Cross Polarized Inmarsat Yagi?

[JamesWest]

I was looking at The Bald Geek's web page and he had a photo of a "antenna shootout" with various Inmarsat antennas. There was a yagi described as "18 dBi and a length of 1,67 meters".

How to build an L-Band ground station.

Rough notes on how to go about setting up the hardware and software for ACARS decoding.

thebaldgeek.github.io

It happens that I have a person who can make that antenna for me for very little cost. (I don't have the actual construction plans for that yagi yet so any suggestions?)

Then I was thinking would a cross polarized version of that antenna work better? And I could have a yagi made that was 3m long if necessary. Like the Death Star of Inmarsat yagis. haha.

obviously there are other options but I was like to be able to decode Inmarsat voice which seems to need more gain than a my patch antenna provides. And building a yagi would actually be more convenient than a helix for me right now.

so what do you think?

 

[thebaldgeek]

For Inmarsat you really should be using RHCP direct and LHCP into a dish. Looking at the numbers, the 'little' 7 turn helix was pretty close to the monster linear polarized yagi. Depending on where you live, where you need to point to to the sat and the yearly weather, keeping something like that Yagi on station could be pretty tricky. (The gent that did the test did not monitor over the entire 23 hour 54 min orbit of the satellite either, so we don't know if such a high gain antenna would need tracking - in my experience, it will, so keep that in mind).

My 4 turn LHCP helix into a 90cm dish works really well and looks just like a DirectTV setup at a passing glance.
There are tons of 3D printing services around these days - so much so I stopped selling helix antennas off the site (that and shipping was too painful), but they really are super quick easy and clean to build.

But to answer your question, yes you really should build a crossed yagi with a phasing harness to give it RHCP (If you have the gear to accurately build the phasing harness at 1.5Ghz, then.... yeah.... go for it I guess).

 

[scudGB]

lo all
re your long yagi ,i built a 26 turn helical to copensate for it having to be mounted in roof space it was set up for best signal when installed
so it was a bit of a shock nxt time i was on aero section signal was way down ...it was after a bit of digging i found my bird alphasat moves around a fair bit so at some times signal great other times ok is but visably poorer made this little vid with stellerium

may be wise for you to have a look at your intended bird movement before the deathstar yagi built scud

 

[thebaldgeek]

Correct.
Here is the orbit on a graph. (Its a lot easier to see here)

Satellite motion charts for INMARSAT 4-F1 based on TLE data - Jens T. Sætre - http://www.satellite-calculations.com

Generate 26 hours short term and 171 hours long term satellite motion charts for INMARSAT 4-F1 (based on TLE data)

www.satellite-calculations.com

You can view any of the 4 Inmarsat birds.
The period is around 23 hours 54 minutes and of course a long drift over months.

I make sure to plot both the lat/lon of the sat and my C-Band and L-Band message rates from Jaero so I can be sure I am also a peak data. The ebb and flow is due to USA <-> Euro flight density, not tracking errors.



I mention the need for tracking of high gain antennas on my Inmarsat and L-Band pages.

Decoding ADSC, ADSB, ACARS, VDL2, Iridium, HF-DL and other aircraft type messages.

Rough notes on how to go about setting up the hardware and software for ACARS decoding.

thebaldgeek.github.io

 

[scudGB]

NICE 1
i wish i had known that before setting my 26 turn up ,i am attempting to get a 8 turn helical direct set up outside for comparison no housing assosiations here in uk just my other half and she is band enough

 

[thebaldgeek]

26 turn?!!! Yikes.
I franked helixed a 14 turn....


It had enough directivity that I ended up sticking it on the C-band dish to pick up some free tracking.



It was also not fun to support, I cant imagine something twice as long - major congrats for building and tuning a 26 turn monster.

 

[scudGB]

it was hell !!
whole story i found a lengh of plastic packaging tube in work stores that was within 1or 2 mm of diamater req by the online calculator so two mins later it was in van
used masking tape to mark my spacings bought some unfortunatly HARD DRAWN COPPER WIRE hell on earth to bend round my former
then after a few attemps got something i was happy with
set up in loft very primative have a loop of string just past the middle of coil passed through a lanyard round roof joist so i can raise or lower front end elevation from behind it and i swing from left to right like a old style cannon and just sat doing that watching sdr# on laptop
once i was happy string was clamped and reflector was wedged in place then connected up to airspy and a laptop just for spyserver
so nxt time i fired up sig was poor back round now to the bird moves but that took a few weeks to fig out
its well out of line at moment as my teenage lad got to age that toys aint cool so he threw all sorts into loft and push coil off centre
at the moment i have the latest rtl patch antenna facing straight up to build a decent iridium heat map then tomorrow weather permitting gona do same with small helical just to see what appature map i get if any scud

 

[thebaldgeek]

Getting off topic, but out of interest here is my current Iridium coverage.
I have 3 stations in the USA and 1 or two in Europe that feed the site.

 

[scudGB]

OFF TOPIC story of my life
am always off topic have that many projects /things to do my brain never stops (exept for delta covid) well aware of your site on git hub
must admit your stuff and cemexcutter dragon os got me back looking iridium had tryed recently to get gr-iridium running on ubuntu 20.04 no luck then tryed a vanilla ubuntu on laptop no luck so now i have dragon os running on a 8 core ex linux game server under bed that i just ssh into worked more or less 1st try ,,prior to starting to build a heat map i had a go with microp11 iridium live not to bad but i prefer my old way of
letting gr run for time to build up a nice big file then work with it later offline upto now not got mutch but only got all working wednesday so not a lot of playtime since but since in uk got two extra days off for queens party scud

 

[JamesWest]

thank you everybody this is very informative and helpful.

i guess i will tell my girlfriend bigger is not always better.

in the beginning i was also unaware of the movement of the Inmarsats and with a V2 patch I could see the signal strength not being constant over time. i began thinking maybe the wind was blowing or the extreme heat was affecting the patch or something like that was causing a problem so i would jump on the roof and tweet the patch position and lock it off but of course it would drift again. but after monitoring the signal for a few days i saw it was slow repeating cycle.

i guess i will try to build a 7 turn helix. i tried to order a prefab from bald geek last year but I am in Southeast Asia and shipping and customs is really expensive. so now i am looking for a 3D printer company.

i did find the pizza pans on Amazon.

 

[scudGB]

hi james
S.E.A what birds can you see ? as iam in uk alphasat is best for me i have no problems with aero voice with the latest rtl patch and a airspy just with it sat on window ledge of 1st floor also just about have line of site to 54W but very little aero on it or spot beams not pointing my way
alphasat is 38858KM away from my location at this time ,,so are you looking at the best bird bird DISHPOINTER site is a good place to check
not sure on foiliage in S.E.A but in uk my biggest problem is trees and as its moving towards summer here they are all in full leaf
re your 7 turn helix try find somebody with a 3d printer i was given a very small printer ,limit to 100mm but works
i downloded a stl of a helical former and after a lot of trial/error learnt how to cut stl into smaller bits then scale up diameter to try tune it
to aero section printed smaller tubes as internal supports for joints then glued it all with black silicon sealant i used pla you may need something different if humidity high in your location, scud

 

[JamesWest]

hi james
S.E.A what birds can you see ?

the only sat i can see is 143.5E 4-F1 POR but i have a perfect clear view all the time.
Alphasat's footprint is setup to exclude my region unfortunately so there is not the overlap you see in other regions. I can see a few very weak signals from Alphasat with my V2 patch.