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The text in red is like listening to Charlie Brown's parents. I can define the words, but haven't a tangible grasp on the concepts as they pertain to antennae. That's why I'm looking for some good remedial introductions on antennae. Then maybe I can ask some intelligent questions.
The text is indeed chaotic
Alright, let me try the intuitive path.
There are some key points you need to take into account. I think it would help to grasp some
First, the length of an antenna matters. An antenna must resonate in some way, and it depends on its dimensions and the frequency you wish to use.
Second, transmissions take a bandwidth necessarily greater than 0, which is why that paragraph you quote mentions "Q" and problems derived from antennas with narrow bandwidth.
For example: a voice transmission on HF (short wave) as used by hams takes about 2 KHz of bandwidth. So, if you are transmitting SSB on, say, 7.100 MHz you are actually radiating energy from 7100 to 7102 KHz. So, your antenna must work well not only for 7100 KHz, but for 7102 as well
If you are using the 433 MHz transmitters for digital data, you need some way to modulate the radio signal with the data. There are so many ways to do it. Morse code is the oldest, just switching the radio signal (called carrier) on and off. But it's not particularly well suited for digital data, so other methods are commonly used.
A simple to understand digital modulation method (which was mentioned in another post) is called "FSK", which uses two signals. One of them is a digital "1", the other one a digital "0". The frequency differences range usually from 170 Hz (typical in amateur radio transmissions) to almost 1 KHz.
Higher speeds require of course much more bandwidth. WiFi transmissions can require between 20 MHz and 160 MHz of bandwidth.
And how do you increase the bandwidth of a simple antenna?
Let's begin with the simple antenna you are considering, which is just a length of wire. That antenna can be considered unidimensional. And it's possible to make an antenna that resonates on different frequencies just connecting together different lengths of cable. Some hams do it in order to cover several HF bands. So, imagine you add together three different wires: you have three lengths availalable, right?
Now imagine that you make your antenna bidimensional. For example, rather than a simple wire you use a rectangular or triangular piece of copper sheet. What happens now? Your bidimensional object can now be seen as the sum of many unidimensional pieces, which is "a collection of different lengths".
I hope it serves as a starting point to somewhat grasp the intuitive aspect of it.
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A comment and a question to try to help you along.
You are having trouble conceptually with bare wires into nowhere. Maybe if you start first with a DC circuit. In that case you will have an electric field that once established consumes no power. There is no current. If the voltage is then changed some charge flows and the change in field propagates out at the speed of light. If you rapidly change voltages back and forth there will be charge flowing in and out of the antenna, and the field strength viewed at varying distances will go up and down based on the history of voltage change. Note that the charge flowing in and out will induce a magnetic field and this magnetic field will interact with the electric field. The details of that interaction are described by Maxwell's equations. These fields have energy and thus power. So power is flowing into the wire and out into the world in the form of EM waves. While this is a fairly easy conceptual start, directly solving these fields from Maxwell's equations is somewhat challenging even with the simplest geometries, and effectively impossible for most real world situations. For this reason simplifications have been developed, which include viewing an antenna as a resonant circuit. An R an L and a C. Q is just another way of saying how small the R is.
The question. Although your current interest is in the antenna and antenna coupling, are you sure that is where the problem lies? There are many places a radio link can fail, and it seems unlikely that your antenna would be bad enough to cause the magnitude of failure you are observing. It may be worthwhile checking the rest of the system just so that you can be sure that your experiments with antenna configuration provide results that are actually due to changes you are making in the antenna. A simple check would be to directly connect the Rx and Tx through an appropriate resistive attenuator. Say 1000:1 or more to prevent overload in the receiver. Which brings up another possibility. At the distances you are describing it is possible your antennas are working too well, the receiver is saturating and therefore nothing is getting through. -
A VNA can eliminate most of the voodoo factor. Of course, looking good on the VNA doesn't mean that the antenna will work. But an antenna looking bad on the VNA most certainly won't
Moveover, mismatched antennas can damage transmission circuitry.
VNAs are really expensive instruments, but luckily there are cheap models adequate for the needs of a radio amateur. For example, I've got a miniVNA Tiny from Mini Radio Solutions (miniradiosolutions.com) and it certainly works. It can measure from 1 MHz to 3 GHz.
There is a very good introductory book on antennas. Depending on how you read (or skim through it) you can learn a lot or at least develop some intuitive knwoledge of what might work and not when designing an antenna.
http://www.amazon.com/Practical-Antenna-Handbook-5-e/dp/0071639586/ref=sr_1_1?s=books&ie=UTF8&qid=1458138978&sr=1-1&keywords=practical+antenna+handbook
Wow... that's a huge resource. The reviews look pretty good except for one moron who gave it one star (who's never read the book) because they are unhappy with the kindle version being only a buck less than the paper version.
At $600 bucks
, it looks like I will have to forego ever owning a VNA.
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So power is flowing into the wire and out into the world in the form of EM waves. While this is a fairly easy conceptual start, directly solving these fields from Maxwell's equations is somewhat challenging even with the simplest geometries, and effectively impossible for most real world situations. For this reason simplifications have been developed, which include viewing an antenna as a resonant circuit. An R an L and a C. Q is just another way of saying how small the R is.
So, the wave is a by-product of the same force that makes a motor armature turn? Electric current in a magnetic field?QuoteThe question. Although your current interest is in the antenna and antenna coupling, are you sure that is where the problem lies?
Nope, not at all. In fact, Howard has all but convinced me that it's likely NOT the problem. While the antenna may not be at the heart of the problem I'm currently experiencing, it's hard to deny the importance of antennas in radio communication (that's an understatement). I've seen lots of electronics tinkerers in videos and blogs that gloss over the antenna aspect. It's obvious that they do so because they lack the knowledge. I guess being a difficult subject, they are often content with a less than cursory understanding of them. I'm trying to avoid that.QuoteA simple check would be to directly connect the Rx and Tx through an appropriate resistive attenuator. Say 1000:1 or more to prevent overload in the receiver. Which brings up another possibility. At the distances you are describing it is possible your antennas are working too well, the receiver is saturating and therefore nothing is getting through.
Sorry. I don't know what that means. Like I said earlier, the software is likely an issue as well. Since the MCU is simply turning the oscillator on and off something within the MCU (an interrupt for example) could be breaking up the transmission sequence. Same goes for the receiver end. The program I have has no filtering no error checking etc. I only just started playing with these little devices so there are PLENTY of weak points in the system. No doubt. -
The text in red is like listening to Charlie Brown's parents. I can define the words, but haven't a tangible grasp on the concepts as they pertain to antennae. That's why I'm looking for some good remedial introductions on antennae. Then maybe I can ask some intelligent questions.
The text is indeed chaotic
Alright, let me try the intuitive path.
There are some key points you need to take into account. I think it would help to grasp some
First, the length of an antenna matters. An antenna must resonate in some way, and it depends on its dimensions and the frequency you wish to use.
Second, transmissions take a bandwidth necessarily greater than 0, which is why that paragraph you quote mentions "Q" and problems derived from antennas with narrow bandwidth.
For example: a voice transmission on HF (short wave) as used by hams takes about 2 KHz of bandwidth. So, if you are transmitting SSB on, say, 7.100 MHz you are actually radiating energy from 7100 to 7102 KHz. So, your antenna must work well not only for 7100 KHz, but for 7102 as well
If you are using the 433 MHz transmitters for digital data, you need some way to modulate the radio signal with the data. There are so many ways to do it. Morse code is the oldest, just switching the radio signal (called carrier) on and off. But it's not particularly well suited for digital data, so other methods are commonly used.
A simple to understand digital modulation method (which was mentioned in another post) is called "FSK", which uses two signals. One of them is a digital "1", the other one a digital "0". The frequency differences range usually from 170 Hz (typical in amateur radio transmissions) to almost 1 KHz.
Higher speeds require of course much more bandwidth. WiFi transmissions can require between 20 MHz and 160 MHz of bandwidth.
And how do you increase the bandwidth of a simple antenna?
Let's begin with the simple antenna you are considering, which is just a length of wire. That antenna can be considered unidimensional. And it's possible to make an antenna that resonates on different frequencies just connecting together different lengths of cable. Some hams do it in order to cover several HF bands. So, imagine you add together three different wires: you have three lengths availalable, right?
Now imagine that you make your antenna bidimensional. For example, rather than a simple wire you use a rectangular or triangular piece of copper sheet. What happens now? Your bidimensional object can now be seen as the sum of many unidimensional pieces, which is "a collection of different lengths".
I hope it serves as a starting point to somewhat grasp the intuitive aspect of it.
Thanks for trying. I got the first part down to just past "Second." Then, it was like listening to someone with a heavy accent where you get every other word or so. lol. You made me realize one thing for sure... I've been conflating (in my mind) bandwidth with data bandwidth. Maybe they're related? I don't know. Morse code is how I think of my little carrier. In my mind, it's the model I'm trying to use. Transmit, don't transmit. By varying the time of transmission against a clock at either end, time on, time off in terms of clock cycles, I could create my own comm protocol. I'm sure there are more sophisticated ways, but this was my first endeavor.
So, bandwidth is the difference between the lower and upper frequency used. So that's FM, no? Does AM also have bandwidth? It doesn't seem to me that it would. My little 433MHz transmitter doesn't have the ability to transmit AM, right?
I might be wrong about my little transmitter now that I think about it. There are three terminals, besides the antenna. Vcc, Gr. and data. Is data doing more than turning the oscillator on and off? Is it transmitting a slightly different frequency when it's high or low? -
Hi donmr, in wire the propagation speed is 100% light speed, 3E8 m/s, if you have a dielectric around the wire like in coax then it slows down to maybe 75% depending on the dielectric.
Its gets down to values like 75% with coax, but a single insulated wire is more like 95%.
And as vk6zgo says its really even more complicated with parasitic Ls and Cs.
But we are getting into fine details and here we should mostly be giving the overall idea: you can quickly calculate the approximate length for a single frequency. -
You make a point I've not seen elsewhere. I've always gotten the impression that at some point, mismatched impedances will result in setting ones hair on fire.
A mismatched antenna impedance results in power being reflected back down the feeder and into the output of the transmitter. When you have a transmitter with very low outputs like the 433MHz modules then you are very unlikely to cause any damage this way. However, as TX output power goes up, the amount of reflected power increases proportionally so matching becomes more critical not only from an efficiency point of view, but also to prevent your power output stage disappearing in a puff of smoke. -
So, bandwidth is the difference between the lower and upper frequency used. So that's FM, no? Does AM also have bandwidth? It doesn't seem to me that it would. My little 433MHz transmitter doesn't have the ability to transmit AM, right?
I might be wrong about my little transmitter now that I think about it. There are three terminals, besides the antenna. Vcc, Gr. and data. Is data doing more than turning the oscillator on and off? Is it transmitting a slightly different frequency when it's high or low?
Every signal that carries information has bandwidth, so AM also. There is unfortunately no such thing as a free lunch.
Until we know what your module is designed to do it's hard to tell what its abilities are, it maybe does AM but it probably is OOK (I know you looked that one up.)
Dit you try to connect the data terminal alternately to vcc and ground and monitor the receiver?
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Is AM bandwidth still a difference between the high and low frequency? Or is it the difference between the high and low amplitude? I was thinking, based on this thread, that "bandwidth" is the method employed to transmit data... low end of the band would be LOW, the high end HIGH in digital terms. Or, are you saying bandwidth is an artifact? A property to be mitigated as much as possible?
Every signal that carries information has bandwidth, so AM also. There is unfortunately no such thing as a free lunch.
Until we know what your module is designed to do it's hard to tell what its abilities are, it maybe does AM but it probably is OOK (I know you looked that one up.)
Dit you try to connect the data terminal alternately to vcc and ground and monitor the receiver?
OOK sounds about right from my understanding of my device, but I could be wrong. I couldn't find my device online (based on the silk screening), however they seem to use the same primary chip... maybe. In any case, mine look similar to others with similar type pcb silk screenings. The one someone posted in a photo demonstrating a dipole antenna set up looks like the same unit as mine from a distance.
No, I didn't connect directly to vcc and gr. I manipulated the output pin programmatically. High for 500 mS, Low for 500 mS. The receiver was set (through arduino board) to output the signal to my computer's serial monitor (USB). I was clearly able to discern a series of 0's followed alternately by a series of 1's. Turning off the tx, the numbers were random. I should note that during transmission, there were fairly frequent out of place 1's and 0's. That's likely a result of poor coding. I initially just followed the tutorial here: http://arduinobasics.blogspot.com/2014/06/433-mhz-rf-module-with-arduino-tutorial.html Part one, only. Though, he connected the receiver to an analog pin in the tutorial, I later tried something similar with the rx attached to a digital pin. Results were similar. I may need a pull down resistor or something too. -
Yes , I think there's something more fundamental than a minor impedance mismatch, although be aware that particularly in some higher power devices, they employ foldback, which detects a bad match and protects the high power RF stuff by significantly reducing power.
Balanced think...
Twin lead
Twisted pair (there are four pairs of twisted pair in a CAT5 cable for example)
Differential
Unbalanced think...
Referenced to ground, earth, common etc.
Single ended
Coaxial connections
Common to both...
Both balanced and unbalanced connections generally have characteristic impedances. You switch between them with a balun which can be constructed in all manner of ways, it might be a transformer, either of the shelf or DIY from bits of tightly wound coax and/or ferrite for lower frequencies, or you can make them out of Ls and Cs (aka a balun filter as they also have filtering characteristics done this way). Mincircuits is frequently the go-to place for off the shelf baluns.
VNAs are not bargain basement, but compared to a decade or so ago they're about an order of magnitude cheaper if you're prepared to go USB such as the miniVNA way. Yes, you're talking about $500 or so, but for a 3GHz VNA that's awesome.
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Quote
Is AM bandwidth still a difference between the high and low frequency? Or is it the difference between the high and low amplitude? I was thinking, based on this thread, that "bandwidth" is the method employed to transmit data... low end of the band would be LOW, the high end HIGH in digital terms. Or, are you saying bandwidth is an artifact? A property to be mitigated as much as possible?
Bandwidth is always the occupied space, the difference between high and low as you will. It is not a 'method to transmit data' but yes, it's more like an artifact. As soon as you change a signal it will have a certain bandwidth, and the faster you change the signal the more bandwidth it will need/occupy. For instance, when you AM modulate a signal (carrier) with a 1KHz tone it will create 2 more signals, one 1KHz below the carrier and one 1KHz above. The harder you modulate the original signal the stronger the 2 other signals (called sidebands) get.
Low end low and high end high would be FSK, Frequency Shift Keying.QuoteOOK sounds about right from my understanding of my device, but I could be wrong.
Yes, your module definitely uses OOK.QuoteI initially just followed the tutorial here: http://arduinobasics.blogspot.com/2014/06/433-mhz-rf-module-with-arduino-tutorial.html Part one, only. Though, he connected the receiver to an analog pin in the tutorial, I later tried something similar with the rx attached to a digital pin. Results were similar. I may need a pull down resistor or something too.
If the receiver module puts out a relative signal strength on the data pin you want to use an analog input, that way you can adjust the 'threshold'
If the output is digital it doesn't matter.
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Bandwidth is lowest frequency to highest. Lowest used to highest used. There are a great many details in how to evaluate bandwidth requirements, but you should wait until you have more of a grasp of the basics before you worry too much about that. In general bandwidth requirements are determined by how fast you are trying to send data. To a first order bandwidth requirement is a fudge factor times the data rate in bits/second. That fudge factor is a number somewhat larger than one that depends on coding methods, error rate requirements and the like. For an application like yours the antenna bandwidth is probably not too big an issue. Your bandwidth requirement is likely to be only a few percent of the carrier frequency (433 Mhz). Antenna bandwidth becomes a big issue when bandwidth is tens of percent of the carrier.
You asked if power from the antenna is transferred in currents and magnetics fields like in motors. Technically the answer is yes, but there are big differences. In motors the current flows in closed electrical circuits of the kind you are familiar with. The magnetic fields are time varying, but fairly uniform over the distances of interest. In a radiating antenna, because of the frequencies involved current (charge) can flow into and out of a conductor from just one end. The electric and magnetic fields vary over the distance between the Tx and RX antennas as well as along the length of the antenna itself. At the frequency involved they will go through a full cycle from high to low and back up again in a little over a half meter, and over the ranges typically used for these kinds of links may go up and down dozens of times.
Saturation of the front end means much what it sounds like. The first stages of a radio have a lot of gain so they can detect a tiny signal. While most have some means to adjust that gain based on the strength of the incoming signal, too much power can overwhelm them. It is much like looking into the sun, and finding that you cannot see anything. Since power doubles each time range halves you placed a very powerful signal on your receiver when you placed the antennas very close together. As others have said it is not likely to have caused permanent harm.
As far as I can tell, all of the answers everyone has given you are correct. Hopefully one or more of the slightly varying points of view can help you get a handle on this.
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Catalina, I won't deign to say I have a grasp on it, but with everyone's help I have a little better understanding of it... aspects of it at least. That's something.
For kicks, I put the scope on the radio to see if I got a better picture. I'm not very proficient with it, but managed to learn a little more. The signal is pretty dirty. Lots of artifacts. I get a pretty good square wave at the receiver but only up to about 2 meters. After that, I start getting little hiccups.
I'll say this again, my arduino sketch is about as primitive as one could hope for. I need to set up a timer interrupt on the carrier to ensure a clean signal to the Tx. Right now, I'm simply relying on a delay() statement in the main loop to control the frequency of shift from high to low and back again.
For those unfamiliar with arduino code, it simply works by using a continuous loop. Start loop, do the instructions one at a time, restart loop. By using that loop, I can alternate between high and low with each iteration, creating a square wave. With minimal instructions, I can get about 35kHz at the pin.
while the wave looks nice and clean at the transmit pin, what I get at the receiver output is kind of echoed. I can still see the basic square wave, but there's like a light ghosting (my term) of that wave on top of the first, more intense wave. I played with a delay instruction at the end of the loop. Just adding a 1 mS delay cleaned up the wave pretty good. 2mS was better. 10 to 20 mS seemed to be the best, but again, beyond 2 meters and I started getting artifacts. That's still with the monopole antenna. Wiring this with jumpers on a bread board isn't helping? Maybe this isn't much help, but I added a 6" jumper wire to the ground connection on both radios to act as a dipole. I couldn't tell that it helped. I'll try to find some time tomorrow to do a proper antenna. -
FWIW, I've been meaning to do a video on how to make an FM transmitter with a $0.50 8 pin DIP PIC on a breadboard. I made a soldered up version and use it as a battery powered signal source for measuring receiver sensitivity at VHF when up towers checking coax and antenna phasing harnesses. It generates a clear 1kHz FM tone at VHF using the PIC's NCO, and I take the 13th harmonic of the NCO and filter and attenuate it appropriately to measure SINAD (a measure of sensitivity).
I made another version which accepted a mic input too. It's low power of course, but it works.
Probably it'd make quite an educational vid. -
I suspect that your Rx/Tx pair is similar to the sparkfun devices discussed at the link below. They expect to see an ascii byte sent serially at a low rate on the digital data pin. With your Arduino code you are sending something which does not correspond to a valid serial data sequence (start bits, data, parity and stop bits) and thus gets lost in the shuffle. The encoder and decoder do the best they can with what is received from the Arduino, but as you have found the results are sort of random.
The fix would be to use the Arduino serial commands to send serial data to one of the output pins, which would then be hooked to the Tx. You could loop to send the same character continuously, particularly while you are debugging.
https://www.sparkfun.com/datasheets/RF/KLP_Walkthrough.pdf -
I suspect that your Rx/Tx pair is similar to the sparkfun devices discussed at the link below. They expect to see an ascii byte sent serially at a low rate on the digital data pin. With your Arduino code you are sending something which does not correspond to a valid serial data sequence (start bits, data, parity and stop bits) and thus gets lost in the shuffle. The encoder and decoder do the best they can with what is received from the Arduino, but as you have found the results are sort of random.
The fix would be to use the Arduino serial commands to send serial data to one of the output pins, which would then be hooked to the Tx. You could loop to send the same character continuously, particularly while you are debugging.
https://www.sparkfun.com/datasheets/RF/KLP_Walkthrough.pdf
The circuits may be the same but they definitely look a little different than what I have. The receiver doesn't expect to see data byte-wise... That's in the code. The transmitter is merely sending the data bit-wise. However, using the serial coding would possible improve the transmission, but I doubt it. Since these are discreet tx and rx rather than transceivers, there's no way to correct errors. Serial may have a parity check but it doesn't do much good if it can't ask for the data to be resent.
Like I said in the OP, my code is very primitive. Serial isn't great, but I could see it may help. If nothing else, I might attempt tweaking the trim pots if I can see the output. Thanks for the link -
Thanks for trying. I got the first part down to just past "Second." Then, it was like listening to someone with a heavy accent where you get every other word or so. lol. You made me realize one thing for sure... I've been conflating (in my mind) bandwidth with data bandwidth. Maybe they're related? I don't know.
Indeed they are Amount of information, used bandwidth and noise. It's called Information Theory, and if you want to learn more about it (without the hard mathematical aspects) there is a very good book written by John R. Pierce.
Now, let's link this relationship between amount of information, bandwidth and noise in an intuitive way
What happens when you are speaking to someone in a noisy place? If the place is really noisy you have to slow down or it's almost impossible to understand you. Right? Information is transmitted as a series of symbols (imagine words, phonemes...) and noise blurs the differences between them, making it more difficult to discern one symbol from another one.
Now, the bandwidth. You have noticed that voices on the phone sound dull. Generally intelligibility is not affected much, because the telephone exploits the fact that our brain evolved to rely on a relatively small bandwidth (200 Hz to 3.5 KHz) in order to understand speech. Anyway, depending on the voice of the other speaker, sometimes the telephone can make it harder to understand, especially when a weird, unusual to you accent is involved.
What happens when speaking face to face or listening to a HiFi recording of a voice? The bandwidth is much higher, say, 20 Hz to 20 KHz (although as you age you lose top end hearing. And there is indeed more information present. You understand better, you can speak much faster and still be understood, and you perceive more aspects about the voice, which, well, are also information.QuoteMorse code is how I think of my little carrier. In my mind, it's the model I'm trying to use. Transmit, don't transmit. By varying the time of transmission against a clock at either end, time on, time off in terms of clock cycles, I could create my own comm protocol. I'm sure there are more sophisticated ways, but this was my first endeavor.
Morse is not very convenient for that. That's the reason why FSK was invented. It's like using two flashlights, one red, one green, for example, for bit values.QuoteSo, bandwidth is the difference between the lower and upper frequency used. So that's FM, no? Does AM also have bandwidth? It doesn't seem to me that it would. My little 433MHz transmitter doesn't have the ability to transmit AM, right?
Any alteration you make to the signal takes bandwidth. On amateur bands it's funny(*) when someone transmits in Morse with an ancient transmitter, they usually pollute half of the band with clicks because the switching is too abrupt. So, AM, FM, SSB (which is a form of AM), Morse, or any other modes, if there is any information transmitted there is a bandwidth used.
The only signal that takes no bandwidth is a continuous signal. But you can't transmit information with it. Whenever you switch it on and off you are "altering it", and using more bandwidth. It is impossible to transmit information for freeQuoteI might be wrong about my little transmitter now that I think about it. There are three terminals, besides the antenna. Vcc, Gr. and data. Is data doing more than turning the oscillator on and off? Is it transmitting a slightly different frequency when it's high or low?
I'm not familiar with them, but someone said FSK.
https://en.wikipedia.org/wiki/Frequency-shift_keying
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At $600 bucks
, it looks like I will have to forego ever owning a VNA.
Not sure if this helps, TI sells this sub 1Ghz RF spectrum analyzer (Link), and for sure even I don't have any clue on RF thingy, but it helps me on tinkering at 433Mhz stuffs, like orienting the antenna or finding the sweet spot.
It created buying frenzy while ago when TI "discounted" it down to $25 (incl s/h)
-> Here, too bad its back to $250.
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At $600 bucks
, it looks like I will have to forego ever owning a VNA.
Not sure if this helps, TI sells this sub 1Ghz RF spectrum analyzer (Link), and for sure even I don't have any clue on RF thingy, but it helps me on tinkering at 433Mhz stuffs, like orienting the antenna or finding the sweet spot.
It created buying frenzy while ago when TI "discounted" it down to $25 (incl s/h) -> Here, too bad its back to $250.
What makes them so expensive? It's hard for me to justify even $250 bucks for something like a VNA when there are so many more essential things I should have... like a bench DMM, proper Bench PS... etc Now, $25 I can handle!
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I know what impedance is in terms of ac circuits, but when I look at my 6 inches of copper wire, all I see is 0 ohms resistance. How do I get to 50 from there? See why I need the basics. I don't even know enough to ask what I don't know?
It all depends upon your viewpoint---- replace your transmitter with a DMM on resistance range,& your piece of wire will read as "infinity".
Am I to understand you're saying only attach one lead of the DMM? Otherwise, I don't see how you get infinity.To RF,your length of wire when used as a part of an antenna possesses both inductance & capacitance,as well as
resistance.
At resonance ,the inductive & capacitive reactances cancel,leaving you with real resistance,& "Radiation Resistance"
This is not a real physical resistance,but looks like one to the external circuit.
Many people get "all bent out of shape" trying to get their heads around radiation resistance,but things which are not real resistors,but act like one are common in Electrical Theory.
One such is internal resistance in a Dry or a Wet Cell,where the internal chemical reaction decreases in activity as the cell becomes flat,looking to the external circuit like an increase in resistance.
Another is in an Electric motor .
Running unloaded,it looks like a high Inductive Reactance,& draws a small current,lagging the input voltage.
Apply a mechanical load,& the current increases,with the lag decreasing towards the resistive case.
It looks like a resistance in parallel with the motor inductance,but is really caused by the mechanical load.
In the same manner,the act of radiating electromagnetic waves from an antenna looks like a resistive loss,in phase with that caused by the real resistance,but is,of course,the whole object of the device.
A resonant 1/2 wavelength dipole is (in free space) about 70 Ohms,that of a1/4 wavelength vertical,half of that.
There is nothing magical about 50 Ohms,it is a standard coaxial cable impedance,& luckily,many practical antennas are closer to that value than the theoretical one.
50 Ohms has become the standard for RF interconnections,as it makes it a lot easier to measure signal levels,etc.
I am guilty of being unable to get my head around it. Unlike the examples you site, motor reactance, battery resistance etc, difficult as they are for a newbie to wrap their head around, those things don't exist outside of a circuit! Antennae require a whole new concept where one end of a wire is simply dangling free. The "resistance" or impedance of that antenna isn't quite so obvious. In fact, I'd venture that antennas would probably be an easier concept in which to get ones head around, if they were taught that before possessing conceptual knowledge of circuits.
In the real big,bad world where antennas live,there is no such thing as "dangling free".
We need to look at a dipole first,as it is the easiest to explain.
A half wavelength dipole consists of two quarter wavelength wires fed from either side of the RF source.
There are reasons why feeding one with coaxial cable is not optimum,but lets ignore them & say one leg is connected to the coax braid & the other to its centre conductor.
There is capacitance between the two legs of the antenna,& inductance along the legs.
(There is capacitance between all objects,& all conductors possess inductance--the familiar coil shape is so we can get increased inductance).
Also,of course,there is real resistance in the circuit.
An antenna is,thus,an LCR circuit,& has a resonant frequency.
Radiation Resistance:-
So-called "radiation loss" occurs in all AC circuits,& efforts are made to minimise it.
Antenna designs wish to maximise radiation so they can do the job they are intended for.
At resonance,XL & Xc,cancel,leaving the antenna impedance (between the two feedpoints) as Z=Rr+Rloss.
Where Rr refers to Radiation Resistance & Rloss to "real" resistance,(which will dissipate RF power as heat,hence it is "lost").
In a well designed antenna Rloss is very low,compared to Rr.
So far,we are talking about a dipole,but what you want is a single 1/4 wavelength element vertical antenna.
But what happens to the missing leg?
Many books show a quarter wave vertical as "working against ground" with one side of the feeder going to an earth stake,so the ground takes the place of the missing dipole leg.
Real "dirt" is a fairly poor conductor,so real resistance Rloss increases,making the antenna less efficient.
This sort of design works reasonably well when the "ground" is replaced by a car body,or a tin roof.but that is not always possible.
What is usually done,is to create a "groundplane" of conductive material to connect to the "earthy"(braid) side of the feeder.
At VHF/UHF this is usually a set of four or more 1/4 wavelength "radials" mounted at the base of the antenna.
Without such a "groundplane" a 1/4 wavelength conductor,only connected to one side of the RF feeder will be very inefficient.
http://www.rfcec.com/RFCEC/Section-3%20-%20Fundamentals%20of%20RF%20Communication-Electronics/07%20-%20ANTENNA/Antenna%20-%20Ground%20Plane%20Antenna%20(By%20Larry%20E.%20Gugle%20K4RFE).jpg
With the groundplane elements at right angle to the vertical element,the feed impedance at resonance will be
approx. 35 Ohms,but there is a sneaky trick--bending the grounplane elents down to about 45 degrees,the feed impedance is close to 50 Ohms.
http://vk6ysf.com/146mhz_ground_plane_antenna.htm
He uses more radials & a different frequency,but he gives design figures for one on 435MHz,which is close to what you want.
The frequency range over which antennas are useable varies in proportion to their centre frequency.
Hams quite often use antennas which cover most of their "70cm " Band,which is around 5MHz or so wide.
(Your 433Mhz device is actually operating in this Amateur Radio Band,but such low power devices are allowed by the Licencing Authorities) -
What makes them so expensive? It's hard for me to justify even $250 bucks for something like a VNA when there are so many more essential things I should have... like a bench DMM, proper Bench PS... etc Now, $25 I can handle!
Probably the manufacturing cost ?
As mine was made and assembled in Germany. 
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What makes them so expensive? It's hard for me to justify even $250 bucks for something like a VNA when there are so many more essential things I should have... like a bench DMM, proper Bench PS... etc Now, $25 I can handle!
Well, getting a VNA just to use it once is certainly overkill, despite being cheap. Some instruments can just be bought by a group of friends/club/hacker space and be shared.
That said, before learning of the cheap VNAs available I took some antennas to the University's microwave lab for testing. Do you know the price tag of the Agilent VNA I used there? I one on eBay right now for $29,000 (Agilent E5061B)
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I forgot. There is such thing as a $25 spectrum analyzer.
Have a look at this:
http://www.rtl-sdr.com
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I forgot. There is such thing as a $25 spectrum analyzer.
Have a look at this:
http://www.rtl-sdr.com
Aside from the price and being a dongle, how do they compare (or differ) to VNRs in function? The Amazon rating is positive. I'm inclined to buy one tomorrow. It sounds like these are perfect for someone like me, too many hobbies, too little time and too little money.
To my untrained eye, it looks too good to be true. .... yet it's only 25 bucks. Anyone here have experience with them? -
The RTL-SDR dongles are out of stock at Amazon in the US
poo. "Expecting" new shipment in a couple weeks. Or, I can order from their China dealer (supplies low).
Before I pull the trigger, I did a quick google search and found other brands (some for even less money!). Since I don't know what I'm looking at to compare them, could someone advise me? Connectors, protocols, resolution... etc. What should I be most concerned with. I think they all use the same chip. Maybe there's a better one in the same price range?