best antenna

[Rooster]

I would try a smaller antenna !!!!! Larger COCOs , if your maths + construction is NOT ACCURATE will only multiply the errors and produce an antenna with lousy performance.

Larger antennae also drag in more interference, so you maybe overloading the dongle with signals you don't want.

Try a simpler antenna "ABCD567's SPIDER" is a good start... place it in the same place as your COCO, on the same feeder and see how it goes.

For measuring range, I use the RTL1090 software together with "virtual radar" this will map only what you see, and leave it running for 12 hours or so to create a decent map of your coverage.

[fungus]

I'd guess the antenna starts at about 1 meter from the flat roof, my house itself is about 3 meters high. You can check the photograph to get an idea.
I don't really want to put it any higher as its already standing out like a Christmas tree. I just expected better performance than the stock antenna...

I think Rooster's advice is valid. I made a collinear using the instructions in the following vid and came up with pretty good performance. Note the different length of the elements. Not all RG6 has the same velocity factor which can be a trap. I used 8 elements. Experimentation with the lengths seems to be an important aspect in hitting the 'sweet spot' for 1090MHz. You'll find a great deal of advice earlier on in this thread if you wish to persist with it.

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

Regards,
Gregg

[thebenchmark]

Thanks for the replies, I'll order some parts and go for a spider and see how that works out. Could I just use 2,5mm² electrical cable for that? I got loads of that piling up here.

One side question on the collinear if found some difference in designs on the web some people shorted their antenna, I found someone putting a resistor on and others made no contact, I went for the last. What's best?
Concerning the maths I have no idea how that works, I got some length from our local coax broadband internet supplier so I thing the quality isn't lacking. It has a VF of 82% and an impedance of 75 Ohm.
Also what kind of accuracy are we talking about? It was a very sturdy cable so I did have a mm of error here and there.

[fungus]

thebenchmark,

I added a quarter wave stub to the top of mine which seemed to help. Some bridge the top which would effectively make mine a 16 element antenna. I also made certain that all the elements were exactly the same length, to the mm. My 1090SSJ sits atop a 30ft mast on my ridge capping as height above surrounding obstacles makes the world of difference, not just with range but number of 'hits.'

All I can suggest is to experiment and see what works for you.

Regards,
Gregg

[abcd567]

.......Concerning the maths I have no idea how that works, I got some length from our local coax broadband internet supplier so I thing the quality isn't lacking. It has a VF of 82% and an impedance of 75 Ohm.....

Your Cable's VF = 82% = 0.82
½ wavelength for 1090 MHz = 138mm
Length of element = VF x ½ wavelength = 0.82 x 138 mm = 113 mm.
You have used 116 mm - an error of 3mm, enough to knock down the CoCo from "Excellent" to "Poor".

Meanwhile try this easy to make Spider CLICK HERE. It does not require any connector like SO239 or any special parts, all parts made of Coax.

Also see the 3 Easy Antennas: http://forum.flightradar24.com/threa...-FOR-BEGINNERS

[thebenchmark]

Thanks for your reply, its interesting to see the formula explained, it might be a tip to make a stickied topic for nebies like me

I think I'm gonna try that spîder in the weekend. What about length for this one is this always 69mm?

[Rooster]

Thanks for your reply, its interesting to see the formula explained, it might be a tip to make a stickied topic for nebies like me

I think I'm gonna try that spîder in the weekend. What about length for this one is this always 69mm?

Yes... doesn't matter what wire you use, as velocity factor does not come into play with simple dipole antennae.
You can use any wire, stripped out mains cable is a good source of stiff copper... I would steer clear of anything apart from copper though. (cheap satellite coax is copper coated steel for instance)

[thebenchmark]

Yes... doesn't matter what wire you use, as velocity factor does not come into play with simple dipole antennae.
You can use any wire, stripped out mains cable is a good source of stiff copper... I would steer clear of anything apart from copper though. (cheap satellite coax is copper coated steel for instance)

Thanks for your reply, this might be a noobish question but shouldn't the reception of that dipole antenna be improved I if for instance would double the length or use a larger gauge of wire?

[Rooster]

Thanks for your reply, this might be a noobish question but shouldn't the reception of that dipole antenna be improved I if for instance would double the length or use a larger gauge of wire?

Very VERY simple explanation...

Radio waves travel in waves, and the name of the game is to capture half of the wave... by grabbing more than half, as the wave is sinusoidal you are actually cancelling out some of the signal... so we aim for half.
Now, before someone says "arrr, but what of quarter or 5/8 wave antennae ?" well, we are STILL aiming for maximum signal, but a 1/4 wave is a good compromise on size, as well as its easier to match to the feeder.... 5/8 are are clever way of saying "half plus a quarter" in an attempt to grab more signal.

If you look at a sine wave, half is positive and half is negative.... so the ideal is half, or your taking away from the first half... of course there's a ton of complicated maths to all this - but a simple half wave antenna is by far the easiest to realise, simply calculate the wavelength and divide by two, then feed it in the centre (two quarter waves...)

[Avsec]

One side question on the collinear if found some difference in designs on the web some people shorted their antenna, I found someone putting a resistor on and others made no contact, I went for the last. What's best?

Any thoughts on this question?

Just cut 9 lengths of LMR400 which has a VF of 85%, works out to segments of almost 117mm.

What to do at the top is the $64 question.

Many thanks in advance.

[abcd567]

Any thoughts on this question?

Just cut 9 lengths of LMR400 which has a VF of 85%, works out to segments of almost 117mm.

What to do at the top is the $64 question.

Many thanks in advance.

First test your Coco with 9 pieces only, nothing fancy at top. If you get reasonably good results, then you can experiment by shorting the top, or adding a whip.

[Avsec]

First test your Coco with 9 pieces only, nothing fancy at top. If you get reasonably good results, then you can experiment by shorting the top, or adding a whip.

Thanks abcd567, will give it a trial and see approach. The talk of a resistor across the top had me a little confused as there wasn't that much information available about size etc.

[Rooster]

Each section of the coco is "expecting" to "see" a certain impedance at the end of it... in order to perform the necessary matching before and after each section, all is dandy until you come to the last section... in order for that one to present the correct impedance to the one before it, there needs to be a "resistance" at the very end... ideally the same as your feeder cables impedance, if you are using satellite coax, then 75 Ohm is "expected" on the end, good quality 50 Ohm feeder wants to see 50 Ohm ...
the great thing about antennae is that the end "resistance" doesn't NEED to be a resistor - it can be either a stub (shorted section or open section, depends on the dimensions)
Another happy coincidence is that an open top section (Often 1/4 wave) can be used with an in line amplifier - say a cheapo satellite amp.

Imagine a line of buckets passing water down a line, too little or too much water lowers efficiency... just the right amount makes the system flow !

[abcd567]

Thanks abcd567, will give it a trial and see approach. The talk of a resistor across the top had me a little confused as there wasn't that much information available about size etc.

.....the great thing about antennae is that the end "resistance" doesn't NEED to be a resistor - it can be either a stub (shorted section or open section, depends on the dimensions)
Another happy coincidence is that an open top section (Often 1/4 wave) can be used with an in line amplifier - say a cheapo satellite amp........

Rooster has given a very good suggestion. I will summarize it:
Add a 1/4 wavelength (i.e. 1/2 x 117 mm = 58.5 mm) at top of your 9-section CoCo.
Do Not short the top section. Do not add a resistor.
Open (un-shorted/no resistor) will make it easy to connect an amplifier, if you want to add one.

[zryder94]

Good Day everyone.

I found something quite interesting. A few months back, I built a CoCo out of LMR-400. I precisely cut my coax, including VF measurements, and sealed everything up inside a fiberglass tube.

I installed this antenna onto a small satellite dish foot screwed to my wooden fence, due to it being winter in Minnesota.

I live 5 miles from MSP (bearing 230), and 3.5 miles from STP (bearing 85) airports. MSP is our main international airport, and STP is a rather busy business / Army Aviation airport.

This feeder was using a Raspberry Pi with a generic RTL-SDR R820T tuner. the base of the antenna was maybe 5' above the ground, 5' away from my house on the south side. 20' of Belden 9913F7 connects the antenna to the RTL.
A 10 day average from this installation yielded 25,000 positions, with 530 unique aircraft seen.

Once the weather got nicer, I moved this antenna to the top of a mast on a tripod on my roof.

The same antenna was installed, with the same coax feedline to my feeder. The antenna is now at 25' AGL. I upgraded to a Raspberry Pi 2, and a Nooelec R8020T2 with TCXO.

My averages have dropped from 25000 to 10,000 positions reported. Same antenna, same coax, 20' higher, on a post in clear space.
Any idea why this might have happened?