433 MHz spring antenna?

Hi. For some playing with 433 MHz radios I bought a couple of small radio modules, They came with 83 mm long wires. I also bought some spring antennas.

Are the spring antennas any good? Or are the wires better? Also, how should the spring antennas be soldered onto the board? How much straight line should be left between the board and the "coil"-part? Or what should be the overall length of the spring antenna? Or does it even matter?

I've been trying to find some info in the web, but google doesn't seem to be my friend on this.

Spring antennas do not work as well as straight wires about 17 cm long, and neither works nearly as well as balanced dipoles, shown below. I get over 300 meters line of site range with the setup in the photo.

Connect one inner end of the dipole to ANT and the other to GND on the transmitter and receiver.

|500x298

The spring antennas will probably work, but as suggested the balanced dipole type is a lot better, maybe as much as 4 times the range.

83mm is a bit short, I would expect around 17cm.

A single wire will work, but its really menat to be used with an effective groundplane, and this is often not the case.

A balanced dipole does not need a ground plane and can be improved further with a choke balun near its centre, see picture. And yes the antenna is steel tape rule, very effective and long range.

|500x375

That's just too long for my purposes. The device with the radio is supposed to be "portable". Hopefully the whole thing could be put inside a smallish plastic box.

Anyway, if a 83mm wire (came with the module) or spring antenna were the alternatives, which is better? (Could it be that the antenna hole is about 3mm away from ground plane, and that's why 83 mm, not 86 mm wire?)

And still, if, for some reason (and out of curiosity), you need to use the spring antennas, how do you solder them? I mean, how long the gap between the ground plane and the "coil part" should be? Or maybe the point is the total height from the ground plane? Or does it matter at all as long as the "coil part" has right dimensions?

And have I been conned, because the measures of the spring antennas are pretty different that web gives you for 433 MHz? (Mostly "coil"-diameter of 5.5 mm is given, my antennas have 4.3 mm diameter, 20 mm "coil" length and 12 mm straight part.)

For low cost modules such as the RFM69 etc, the best length for maximum signal is going to vary a bit anyway, the modules do not use precision components in the matching circuits.

The coil antenna, or single wire, is meant to be at right angles to the ground plane, i.e a horizontal ground plane and vertical antenna. Arange the antennas any other way and you will loose range, which may or may not be important, you did not say what the application was or the range you were after, so we can only guess.

The applications are still somewhat open, but it would be enough if it works about 20 meters away and through a couple of thin walls - say 8 mm plywood...

For some other things (like a greenhouse monitoring) I'll probably take the advice and use a half wavelength dipole (both lines 1/4 wavelength). More distance and thicker walls.

And I'm still curious: how are the spring antennas supposed to be mounted. In the link you can see that the end of the antenna is not bent. There is no indication or instructions how "deep" to mount those. This way: https://www.dropbox.com/s/4u5jpmoavgkr0i3/20170205_211453.jpg?dl=0 or this way: https://www.dropbox.com/s/61zhh5a6ev25k9g/20170205_211554.jpg?dl=0

Best to keep some distance between the coil and the electronics, although the tip of the coil is the most active part. You could also cut the straight part in half if space is a problem.

Both pictured dipoles are electrically "wrong". A dipole should be driven "ballanced", not with one half grounded. A balun should be included. Practically, mis-matching at that frequency/power/indoors is not going to make a huge difference.

I vote for the "sleeve" aerial, made from a piece of thin coax (with braided shield). Outer plastic stripped ~25cm, and braid folded/pushed back ~17cm over the outside of the coax. Then the core (not stripped) also cut back to ~17cm. A ~35-40cm piece of heatshrink keeps it all in place. That type of aerial does not need a balun, and can be used for TX and RX. Easy to hang up (vertical polarization). Leo..

Wawa:
Best to keep some distance between the coil and the electronics, although the tip of the coil is the most active part. You could also cut the straight part in half if space is a problem.

So the length of the straight part doesn’t matter (as such)?

Both pictured dipoles are electrically “wrong”.
A dipole should be driven “ballanced”, not with one half grounded. A balun should be included.
Practically, mis-matching at that frequency/power/indoors is not going to make a huge difference.

I vote for the “sleeve” aerial, made from a piece of thin coax (with braided shield).
Outer plastic stripped ~25cm, and braid folded/pushed back ~17cm over the outside of the coax.
Then the core (not stripped) also cut back to ~17cm.
A ~35-40cm piece of heatshrink keeps it all in place.
That type of aerial does not need a balun, and can be used for TX and RX.
Easy to hang up (vertical polarization).
Leo…

That I must remember for the non-portable things! Elegant.

So the length of the straight part doesn't matter (as such)?

Yes it does.

Grumpy_Mike:
Yes it does.

How?
I mean, how can I find out how much the straight part should be left there.
I haven’t found any hints.
I have seen some stuff about designing helical antennas, but those ones that are sold dont’t seem to match.
On the other hand…
https://www.circuitsonline.net/forum/view/121460
https://www.alibaba.com/product-detail/Factory-Price-Phosphor-Copper-Wire-Antenna_60485517656.html

https://github.com/OpenHR20/OpenHR20/wiki/2.1)–433-MHz-and-868-MHz–Antenna-Design-Examples
http://www.banggood.com/10-Pcs-433MHZ-Spiral-Spring-Helical-Antenna-5MM-p-932781.html
And so on… Can’t make any sense out of those…
How can they all be 433 MHz antennas…

Antenna theory and design is in fact quite a challenging topic, ranging roughly from simple to impossible.

Take a look at some of the antenna design textbooks (and their prices) if you have any doubts.

Overview here.

Those little spring type antennas are normal mode helicals and are narrow band, so only operate over a narrow frequency range. The length of the straight wire at the bottom is not critical. What is critical is the space around the coil part and the open circuit end must be free from metallic objects like wires or the sides of a metal box, or the coil detunes and becomes an extremely inefficent antenna. Allow at least 2 in of space around the coil. These types are common in the rubber ducky types of antennas on hand held walkie talkies.

Antenna design, as others mentioned, is an in-depth topic which people spend months or years of their time in college mastering - it's one of the "hard parts" of electronics, not typically something considered part of hobby electronics.

I empirically tested spring antennae vs wire antennae a year and change ago, and the wire antennae of the appropriate length (even just one, not the dipole antenna) absolutely buried the el-cheapo spring antennae I got on ebay.

Wawa: Both pictured dipoles are electrically "wrong".

Not ideal would be a better description.

If described as "wrong" the OP might incorrectly assume they are setups that should not be used.

A spring antenna is not the same as a helical antenna. A spring can be considered to be simply a tube rather than a wire. A tube used in an antenna has a wider bandwidth than a thin wire. Note that the spring must have its turns touching for this to apply, otherwise it is simply the total length of the wire but with an inductor included.

You can do all the theory you like but it comes down to the velocity factor, that is the actual speed of the electromagnetic wave in the material. This is something that can only be estimated so it throws simple calculations off. The normal figure taken is about 0.6C where C is the speed of light.

For good information about home built antenna see the ham radio web sites. In practice you will need an SWR meter to tune your antenna.

In the practice that is tuning antennas for these low power transmitters (100mW or less) most all SWR meters are of little value, they dont work at such low power levels, an alternative method of tuning is needed.

Get the transmitter to transmit and measure the signal strength (RSSI) with a receiver or scanner at a standard distance. Continue to cut bits off the antenna and measuring RSSI.

You will soon have the length that gives the lowest RSSI, which means the antenna is sending out the maximum signal.

Some hints here;

http://www.loratracker.uk/?p=177

You will probably find that the TX antenna length for maximum received signal level is often quite different to what a calculation might suggest.

Is helical, is not helical... You see what I mean. Pretty confusing.

Anyway, The spring antenna should be soldered so that the "coil" is as far from the board and other components as "conveniently" possible?

I know that the higher frequency stuff - including antennas - are a field of study of its own, and that's why I asked here. Maybe some people here have studied that field and also become interested in Arduinos.

I think it's a bit extreme to study the subject for several years just to figure out how to use those little spring antennas. I think I can understand a simple monopole or dipole for "moderate" frequencies, but that's pretty much it. I'm more of a SW guy.

...otherwise it is simply the total length of the wire but with an inductor included.

I think that's the case here, because there is a small gap between the turns. That would also explain the different numbers of turns, different "coil part" diameter etc., and also the fact that the dimensions differ from the ones given by helical antenna calculators.

Get the transmitter to transmit and measure the signal strength (RSSI) with a receiver or scanner at a standard distance. Continue to cut bits off the antenna and measuring RSSI.

Unless you have an RF anechoic chamber that is not going to work very well, but it is the best a beginner can expect.

the dimensions differ from the ones given by helical antenna calculators.

A helical antenna has the pitch and diameter of the wire as control parameters.

In practice you can tune up anything to act as an antenna. In the war ( second world ) they used to wrap a coupling coil round the base of a tree trunk and tune it up so that the tree was a hidden antenna. But you need the measuring equipment, you can't do it blind.

most all SWR meters are of little value, they dont work at such low power levels,

Insert the word cheap and I will agree with you. You can get SWR meters to work at tiny power levels as I have worked with several. However they are not cheap.

Wawa:
Best to keep some distance between the coil and the electronics, although the tip of the coil is the most active part. You could also cut the straight part in half if space is a problem.

Both pictured dipoles are electrically “wrong”.
A dipole should be driven “ballanced”, not with one half grounded.

Doesn’t matter at low power, the antenna works pretty much the same since you are driving it
at a low impedance point, where the voltages are much smaller than the tip-to-tip voltages on the
antenna - a few percent unbalance. The extra losses of a balun are probably worse than the imbalance.

A folded dipole is a different matter though.

Are those small antennas known as "coil loaded antennas" then?

Hmm, after having another round of googling with the info I got from here, it looks like these antennas are "short whips". http://www.hoperf.com/upload/rf/ANTENNAS_MODULE.pdf