Using the outernet ku-band dish for L-band


#1

First: this isn’t completely done but it is working well enough to report on.

I made a helix antenna for L-band and mounted it in the outernet ku-band dish

Here are the main references used:
http://www.uhf-satcom.com/lband/
http://jcoppens.com/ant/helix/calc.en.php
http://www.rtl-sdr.com/building-a-l-band-helical-antenna-for-inmarsat/

I was getting the outernet signal 2-4 decibels better than the patch antenna, generally. I wasn’t being strict about the testing and was also swapping in different lengths of coax and checking differences between the E4000 and RTL-SDR, and I found out later there was something wrong with the CHIP I was using, so even with all of that messing around the helix-in-dish worked at least as good as the patch.

Notes:
-the outernet signal is RHCP (right hand circularly polarized) which gets reflected in the dish so the helix is LHCP

-coax length made a difference. I ended with reusing the 25 ft coax running through the pipe by cutting it down to about 12 feet.

-the better focal point for the helix in the dish seems to be further out than where the LNB was (from just holding it in different positions) so I’m planning on (in a few weeks) extending the arm by about 6-8 inches and adjusting this tuning using SDR#

-I didn’t move the dish from pointing at galaxy 19 (the ku-band satellite) to inmarsat since there is only 1 degree difference between them–97 vs 98. I figured if I could get a signal where it was I’d better not mess with it.


#2

I was wondering when you compared SNRs against the Patch, was the patch free stand like it would normally be or mounted in place of the helix facing the dish?

I was thinking about trying something like mounting my spare Patch facing the Ku dish here. Ken


#3

The Patch was freestanding.
I meant to test it in the dish out of curiosity but just plain forgot to–I guess I had ruled it out as I hadn’t yet figured out a good way of housing it for outdoor weather installation. Here’s a reference I was looking at earlier on mounting l-band patches in dishes: http://jontio.zapto.org/hda1/jaero.html
Halfway down the page there’s a test and some speculation on how well it works with polarization etc.


#4

Thanks - - I’ll try it this weekend and see what happens. Will report then. Ken


#5

I used my C-band Dish for L-band. I tried it with a helical and a patch antenna as the feedpoint. (I had to modify the patch antenna to switch it from RHCP to LHCP).

Using the modified patch antenna as a feed worked as well as the 3-turn LHCP helical coil. Here is a link to the SatelliteGuys thread where I posted photos and screen shots showing the signal strengths: http://www.satelliteguys.us/xen/threads/experimenting-with-l-band-antennas.363533/#post-3952451

I am also planning to try this with a small offset dish. And for the feedpoint I plan to try some modified GPS antennas.

Brett
N6BY


#6

I can confirm that modified GPS antenna (25mm ceramic) do work.


#7

@Syed - I am wondering what brand of GPS antenna you were able to modify? I ruined my first one trying to bypass the filter. Would like to buy another for a second attempt.

My plan is to use the 25mm GPS antenna as the feedpoint on a 12 inch prime-focus parabolic dish. The appeal of the GPS antenna is that it won’t block the signal to the dish like the air gap patch.


#8

We had used a modified Abracon patch. It was retuned for a CF of 1542MHz.


#9

A good example here, on L-Band use the parabolic dish illumination is the problem:

“…For example after discovering that he couldn’t find a 1.2m offset satellite dish which was recommended by the experts on #hearsat (starchat), he went with an alternative 1.5m prime focus dish. Then after several failed attempts at using a helix antenna feed, he discovered that his problem was related to poor illumination of the dish, which meant that in effect only a small portion of the dish was actually being utilized by the helix. He then tried a “cantenna”, with a linear feed inside and that worked much better.”

This augments the problem if some trying to use a prime focused dish for L-Band (instead an offset dish)