How to measure RF Resonance of objects

Still working that same old problem, eh?

It's not that different from the dipole. In order for a metallic object to be resonant, it has to have dimensions on the order of a half-wavelength; because that's how you get significant rf current to flow when the object is illuminated.

However, the effect of that resonance on the local field intensity will fall off with separation from the resonant device faster the smaller the device is. So it's only likely that the house wiring could cause an effect that you would measure at any reasonable separation.

In other words, if your fridge or filing cabinet were resonant, once you got a few feet away, I would expect the effect to be almost unnoticeable, whereas if it's your attic or wall wiring, you might see that pretty much all over your home.

That makes a lot of sense.

I'd still like to figure out a good method of measuring the resonance of random, even small metal objects, though, simply because I work with metal and I'm curious about making metal art that has resonance. I guess I won't be doing that any time until I figure out how to clean up the home signals environment, though.

If you want something to really resonate, you work from the inside out. You build a resonant chamber. It's made of metal and it shields against signals coming in from outside, so that problem totally goes away.

Not sure I'd call it art, by any stretch of the imagination, but here's a shot of the interior of mine...

This chamber will resonate at very well-defined frequencies related to the speed of light and the inner dimensions of the chamber.

Wow!!! Sounds like a whole other genre of art, to me.

How did you choose the design resonance of the chamber?

Actually I didn't build it to resonate. It's just an artifact of an all metal room. Sometimes it can be useful, other times it is a problem. If you get to where you want to build one, I will send you the equations. In the meantime., the smaller something is, the higher the resonant frequency. Something like your pendant would resonate in the microwave bands.

For your purposes, acoustical resonance, as in a tuning fork, would be much more readily achievable and possibly noticeable by your customers. No one is going to know, nor care, at what radio frequency their artwork resonates, but if they accidentally strike it against a hard object and it gives off a pleasing sound, that would be added value.

Here's an example of the small metal objects I whack out with a hammer to earn my daily bread.

So you're hammering this out, thinking about the RFI, naturally you can't help thinking, this metal too has a resonant frequency of some kind. Puny, of course.

Then I think about wind chimes or big metal hangings... I'd want to know what antenna or other effects they might have. Some version of "..life gives you lemons resonance, make resonade..

Resonant architecture.. that is a higher art.

In the case of the item shown, you need to consider the shape as well. To me, this looks like a shorted loop.

Another consideration is the location of the feedpoint. Most loops are dipoles where the element are curved and a gap is left between the ends. But a loop can be a single element with the ends attached to both sides of the connector or feedline.

Looks to me like you might benefit from one of the antenna books available from the ARRL.

That's a good link 3Doug. I did some googling for "shorted loop" antenna as well. And read some about the tuning features of other loop antennas. My grasp of the basics is not great, I should get the book you recommended, study for a few months...

I might experiment with making some tuned loops/shorted loops, see if I can get clean reception of specific frequencies out of the broadband. That would be interesting, if I can come up with a rational "hit and miss" iteration approach, to compensate for uncertainties... Start with the lower end of the confidence interval, I guess, and cut down to size...

OH well, back to work...

Interesting Topic,

If it is art that you want to reproduce over and over with the same frequency responce, i suggest once you have working models of various designs that you are happy with, you should make solid machined templates for hammering out sheets of metal over for consistency. I would also choose a foam dielectric filler for the 3d jewelry and even consider the length of the loops for each link in the chain of the necklace and be sure the chain has the correct responce characteristics.

However, i think keeping the supporting necklace neutral as to not interfere would be the best bet. I assume you want to increase the rf signals for various 802.11 devices like bluetooth or cellular to give someone a camoflaged antennae that also serves as a piece of jewelry or art. Even making larger art for the house that can increase cellular signal near your bed or recliner to give the house hotspots for better reception would be cool stuff to work on. Inconspicuous items like lamps or tables would be great focal points for inside the house. Anyway that an antennae can be made directional should be a large consideration along with making the opposite side sheilded to direct the focus on a particular signal source if you know the direction of the incoming signal you want to pick up on. Don't forget every signal has harmonics that carry the same information if you want to squeeze every last wave length out of the surrounding area. LOL

good luck

Bill

I can see a potential market for RF resonant art beyond what you are talking about. Most new housing developments have restrictive covenants that prevent hams from putting up towers and antennas. Some get by with disguising antennas as flagpoles, or using wire elements hidden by tree foliage or tacked to the roof edge, or placing the antenna in the attic (especially for VHF and up). RF resonant art would have a better chance of being accepted.

BTW, for years the ARRL magazine, QST, has run a bogus article in their April issue, as an April Fool's Day joke. I remember one about a group of hams that used an old iron bridge as an antenna. The first one I seen was about preparing coax by stretching it. Another was about a new type of coax that had gain instead of loss. The most famous such article was about how all electronics actually run on smoke, because they stop working when you let the smoke out. That article (artickle?) even featured a device for putting the smoke back in.

Ah.. product development.. my favourite topic! I was thinking today about the "teeny tiny 'tennae " for wifi. Why not. I've thought about the use of insulating materials as well: and this is not unheard of at all in the jewelery scene (rubber tubing immediately springs to mind as a non-interfering chain.) Very doable. I had a quick look at foam dielectric filler online, bookmarked for future consideration.

We are in the wireless era here, everybody needs a hotspot. Conductive 'art' that does the trick could be easy to market. I guess the ham operators can make amazing big stuff for their 'secret rig'. "It's a balcony". "it's a fire escape". "It's folk art".. . Right.

There may be markets for the selective shielding effects possible with conductive art, as well. A window covering, if designed 3D could, for example, use polarization to remove some unwanted frequency, then enhance reception of what you do want in the next layer, without blocking all the light either (in my fantasy, it's all possible of course ).

I suspect the people you wish to consult with are radar specialists. This leads to the realm though of what may or may not be considered classified material.

Good Luck and Be Careful

HAMs often use a special device, the GDO (Grid Dip Oscillator) to find resonances.

http://en.wikipedia.org/wiki/Grid_dip_oscillator GDO is a small variable frequency oscillator combined with a level meter. When you tune the GDO resonant objects draw energy away from the oscillator and the oscillator's signal level falls back. This is the "dip". With a GDO you can find all the resonances in the frequency range of the device. GDO also can be used in passive mode and can detect - and measure the frequency of - any emanated electric signal.

Looks like exactlly what I need. Thank you

Well, after searching for a GDO, I found that they are vintage instruments and the range is only up to about 300 Mhz.

Is there an updated instrument that has a full frequency range?

A Zapchecker might be what you are looking for.

GDOs are usually home-made HAM instruments and cover the HAM bands but that's not the technical limit of the principle. Look at these ones (scroll down):

http://www.agder.net/la8ak/m11.htm

That's a good link. Thank you.

To be a "receiver", you need two terminals. If you open a circlular object at some point and attach two wires, you can receive an RF signal (picture a TV UHF loop antenna). Measuring the frequency response can be done with a small RF anechoic chamber, a broadband transmitting antenna (a TV antenna), and a scope to capture the output of your two terminal device. The fast way is to drive the transmitter antenna with a square wave, and capture your device's response. Differentiate that signal to get the (approximate) "impulse response", then take the magnitude of the FFT to see the frequency response.

If you are interested in only the absorption of RF energy of any small piece of art, perhaps the same experimental setup , but use two similar antennae. Capture the square wave on the receiving antenna and measure it's response. Then hang the object d'art in between and measure again. The difference will show how the art affects the RF transmission, and specific frequencies should show up.

Something along these lines would work well for an installation. Hanging art, transmitters, receivers, meters, real time signal images... much entertainment.

Some signals are pretty amazing looking as well IMO. I don't know how to make this one, but I robbed it from the airwaves somewhere in my journeys:

A signal artist could make this a string of birds, flapping their wings... betcha...