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

Joe and Domenico,

I can agree generally with what Dom has said.

For the current crop of Leo sats, one does not need large-long, high-gain
antennas for good operation (except maybe for good reception of AO-7; I've
not operated on it so others can comment what it takes).

Up until recently, I had a 3-element yagi (the VHF portion of my Arrow
antenna) and a 11-element UHF yagi on my az-el crossboom (using a B5400
rotator).  One could even mount a fixed crossboom with yagis tipped up
about 20-degrees in elevation and it will work pretty good over most of Leo
passes (see Bob Bruninga's comments on this approach).

I mounted my two small yagis in vertical polarity for ease of mounting on
the crossboom.  I can understand the suggestion to mount them horizontal if
you might use them for non-satellite operations in parts of the world where
terrestrial operation is horizontal (in Alaska all VHF+ is vertical
polarity because most folks migrate from FM to SSB and vertical antennas
predominate on FM).

For satellite operation with a linear pol. antenna it matters not whether
you chose horz or vert.  If the satellite is circularly polarized you will
experience up to 3-dB penalty using linear antennas but that is really not
a big deal with strong signals from Leo's (evidence all the HT crowd using
whips).  At times when the satellite squint angle is high the signal will
be elliptical pol and you may get a little less than 3-dB loss at times.
You probably will see QSB on the signal using linear pol as most satellites
are spinning slowly and that can impart "spin modulation" (fluctuation in
signal strength).  But even folks using CP antennas see this at times.

I still have a 2m 1/4 WL mag-mount whip on a square sheet of steel on my
roof for reception either at 145 or 435 MHz.  It works pretty fair.  It
will work for uplink as well (be sure to remove or switch around any
preamps if you use it for uplink).

I have now begun to change over my satellite antenna system to prepare for
Heo operation later on P3E:  I now have my M2 436CP42 (21-element x-yagi -
RHCP) back up; took down the 3-element 2m yagi and will replace it with a
KLM-22C (switching CP) later this month; retained my 33-inch dish for 2401
MHz.  I will (soon) remove the 1296 45-loop yagi and it will be installed
on my eme tower for better terrestrial use (it still will be usable for
satellite as the eme array is az-el).

But I digress!  My point is using small linear yagis is perfectly
acceptable for Leo operation.  And a fine move up from vertical omni
antennas.  Later one can consider high-performance CP antennas if they have
the funds and real estate.  Rather than investing in switchable pol
x-yagis, I would suggest one "first" invest in good preamps at the antennas
for good reception (they need to be switching types if you also plan to
transmit on these antennas).

When you can hear the satellites well, you will work lots of stations!
High-gain switching-pol x-yagis will be needed when we get Heo's back in
orbit (if one has the money, then that will certainly prepare you for P3E
and Eagle) and still be great for use with Leos if you also have a good
az-el tracking system.  But one does not need them with the current sats.

73's Ed - KL7UW

At 04:47 PM 7/2/05 +0200, i8cvs wrote:
>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
>> 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
>> 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
>> ----
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