ulianov said:
I put the pictures from the antenna at **broken link removed**
The circuit is non isolated and the antenna is the ANT-433-SP (splatch). Any sugestions? The third antenna I am using (as shown in the web page) uses lambda/10. Anything bigger does not fit in the space I have for the project. The entire reception circuit has to fit in the wall plate. So lambda/4 is too big. Any other antenna I could use? or any way to improve the ones I have?
In the first picture it is not clear if there is a ground plane on the other side of the board. Since I see no connection from the visible ground plane area around to the other side, I must assume there is no back side ground. In this case, it is obvious that your antenna counterpoise is awfully small which would contribute quite a lot to poor radiation performance. The other problem may also be that the trace leading from the receiver chip to the antenna pin 1 is not a 50 ohm transmission line. In this case, the impedance mismatch between antenna and IC may be very severe, causing considerable loss of signal.
In the second photo, I cannot tell how the connection is made to the receiver, an important issue. The ground plane provided for the antenna is getting better, but still is not as large as recommended by Linx. Since the antenna element of this type is only one half of what is needed for resonance and for adequate impedance matching, your ground plane must provide the other half, or the "counterpoise". This is necessary for RF currents to flow most strongly and therefore for radiation coupling to be the strongest. Your counterpoise should offer a quarter wavelength of conductor in any dimension projecting away from the Linx antenna. So you have the right idea with the shape in picture 2, but the length may still be inadequate. However, I am concerned that the bigger issue is that you have made no effort to correctly impedance match the antenna to the receiver, and so may be losing quite a bit of performance without knowing it.
The third photo is not useful. I cannot understand what is shown, as it appears to be a piece of metal with solder around the edge and some dark spots near the center.
To best understand your problem, I need to see exactly how you have connected the antenna to a pcb, and exactly how you have connected the antenna pin 1 to the IC. When I see this I refer back to the Linx data sheet which shows that you need a 50 ohm transmission line connecting the IC to the antenna, and the antenna should have about 3.3 inches of ground plane projecting away from it.
Another point to consider is that if you are putting this assembly inside a metal outlet box the metal of that box will severely interfere with the impedance of the antenna and so will degrade its performance. A plastic outlet box won't have this problem. It is best for the antenna element to project out and away from the majority of nearby metal.
What kind of range are you hoping for and what are you transmitting with?
Without any RF instrumentation to measure impedance matching and radiated energy, the best way for you to try and get improvement is to cut-and-try with this antenna and perhaps various shapes of wires. Are you planning to install into a metal box?
One idea that sometimes works (and sometimes doesn't) is to use two planar antennas set up to form a dipole. The antennas would point in opposite directions and share a common feedpoint, kind of like a butterfly's wings. In this case, there should be no ground plane under either antenna, and the IC would be in the body of the butterfly which must be kept as small as possible, and maybe even put off to the side and not between the antenna elements. Unfortunately, even this might not work if it is inside a metal box.
Small antennas are often made small even though they are electrically similar to a quarter wavelength by surrounding the antenna metal with a dielectric with a higher dielectric constant than 1.0 For example, the Linx antenna has a conductor embedded inside or on the surface of ceramic which has a dielectric constant of anywhere from 3 to 10 or more. The electric length of the antenna would be increased by the square root of this dielectric constant, so that might be able to make a "quarter wave antenna" that is 1.7 to 3 times smaller than a wire in air.
You can also get away with bending the end of a quarter wavelength copper wire to right angles and still have it work pretty well. You can do this with up to half of the overall length, so it can fit into a smaller space. Another trick that we use to shorten an antenna is to place a large area of metal on the open tip of the antenna. This is something we call a "capacitive top hat". If you had a quarter wavelength copper wire, you could shorten it to half that and add a one inch round disk or ball of metal at its tip and it might work well.
As I just typed this, I realized that it might be possible to use a metal switch box and use the two sides of the box as a pair of oppposing capacitive top hats. In this case, you would have to place the antenna feedpoint at the center of the face of the box ( a very inconvenient spot) and run copper wires out from that point to touch the metal sides of the box. I can't explain this very well in words here, but perhaps you get the idea.
One last comment and I'll stop. An antenna is very sensitive to its nearby environment. You cannot put an antenna inside a conductive box, or beside a bunch of cables, and expect it to work as if those things are not there. Most of the antenna work we see in the industry is in adapting common types like dipoles, monopoles, PIFAs and so on to fit their environment. Anything metallic and most dielectrics that are closer than, say, two wavelengths, to the antenna will have a strong effect on that antenna's impedance match and on its radiation. The closer, the worse. So, installling an antenna inside your box has to be done with some knowledge of antenna design, if you want the best possible performance. Without the knowledge, you can still go for it, but settle for something less than perfect.