Two antennas one SDR

SatNOGS Hardware
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Hi,

Total noob question, but how are people running multiple antennas on one ground station? Im assuming some type of coax T or splitter but I want some clarification as I am building a UHF antenna to add to my VHF ground station.

Could some of you please share your setups?

Thanks in advance!

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Hi @fobter, good question.

You use a diplexer. It splits the path and has a filter for each frequency. It is also setup so that each port is at 50 ohms impedance for the given frequencies. A T splitter won’t do that and won’t be as good.

Here is one I built some time ago.

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You can see they are quite simple but you need suitable test equipment to align them or to optimize the parts if surface mount components are used.

You can of course buy them online and they are cheap for low power, which is all you need for receive. You might see them incorrectly called a duplexer. Arrow antenna does that as I remember.

73
Chris
G0KLA / AC2CZ

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Thanks a ton!

Do you know of any papers that could provide some more information? I would like to go down the DIY route and actually understand what I’m doing. If not do you recommend any diplexers available in the US for VHF & UHF?

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I followed a design from PA2JT: 2m / 70cm diplexer

But if I was building it now I would use surface mount parts. Here is a more modern design that was in the AMSAT journal: DIY 2m / 70cm Diplexer for Satellite Work - KW4FB. You can order the PCB or could probably make it on a prototype board for SMD parts, as long as it has a ground plane.

Note that if you do not follow the layout exactly then you might have to tweak the part values for best performance. If you don’t have suitable SMD parts then order a “sample book” of inductor and capacitor values.

73
Chris
G0KLA / AC2CZ

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Thanks a ton, I think im going to try out JLC PCB. Haven’t worked with them yet but this gives me a reason haha.

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I had a spare diplexer so it was quick to hook up.
But then spent hours and hours trying to get the software side to work. I needed different gain values for the different antenna setups and SatNOGs would no way switch settings depending on the pass.
Perhaps things have changed with the new updates?

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Doesn’t look like anyone has implemented support for different gain. The feature is tracked in Different Radio RF Gain on VHF and UHF (#417) · Issues · librespacefoundation / SatNOGS / satnogs-client · GitLab

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If you have 2 antennas, the probably more useful solution (from a network perspective) is to turn them into separate SatNOGS stations. Obviously only really useful for Omnidirectional antennas, not a shared rotator system.

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Hmm. And there’s the challenge for the SatNOGS client development team:

  • recognise the frequency band in which the next pass has been scheduled (i.e. VHF, UHF, S-Band … etc.)
  • include the settings option of controlling a diplexer using either DiSEqC commands, a control voltage or some other digital message, depending on the scheduled frequency band AND/OR
  • directly selecting one of two (or more) connected SDRs with their own dedicated antennas for that particular pass
  • either simultaneously controlling just one rotator or selecting and controlling the appropriate rotator via USB for that particular frequency/rotator/antenna/SDR combination.
    If I could program, I’d willingly help, my coding ability stopped at BBC basic :wink:
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Hey, I’m using an Elk Antenna. Dual-Band Log Periodic Antenna or LPDA (log-periodic dipole array) VHF/UHF. 2 Meter gain is 6.8 dbd (8.9 dBi) and 440 Gain is 7 dBd (9 dBi). Set up on a 360° Rotator, Azimuth only with a set angle of 30°. There is no diplexer or switching needed. Station 1775

When you set up your account with the antenna in edit you can add the field of frequency range. When you schedule passes only those Sat frequencies will be given as a choice to schedule. I also have a separate station with its own antenna for Noaa, which gets little attention from myself so far. The Azimuth is controlled by wifi interface manually and/or soon to be implemented PstrotatorAZ on com port through SatNOGS client. Keep asking questions, it’s a great place for ideas and continuing contributions to the community.