[amsat-bb] Re: Help for Humber College Students with ISS Contact
Roger Kolakowski
rogerkola at aol.com
Wed Nov 26 15:38:06 PST 2008
Nice explanation...Thank you!
Roger
WA1KAT
----- Original Message -----
From: <AJ9N at aol.com>
To: <gordonjcp at gjcp.net>; <amsat-bb at amsat.org>; <paul_je at hotmail.com>
Sent: Wednesday, November 26, 2008 11:04 AM
Subject: [amsat-bb] Re: Help for Humber College Students with ISS Contact
> Hi all,
>
> Let me clear up a little bit of what ARISS wants for a school ground
> station. What you do for your own home contact is your business but here
is what we
> want for an ARISS school contact:
>
> 1. We require two complete radio stations, each one 75 watt class or
better
> (we actually prefer over 100). RF amps are OK. The radios should have
the
> ability to go in frequency steps smaller than 5kHz so that the Doppler
shift
> can be corrected (at 2 meters it goes about +/-3.5 kHz).
> 2. The primary radio is to have a circular polarized beam with azimuth
and
> elevation control rotors. We prefer computer control of the rotors.
> 3. The backup radio is to have a vertical and/or eggbeater style
antennas.
>
> Now for a short explanation of why for each:
>
> 1. The need for two complete radios is so that if one radio fails for
any
> reason, the school contact can carry on (it is recommended each radio be
on
> its own 120VAC circuit and UPS if possible). The reason for the 75 watt
class
> is that we want to have as much signal to reach the ISS as possible. The
ISS
> is actually pretty noisy and the radio footprint is very big and it picks
up
> all sorts of interference. So it helps to have as much signal get to the
> astronaut. Throw in the fact that the ISS superstructure is so big now
that we
> have had schools have the signal dropout to almost nothing and you can
see
> that every little bit helps.
>
> 2. The circular polarized beam helps because the signal to and from the
ISS
> can be bouncing off of the superstructure itself and in some cases the
> surrounding ground terrain. As I mentioned above, we have had some
schools where
> the signal dropped out almost to zero. Luckily the signal (sometimes
about a
> minute later) came back up as the ISS changed its orbital position
relative
> to the ground station and thus some of the blockage was reduced. I have
done
> 4 school contacts as control op and I use 5x2 LHCP and 10x2 RHCP circular
> polarized beams with an antenna switch. Most of the ARISS telebridge
stations
> are using something similar. The ISS antennas are basically vertical
> antennas but the signal can be deflected all over the place because of
the
> superstructure. I tend to run my contact on the RHCP beam (but I am
ready to
> switch) but we at ARISS have had some reports where the signal did come
up a bit
> when using LHCP. Those who are really into satellite work know that the
RF
> pattern does change during a contact so it makes sense to be able to
switch
> polarity. And don't forget the ISS radio is running maybe 25 watts (or
maybe 5
> depending on the radio used) and can not do any Doppler correction.
>
> 3. The backup radio is to have a non-directional antenna so that in case
of
> rotor or computer failure, the contact can carry on although it will be
with
> a shortened contact time and the quality may suffer. I have an antenna
> switch to switch between the 2 antennas during a pass as the RF pattern
between
> the 2 antennas is completely different.
>
> The biggest reason for doing what some may think is overkill is this.
The
> hams involved with a school contact are just the messengers. The school
kids,
> teachers, and parents are the ones we have to satisfy and they don't
> understand this ham radio business. They do understand good audio and no
screw ups
> on the part of the ham crew. I always tell the schools that I mentor to
plan
> on 600 to 800 people-hours for 10 minutes of contact time. They usually
> think I am nuts until they do the contact and they often tell me that my
> estimate was too low. Think of a school contact as your worst case Field
Day; not
> so much because of the equipment issues but because of the 600 or so kids
> watching.
>
> Hope this helps a little.
>
> 73,
> Charlie Sufana AJ9N
> One of the ARISS mentors
>
>
>
> In a message dated 11/26/2008 4:15:52 A.M. Central Standard Time,
> gordonjcp at gjcp.net writes:
>
> Ken Owen wrote:
> <snip>
> > From: Paul Je [mailto:paul_je at hotmail.com]
> > Sent: Monday, November 24, 2008 1:17 PM
> > To: Ken Owen
> > Subject: RE: ISS contact
> >
> > Say Ken, we've set up our primary station just fine, but I was
wondering if
> > I could ask for your advice. Well, you see, we've tested the
transceiver
> > that we have (the ICOM IC-V8000), and we can transmit and receive just
fine
> > with it on our circular-polarized HyGain 2m antenna. Also, we did a
VSWR
>
> What kind of antenna? Anything more than a 3-element Yagi will be more
> trouble than it's worth. Bear in mind that I've successfully sent and
> received APRS with the ISS using a homebrew vertical. The higher the
> gain of your Yagi, the more directional it is, and the more accurately
> it needs to be pointed. I find that a 3-element beam is okay for
> handheld use when working portable, and has more than enough gain to hit
> the amateur satellites with 5W from an HT.
>
> > test and our loss is minimal with the 75W transceiver that the ICOM
>
> 75W sounds a bit much, especially into a very directional antenna.
> You're trying to talk to the ISS, not etch your name on the side.
>
> > produces. Ok, so here's the problem. Even with all the proper testing
> > done, we still can't seem to pick up or hear the 166MHz beacon that the
ISS
> > produces.
>
> Are you using a 166MHz aerial for this? Are you sure the beacon is even
> transmitting when you think it is? Your high gain Yagi might well be
> very very deaf outside its intended band. Try making a simple dipole or
> even a two-element beam for 166MHz. With two elements, it will have a
> more-or-less cardioid pattern, so you shouldn't really even need to
> steer it much ;-)
>
> > My classmates and I are a bit worried/stressed out. I mean, just on
last
> > Friday, we did a test and someone drove at least 5km away from out
college
> > and heard us fine with the handheld radio he had. We had a signal
strength
> > of 3+ out of 5. He could've drove out even further, but we felt that
we
> did
> > enough testing to know that any attenuation losses were very minimal.
>
> The ISS is pretty much the classic case of line-of-sight. There's
> nothing in the way, and it's only 200 miles away. There's nothing to
> stop the signal anywhere.
>
> > Well, do you know what the problem could be? Have you heard the
beacon?
> > What does it sound like? Maybe we should delay or advance the rotor by
a
> > few seconds? We're using NOVA software, and it allows us to send our
> > transmission a few seconds ahead or behind.
>
> Use a wider beamwidth.
>
> > Ok, so we have a circular polarized HyGain antenna hooked up to our
Yaesu
> > G5500. Uhm, this might sound dumb but do you know whether we should
be
> > right hand circular polarized or left hand circular polarized? Is the
ISS
> > right hand or left hand on 144.490MHz?
>
> This I'm not sure about. I thought about building a circular polarised
> antenna for ISS and amateur satellite work, but it seemed more trouble
> than it was worth. If you've got the polarisation wrong, it will be
> incredibly deaf!
>
> > I'm trying to research this, but I'm having the hardest time to find
this
> > information out. Oh, also, since our antenna is circular-polarized,
does
> > the way we set our antenna have an effect on our transmission? I know
this
> > sounds confusing, but let me explain:
> >
> > If you looked at our antenna from the front so that you could see all
the
> > dipoles/elements both vertically and horizontally to your view, well,
> should
> > they be perfectly aligned with one set horizontal and one vertical?
Both
> > the vertical and the horizontal are perfectly 90degrees to each other,
> > however, instead of being a perfect cross to your view, the elements
are
> > more like an "X" to your point of view (even though both are perfectly
> > 90degrees to each other).
>
> That shouldn't make much of a difference. Imagine the signal arriving
> like a big corkscrew - the key to the circular polarisation is that the
> signal arrives at one set of elements and then a quarter wavelength
> later arrives at the second. Now, let's imagine we've made our
> circular-polarised aerial by putting two dipoles on a boom, 1/4
> wavelength apart, and connected them by two equal-length lines. The
> vertical one is at the "front" of the boom and the horizontal one is to
> the "back", and the up and left elements of the dipoles are "hot".
>
> Let's pause reality just as a "vertical" peak hits the vertical dipole.
> That dipole now has some signal. Using the single-Planck-time advance
> button on our Worldivo (it's like a Tivo for the fundamental nature of
> the Universe), we'll step through - tick, tick, tick, tick - until a
> quarter wavelength has passed. Now the vertical peak is somewhere above
> the centre of the horizontal dipole - it's picking up no signal - and
> there's a horizontal peak about the centre of the vertical dipole - no
> signal there either.
>
> Step forwards another quarter wave, and there's a vertical dip at the
> cold end of the vertical antenna, and the horizontal peak we just saw
> came in is at the hot end on the horizontal antenna. We now have a
> negative signal on the cold side of the antenna connection (remember,
> both dipoles are effectively in parallel) and a positive signal on the
> hot side of the antenna connection - loads of signal!
>
> If we reversed the direction of the corkscrew, or reversed the phase of
> *one* of the dipoles, then the two signals would cancel out almost
> completely. You can have two signals transmitted in left and right
> circular polarisation on the same frequency, and have *phenomenal*
> rejection between the two.
>
> I should point out that there's quite a lot in that explanation that's
> not entirely true, or at least terribly inaccurate. It's still a useful
> model for getting your head around what seems at first to be a very
> confusing polarisation mode.
>
> HTH,
> Gordon
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