Home made Helix Antenna

Inspired by @rtlsdr4everyone 's blog post, I decided to throw together a Helix antenna for L-Band Outernet (and Aero etc.) The post is here http://www.radioforeveryone.com/p/easy-antenna-for-outernet-l-band.html

I looked at the calculator that is referenced there and I had:

  1. some copper wire (from 240volt electrical cable) ,
  2. and I happened to have a plastic tube that used to hold a map that was 6cm in diameter.
  3. I bought a 20cm pie pan from the 1 Pound store to use as a counterpoise.
  4. I had a very old external wifi antenna that had (reverse SMA connector on a short cable )

So I wound the wire on the tube ( by the dimensions in the calculator) I got 5 turns with my scarp wire
I used double sided sticky tape to stick the bottom of the plastic tube to the pie pan.
I soldered the coax on the copper wire ( Sorry but I had to solder!)
I simply punched a hole in the pan and put a self tapping screw with a washer to connect the shield.

I have reasonable quality “wifi antenna” extender cable that cost £8 from Ebay that is 5 meters long using low loss CDF-200 coax ( TP-LINK TL-ANT24EC5S 5m Antenna Extension Cable ) - Not that this has REVERSE POLARITY connectors so you may need adaptors! )

I plugged the cable in to the Antenna ( yes I know the LNA should go at the ant end, but the RP-SMA stopped me!) with the CHIP etc. INSIDE the house, pointed it, and lo & behold 4db signal! I left it out over night ( this the frost on the photos I took this morning! )
Once I get the correct adapters I will try the LNA out there & compare the patch antenna too.

This took me all of 30 minutes to build BTW, and it can see the Aero signals from the bird as well.
Photos below… Note that I have not cut off the extra tube yet… I will!

Neil this is a great antenna. This is exactly what we need people to be doing.

It was fast to make and low cost and yet it works and downloads data.

Fantastic achievement.

Can you try pointing the Hellix antenna straight up and see if it still receives packets?

I will try later, as I have disconnected things for a while… The other connector adapter arrived & I am testing the patch ant with the LNA at the antenna and the chip & SDR inside :slight_smile:

Ok, since we are building… Here’s what I threw together in a few minutes using:

  1. A piece of cardboard
  2. Some aluminum foil
  3. Some tape
  4. Some 10GA solid copper wire
  5. Some EPP foam
  6. Some zip ties
  7. A banana plug
  8. a SO-239 to SMA adapter
  9. A SMA jumper

The last three items were of convenience, and could certainly be replaced with just a piece of coax with a SMA connector. Used a shampoo bottle for the coil form. Just happened to be about 6cm diameter. Tweaked spacing and diameters with ruler measurements.

In place of the Outernet patch, and using the LNA, initially, it gets a 2 dB better SNR, but about 2dB less RSSI than the patch. More directional, and finicky about placement. (subsequently, I found that actually in side by side with antennas in exactly the same position, the RSSI is virtually the same, with the helical getting about 2dB better SNR. I suspect this is due to the narrower beam width)


I’m so delighted for for the posts above! The exact reason I’ve written the post referenced on top.

looks like it would bounce like a pogo stick if you dropped it :slight_smile:
I have yet to do proper side by side tests , I have too much happening … I will though.

@rtlsdr4everyone I have always loved making knocked together antennas :slight_smile:
Usually they are HF, so big & ugly ( you should see my back yard LOL! … ) been a long time since I plaued with these frequencies…
Thanks for the “nudge”

Ok, here’s a comparison with the antennas outside and pointed for best SNR. As you can see, not much difference in RSSI.

  1. The helical

  1. The patch

Interesting … The actual signal strength as measured by the Antenna is marginally different BUT the noise level from the helical is SIGNIFICANTLY less, so the signal strength above the noise is much, much better!
From memory (which is old & faulty!) 3db is about twice, so the noise floor is more that twice as low … My guess is that the patch is picking up some local noise the that helical isn’t :slight_smile:

You should try looking at the spectrum in SDR# ( whatever it is called now), … I can see the outernet signal clearly when I look. (not as strong as the Aero signals though!)

Neil, you are correct. 3db is twice. Thats a pretty substantial gain in SNR k5ted! nice comparison. Thanks.

Very interesting.

Could you do a comparison of offset angle from Satelite direct pointing and measure signal strength.

Why did you choose so many turns?.

The beamwidth is probably around 50 degrees. I just used the measurements given in the article.

4 turns, 60mm diameter, 50mm spacing. Randomly cut reflector more than the minimum required. Feedpoint is about 2-3 mm above the reflector.

Overall, not entirely scientific and precise. That’s what makes it fun when it works!

I think you are trying to find out if it will work pointing straight up . The answer is no. With the antenna turned about 20-25 degrees off left or right, the SNR drops to 1 or less.

It’s possible a single turn helix might work as long as the LNA is good. The gain would be around 0 and 100 degree beamwidth. Not much to work with. Two turns brings the beamwidth to around 75 degrees and gain up a bit.

The good thing is this sort of antenna is pretty easy to build and experiment with.

Take your pick of calculators. They all produce slightly different results. Google “helical antenna calculator”

Thanks for the feedback. I am a bit snowed under at the moment with hijacking jobs but as soon as I get some more time I intend some more antenna experiments.

Here’s Outernet and adjacent carriers with a SDRPlay and the helical antenna. No external LNA or filter.

@k5ted I have an SDRplay too, and I am amazed at how well it works “barefoot” at these frequencies - It does have a built in LNA, but…

A quick update on my original post on this…
I replaced the “reverse polarity” SMA connector with a normal one (as my adapters arrived for the cable!) so I could put the LNA at the antenna end of the cable (where it should be), and cut off the excess length of tube
Using the same cable the patch antenna gives a good 5db SNR and the Helix give me 8-9db… more than I was expecting LOL!
Well worth the 15 minutes to make, and I am not worried about it being out in the weather ( after hot gluing all over the end of the coax, and putting the connectors & the LNA into a plastic bag) It was -5C outside this morning!

Fantastic Result.

Can you please send a updated picture. Please also do a close up of the coil wire to Coax connection .

I have SMOTHERED the connection in hot glue now, sorry :frowning:
it is the same as it shows above… the end of the coils goes parallel to the counterpoise for a CM or so ( stuck down with 1mm thick double sided sticky tape, that I happened to have ) and simply soldered to the centre conductor of the coax. the I put a screw in the pie tin close by and simply put the braid under it…