Question on wireless power transfer !

[farishamam]

i am attempting to transmit a signal using loop antenna transmitter and receiver.
i am using two square loop antennas as transmitters each connected to a function generator (24Vp-p and 20mW power output), one is set to 200Khz and the other to 1500Khz.
the two transmitters are set opposite to each other and the receiving antenna is between them. when only one generator is turned on, the received signal is normal. however, when i turn both generators on, the received signal power is actually higher than that transmitted. (at one point, the total transmitted from the two generators was 40mW and the received signal was 60 mW !)
can anyone please explain this?

 

[biff44]

Patent it, quick!

Are you sure about your power measurements? Like are you actually using a power meter?

 

[farishamam]

Patent it, quick!

Are you sure about your power measurements? Like are you actually using a power meter?

for now, i am relying on a digital multimeter to measure the received current and voltage. this is the only equipment available.

 

[jiripolivka]

It sounds to me that by using loop antennas, possibly tuned and fed with medium-wave signal source(s), you created a really "unknown" device named a band-pass filter. Such filter consists of two COUPLED LC circuits. In resonance, it may be possible that the WAVEFORM you pick up from a receiving loop has a higher-than-expected VOLTAGE. And as you use the voltage as a power measure, the error is created due to this.
You should use a power meter, or, an oscilloscope to see what exactly happens.
Using medium-wave frequencies as you did is also illegal as those are "licensed" frequencies.
Other attempts to "transmit power" over a short distance used "Industrial" frequency band like 13.5 MHz. There the loop antenna can have a single turn and the tuning capacitor is small. Usually you can transmit some power over a distance of loop diameter or so. Tuning the loops into resonance improves transmission efficiency. Recently there were attempts to apply for a patent while equally good filters were designed and patented around ~1900-1910.

 

[DXNewcastle]

I agree with jiripolivka that there are resonances in any r.f. circuit (including the antenna etc.) which are likely to appear as peaks in the frequency spectrum. Sadly, the enhanced efficiency of using a resonant circuit isn't anything new, but is the feature of well-designed transmitting and receiving radio equipment.
I also agree with biff44 that your measurement technique is likely to give you an unreliable indication of received power. I suggest that you try to obtain as accurate as possible measurements of of the actual power transmitted and received.

There are also other factors to consider:
- Any reflections (of the radio waves) will lead to standing waves and also to localised 'peaks' and 'nulls' where the reflections add or subtract from each other.
- When combining transmissions on different frequencies, there will be intermodulation products - these are other frequencies which will be received as signals but which are not transmitted; they are mathematical products of the two (or more) transmitted frequencies.
- There will always be other radio frequencies being received by any radio receiver (from broadcast, private communications and many other users of the radio spectrum). Some of these will be received by your circuit before they are fully "tuned out" and may lead to errors. - - These other transmissions may also combine with your transmissions to produce even more intermodulation products, and these will be received, detected and measures, incorrectly, as your received signal strength.
- And there might be other factors.

I'd be interested to hear how your work progresses, but I suspect that there may be several good explanations for your results that are less exciting than a new discovery! Sorry.
But please don't stop looking!

 

[farishamam]

"I suspect that there may be several good explanations for your results that are less exciting than a new discovery! Sorry."

thank you for your response.
I never said that it was a new discovery ! The reason i posted this thread was to understand what was happening so i can correct whatever it is that's wrong.
As for the interference from signals other than those i transmitted, i checked the output from the receiving antenna with both sources turned off. The signal was a 50mVp-p distorted sinusoidal at a frequency in the Khz range. Therefore, with that low of a voltage, i dont think that outer noise is the issue.