ADSB Ground-Based Antenna

[PaulSS]

I know here be dragons and talks of Raspberry Pi codes etc but I thought I would venture here with a quick question.

Just for a laugh I am going to attempt to set up an ADSB receiving station at my house (in Australia). I've ordered a Pi 3B+ and I have a nice ADSB radio/dongle thing but I'd like to know if I can modify an antenna to get a decent external aerial for my house instead of (a)spending more $$$s on eBay/Amazon (b)bending bits of 69mm lengths of wire and scaring everyone with spiders on the roof (there's already enough of those buggers in these parts).

I have a spare GME VHF marine antenna. Obviously it is too long but I was wondering if this could be cut down and a suitable connector welded/soldered/glued to the base with which to connect the coax to the RTL dongle thingy. I think I may need to install an amplifier in there somewhere but now we're getting high-tech and I'll just take one step at a time.

As it's started life at 1.8m I assume I'll have to cut it to 276mm. Does that sound right?

Anyway, not a huge deal if it can't be done but I'd appreciate any thoughts you might have on my hobby project 

This is pretty much the antenna I have: https://www.gme.net.au/catalogue/antennas/vhf-marine-antennas-and-whips/ae346v.aspx

[exfirepro]

Hi Paul,

Bearing in mind the high power of 1090MHz Tx from aircraft, you can usually receive them from hundreds of miles away on the proverbial piece of wet string, so why not give the antenna a try ‘as is’ in the first instance and see what results you get. All you will need is a suitable adaptor to match the SDR socket.

Not my area exactly, but there’s loads of info out there on the net about setting up 1090Rx stations. Take a look at our partner site for the Mode-S/3D project for instance....

https://radar.lowflyingwales.co.uk/

You can probably skip the bits about feeding in to Phil’s site. Not sure it will help us over here  Just start with the dump1090 part.

Let us know how it goes.

BR

Peter

[AlanG]

Hi Paul

You say you've got the "dongle thingy", did that not come with the usual magnetic twig antenna?  If so stick that to the top of an empty coke can (other cans are available) and bung it all up in your loft running off a power bank to start with and you'll be amazed at what that brings in.  Just get it as high as possible into the apex if you have one.
But a wee spider made out of satellite coax and sealed with superglue works even better on the roof.

Alan

[PaulSS]

Thanks guys for your replies.

I shall take a closer look at the lowflyingwales website and see what information I can glean from there. A lot of what I've been looking at has been on the Flightradar24 and Flightaware websites. I expect a lot of it is repeated but it is interesting to read, nevertheless.

Alan, I do have a couple of ADSB 'whip' antennas but they are in a box in the UK. I already have the Flightaware Pro ADSB dongle, so I thought I'd buy another one but it comes without an antenna. I expect I will end up constructing a spider (like yours) as they seem pretty simple to do and their popularity must mean they do a pretty good job. I was just wondering about modifying my spare boat VHF antenna just because then it would look semi-professional, like the outdoor ADSB antennas you can buy. Just in passing, what sort of reception range do you get from the spider antenna you've built?

It's all a bit of fun, this project and I'll probably feed the results to Flightradar24 but it's all good practice for when we go to the UK and I set up my OGN-R station (one day).

[PaulSS]

Just to bore a little further, I thought I'd share my thoughts of being 'ripped off' with the marine VHF antenna. Today I cut it open to see what was what and found that my 1.8m aerial was, in fact, 2/3 full of air. The fibreglass tube extended the full 1.8m but the coax inside was a measly 0.6m of RG58. I always thought it was mighty impressive that I was getting better VHF ranges on the boat because of the length of my antenna but, alas, it would seem I fell somewhat short (I seem to recall my wife saying something similar) 

Anyway, it has given me a couple of bites of the cherry for when I cock something up with the 275mm ADSB antennas. I am going to procure a couple of N-type connectors tomorrow and then have an N to SMA coax cable to my ADSB dongle.

If the soldering on it is anything like the bodge I did of soldering my old MacBook's charger today I think I'm going to need a LOT of heat shrink to hide the blobs and mess 

Now I'm wondering if I need some sort of ground plane, since it isn't anything fancy inside the fibreglass but a length of coax. As I understand it, those vertical antennas you see (with no bits sticking out the sides etc) are more complex inside than a straight bit of coax cable. I get the feeling I'll need some sort of plane twixt the antenna and connector but, now I've gone for the 275mm antenna length (1 wavelength) I think the ground plane will have to have the same radius.

[Admin]

Just to bore a little further, I thought I'd share my thoughts of being 'ripped off' with the marine VHF antenna. Today I cut it open to see what was what and found that my 1.8m aerial was, in fact, 2/3 full of air.

unbelievable

[Paul_Sengupta]

Right, before you start hacking antennas around (oh, sorry, too late!  ) or spending lots of money on e-bay, a little bit of antenna theory.

The basic antenna is a half wave dipole. You have the two conductors going into two quarter wave sections which are lined up in a row. You normally feed this at the centre. An alternative is to use just one quarter wave section and a ground plane. The ground plane then "reflects" the quarter wave to make it look like a half wave dipole, but the antenna will be half the size. This is normally what we use on aeroplanes with their metal bodies as it's a natural groundplane. It won't be quite as efficient as a half wave dipole, and the radiation pattern will be slightly different, but it won't be far off.



If you want to end feed an antenna, you can either use a ground plane, or you can centre feed it but with the "lower" half of the dipole folded back over the coax. This is known as a sleeved dipole and is what is inside the PAW half wave antenna which is screwed on. It's a cross between the middle and right hand antennas shown below.



If you want a higher gain antenna, you must then add elements. You can do this in the shape of a yagi, like a TV aerial, but then it becomes directional. If you want to have an omnidirectional antenna with more gain, elements must be added in-line with the first. This is known as a colinear antenna. You have to either feed the elements separately in phase (a stacked dipole) or you "hide" the out of phase half wave in a section of transmission line or a coil.



If you google "ADS-B antenna" there are hundreds of variations on a theme, including coax colinears which are fairly easy to make.

https://www.youtube.com/watch?v=TkUYdCPFXXs

[PaulSS]

Thanks, as ever, Paul (I think  ) I'll get to the bottom of all this stuff one day but I seem to be gaining a little understanding each time I ask something, so it's not all bad.

If you want a higher gain antenna, you must then add elements. You can do this in the shape of a yagi, like a TV aerial, but then it becomes directional. If you want to have an omnidirectional antenna with more gain, elements must be added in-line with the first. This is known as a colinear antenna. You have to either feed the elements separately in phase (a stacked dipole) or you "hide" the out of phase half wave in a section of transmission line or a coil.

I think what you describe is the one I thought I would get by sticking my 275mm bit of coax inside the fibreglass tube (it looks nice) and connecting it to the N-type connector (see picture of proper one below and what I thought I was going to get). It would seem that I am somewhat off the mark 

I did see that antenna with the three bends in it on YouTube and thought that was 'doable'. I haven't yet watched the video you linked to but the one I watched was an English voice in South Africa and he talked about it being a directional antenna.

In summary, I think I should abandon the idea of a nice-looking 275mm bit of coax inside a fibreglass tube and concentrate on making a spider to sit atop a pole on my roof 

(I do like the animated explanation......it actually makes a lot of sense).

[Paul_Sengupta]

I think, from what you describe, you're talking about a sleeved half wave dipole. You want to strip the sheath off 68mm of coax, then, if you can, fold back the braid down the outside of the coax. What you end up with are two 68mm (quarter wave) sections pointing in opposite directions, the coax inner pointing up, and the coax shield pointing down, folded back. This gives you a 136mm antenna in total. You can stick this up a plastic pipe and Bob's your uncle.

It's effectively the right hand of the three in the picture of the three antennas above.

It should work fine out to 100 miles or more for airliners and is quite simple.

Forget the connectors, just carry on your coax run to the receiver! 

[PaulSS]

OOoooo, that sounds easy and within my capabilities 

SO, just one question; why do half a wavelength and not a whole one e.g 136mm of inner core of the coax and 136mm of the braid folded back down the outside of the coax (all inside the fibreglass tube)?

[Paul_Sengupta]

A full wave could work for receive but it would have a strange reception pattern.

Why full wave antennas don't tend to work very well is that the two halves of the wave are in opposite phases and while not cancelling each other out, they can cause weird interactions when they interfere.



Imagine one half of the wave pulling while the other half pushes. A half wave antenna resonates at the tuned frequency with only a single phase.

This is why, as in my explanation for a colinear above, if you want to add half wave sections, you have to somehow git rid of the anti-phase sections. Have a look at the diagram below. A J pole is essentially a half wave antenna fed with a quarter wave matching stub (don't worry about this for the moment). If you then want to add another half wave, you effectively take another full wave and add it to the J pole...but then for the anti-phase half wave, you fold it back on itself creating a transmission line which doesn't radiate. So you have a half wave, then a quarter wave stub sticking out (being a half wave there and back but cancelling itself out as it goes) then you can have another half wave. You can add in as many extra sections like this as you like, but there are diminishing returns. You can also hide the anti-phase half wave in a coil, like in the right hand antenna:



You can see that you then radiate two half waves which are in phase:



As an aside, I also found a good diagram for a quarter wave whip with a ground plane:

[Paul_Sengupta]

Glad you asked? 

[exfirepro]

Probably not Paul,

But the rest of the Forum Watchers will be   

BR

Peter

[PaulSS]

I am very glad I asked and your explanation is both easy to understand and very helpful. Thank you 

The sleeved dipole looks like the easiest to make.

The one in the YouTube video is also very simple and looks like it gives very good results if you use 16 sections of 11cm coax (I knew I shouldn't have chopped that 1.8m antenna cover in half  ). There are various comments about shorting things out and resistors but the OP says to ignore all that and just use RG6 cable as it works.

The spider looks like fun to make 

I think today I will investigate local suppliers if coax.

Thanks again, Paul 

[Paul_Sengupta]

A sleeved dipole is essentially what's inside one of these:



As can be seen here:



You can in fact use one of these if you like. Stick it up a bit of plastic pipe:

https://www.ebay.co.uk/itm/2pcs-5dBi-1090MHz-978MHz-Antenna-Bundle-2pcs-MCX-male-to-SMA-female-Connect-T4D3/183812168508

Alternatively you can get a PCB version already stuck up a plastic pipe:

https://www.ebay.co.uk/itm/1pcs-ADS-B-PCB-Antenna-1090MHz-Antenna-SMA-female/273518424463

With that spider antenna, the radials act as a ground plane and the angle of them affects the impedance and radiation pattern. The top radiator is still a quarter wave.

https://m0ukd.com/calculators/quarter-wave-ground-plane-antenna-calculator/

Here are some easy to build antennas:

https://discussions.flightaware.com/t/three-easy-diy-antennas-for-beginners/16348/7

Note that the coax colinear mentioned earlier and linked in the video should, theoretically, be much better, but it's subsequently much longer. I have one in my attic shoved up inside a bit of plastic pipe. I also made an 868MHz version which I was using for my OGN-R station's Flarm reception.