No signal with Helical antenna, at all

Hello guys!
I’ve just get my Dreamcatcher and installed Skylark OS.
I’m HAM, but i dont have any l-band antenna, so i make for test easiest and fastest Helix antenna from stuff wich i have in balcony: Pve pipe, ф3.6mm copper wire, copper plate and BNC connector. Antenna is not perfect, but it pretty wideband so must work.
When i connected all stuff to my dreamcatcher its shows me random SNR -0.5 to 2.0 fluctating. Antenna mode swiching or swiching the active/passive connector don’t help.
To test antenna i connected it to rtl-sdr v3 dongle, set up gain to maximum and i can see some strong sats on 1.5ghs, not outernet on 1546.25 because no LNA here.
Tell me please, where can be an issue in my case? And maybe i can somehow run rtl_tcp to connect from sdrsharp and test receiver?

Best regards, Sergii.
Crimean peninsula.

What polarization do you think your helical antenna has? RHCP or LHCP?
Why might this be important?

–Konrad, WA4OSH

Take a close look at this 9A4QV patch Antenna
It works well on Outernet. Is it RHCP or LHCP?

–Konrad, WA4OSH

Thank You, Konrad.
Mine might be circular left hand polarised :confused:
Dont know why i did it, but i remember that read our signal is lhcp, so now there is about 30 dB loss :frowning:
I dont need compact one as patch, so ill try to make 13 dBi, it might be +6-7 dB more than patch one, isnt it?

Exactly! Just wind it again RCHP. I think it will work much better.

Should you want to refine the design, include a 1/4 wave stripline as a matching section to match the helical antenna, which has an impedance of about 150 Ohms. I have not done this myself or the complete research to get the spacing from the reflector - ground plane as well as the thickness of the conductor.

–Konrad, WA4OSH

Here is an interesting article that discusses antenna polarization:
http://www.qsl.net/sv1bsx/antenna-pol/polarization.html

It also explains the matching section between the 50 ohm feed and the antenna that you are trying to match.

–Konrad, WA4OSH

Or just gradually bring the last 1/4 turn of wire very close to copper reflector before soldering to center pin of connector. Be sure connector flange well grounded to plate.

1 Like

@donde Agreed. It’s hard for someone without a VNA (Vector Network Analyzer) to set that matching section perfectly. This will work.

Better impedance matching means better reception (as well as transmission).

–Konrad, WA4OSH

donde, Konrad_Roeder, thanks guys. I have a lack of time last days to rewind my helix, and i know that i need to solder copper foil triangle for 1/4 torn orput it parallel (1.2mm) to ground (reflector) for mach 50 ohm resistance. Ill try to do it soon as possible.
Also, today i got 32db Cirocomm caeramic pach based GPS antenna and removed filter with fan. And… nothing.
I dont have any reception with my dreamcatcher, but on 1575 it works great on my v3 dongle :frowning:
Can you tell me how i can test dreamcatcher receiver? Somethig like rtl_tcp will be just great.

Buy the way, ive ordered limesdr_mini, and i hope i can make the VNA with it, bacause people did it with full version.

@donde
I left my DreamCatcher outside during a rain shower. It’s not worked since. I suspect that it shorted out.

With that aside, a VNA is usually used to analyze passive components with two ports, or an active component whose net gain is zero (because some attenuators were inserted). Be VERY CAREFUL in hooking up a VNA to an active circuit because VNAs ports are very sensitive to a DC bias on the input and too much power. The commercial ones are very expensive. their annual calibration is expensive, and their repairs are astronomically expensive.

I have not used my signal generator and scope to my dreamcatcher because I don’t have a schematic. I have back-burnered the project because I don’t know the outcome of the big announcement today.

But assuming I wanted to fix it, there are several steps that I would take.

  1. Is the power supply working?
  2. Do the chips all have the needed power supply voltages?
  3. Does the CPU look like it’s working? There’s a heart beat LED that tells you if its running.
  4. Verify that it’s running the right software.
  5. With a small signal, follow the receive chain from the SMA LNA_BYPASS antenna connector past the mixer and the A/D sampler. The rest is software. The ARMbian load can run several programs that could help you debug the hardware, but this too is a rather closed system.
  6. If this works, debug the SMA_LNA input and see if the LNA and SAW filter are working. The LNA is extremely static sensitive. It’s easy to destroy with an accidental ESD discharge.

Depending on what today’s announcement is, my DreamCatcher may become a wall decoration and replaced, or something I will want to repair.

–Konrad, WA4OSH

PS … I will watch the VNA YouTube when I get home tonight. There’s a lot more to a VNA than just an SDR.

–Konrad Roeder, WA4OSH

Well, the YouTube did not surprise me much.

  1. It’s not a two-port VNA, but a one port VNA.
  2. The reference cal is against a short. Real one-port Network Analyzers use three standards so they can solve three equations with three unknowns.
    a) A short
    b) A 50 Ohm load, and
    c) An open connection.
    (A two port analyzer requires a forth calibration – a through measurement)

Hopefully, the Python program can be tweaked to incorporate this.
One port calibration equations

  1. Normally, the feeder coax to the antenna is calibrated out. The standards are applied at the end of the coax so that the base of the antenna becomes the measuring point.

It’s a matter of understanding that the feeder coax is not part of your antenna and can skew your results.

  1. It was not clear to me if they used a directional coupler or a return-loss bridge. I suppose for the way they are making the measurement, they will only need a return-loss bridge. The forward measurement from a directional coupler is not needed.

It will serve as an antenna analyzer within a fair degree of accuracy. Finding a coupler/retturn loss bridge that is flat across several (3-4) octaves can be difficult. They’re around.

–Konrad, WA4OSH