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Old 17-11-2015, 16:04   #1
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KA4WJA - Iridium question...

Hi John,

Some sailors complain on these forums that IridiumGO in the current package give spotty performance.
So a commercial external antenna (active or passive) might help.

OR,

Us hams like to build antennas...

I am thinking a 1.6 GHz handheld Yagi.
Use old AMSAT skills like in the link below.

Question:

A. Would this work ? (Satellite voting concerns)

B. Are there any serious legal concerns ?

http://youtu.be/liqqIweZ3eQ

Hamsailor.
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Old 17-11-2015, 16:20   #2
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Re: KA4WJA - Iridium question...

Iridium satellites move very quickly (100 min orbit) and each satellite divides its coverage area into 48 spot beams each of reasonably limited size. Depending on how directional you make that antenna, it might be darn near impossible to aim it and keep it aimed given the rapid spot beam motion. It also may fail to handover as the spot beams go past you since the handover from spot beam to spot beam is typically once per minute or less. I think you're much better off having an external antenna away from blockers that's reasonably omni through a half sphere.
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Old 17-11-2015, 16:57   #3
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Re: KA4WJA - Iridium question...

Quote:
Originally Posted by gjorgensen View Post
Iridium satellites move very quickly (100 min orbit) and each satellite divides its coverage area into 48 spot beams each of reasonably limited size. Depending on how directional you make that antenna, it might be darn near impossible to aim it and keep it aimed given the rapid spot beam motion. It also may fail to handover as the spot beams go past you since the handover from spot beam to spot beam is typically once per minute or less. I think you're much better off having an external antenna away from blockers that's reasonably omni through a half sphere.
A 10 element Yagi beam for 1.6 GHz would be only about .5 meter (1.5 ft) long.
It would give a 60 degree fan beam. (Fan, not cone.)
Not too sharp to be impossible to point....
The Iridium satellites move North to South and South to North in a polar orbit.
Once you found one (5 bars on your smart phone app), just follow it north or south as in in the above video.
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Old 17-11-2015, 17:20   #4
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Re: KA4WJA - Iridium question...

Quote:
Originally Posted by Hamsailor View Post
A 10 element Yagi beam for 1.6 GHz would be only about .5 meter (1.5 ft) long.
It would give a 60 degree fan beam. (Fan, not cone.)
Not too sharp to be impossible to point....
The Iridium satellites move North to South and South to North in a polar orbit.
Once you found one (5 bars on your smart phone app), just follow it north or south as in in the above video.
But why would you want to? A standard quadrifilar antenna gives you adequate gain and a nice wide pattern directed towards the sky. No aiming necessary, just put it in a reasonable location.
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Old 17-11-2015, 17:49   #5
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Re: KA4WJA - Iridium question...

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But why would you want to? A standard quadrifilar antenna gives you adequate gain and a nice wide pattern directed towards the sky. No aiming necessary, just put it in a reasonable location.
Would you receive a constant 5 bar signal ?
Important for constant high data thruput.
Does the pug on the GO give that for a full satellite pass ?
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Old 17-11-2015, 18:08   #6
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Re: KA4WJA - Iridium question...

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Originally Posted by Hamsailor View Post
Would you receive a constant 5 bar signal ?
Important for constant high data thruput.
Does the pug on the GO give that for a full satellite pass ?
It may or may not help. I'm talking as someone who was one of the engineers who worked on Iridium BTW (20 years ago). A 60 degree dispersion angle makes it easier to aim, but it may not really give you much gain uplink.

The real issue downlink. It's not just the satellites themselves - each satellite breaks up its signal into 48 spot beams, each of which is roughly analogous to a cell in a cellular system. As a satellite passes overhead you handoff from beam to beam, eventually handing off to a beam on a different satellite. This happens every 50-60 seconds. The algorithm for directing handoff is driven by reports from the handset of adjacent spot beams. So artificially attenuating adjacent spot beams via a directional antenna can cause a connection drop if the satellite thinks there is no other viable beam to hand over to.

The upshot: I think best case any improvement would be very marginal, and that's assuming you can hand track the motion of the satellite and the beams optimally and don't mess up the handover algorithm. If not, you may find it works worse.
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Old 17-11-2015, 18:23   #7
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Re: KA4WJA - Iridium question...

Mess up the handover algorithm...
That is what I was afraid of.

So the helical antenna looks best as designed...

Some active antennas cost $1000.

I am suspicious that the Yagi/quad/dish antennas might have been tossed in R&D due to consumer acceptance concerns.
Hams will accept those solutions.
Some have old AMSAT skills.
Enough reason to keep experimenting with this.
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Old 17-11-2015, 18:44   #8
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Re: KA4WJA - Iridium question...

Quote:
Originally Posted by Hamsailor View Post
Would you receive a constant 5 bar signal ?
Important for constant high data thruput.
Does the pug on the GO give that for a full satellite pass ?
I don't know about the signal strength difference between a handheld yagi and a fixed quadrifilar or turnstile antenna. If you can truly aim the yagi properly it's going to be better, but by how much? Go ahead and try, but when I'm on a pitching sailboat just trying to hang on, the last thing I want to do while I'm working at the computer is simultaneously trying to keep a beam antenna pointed at a moving target. In the rain, probably. I'll settle for the nice broad pattern of my fixed antenna, thank you very much!

But you seem to be saying that the bigger issue if the built-in antenna on the GO. If you can connect an external antenna then even the fixed external antenna is going to give better performance.

And according to what gljorgensen is saying about handoff issues, the narrow beam of the yagi might make things worse.

By the way, after reading your comment about tracking (in another thread), I turned my Raspberry Pi into a WSPR transmitter! Lots of fun. I will post more in that thread.
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Old 17-11-2015, 20:00   #9
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Re: KA4WJA - Iridium question...

The GO is a good product.

I think consumers are expecting it to work as good as a cell phone.
More interested in achieving the full advertised data transfer.
(2.4 Kbits/sec)
Need 5 bars for that, I presume.

Thanks.

BTW, WSPRRy Pi *IS* fun !
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Old 18-11-2015, 09:55   #10
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Re: KA4WJA - Iridium question...

I have used a GO extensively over the past year for coastal sailing in Central California (including in Santa Cruz Harbor and SF bay) as well as during this year's Transpac. It was spotty with just the built in Antenna.

Once I got around to installing the external antenna (that came with this bundle: Iridium GO! Bundle) it worked like a dream. During the Transpac, I sent several photos a day that were posted on Facebook by the Admiral.

The external antenna makes the difference between the GO being a so-so unit and it being stellar to use.
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Old 18-11-2015, 10:00   #11
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Re: KA4WJA - Iridium question...

Quote:
Originally Posted by Hamsailor View Post
More interested in achieving the full advertised data transfer.
(2.4 Kbits/sec)
Need 5 bars for that, I presume.
I will tell you that with the regular iridium handset - the actual achieved data rate is not highly correlated with 5 bars. This was confirmed to me by the folks at GMN when I asked about it. I am sure some of you technical folks could explain why this is - but it is a fact that chasing 5 bars (which obviously not bad) is not the sure path to max data rate.

There are programs that track the iridium constellation, and there are suggestions that it is more important to time your data bursts to moments of attractive geometry. I dont know for sure if that is true or just myth, but it has been suggest to me by some 'experts'.

In my case, the condition of the antenna coax and its connectors made the most obvious practical difference to data rates.
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Old 18-11-2015, 18:46   #12
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Re: KA4WJA - Iridium question...

Hamsailor,
1) Since you posted a question to my callsign, I thought I should at least respond...

2) As others have already answered, your idea is a going to be an effort in futility!!
Quote:
Originally Posted by Hamsailor View Post
A 10 element Yagi beam for 1.6 GHz would be only about .5 meter (1.5 ft) long.
It would give a 60 degree fan beam. (Fan, not cone.)
Not too sharp to be impossible to point....
In brief, gjorgensen gave you good advice...you're best bet is to get a broad-beamwidth antenna (Omni above the horizon) with a clean pattern, mounted out in the open, away from other metallic objects / structures...
L-band patch antennas are very cheap to make, and are surprisingly good at this application....hence why they're recommended / sold for this...


3) And, as Evans (and ssscruz and gjorgensen) elegantly put it....it is this small, simple, wide-beamwidth antenna, out in the clear AND the coax / connections, that makes or breaks the system!!
And, unless you have a really big boat, requiring more than 50' of coax....there is no need to use an "active" antenna with Iridium.....this is, of course, not the case with INMARSAT, but for Iridium, passive antennas work very well!!!



4) As for 1.6Ghz yagi's....
I built some 1691mhz "loop-yagi's", about 30 years ago for GOES weather sat reception....and they worked great....but that's a stationary target, and I was on land!!!
(I also built 12-turn helix's for 432mhz and 1296mhz, many years ago...with good results....but these too, are way too high-gain / too narrow pattern for Iridium use on-board...)


With the multiple beams, having an antenna with a clean pattern (low sidelobes), becomes important as well....and HB yagis are not known to have the cleanest of patterns...

And, I'm not sure how you calculate a 60* beamwidth for a 10-ele yagi?
'Cause the real figure is about 35* - 40* (3db beamwidth), depending on design / construction....




5) Evans, as for using on-line calculators to determine the best time to use Iridium?? This is a new one to me!
(I know this is an absolute for Globalstar....but never heard of anyone propagating this for Iridium...)




Fair winds...

John


P.S. On a side note, I've also designed/built antenna systems for AMSAT use, etc. (as well as for 2-m EME)....and my cheapest/simplest antenna was ironically one of my favorites....
Over 30 years ago, I built a 2-m turnstyle using 8 ga copper wire, over metal window screen, all supported on a 2x2 wooden frame...
I used this antenna (and if you don't mind some bragging, my operator skills) to successfully complete 2-way contacts with the first ham in space, W5LFL, in the SpaceLab in the Shuttle....one of only 300 stations worldwide to do so, and one of < 100 that did so "on random, non-sked"....and one of only 3 stations in Florida to do so...(and one of those others was a sked with Motorola in Ft. Laud)

The reason I'm posting this is not be a bragging, obnoxious, know--it-all....but rather to point out that fast moving comm targets, using line-of-sight paths, require one of two things:
a) clean, wide-pattern antennas....(cheap and easy to do on-board a boat)
---or---
b) precise, accurate, instant, aiming of antennas....(a pain in the ass to homebrew and do on a boat)

I hope this helps!!
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Old 18-11-2015, 19:05   #13
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Re: KA4WJA - Iridium question...

Quote:
Originally Posted by ka4wja View Post

5) Evans, as for using on-line calculators to determine the best time to use Iridium?? This is a new one to me!
(I know this is an absolute for Globalstar....but never heard of anyone propagating this for Iridium...)
I primarily heard this from johnathan selby - the guy who wrote weatherfax2000 and skyeye. As I said, I have no idea of its validity, but Johnathan is a damn bright and practical guy.
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Old 18-11-2015, 19:38   #14
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Re: KA4WJA - Iridium question...

Evans,
Not disputing what Jonathan said....I've never talked to him, so I have no knowledge to make any judgment about that...


But, I do know that Iridium satellites shutdown some beams as the near the poles, in order to keep the satellites and beams from overlapping / interfering with each other...
And, I'm wondering if this is where his recommendation comes into play??? As this instance / use at very high latitudes, is where handoffs can become more critical...


BTW, this Iridium operational design of shutting down some beams when the satellites near the poles, is why the "myth" of Iridium working better near the poles and worse nearer the equator is really a myth!!!
Actually, those old Moto engineers designed a very elegant system....and remember this was all designed > 20 years ago!!!
And still works great today!!!

System concept and initial design in 1987....build-out in the mid 90's!!!
About the same time frame of HF-DSC in the GMDSS, in the mid to late 90's!!
Always makes me smile when I read sailors comment on the "modern" sat comm technology and the "antiquated" SSB technology....


Fair winds...

John
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Old 18-11-2015, 21:36   #15
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Re: KA4WJA - Iridium question...

Thanks for all the insightful responses.

Here is your Iridium satellite position map.
Yes, you need the Internet for this.
Kind of catch 22....
Notice how Earth rotates inside the Iridium cage.
The satellites travel from North to South at first, then travel South to North later.
Yep...

Iridium position map:
https://in-the-sky.org/satmap.php
Play with Select Satellites.

Hamsailor.
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