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Is there an antenna that absorbs all wavelengths?

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May 6, 2010
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From my understanding, antennas are designed to function on wavelength, whether it is quarter, half, full, etc... and some wavelengths are very big, as in meters... so I'm interested in an antenna that can absorb all wave lenghts, my hope is that this does not necessarily mean a large (very large) antenna.

I was thinking about a stacked system, almost like a filter eg. layers... where different ranges of electromagnetic radiation were captured / filtered... like siftings rocks, from pebbles, from sand...

then getting a total wavelength...

the idea is a sprayable black body that generates energy from the sun, by absorbing all forms of electromagnetic radiation and channelling it through a cathode/anode which can be attached anywhere... thus a floating "solution" is required and wired parallel similar to Christmas lights... and essentially the energy just finds somewhere to go in this case the wires and thus... you can spray this material on any surface exposed to the sun and start generating energy...

I'd appreciate any thoughts

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Sorry I meant to say "...then getting a total voltage/current... from the collected attenuation..."

I would give up on that idea right away.

There is a general misconception that we walk around in a 'soup' of high intensity energy and all we need to do is tap into it to get a free power source. The amount of electromagnetic energy is very tiny unless you are very close to to a radio transmitter. Even then you would be lucky to harvest more than a few milliwatts, even if you used a resonant antenna.

The other problem is that as you rightly assume, an antenna works optimally at one or more frequencies. A single wire has one natural resonant length although it is possible to give it more than one resonance by fitting tuned circuits to it (aka 'traps'). This principle works from very low frequencies up to a few hundred MHz where the physical size of the tuned circuits starts to become proportionately too big to allow them to be added without upsetting the antenna as a whole. Beyond those frequencies, up to to millimeter waves it is only possible to use to specifically tuned antennas. When you go beyond radio frequencies the whole wire/plate antenna system stops working and you start moving into the realms of optical radiation where photovoltaic or thermal capture systems are already commonplace.

Your 'wiring similar to Christmas lights' idea also isn't practical. Consider that your lights have two wires and the electricity flowing up one wire and down the other is what makes the lights illuminate. There is a 'feed' and a 'return' path between the power source and the load (lights). In the situation you propose where electromagnetis energy is captured you have an antenna and only one wire connected to it. The other 'side' of a radio antenna is either an antiphase antenna signal from the second half of a balanced structure or in the case of a single wire antenna, is the Earth below it. You would not have that second connection in your system.

You could simply push two wires into the ground some distance apart, it would capture the same energy as though it was raised in the air and would get the same radio and solar insolation as your raised black body. Try it and see what you get. You might be surprised, you almost certainly WILL measure a voltage and it will be comparable to the system you propose but you should also see how little energy is available to use. Don't cheat by using different types of ground probes! That creates a chemical battery which is not the same thing at all!

Wow thank you for that. Very nicely explained.

This seems to be the common pattern of my "great ideas" some common flaw that I overlook.

It's not so much "free energy" I remember reading this book in the basement of the Physics Department that said something like "A square mile exposed to light can produce 5 million horsepower" which... I may have mis-remembered but we do orbit a natural Fusion reactor.

( sorry to get somewhat off topic )

How much of a gain, for example, if a space bridge was created and this received energy from orbiting solar panels aimed towards the sun? As opposed to natural solar panel setup eg. mounted on the Earth where the radiation from the sun has passed through the atmosphere.

Thanks again for your post.

Don't worry, it's a frequent topic on here and always ends in the same disappointment!

The general 'rule of thumb' is one square meter (just over 1 sq. yard) receives about 1,000W of solar energy at sea level. You can see why your harvesting idea falls short when in the same area it would be lucky to recover microwatts. A good PV panel is about 10% efficient so recovers around 100W per meter when directly illuminated. I have 8 large panels and 6 smaller ones providing my electricity right now and on a good day they produce around 2.2KW. I also have 25 square meters of heat collectors which nicely warm up my 36,000 gallon outdoor swimming pool! 28C (~80F) right now in early spring and lousy weather.

Going above the clouds is a great idea, especially if you can also avoid the Earths shadow so you get constant insolation. The drawback is of course how to get the electricity to where it's needed. There have been several proposals to launch orbital solar collectors but they have all failed on safety grounds. The idea was to send a highly focussed microwave power beam to a huge collecting dish on the ground. In theory it works, the problem is what happens if the orbital transmitter goes off target (it happens with satellites quite often) and the energy of say 100,000 microwave ovens is pointed into a residential area. A small slip could kill millions and the risk is just too high. Positioning the collector dish away from civilization, for example in mid ocean is also not practical because of the costs to carry the power and the losses involved are too expensive.

For the forseeable future I think microgeneration schemes using PV, wind and water are by far the best options.

Thank you very much.

Perhaps if a space bridge was built, the collector would be on top of this bridge, possibly one on each poles where rotation is approximately 0. But then the bridge poses as a target to be attacked...

I'll have to research on solar panels.

Thank you very much for your time.

Your idea for space based solar panels is not new. But it has no shortage of challenges. This podcast is dedicated to the subject.

**broken link removed**

20MHz to 520MHz is a tiny fraction of the electromagnetic spectrum!

It would be interesting to see how they match the impedance to 50 Ohms over that range though. My first thought was it was an untuned vertical with an impedance matching amplifier in the base, something like an 'active antenna' but then I noticed it was rated at 100W so I guess that rules thought #1 out.

I think quoting VSWR =< 3.5 and 'nominal impedance' might be clues that it isn't really as good as it sounds.


so I'm interested in an antenna that can absorb all wave lenghts, my hope is that this does not necessarily mean a large (very large) antenna.

Just make it very lossy (resisitive), then it can be wideband.

For IEC /FCC radiation field tests from 30 ~500MHz they use dual dodecahedral dipole antenna because of flat response into a constant BW spectral density analyzer.

Yes also like a bi-conical which is varies in quality. Those are most common.

The one you showed seems to have been tested in a small room with fading loss at 90MHz. There are other fractal and butterfly types too.

In a Faraday screen room they use a 2GHz wire for measuring E-fields with optocoupled outputs outside room and filter to equalize the frequency response from 1MHz? to 1GHz and use for susceptiblity testing with a 1kW RF Amp to radiate.

Other details...
When I added 1kHz modulation, I decided on this as a corporate standard in the 80's but found the feedback indicated a null at the room's 1/4 wave dimensions, so the max headroom on AGC of the emitter RF had to be reduced to low values to prevent blasting the cables with 1kW /m RF and resulting false failures.

Yeah, but let's stay on topic. The gain of these antennas is not flat over the specified frequency range.

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