[AllyCat]

Hi,

You seem to have come to the correct conclusions, but perhaps for the wrong reasons.

...only 40mm in length with an apparent 13dbm of gain.

dB are a relative measurement, e.g. 0dB means that two signals have the same power (maybe transmitted or received). It's a logarithmic scale so +3dB means that a signal is double the compared value and +6dB is four times the reference. The suffix indicates the reference, i.e. the "m" means a 1 mW reference, the "i" an "isotropic" antenna (which radiates equally in all directions and hardly existis in practice).

An antenna is a passive component so it cannot "amplify" signals except by being directional, i.e. transmitting/receieving in one direction at the expense of other direction(s). A "monopole" antenna (e.g. a 16 cm piece of wire in the case of 433 MHz) transmits/receives most power at right angles to its length (and theoretically nothing in a direction along its length) and thus has a maximum "gain" of +3dBi (relative to that theoretical isotropic antenna).

In a very simplistic fashion, antenna design is all about resonance at the required frequency. Basically, electrons (i.e. a current) rush up the antenna (wire) at nearly the speed of light and at the end find there is nowhere to go! So they rush back again to meet the next "wave" in phase. So you might think that the optimum length would be half a wavelength, but in practice the end of the antenna looks like a "mirror" (to the electrons) so they are turned "upside down" (180 degrees phase shift). Therefore the optimum length is a quarter wavelength. There are various tricks (see later) to "slow down" the electrons but they cannot travel faster than the speed of light so an antenna even only slightly longer than a quarter wavelength works very badly.

The other requirement is to correctly match the impedance at the "feed" end so that the "new" and the "returning" electrons assist each other (to increase the current in the antenna wire and radiate more power). A tuned monopole is between 50 and 75 ohms so any connecting cable used (and the output of the transmitter) must be similar. However, just as the electrons can bounce back and forth along the antenna they can do the same in the feed cable (except their effective speed is typically nearer 2/3 the speed of light). This can cause an unmatched feed cable to change the apparent end impedances, so to avoid "unexpected" effects (unless you know exactly what you're doing), it's best to make the feed cable an exact integer number of wavelengths long.

Now for the "tricks": A quarter wave monopole can be made shorter by putting a "loading coil" (inductor) at its feed end, which has the effect of correcting its impedance. Also, a "rod" (i.e. larger diameter than a wire) antenna has a wider bandwidth, so the length is not so critical. I guess the "helical wire" antenna you describe basically uses these principles. Finally, a monopole should ideally be used above an "earth plane", but alternatively a second (inverted) monopole can be used to create a "dipole" antenna, as is often used for radio and TV broadcasting.

Cheers, Alan.

[werk_ag]

20cm - unusable
1.5m - unusable

... the last two antennas are not applicable at all, even I bought them for 433MHz band!

Regards

Hi Zdenek

Thank you for your labor and time spent to elaborate this analysis. At least for me this matter is always complex.
I have all types of antennas that you used in your tests, and your results seems to coincide to my practical observations, which leads me to make a question: As said, and just based on my practical observation, the last two types of antennas indeed proved to be very bad for transmission purposes, but for reception, are the ones from where I get better results... interesting I think.
If you have the time, could be interesting do the same tests but using the 20 cm long antenna in the receiver?





My main receiver is using one of those 20cm antenna, and the backup receiver, is using the other type (1.5m cable), with the antenna placed outside.

[hornychz]

If you have the time, could be interesting do the same tests but using the 20 cm long antenna in the receiver?

Hi, Werk_AG,
yes, of course, it's a great idea!

I've used in my RX units 16 and 20 cm antennas. Now I've exchanged the both them for 5 cm and both RX units work perfectly on. But at this moment they are in the basement in my office. By default, the main RX unit is in the engine room and, unfortunately, almost touching the metallic copper pipes. And there I witnessed only by the 16 cm antenna, nothing else here does not work.

I'll prepare another test

Regards

[werk_ag]

Hi Allan

A "monopole" antenna (e.g. a 16 cm piece of wire in the case of 433 MHz) transmits/receives most power at right angles to its length (and theoretically nothing in a direction along its length) and thus has a maximum "gain" of +3dBi (relative to that theoretical isotropic antenna).

I think this applies to a helicoidal antenna too, and it can explain why, when the receiver is placed right below the transmitter, which happens with almost all installations where the transmitter is on the roof, placing the transmitter antenna in horizontal position seems to give better results.
Now imagine the z axis as horizontal.

Example of the radiation pattern for a quarter-wavelength monopole antenna (oriented in the +z-direction)

If I'm wrong, feel free to correct me.

[Palmyweather]

Wow, what a great lot of useful information that has been posted in the last couple of days, thank you hornychz and AllyCat.

As Werk_AG is to well aware, I have been having issues receiving data over the last 2 months. This problem started out of the blue in February after no previous known issues from when I started using this system in December.

Together, Werk_AG and I tried almost everything we could think of for a solution, but nothing seemed to work. After Werk_AG questioned the integrity of the antenna itself, I thought I would give the little tiny 40mm antenna a go (which I described in an earlier post) which seems to have done the trick. Everything now seems to be working as it should once again.

The antennas I were using were sold as 433MHz antennas. But now I am starting to doubt this, or there might be something wrong with them. But the information provided here explains a lot too.

Thank you all for this information and a big thank you to Werk_AG for all his help and ideas over the last few months; the support on this website is fantastic!

Kind regards,
Palmy Weather

[hvalentim]

in the last two days I measured the antenna gain in the band of 433MHz. The results are quite interesting and maybe answer a range of questions:

Wonderful work. Thanks for sharing.

I have been using the 20 cm model on the TX side and a 13 cm model on the RX side (both placed vertically).

Works well enough for me (~7 meters apart, TX on roof, RX close to a window just below).

But I am now convinced it is perhaps not the best possible combination.

[hornychz]

Hi everyone.

The technology antenna and wireless transmission of signals is very complex and complicated problem. E.g. - Theoretically it does not matter which antenna at which end of the transmission chain use, but in practice it is not at all true. When we don't give to the transmitting side powerful enough antenna, terminates the signal at the receiving end in noise. Using too powerful antennas can exceed the legal limits and extra "get" lots of reflections from the surroundings. At too powerful antennas on the receiving side we can receive even much more interference.

Neither antenna polarization is not a simple problem. When using 1/4 wave antenna theroretically we should always use vertical polarization, at least a little to emulate half-wave dipole antenna degraded second half. Theroretically, it is also the largest reach in a horizontal field. However, when the transmitting antenna is on the roof, are below the theoretically completely no signal.

And the influence of the surrounding area is also essential - eg. in my measurement, I have to respect exactly where I stand and where measuring apparatus is lying that the results of measurements at least approximately correct.

Therefore, always leave a part of some kind of experimentation and testing, as in the present case and the situation best. But perhaps that is why this part of the building weather stations so funny ???

[edr1924]

Sorry for reviving an old thread but while waiting for the part to arrive I'm reading about the 433Mhz stuff and antenna's

This thread is very informative, thanks to all the participants!



After reading for 2 days now on several fora and watching numerous video's, I came to the (obvious) conclusion that a very good antenna is key to the success of the WeatherDuino setup in my particular case because the temp/hum/rain station will be 20 meters away in the garden

I came across one antenna which had very good reviews in difficult situations for numerous users, appearing on several fora and is not mentioned here so maybe other will benefit:

It from the well known company Aurel, they also make very good Tx and Rx receivers which I already use in my Domoticz domotica system.

This is the antenna: CLICK

Aurel GP 433 Ground Plane Antenna. cost is 36 euro's

  someone on the Domoticz forum mounted it upside down on the top floor!

Any thoughts / experience with this kind of antenna?

[hornychz]

Hello,

This antenna is only suitable for vertical installation, which may be a problem as Werk_AG wrote yesterday in another thread
(TX on the roof and below it RX). On the other hand, it can be quite effective in field use. The basic problem: the datasheet
does not show the profit/loss of the entire assembly. And I'm afraid that the lineup will have a loss rather than a gain because
of the length of the cable. But without the manufacturer's data or measurements, it's just "divination from the ball". Positioning
upside down seems completely nonsensical - the antenna radiates minimally in its vertical axis. And the wire-simulated ground
should always be a ground not a ceiling.

But: I never used this antenna, so this information may not be guaranteed, it results from my experience. The antenna is very often
used to connect radio stations (HAM), eg in the 27 MHz, 134-174 MHz, 430-490 MHz band, ... But the transmission power used starts
at tens of W and ends at kW ...

   

Our recommendation for small antennas with a gain of 2 dBi with their correct polarization still applies (all antennas point in the same
direction, perpendicular to the plane interspersed with all transmitters and receivers).

[hornychz]

If you have the time, could be interesting do the same tests but using the 20 cm long antenna in the receiver?

I am sorry, I haven't done these experiments anymore, I'll try to come back to them ever again.