Antenna comparisons

SatNOGS
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Summary (tl;dr): This week I’ve been testing various antennas from the same location on the same observations, directional az/el yagis vs fixed low-gain yagi vs discone. A fixed yagi pointing up produces great results but still not yet strong enough to decode much on data links in my configuration. Keep reading for more.

I stumbled across station 488 by @W7KKE a while back, and he seems to have great success with a 3 element UHF yagi pointed straight up (plus a low noise amplifier), using a high “minimum horizon” of 40 degrees. This led me to wonder if our conversations about fixed stations using omni antennas such as turnstiles is not the best way to go about things.

So I got to testing this past week, with the following configurations (as of 2019-05-27 if you’re looking through my observation history):

Station 2: Az/El rotation, M2 cross-element yagis on UHF and VHF fed into an MFJ diplexer before a low noise amplifier, then 100ft of LMR-400 into the SDR (which is typically USRP B200 Mini, but more on that later)

Station 300: A Cushcraft A270-6S 3 el UHV + 3 eh VHF yagi antenna, pointed straight up (like station 488), fed into an rtlsdr with 5 ft of RG8. No LNA until 2019-06-11 15:00, more on this later…

Station 767: A Diamond D130J wideband discone antenna fed into an rtlsdr with ~25 ft of RG8. This is my “Control” for the experiment, and should under-perform the rest.

All 3 stations set to a 40 degree minimum horizon, and initially all 3 stations set to capture the same observations from the same location.

Some initial results (Az/El M2 yagis, usrpb200, LNA … vs dual vhf/uhf yagi fixed … vs discone):

Unisat-6, 3 min 75.0° pass
Station 2 (18 pkts decoded)
Station 300 (clear visible packets, no decode)
Station 767 (barely visible)

NOAA 18, 4 min 81.0° pass
Station 2 (clear image decode)
Station 300 (some artifacts from the beam edges)
Station 767 (artifacts throughout)

FOX-1B, 3.5 min 64.0° pass
Station 2 (38 DUV frames, good audio almost throughout)
Station 300 (2 DUV frames, some spots of good audio near center of beam)
Station 767 (no DUV, one spot of weak audio near horizon)

There are a lot more observations from this portion of the test, click here.

Is there a USRP B200 advantage over the RTLSDR?

At this point I took the discone offline so I could use its rtlsdr on Station 2 to see if there was an unfair advantage with the USRP B200? Summary: No noticeable difference.

RTLSDR vs RTLSDR (Az/El M2 yagis, LNA … vs dual vhf/uhf yagi fixed):

Unisat-6, 3 min 69.0° pass
Station 2 (15 pkts decoded w/ rtlsdr, compare to obs 706516 above for usrpb200)
Station 300 (clear visible packets, no decode)

Is the LNA an advantage?

Most certainly yes. I dug up a PGA-103 based LNA and put it in front of the rtlsdr on station 300 as of 2019-06-01 15:00, dropping the gain back down to default. However, I’m still not decoding data frames as well as station 488 is. At this point given the small horizon I’m hitting a bit of a drought before more data downlinks pass, but here are some initial results:

SO-50, 4min 75.0° pass
Station 2 (good audio almost throughout)
Station 300 (decent audio through about 75% of the pass)
(resist the urge to compare this with the audio of FOX-1B above, as they downlink on different bands)

NOAA 19, 3.5 min 57.0° pass
Station 2 (clear image, no difference from the results of NOAA 18 + usrpb200 above)
Station 300 (a little bit of artifacts, but better than the NOAA 18 pass with no LNA above)

I have some other observations between the two in this configuration that you can dig through here.

At this point I’m anxious for another Unisat-6 pass to see if it would decode now but at such a small window/horizon that’s not going to happen for a while. Hopefully I can get @W7KKE to chime in with the secret sauce to his fixed setup.

Interested in your thoughts and ideas here, and other possible combinations for testing. I don’t have much experience with the PGA-103 LNA so at some point I may want to swap it in on Station 2 to see if it under performs compared to what I’ve got in there.

Cheers!

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The only difference with my station 488 is the SSB preamp. I had a spare from another project so installed that with the old UHF Yagi that I pared down to three elements. So, not too many secrets in my “secret sauce”. For a fixed antenna, I’ve been pleased with the results.

For some reason I can’t decode any packets from STRAND-1 even though the signal looks good in the waterfall.

(I’m also feeding data to SatNogs using a steerable Yagi and 2 meters and 70 cm, but use DK3WN’s Online Telemetry Forwarder.)
Ken, W7KKE

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My experience with receiving antennas over the years have come to a very similar conclusion. Discone pretty much useless for satellites. High gain verticals OK only to hear the satellite when it first pops up over the horizon. Omnidirectional antennas with a good preamp seems to be the way to go.

I experimented with a few. Turnstiles work but Eggbeater is better. Eggbeater with a reflector slightly better. Better still my home-brew QFH antennas. I use these on my ID-568 and ID-724 stations with LNA4ALL pre-amps. Worth the money. Home-brew pre-amps not successful.

I also found my results using the same antenna, LNA, SDR are much better using them in stand alone mode without SatNOGS on Gqrx or SDR#. I can’t understand why this is the case? Also in this mode I only need 10-20db or so of gain for good decodes.

For example I get an almost perfect weather map on NOAA Satellites from 10 degrees above the horizon. (Tall trees). Better still with SDRplay and SDRuno.

Bob, vk2byf

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Really interesting comparison. I will have to look at my own antennas now :slight_smile:

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Thanks for this post, will look at my antenna choice for an upgrade

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Good afternoon Bob,

For STRAND-1 you have to disable “non-AX25 filter” if you are using UZ7HO.

73 Jan PE0SAT

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USRP B200? Summary: No noticeable difference

Very interesting. I’ve been playing around with an AirSpy HF+ on my station (unfortunatly VHF only), which is limited to a pretty bad antenna due to asbestos-issues on the roof. Still, I’m getting some decent results, which I can’t say about the previous RTLSDR+LNA setup.
You don’t happen to have (or can borrow) an HF+ to do some more testing with?

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I believe that if the station is located in a quiet environment, it is the LNA, either external or tuner, that makes the difference. It might be that Airspy HF+ is better in this aspect.

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I swapped in an Airspy R2 in #300 a couple of days back - no real improvement over the rtlsdr, in fact a couple of modes don’t work out of the box.

(note - to do this on a rpi I had to edit the sample rate down to 2.5msps which airspy claims is ‘experimental’)

Going to test the gr-soapy scripts next.

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There’s quite some difference in the small signal performance between R2 and HF+… I had both in my station and the HF+ is significantly better… (after removing the LNA, it’s noise was higher than the SDRs…)

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