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Planegadget External Aerial

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Re: Planegadget External Aerial
« Reply #15 on: May 02, 2010, 12:50:04 PM »
Thanks


Just purchased a 10M USB cable of ebay for ?7.

Will be interesting to see if a 10M cable works! In theory this breaks the USB spec! I see ebay has some 10m cables that it says are USB 2.0...interesting. I noticed there are also some repeater units that are a 10m cable with effectively one hub combined that give you 10m, technically that is within the spec.
http://www.usb.org/about/faq/ans5
« Last Edit: May 02, 2010, 01:03:05 PM by radargadgets »

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RobinB

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Re: Planegadget External Aerial
« Reply #16 on: May 02, 2010, 01:15:29 PM »
Thanks


Just purchased a 10M USB cable of ebay for ?7.

Will be interesting to see if a 10M cable works! In theory this breaks the USB spec! I see ebay has some 10m cables that it says are USB 2.0...interesting. I noticed there are also some repeater units that are a 10m cable with effectively one hub combined that give you 10m, technically that is within the spec.
http://www.usb.org/about/faq/ans5


Same here, it claims to meet the USB 2 spec, so we shall see.

?7 not going to break the bank and I did not want a simple extender as that would mean a join in cable, if this does not work then I will have to get a repeater type.


Question for you, what is the optimum size of ground plane needed ? or is it a case of larger the better ?
If I knew how to make money easily I would be a multi millionaire by now.

Re: Planegadget External Aerial
« Reply #17 on: May 02, 2010, 02:11:55 PM »
Well, size of the ground plane is interesting from a theoretical point of view.  Ive heard many old hands say the bigger the better.  But in practice I think that its perfectly fine to have something the size of a biscuit tin.  On the bench you can see an improvement on top of a tin, but it still works pretty well without anything.
So something fairly small and convenient would be fine.

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RobinB

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Re: Planegadget External Aerial
« Reply #18 on: May 02, 2010, 02:22:45 PM »
Thanks

also

Would it be better if the mounting pole is isolated from the ground plane ? (biscuit tin) :-)
If I knew how to make money easily I would be a multi millionaire by now.

Re: Planegadget External Aerial
« Reply #19 on: May 02, 2010, 03:26:55 PM »
Quote
With something like ADS-B we are talking lots of small transmissions that are very short, of a large dynamic range (big difference in signal strengths) from multiple transmitters (aircraft). This means that its impractical to try to automatically adjust the gain of the receiver, if you adjust for a big signal from a local aircraft you would miss the small signal from a far off aircraft...in fact you would never know it is there so could not adjust for it.  So an ADS-B receiver needs to handle a wide range of input signals keeping the small signals just about detectable and the large ones below a point where they overload.

Interesting discussion. A bit needs to be added with all due respect for this design.
Typical input voltages on a 50 Ohms antenna for ADS-B signals are

-90 dBm equals 7 ?V for aircraft farthest away
-60 dBm equals 225 ?V for aircraft within 20 NM
-30 dBm equals 7 mV for aircraft  2000 ft overhead
+0 dBm equals 225 mV within 100 feet from the transmitter antenna

This shows how wide spread the voltages are that the receiver must orderly process.

Other ADS-B receivers (than PGR) use a combination of static amplifiers and one logarithmic amplifier to process the input signal. The advantage of a logarithmic amplifier is, that a wide range of input voltages will be translated to a small range of output voltages only. For a typical SBS-1 setup with a 20 dB preamp and the commonly used AD8313 logamp chip the translation scheme is about

Input: -90 dBm equals 7 ?V => output 0.7 V => Gain 100000
Input: -60 dBm equals 225 ?V = output 1.45 V => Gain 6444
Input: -30 dBm equals 7 mV = output 2.1 V => Gain 300

It can be seen that there is an automatic gain control inherent to these systems by means of the logamp chip. The detector must only process signals in the range of 0.7 and 2.1V. If the rest of the design is well selected this covers almost all available ADS-B signal levels. If not, then the typical black spot overhead the station appears or there is a degradation in range or when the reception path is blocked by obstacles.

The PGR design does not provide a logamp and there is a provision for a variable gain, but it cannot be controlled from the signal strength of an actual signal. This is due to its frontend nature using a satellite receiver design, which has very little fluctuations in signal strength. It uses an average acceptable gain as a compromise. For installations that are dedicated to special purposes as traffic surveillance on an airport apron this must not be a disadvantage.

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RobinB

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Re: Planegadget External Aerial
« Reply #20 on: May 02, 2010, 03:29:31 PM »
Here is what I was thinking of doing with setup.

Any thoughts on it and should the (metal) pole be isolated from the ground plane.?

Obviously everything will be sealed to stop moisture getting into the aerial and the receiver.

The receiver will be mounted at distance from aerial so the cable will be more or less straight.

USB cable will be 10 M in length so can get into house.

Pole will be mounted about 2 meters off ground level so should get at least 1 meter clearance above roof line.

Anything else ?

Thanks
If I knew how to make money easily I would be a multi millionaire by now.

Re: Planegadget External Aerial
« Reply #21 on: May 02, 2010, 03:48:29 PM »
Thats  a great explanation from junglejet.  I would say that a sat front end does have to work over a fair range, as clouds, rain and different gain dishes have to be catered for. However there is a constant carrier in sat TV and you are looking at one set of transponders at a time, so its not the same as simultaniously trying to resolve small signals and large ones as you do in ADS-B.
However, a log amp as used in the other receivers is inherently more tolerant of overload.  Though it does make filtering more important at the front end as you have to not only provide good filtering at Ghz, but also the log amp is happily more sensitive (has more gain) to lower signals...that is also the out of band interfering ones you just attentuated through your SAW filters.  So its a bit of "swings and roundabouts".

Regarding your metal pole Robin, I cant see that grounding it will hurt anything, as its below the ground plane anyway.  Might provide a great lighting conductor?!

If you have a laptop I would try the antenna at that height first.  There is always the posibility that the extra height puts in clear view an interfering transmitting tower (like a nearby mobile phone mast) and degrades your signal.  You never know, and better to check first!
« Last Edit: May 02, 2010, 03:53:41 PM by radargadgets »

Re: Planegadget External Aerial
« Reply #22 on: May 02, 2010, 05:07:31 PM »
Agreed. It's all about a compromise in electronics. Everything has its place.


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RobinB

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Re: Planegadget External Aerial
« Reply #24 on: May 02, 2010, 05:57:25 PM »
The picture on this site and the linked site are different.

On the linked site the coils are 1 1/2 turns but on this picture 1 turn.

Have you made this ?

What results ?

thanks
If I knew how to make money easily I would be a multi millionaire by now.

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pelle900

Re: Planegadget External Aerial
« Reply #25 on: May 02, 2010, 06:29:24 PM »
The modified versions origins from here:

http://www.kineticavionics.co.uk/forums/viewtopic.php?f=1&t=7489&sid=3a891166c8ff5dfa87288cb159b9b619&start=15

i have not stepped into this world yet so no ads-b gears so far, just a lot of amateur radio stuff  ;D
« Last Edit: May 02, 2010, 06:31:34 PM by pelle900 »

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evilv

Re: Planegadget External Aerial
« Reply #26 on: May 10, 2010, 07:45:00 PM »
Thinking about this, I suspect that putting even the standard Plane Gadget antenna outside will create the same problem as having one with more gain. Considering the comments of RadarGadget and Junglejet, the issue is that the design using a satellite receiver front end doesn't have an Automatic Gain Control to level up the weak signals of distant planes and level down the strong ones from planes nearby. The gain would seem to have been set to optimise performance with the provided antenna in the recommended situation (on a second floor windowsill). The optimisation involves setting the gain of the system so that strong signals don't overload the analogue to digital convertor and weak ones still get detected.

So, fitting a high gain antenna will undo that careful adjustment and risk overloading and failure to receive, but so should putting the provided antenna in a high vantage point maybe?

Also, radargadgets points out another risk - interference from strong inband signals like maybe GSM towers on 900mhz. Depending on how many saw filters the sat system has, you might well swamp the receiver with unwanted signals that spoil reception. I noticed this just today when I connected a 2mtre collinear antenna to my 2mtre handy talky. With its supplied rubber duck antenna it receives cleanly, with a much higher gain outdoor one, I had all kinds of breakthrough including a mixture of FM broadcast band and some kind of PMR broadcast.

This is not a criticism of the PlaneGadget device, Hats off that they produced a receiver at a sane kind of price, but by using the satellite front end, and keeping down costs, there might well have to be a sacrifice on sticking other antennas on. Of course, you can adjust the gain, but once you start with that, it is up to you to see it through, and also to set it back again if you revert to the original antenna.

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pelle900

Re: Planegadget External Aerial
« Reply #27 on: May 10, 2010, 10:58:34 PM »
Better antennas, preamps or low loss cables is always an win-win or lose-lose situation. If you gain up the reception you also gain up the signals you don't want which render either in good or bad result.

My location where I live has 2 pcs of 400KV power lines quite close that interfere with my radio amateur hobby to the level that i have put a lot of stuff into the garage. The 2m (144Mhz) stuff and higher works more or less OK but the HF bands is just full of interference and the better antennas and towers I put up the worse the interference becomes so i boxed my HF equipment.
My plan is to persuade my wife to get a new location without radio interference but...  :-X

There are some 1090Mhz filters to home-brew (or even buy ready made) but then, as you outlined, the tuner in the RG device may have problems with strong and weak signals combined. A normal SAT receiver has a AGC control but this may have been omitted in this device or may not be quick enough. The human ear do the AGC by default and the human super-CPU easily filters out the bad signals so we understand the weakest/strongest information that is transmitted. But when this should be done digitally it's a different matter so yes a better antenna can get worse result but shame the one who doesn't try :)

I consider the stock antenna to be a "included for free" in the package and for those with a good disturbance-free location a bigger antenna will do wonders to catch weak signals. I also believe that I read that the stock antenna was not weather-proof either so to put it outdoors will ruin it unless it's modified.

Cheers,

Pelle
 

Re: Planegadget External Aerial
« Reply #28 on: May 11, 2010, 04:55:32 PM »
My location where I live has 2 pcs of 400KV power lines quite close ..........
...........My plan is to persuade my wife to get a new location without radio interference but...
The way to sell the move is to tell of the risk of cancer from living close to those lines.
Fact not fiction.