calling on VO-52... what happened?
>
>
>
> > -----Messaggio originale-----
> > Da: owner-AMSAT-BB@amsat.org [mailto:owner-AMSAT-BB@amsat.org] Per conto
> > di i8cvs
> > Inviato: venerdì 1 luglio 2005 4.27
> >
> > I already anticipated to you that following my observations
> > the polarization of downlink for HAMSAT is LHCP in 2 meters.
> >
> > Best 73" de
> >
> > i8CVS Domenico
> "amsat-bb" to Majordomo@amsat.org
> ----
> Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
> Not an AMSAT member? Join now to support the amateur satellite program!
> To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org
----
Sent via amsat-bb@amsat.org. Opinions expressed are those of the author.
Not an AMSAT member? Join now to support the amateur satellite program!
To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org



AMSAT Top AMSAT Home