If an antenna acts like a tuned circuit, how can I be sure it has the necessary bandwidth?
Antennas are resonant, so they have a Q and related bandwidth (BW). For most antennas, this bandwidth is roughly 10% to 15% of the resonant frequency. It’s important that the antenna has a broad enough response to pass all of the necessary sidebands to avoid distortion. Most antennas are selective so they can get rid of noise and some harmonics, but you don’t want sideband clipping. If you’re using a commercial antenna, look at the selectivity or BW specification to see that it fits. In antenna construction, the physical dimensions affect BW.
So you have a TX module and a RX module, both with ~ 17cm antennas and you only get 1-2M range?
Are the antennas oriented the same way? So like | | and not like | _ ?
It could be that you have something else on that exact frequency that spoils the fun, but not very likely.
Are these FSK units or OOK or anything else? Can you post a link to the modules?
How did you test the range?
On a more general note to the OP, it took me many years to really think I had any grasp of antennas despite a huge amount of effort during that time and several shed loads full of bent aluminium. The day the penny really dropped was the day I used a VNA, everything just sank into place in a matter of minutes. You can do stuff without a VNA, but it's a bit harder and often much more time consuming. The visual nature of it and the immediate feedback adds so much value to the understanding.
Luckily nowadays you don't have to remortgage your house to get a VNA, there are USB based devices commonly available.
Note that a VNA will only tell you about impedance matching and resonance, not radiation patterns.
Doing antennas is a life long class, it's such a big subject that most aficionados tend to specialise in a particular area.
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.
The cheap 434 MHz modules work very well with a balanced dipole, like those shown below. Connect one end of the dipole to ANT and the other to GND on each module.
On a more general note to the OP, it took me many years to really think I had any grasp of antennas despite a huge amount of effort during that time and several shed loads full of bent aluminium. The day the penny really dropped was the day I used a VNA, everything just sank into place in a matter of minutes. You can do stuff without a VNA, but it's a bit harder and often much more time consuming. The visual nature of it and the immediate feedback adds so much value to the understanding.
Luckily nowadays you don't have to remortgage your house to get a VNA, there are USB based devices commonly available.
Note that a VNA will only tell you about impedance matching and resonance, not radiation patterns.
Doing antennas is a life long class, it's such a big subject that most aficionados tend to specialise in a particular area.
I really appreciate the story. It was helpful... up until the "VNA" part. What the hell is that? I'm guessing something something analyzer? I'm probably overstating my knowledge of radios when I say I don't really know much about them.
If VNA's can be had for little money, what you describe sounds like a HUGE help for me. Right now, I don't know what the impedances of the Rx, the Tx or my antennae are. That's one of the disconcerting things for me.
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?
Do the helical antennas glued down on a spindle perform much better than the loose wires ones?
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".
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.
As I mentioned earlier, you can spend a lifetime studying antennas. There are a lot of abstract conceptual things to understand if you want to go the theory way. Equally, there are an awful lot of rules of thumb to learn if you go the empirical/practical way. Typically most antenna gurus have a hybrid understanding, in its simplest for there's the calculation for a dipole resonant length... and then the 95% end effect thing would be a rule of thumb.
The antenna itself is just one part of the equation. The matching of the antenna is a whole topic in its own right, but it's a key part of any antenna design, so here's a worked example for a simple dipole.
The dipole is a reasonable starting point except that it's a balanced antenna, and most (but not all) presentations are 50 ohm unbalanced (ie, coaxial). Coaxial is generally favoured over balanced twin-lead because it's easier to route physically and its performance isn't affected by other adjacent objects, like other feeders, pipes or walls for example. To terminate (ie connect the coax to the antenna), you could simply put the outer of the coax feed to one side of the dipole and the inner to the other side, and in practical it will usually work apparently reasonably well. The downside is that because you're feeding a balanced antenna with an unbalanced feeder, some of the power will also be radiated back along the coax outer as well as the antenna itself, affecting the radiation pattern. To fix this, you use a balun which is simply a high frequency isolating transformer, with two windings, one you put across the coax and the other goes to your dipole. This can be a simple off the shelf part you can mount on a PCB for low power applications.
Practically speaking, just like the balanced/unbalanced thing, you might well have difficulty in telling any difference in most circumstances, as the impedances aren't a million miles away from each other, but frequently that is not the case, and anyway it's bad form from an engineering perspective to throw away power. So to fix the impedance mismatch, you could use a 25 ohm series resistor to convert that 50 ohms to 75 ohms at the antenna... but then resistors will throw away that power as heat, we want to transfer all the power to the antenna. So a better way is either to use an inductor/capacitor matching network, or as you're already using a balun, use a balun with a different windings ratio, ie 1.5:1 instead or 1:1. Typically to reduce losses, if it's economically effective, I'd rather use a single part than several.
Say, though, you do want to match 50 ohms to 75 ohms in a relatively lossless way at a given frequency with an LC network rather than a balun, how do you do that? The internet is at your service! There are plenty of online calculators for this, for example http://leleivre.com/rf_lcmatch.html
I plugged in a frequency of 433MHz, source impedance of 50 ohms and load impedance of 75 ohms. We are only interested at resonance, so in theory there is no reactive component on source or load, so we set the j ohm entries to zero. (Note that the j notation is to do with complex numbers: don't worry about this for now as it'll just confuse the issue, but if you do want to understand antennas in any depth you'll have to engage with the topic at some point.)
The calculator came out with two solutions (13nH & 3.5pF or 39nH & 10.4pF) and two "Nan" (not a number, ie unsolvable) solutions which simply can't be made with the topologies given. For the two solutions provided, you could use either one, but for this I'd choose the one with the most practical of components and easiest to fabricate, which would be the second one. Why? It's easier to fabricate the second one because parasitics (inductance and capacitance from PCB and other construction effects) on those larger value parts will be relatively less than on the first option. Sometimes though, there may be an engineering reason to choose the first option, for example you might want a DC path which you get with the first option. Particularly at UHF and higher, rather than using "lumped" parts frequently you can fabricate the matching on the PCB itself, using those parasitic characteristics to your advantage, but that's yet another topic.
As with many engineering topics, knowing what matters and what doesn't in a practical sense is frequently a matter of experience, and no matter how many theory classes you do, you'll never know what can be discounted and what's really important, how things interact, and how to prioritise things given a set of design criteria.