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

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@gjcp.net writes:

Ken Owen  wrote:
<snip>
> From: Paul Je [mailto:paul_je@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
_______________________________________________
Sent via  AMSAT-BB@amsat.org. Opinions expressed are those of the author.
Not an  AMSAT-NA member? Join now to support the amateur satellite  program!
Subscription settings:  http://amsat.org/mailman/listinfo/amsat-bb


**************Life should be easier. So should your homepage. Try the NEW 
AOL.com. 
(http://www.aol.com/?optin=new-dp&icid=aolcom40vanity&ncid=emlcntaolcom00000002)
_______________________________________________
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author.
Not an AMSAT-NA member? Join now to support the amateur satellite program!
Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb



AMSAT Top AMSAT Home