FlightAware Discussions

Attn: Anyone who has bought a PCB aerial for ADS-B

#1

I’d completely forgotten that I ordered a PCB aerial from eBay a few weeks ago until it turned up from China this morning. It looks simple enough and upon examination, I can see that it’s effectively a vertical half wave dipole with a female SMA nailed on the bottom.

As I tend to do with aerials, I put it on the cheap and cheerful aerial analyser I have rather than the full VNA. It’s easier to just screw the aerial on and see what it shows.

It’s not good with an SWR of around 6:1

6-to-1

However a quick visual inspection of the aerial showed something odd. The bottom half of the dipole didn’t appear to be soldered at all and I could see daylight between the two pins and the PCB itself. I bleeped it and I as right, they weren’t actually connected at all (excuse the scratch marks, they’re from my DVM probe).

wheres-the-solder

I fired up the soldering iron and two minutes later, it looked a lot better.

soldered

Back on the analyser and bingo!

one-point-two-five-to-one

still-one-point-two-five-to-one

I’ve not connected it to a receiver yet but will do that over the next couple of days to see how it performs but be aware of this - If you’ve bought one of these, please double check the connections to make sure that they’ve been made properly. I’m sure that a full half wave dipole will work somewhat better than what was effectively a quarter wave vertical with no groundplane!

The point of this thread is to draw your attention to the fact that the connections may not all be soldered and if that’s the case, you’ll need to do that yourself because if you don’t, the aerial will not work as expected.

3 Likes
#2

Thanks for the measurments.
Yep, I’ve got three - slightly different to yours.
One was for a site I wanted to up and running quickly.
The other two are for side-by-side system comparisons. I built the radomes for both at the same time. They are 1.2m apart, but up the same mast. The coax feed for each came off the same roll.
When I run two similar receivers, they give very similar performance
I keep one as a reference and make changes to the other:

  • play with the gain
  • add an amp
  • add a filter
  • etc.
    it’s a great way to stay sane while trying to improve performance

image

They are obviously not a high gain antenna, but do perform well.

1 Like
#3

Well technically the best it can ever be is 0dBd :smiley:

I’ve got it in my loft now along side another receiver which has a home-brew two element j-pole attached to it. It’s the same model Pi, same model dongle, the only difference being that this one is about a foot lower.

Needless to say, the existing installation is hugely outperforming it although I’ve still got the gain set very high on the new setup so it could very well be overloaded. I’ll tweak it over the next day or so to see.

It’s a handy aerial for experimentation though because it’s so nicely self contained, no spiky legs from the traditional spider and none of the fragility of the home-brew collinear.

#4

About a week ago, I have ordered this antenna plus pigtail (with SMA-male connector at both ends). Awaiting delivery from China.

https://www.ebay.com/itm/253915016420

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#5

Run your swr analyser at 978 mhz and see what you get. I was the same way that I completely forgot that I ordered one and it came in two weeks ago.

#6

@keithma
Your VSWR vs Frequency curve is a slanting straight line, while normally it is of parabolic shape, like below.

Dipole_fig2

Source: https://rsgb.org/main/get-started-in-amateur-radio/antennas/your-first-antenna-the-half-wave-dipole/

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#7

My first 1/2 Walength Dipole with 950~2050 Mhz Satellite Amplifier.

Was a good performer inspite of its tiny size and simple build. Length of RG6 coax to DVB-T / Windows Desktop = 50 ft (15 m)

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#8

The picture is zoomed in, i’d say you are seeing the left center part of the curve.

#9

That is what i also guessed. For a well tuned antenna, the tip of parabola should be at design frequency. In this pcb antenna case, it is not. This antenna is actually tuned to a frequency > 1100 Mhz

#10

At 978MHz the SWR is over 8:1 which is no surprise.

Expanding the frequency range outwards, the aerial is resonant at pretty much exactly 1100MHz but that’s really not an issue here. When tuning aerials for amateur radio, if I get down to <1.3:1 SWR then I’m generally not going to bother adjusting it any more.

1100

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#11

Thanks for posting the expanded range. Now it is clear.
Yes, you are right. At 1090 Mhz, the SWR is about 1.1, so no need to bother.

#12

For completeness, I did fire up the VNA which pretty much matches what the cheaper (but prettier) analyser showed me. You can click to see it full size.

1 Like
#13

Thanks for taking the trouble.

I also have the “prettier analyser” N1201SA.
I will test with it my PCB antenna when delivered.