Antenna Coupling Problem

In summary, Rob designed a second antenna that was resonant at the same frequency as the transmitter and placed the transmitter in a metal box with the second antenna protruding through the metal box. He was able to make this work, but the problem he is faced with is the math behind it. He would like to put together information on how this is done. This is basically like a transformer coupling, but done at 434 MHZ. Can anyone provide some information.
  • #1
I am looking for some information on antenna coupling.
I am looking at coupling energy from one transmitters antenna into another antenna. Basically instead of using the antenna on the transmitter itself, I need to use the second antenna.

This is for a project at work. I designed a second antenna resonant at the same frequency as the transmitter. This second antenna was basically a number of loops of wire wrapped around the first antenna (they were isolated by plastic).

The transmitter is placed in a metal box with the second antenna protruding through the metal box. I was able to make this work, but the problem I am faced with is the math behind it. I would like to put together information on how this is done. This is basically like a transformer coupling, but done at 434 MHZ.

Can anyone provide some information
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  • #2
The kind of RF transformer you are talking about was common when vacuum tube amplifier stages were transformer coupled. I'd go to your local university library and take a look at some of the old books like RCA's Audiotron (or was it Radiotron) Handbook, or Terman's books on radio engineering. You'll be looking in the 1940's and 50's, maybe into the 60's.

Another good general source is the ARRL Radio Amateur Handbook. Again older editions may have more on this than new editions that are digital-oriented.

You'll find that the frequencies discussed were generally lower but the issues of inductance calculation, coupling, stray capacitance, etc. will be the same.

As for getting more help here, you'll need to provide much more information about you're device and application.
  • #3
RF design is obsessed with impedance matching to ensure max power transfer.

I'm assuming the output impedance of the transmitter is 50 ohms, so you have to make the antenna look like 50 ohms by placing a matching network between the transmitter and the antenna.
  • #4
Thanks for the replies
Actually I have a 434 MHZ transmitter I designed last year (the antenna was a trace antenna) This is one product we have. The idea was to be able to couple the energy from this antenna into another antenna.

The problem is I can't give to much details as we are awaiting a patent for this design.

But I can say that by designing another antenna that closely loops around this first antenna (and matched to the correct frequency) I was able to couple enough energy into the second antenna and re-radiate the signal out from the second antenna.

Does this make sense?
  • #6
Look into how signals are coupled into and out of waveguides. Also, look into how helical resonators and resonant cavities are designed. Energy is coupled into and out of enclosed cavities all the time with minimul loss.
  • #7
Any waveguide/cavity at his low frequency will have large dimensions. A half wave is around 35cm or about 1 foot.

Rob, can you provide information about what you are trying to accomplish? (For example, why wouldn't a cable work?)
  • #8
Thanks guys
Thats the information I was looking for.
Sorry, I couldn't tell you exactly what I was trying to accomplish as we are in the process of getting a patent and can't share the information yet.
  • #9
By the way. I like this forum.
I have tried others, like rf globalnet but the responses are quite slow and sometimes no responses at all.
This site is setup very well.
Whoever set this up. Excellent work!
  • #10
I can mention what I was designing now.

I design the RF for a tire monitoring sensor for measuring the pressure and temperature in a tire.
We have a sensor that mounts on the inside and just designed one to mount onto the valve stem of the tire.

The objective now was to use this same sensor and mount it on the inside of the tire (customer request). I planned on coupling the energy from that transmitters antenna into the valve stem of the tire such that the valve stem would act like the antenna. The signal level transmitted would be higher as the signal is now on the outside of the tire (rather than inside).

I think the easiest way then is just to work out the math for the mutual inductance to determine how much energy is transferred and then relate that to how much power I am now transmitting outside the tire.

  • #11
One question: will antenna coupling problem affects the superposition of two antennas when they are placed two close, thereby I can not really apply the principle of superpostion here?

Looking forward to your reply!

  • #12
Two antennae placed close together can have a significant mutual impedance and this will affect the input impedance of the driven one. Take, for instance, a Yagi antenna. You could expect an input imp-edance of around 50Ω when an isolated dipole has about 75Ω impedance.

I like the idea of an affordable tyre pressure measuring unit. I am really surprised that such a safety-of-life system isn't a requirement for all new cars. This is a really interesting project.

I am wondering why you have chosen such a high operating frequency for this telemetry. I would have thought that it should be possible to have an inductive coupling system at a much lower frequency. But you will have gone into it a lot more than I, so there's probably a very good reason.
  • #13
If the design is new (and this is for commercial use), empirical measurements are probably a faster (and cheaper) way to go to gain some understanding of the problem space. You can use a "low frequency" network analyzer and treat the two antenna coupled system as a 2-port. This is how antennae are characterized in the "real world" anyway. Contact Agilent - they have systems for this.
  • #14
But how do this for 4G LTE ? Is there any differences ?
  • #15
The system of which the antenna is a part should not affect the RF situation with the antennae.
  • #16
I've designed antennas professionally. I have some advice.

1) hire an antenna consultant. There's no way you will stumble into a good design the way you are going
2) the valve stem is a lousy antenna at the 400 MHz range
3) rubber is easily penetrated by the signal. Your problem may be the steel belts in the tire.
  • #17
I don't understand why the antenna needs to be inside the tyre. To be economic, the transducer and comms would not be part of the tyre, surely. They would be part of the wheel (hence anywhere you wanted)l and a slot antenna as part of the wheel pressing / casting would be an obvious (?) solution. Many mobile phone antenna are of this type. But I still don't see the attraction of 400MHz. A lower frequency inductive link would seem very attractive to me.
I could see how the solution for a retro-fit could be different, however. But the way tyres are removed and replaced is very traumatic. A fragile bit of electronics inside the rim would have a short life!

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