100Mhz noise

[madshaman]

I wonder if something like this might help in your hunt:
http://www.rc-cam.com/forum/index.php?/topic/3891-20-spectrum-analyzer-for-testing-fpv-systems/

For $20 you can't expect spectacular quality (don't see much mentioned about sensitivity/noise...), but might be good enough for DF.

Yes, I do believe it would, spaghetti monster praise the internet!  That link went right into my "cool" bookmark folder.  Thanks!

[madshaman]

Can anyone recommend a good reference/survey/starting-point/you-tube-videos for me to get familiar with the principles of and start learning the specifics of antenna design?

[C]

madshaman,

In the past I had access to all the fancy tools to find sources of RFI. I found that unless you can quickly rig a better tracker,(IE get the fancy stuff to show the RFI) to go with what is working. I could get better results faster using a pile of crud that showed the RFI vs fancy RFI tracker that did poorly.  In your case, it's money spent on might be better vs time with what you have.

If moving your scope &  function gen around in the room and rotating it 360 shows no differences then it will not work and you need to try something and have wasted a few minutes of your time, but if it does then you have more money for the good things.

C

[C]

Antenna's
There is some good info on the net and a lot of very bad as far as antenna's. I saw a news item the other day where an Italian took a good wifi dish antenna sliced it from edge to center to create a spiral dish if that's the word. He has a radio link setup with regular wifi dish an beside it his spiral dish. States he now has two wifi links that are independent. I can see it working, a lot of antenna designers will probably say "no way" until they see it for themselves. 

You have probably seen the saying " electronics is F**** in magic" well antennas are a magic all their own. I have often seen a description of how an antenna works be wrong. Also sometimes, something that works, is very hard to enplane.

So here is something to get you started with. If you look at a sign wave on your scope, antenna is trying to make something that may look like that but is very weak, large enough for electronics to see it. A dish is like a mirror, while most antenna's are trying to act like an oscillator. The idea being that it takes very little energy to oscillate and in the process of trying will make the signal larger.  You want the peaks at the antenna ends while you hold it at the zero point. With light you use lenses and mirrors, an antenna does the same normally with wires, rods, pipes acting like the lenses and mirrors.

Good old wikipedia is not a bad start point, then ham sites. If you want to see large, google "cdaa" and to to wikipedia 

C

[madshaman]

Thanks C.  This is usually why I ask for recommendations of resources.  I'm not an expert at anything myself but when I look for a text or other source material I try to find what's considered the best of the best.  The instruction is usually more coherent and is, shall I put it kindly, on the other end of the spectrum from "I'm getting free energy by electrolysing water".

A couple of my favourite authors are Jim Williams and Douglas Self, but personalities are independent of truth in science, and any good reliable source is good.  More often with ACM articles than IEEE (I'm a computer scientist by trade) I find hidden brilliance in random papers, sadly wishing these people knew how good they were.

Re antennas: what you've said I pretty much understand from basic principles and my experience with audio, computer simulations of the wave equation, and to a laser degree lasers, however, at ~= 100Mhz I'd have a resonator of 3 metres in length; hardly portable.  I know that there are antennas that operate efficiently at fractions of a wavelength, I know there are antennas that are efficient over a broad band of wavelengths and I have a basic understanding of things like a parabolic reflector.  I know there are "fractal" antennas.

I don't mind stuff that "seems" magical, but I like a good solid explanation of why the magic works (preferably with equations and their derivation).  I don't mind stuff that works but isn't fully understood why it works, just so long as that's really the case and not just lack of knowledge.

When I build stuff, I like to be able to design the thing myself from the ground up, or at least *fully* understand an existing design, and that requires really knowing what's going on.  I'm a bit of a goof and let my imagination run wild, put when it comes to a task, I'm pretty serious, even for fun projects.

Hopefully that gives you an idea of the kind of resource I'm looking for, and I will hunt around!

It's probably obvious my next thought was to buy one of those DVB-T TV dongles, turning much of the task into a software problem, leaving me with the task of building an antenna for it and maybe rig myself some kind of broadband direction finding tool, eventually making it portable (not tied to the PC or laptop)

I'm putting up new shelving in my lab right now, so I haven't gotten back to the noise hunt yet, but I am going to follow the common sense suggestions first.  I am thinking to the future though for the next time I need to hunt for a noise source, it would be nice to have a quick and dirty tool to help me out :-)

[C]

If read between the lines so to speak, nothing says a very short or very long wire will not work as an antenna for _____Freq. Less than perfect just means less range, weak signals not heard. Some things you can counter act, others you live with. Those rubber duck antennas you see are very very poor, but if you can stand shorter range and the transmitter can protect it's self, rubber duck = short range while better antenna = more range. It could also mean better antenna = longer on battery.

C   

[madshaman]

If read between the lines so to speak, nothing says a very short or very long wire will not work as an antenna for _____Freq. Less than perfect just means less range, weak signals not heard. Some things you can counter act, others you live with. Those rubber duck antennas you see are very very poor, but if you can stand shorter range and the transmitter can protect it's self, rubber duck = short range while better antenna = more range. It could also mean better antenna = longer on battery.

C   

If I get what you're saying, I should imagine an antenna a bit like a band pass speaker enclosure (or any resonator really, laser cavity, flute, etc.), where the antenna length isn't an integer multiple of the wavelength I won't get maximum gain and in the worst case near 0 gain where the length is an odd integer multiple of half the wavelength.  That leads me to think there must be software which given an antenna model, returns the frequency response of the antenna (obviously I could get this easily for trivial ideal antenna models myself using Mathematica).  Anything open-source (I'm not a simulation guy, but a half simulation, half real world guy ;p)?  Like anything domain specific I imagine commercial software must have ridiculous prices like $50,000 per seat.

[C]

I do not know any free software for this, I have not looked, A while back I was looking a site where they were talking about some commercial software. From what I was reading, it sounded like that software would have said the spiral dish would not work.

It is a shame, but some of the early people's designs are unheard of today. You find a lot of cap tuned radios. If you dig, you can find some old ham radios that are inductor tuned. I have worked with both and no tuned ones, but I have only seen one that did both when tunning it.

In the different category was an IF band pass filter. There was a long L shaped U in the bottom of the chassis. At one end was a coil paralleled by a cap connected to an amp output, with the coil paralleled by a cap connected to an amp input at the other end, Normal almost. There were no other electrical connections between the ends. Spaced along this L shaped channel were (coil paralleled by a cap) on insulators and no connections.  They covered the U with a medal cover that had small holes you could uncover to tune each coil/cap pair.

If my description is good, can You understand how it worked?

C

[madshaman]

C, I have sort of an idea, but I'll admit my mental picture of the setup is a lottle hazy.  Was imagining it worked much like a recorder does.

[madshaman]

Also, with what I've got on my plate right now I might *never* get around to it, but if this kind of simulation software doesn't exist, I'm tempted to try to write it.

If I do start giving it a whack (atm the timeline is "real soon now"), I'll be sure to post some video of prototypes.  Don't expect anything until at least the fall, already have 6 full fledged projects on my bench.

Anyone care to give me a couple million dollars so I can quit my job and tinker full time?

[C]

Think backward.
In stead of finding someone to give you money,
Be Mean,
Thinker up something so that,
They are begging to give you money

which could equal
Going to work to Play.

C

[madshaman]

Think backward.
In stead of finding someone to give you money,
Be Mean,
Thinker up something so that,
They are begging to give you money

which could equal
Going to work to Play.

C

Wheels have been turning ever since!

[vk6zgo]

A 100Mhz half wave dipole is actually around 1.425m long,as there is a correction of around 0.95 applied to real antennas.
Full wave antennas  are not commonly used.
If,as you believe,there is some 200MHz involved,you can halve the dimensions.

Back in the day,you could probably wait till the "wee small hours" of the morning when the FM stations went off air,& see if the signal went away.

If there is enough 100MHz signal to see on an Oscilloscope,buy a cheap portable radio & tune around--you should hear it go silent as you tune through the carrier.
Crappy little radios are much more sensitive than an Oscilloscope!

[madshaman]

A 100Mhz half wave dipole is actually around 1.425m long,as there is a correction of around 0.95 applied to real antennas.
Full wave antennas  are not commonly used.
If,as you believe,there is some 200MHz involved,you can halve the dimensions.

Back in the day,you could probably wait till the "wee small hours" of the morning when the FM stations went off air,& see if the signal went away.

If there is enough 100MHz signal to see on an Oscilloscope,buy a cheap portable radio & tune around--you should hear it go silent as you tune through the carrier.
Crappy little radios are much more sensitive than an Oscilloscope!

Thanks, I will try!  Been absorbed in other stuff recently including a minor lab reno :-), I've yet to get back on the noise hunt...  Now, if I can actually *find* a cheap FM radio any more these days..

[vk6zgo]

Some cellphones have a built-in FM radio function.
Never used it,don't know if it actually works!