I put the receiver unit into a metal box, and left 20 mm length of the antenna outside the box, still the signal be shield, there is some communication though, what can do with this?
Thanks

 

I put the receiver unit into a metal box, and left 20 mm length of the antenna outside the box, still the signal be shield, there is some communication though, what can do with this?
Thanks

This looks like the middle of a conversation that the rest of us have not seen nor heard. Without any context all responses will be guesses, at best.

 

This looks like the middle of a conversation that the rest of us have not seen nor heard. Without any context all responses will be guesses, at best.

Thank you, I edited it.
It is the metal box, shield the RC signal, how can improve it?

 

I think your analysis is flawed. Have you measured the signal strength in the vicinity of the receiver? Have you computed the efficiency of the receive antenna at the frequency of the transmitter. Without some empirical data there is no help for you.

 

I see a problem in that 20mm = 2 cm, which is far too short for an antenna for any common radio control frequency below about 2 Gigahertz. In addition, depending on how the receiver is built, adding a shielding box may alter the tuning so that it is no longer tuned to the desired control signal. Beyond that, a shielded box may also be short-circuiting some portion of the receiver preventing it from working at all. So you do need to describe the project a bit more in order to get answers that make sense and actually relate to the problem. Otherwise all you will get are guesses based on guesses.

 

I see a problem in that 20mm = 2 cm, which is far too short for an antenna for any common radio control frequency below about 2 Gigahertz.

Except that the coiled spring like aerials that come with 443MHz TX/RX are about that long and work well.

 

Except that the coiled spring like aerials that come with 443MHz TX/RX are about that long and work well.

Most of those coiled antennas are resonant and none of them are mounted in locations that they were not designed to be mounted in. That matters a great deal. And so far I have not seen even one word about what sort of receiver is mounted in a shielding box, nor a single word about why it is in the box. Thus most responses include a fair amount of guessing. And one guess may be as good as another, meaning not much.

 

At church we had microphones with a wire antenna about 24 inches (going from memory of a very long time ago). The issue with that antenna was that it would often get pulled and broken. They wanted a shorter antenna. Now, I'm no antenna expert, and again, I can't remember all the details, but I think I made a coil antenna. The coil was constructed from either a 2X length of copper wire or maybe it was 4X length. As I recall, they worked. However, a new problem manifest itself - the stubby antenna's were getting broken off.

Once I set up an antenna for my radio. Regular AM / FM radio. It was in a metal building inside a metal building. Only one radio station had the strength to be picked up in that environment. I had a spool of coax cable and routed it from my lab through the ceiling into the main building then along the roof of the inner lab to the outer wall where there was a small hole. I pushed the coax outside the building. But I didn't get any better reception. So I cut a length of (about 4 feet) coax and soldered the inner conductor to the outer shield turning the four foot section into an aerial. I got better reception on the low end of the dial but poor reception on the high end. So I shortened the antenna an inch at a time and checked for reception. When it seemed balanced I had a tuned antenna outside the building that could receive radio signals and my radio was the only one in the entire building that could get all the stations. One trick other people were using was to use the phone lines. There were often several wires in a cable that were not connected to anything but were run throughout the building. Pick the right one and you could get "More" stations, but that was because in places the wires ran close to the outside of the building. Still was shielded and results were dismal, but were better than no antenna at all.

So in short, the length of the antenna has a lot to do with the signal it's receiving. If the antenna is the right length it will do its job for an intended frequency. Keep in mind that a LONG antenna receives low frequencies whereas a SHORT antenna receives high frequencies. Vary the length and you vary the received frequency. Resonant frequencies can also occur. Suppose you have a 2 foot antenna working well. A 4 foot antenna will do just about the same job because it's receiving a resonant frequency. However, the longer antenna will also pick up lower frequencies as noise. A 1 foot antenna will work but be less efficient because it's only receiving half the frequency. Coil loaded antennas overcome some of these problems. CB radios operate on a frequency that can require a very long aerial. Back in the day we had "Quarter Wave" antennas that were 1/4 of the wavelength of the transmitted frequency. They worked well enough but were somewhere around 8 or 9 feet in length (if memory serves). Shorter antennas were possible because of base loaded coils or mid loaded coils. CB radios may be before your time but you still see them on Big Rig trucks. They can get pretty fancy too. So your antenna problem is largely due to an incorrect length antenna. And as someone before has mentioned, shielding the electronics will change the way it receives signal. My garage door opener has a short antenna wire, about 6 or 7 inches in length and the unit is housed inside a plastic case. If I want to improve reception I'd have to increase the length of the antenna in multiples that are equal in length to the original antenna. But again, a longer antenna can come with unwanted results. I'm leaving my antenna alone.

 

A company I worked for made transceivers for various purposes including race cars. They were sponsoring the Le Mans 24 race and as I was known as bit of a fan (see picture left) I was sent as part of the technical support. We had a problem where one car could communicate perfectly with the pits from the pit lane but not when it was out on the track, even the section that runs close to the pitlane.

This was eventually tracked down to the antenna bending flat to the bodywork in the 100mph+ wind when the car was out on track. It's not just the length that matters!