[amsat-bb] EsHail Microwave transponder satellite

[Edward R Cole]

Comments *** inserted, below:
---------------
Date: Thu, 2 Jun 2016 12:05:13 +0200
From: Remco <pa3fym at amsat.org>
To: amsat-bb at amsat.org
Subject: [amsat-bb] EsHail Microwave transponder satellite
Message-ID: <201606021205.13732.pa3fym at amsat.org>
Content-Type: text/plain;  charset="us-ascii"

Having posted my setup at the end of last month and
seeing that my mail program wrapped line feeds, decreasing
readibility, here again my description and perspective.


Es'hail2 will not differ from any other geostationary broadcast satellite
in a sense that it's a 'full duplex' repeater with some time delay.


I reckon this time delay inhibits actual full duplex operation,
just like the TV reporter for a 'live stand up' in the TV news via
a satellite link having an earplug with all audio except himself ('N-1').

***Why?  Duplex is totally possible - just listening to your own 
retransmitted voice might mess up your mind too much.  I suggest 
probably muting the receiver when you talk (simple VOX controlling Rx 
audio).  Rx resumes immediately upon cease of talking.  Disable to 
tune in your signal using carrier.

====snipped


Feed:
 From my perspective the only challenge lies in the 2.4 GHz uplink.
That is, to make an S-band feed which phase center coincides with the
10.5 GHz LNB (which has its own 'feed' ;-)

***LNB "is" a feed with block down-conversion (e.g. Low Noise 
Block-down-converter)

I've no fear for isolation issues due to the large frequency difference.

***Probably true as wg for 10-GHz LNB cuts off way above the uplink frequency.

I am told that circular polarization (CP) is 'mandatory' for S-band
uplink's in general but can't find a verification for this,
perhaps somebody else?

***Generally, I am hearing that the s/c will have linear antennas 
whose orientation will change depending on your lat-long on 
earth.  Common issue for eme'rs separated by large differences in 
coordinates (term is called spacial loss but refers to polarization 
angle shift with apparent location in respect to the other station).

***CP should solve this for the ground station; but designing and 
building a mw CP feed is not easy.

Anyway, we're amateurs and creative. So when RHCP uplink is too
difficult we make a linear antenna and increase uplink power with 3 dB ;-)

***Huh?  That would only be true if s/c is circular pol.  If it is 
linear you could have a loss all the way to 20-dB depending on 
polarity mismatch (see spacial loss referred to above)

But, I follow the published receiver specifications . . .
Last week I made a 6 turn LHCP 2.4 GHz helix.
(Rule of thumb seems to be 1 turn per 0.1 f/D)
Phase center of such a helix is a bit 'foggy' but from what I read lies
somewhere between the first and second turn.

I've some issues with matching it due to some mechanical boundary conditions
of the helix feed point in conjunction with the mounted downlink LNB and
relative position of the helix.
There is an additional 'transmission line' involved and
a 'normal' 1/4-wave stub doesn't give me the desired return loss.
So, this needs some fiddling, perhaps with a gamma match or so.

***I used a 33-inch offset fed dish on 2.4-GHz with a 6-1/4 turn 
helical feed for AO-40.  I used brass strap for the first 1/4 WL to 
match the feed to coax.  Matching is done by adjusting the height of 
the matching section from the ground plane.  I ended up with 3/16 
inch separation at helix end of matching line.

My approach is, the downlink LNB is mounted in the dish 'as usual' and
looks through the centre of the uplink helix.

***If the 10-GHz feed is generally cylindrical and smaller diameter 
than the Helix this will work fine.  I used a metal tube with heat 
sink covering as center support for my helix.  0.7-inch ID tubing is 
proper diameter for 10-GHz wg.  But not sure how well the typical 
12-GHz LNB will work inside a helix.

***Other geosat designs are for 5-GHz/10-GHz so not sure how well a 
Helix will work there.  Perhaps the 10-GHz cylindrical wg could serve 
as center conductor of air-insulated coaxial feed with a simple 
dipole feed on 5-GHz and 10-GHz horn extending a small way beyond the 
dipole.  This would work better with center-fed dishes of f/d ~ 0.35.

Pictures of the prototype feed arrangement can be provided soon.
They are on a camera not present here at this moment of writing.

***Attachments to Amsat-BB probably will not go thru; post http link 
to the photos, instead - please.

Remco PA3FYM


73, Ed - KL7UW
http://www.kl7uw.com
     "Kits made by KL7UW"
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