[amsat-bb] Re: Help for Humber College Students with ISS Contact
Gordon JC Pearce MM3YEQ
gordonjcp at gjcp.net
Wed Nov 26 02:13:44 PST 2008
Ken Owen wrote:
> 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
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
> 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.
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