Yet another Helix experiment

Got a wild hair yesterday and decided to go out to the shop and look for materials to attempt a Helix antenna.
fired up the calculator and started building. Here is what I came up with real quick.


As you can see it is not very pretty.
I didnt have the SMA bulkhead connector that I wanted to use so I used the only thing I had. an SO-239 with the mating PL-259 coming out of the back of the antenna. Had 3 foot of junk coax on hand and had to use that as well. So all in all there are 4 adapters in line between the LNA and the antenna. And crappy coax. Disconnected the Panel antenna that comes with the kit and put this one inline. It has a beamwidth of about 24 degrees so its a bit more precise of an aiming experiment than the fantastic antenna that comes with the DIY kit. Having said that, I did not spend a lot of time aiming precisely. plugged it in and aimed it in the general direction of the satellite. Here are the results from the OUTERNET UI.
Side note: the refresh on the UI lagged a bit so it was hard to get a good idea of when I was actually pointed in an optimal direction but it worked good enough.

I will connect the antenna to my analyzer a bit later and do some fine tuning. I will also plug it into SDR# . I will post results a bit later…

1 Like

Nice work :slight_smile:
Is that 9 turns? what was the SRN from the same spot with the panel, BWT?

Neil,

Yes sir its 9 turns. my connectors and cable choice are degrading this by about 3dB. the SNR of the kit panel in the same spot was average 5-7 db. (These readings were while on the RPI micro controller. They differ greatly on the CHIP with Skylark.) After the move to Skylark on the CHIP I am lucky if I get 3-4db and the RSSI is a lot worse. I assume it is due to noise but have not tracked it down yet. I believe the CHIP is somewhat susceptible to the noise of my Powered USB in the box with it.

1 Like

@ki4its I found that the USB on the CHIP was just a giant noise source. I didn’t put a spectrum analyzer on it but when I added a 3ft USB A-B extension wound three turns on a snap on core it really choked the noise off in the receiver. I was having issues getting a lock with the patch antenna and SNR in the 2-3 range, after the hack I am getting 6-7 SNR in the same spot indoors looking through a window. I had tried several USB power connectors including a true apple part and it did not make any difference.

I also did a quick and dirty shielding job on the PA. 2-3 wraps in packing tape and bubble wrap then covered the whole mess in two layers of thick Al foil. I thought of doing the same thing with the CHIP but I was holding off until I am sure Skylark is stable.

-Cecil
AA5CE

PS Did you use an online calc, or just the ARRL antenna book?

1 Like

Smart move on the ferrite. I had planned to use a 6" USB cable just to keep the SDR heat away from the CHIP. Will add a ferrite also. I have a SDRPlay that I have copper foiled the case and added ferrites to the USB cable and really simmered down the noise.

1 Like

A agree, I think NOISE is a big issue. How are you measuring noise . What sort of equipment are you using. Or is it trial and error.

I will put some ferrites on my USB power lead to the CHIP.

Why is the noise filtering not built into the CHIP and the RTL Dongle?

Seasalt

@CRCasey
It is indeed the USB on the CHIP. Extension cable and ferrite has eliminated the noise.
I used this calculator to construct the antenna. http://jcoppens.com/ant/helix/calc.en.php

What was your SNR before and after the ferrite on the USB?

Before it was ~3
After the ferrite it is currently sitting at 8.89

1 Like

Quick screenshot of the SNR of this antenna without the LNA attached. The results fall in line with what the design calculator stated I should achieve. I need to tune the spacing of the coils to get it better. (and put it on a good mount. I was holding it in my hand for these readings)

setup was SDR# with an AIRSPY SDR attached directly to the antenna connector. No adapters or coax.
I will do it again later with the LNA inline.

I would be really interested to see how the LNA changes the noise floor.

I have an SDRPlay, I didn’t remember it having the ability to output bias voltage on the SMA. Have you modded your receiver?

-Cecil

@CRCasey

no mod needed for the airspy SDR

Cecil, did you connect the core wrapped 3 ft USB cable directly to a USB wall wart, or plug it into a longer extension USB cord? Ken

Put the core on a USB A-B cable between the CHIP and the SDR receiver.

-Cecil
AA5CE

Oh - - OK. I was looking at the solution backwards. In the Alpha Lantern, I would put the core on the pigtail that comes from the hub and connects to the CHIP.

Making note of this for @zoltan.