Nice explanation...Thank you!
Roger WA1KAT
----- Original Message ----- From: [email protected] To: [email protected]; [email protected]; [email protected] 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:
- 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:
- 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.
- 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.
- 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, [email protected] writes:
Ken Owen wrote:
<snip> > From: Paul Je [mailto:[email protected]] > 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 _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite
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