antenna-theory.com

[kochevnik]

I am a newbie to electronics and since this is an antenna system albeit a strange one I thought you guys might have some answers for aproject I am working on.

I am working on a project that uses two parallel 'leaky' coax cables to determine when a human being crosses the wires. This is based on student paper (Simon C Wong) that I found on the internet :

http://people.ee.duke.edu/undergrads/IndStudy03/WongS2003.html

There is a more complete pdf description at the bottom of that page.

In a nutshell, if you run an RF signal thru coax that is designed to propagate signals, then place a second coax wire about a meter away, the signal in the second wire will be altered by the presence of a human - my basic understanding being that the human acts as an antenna. In the paper Wong used a 150 mhz signal (5 watts ?) and then the human body would be act halfwave antenna.

I'm basically a software guy - I've gotten into digital hardware mcu's and stuff in the last year, this is my first venture into the analog realm. So I bought an old Tek analog scope off ebay, learned how to use it, got an old eico 324 rf signal generator also off ebay and set up two coax wires parallel about 3 feet apart. The first wire (I call the primary) is hooked to the eico red lead and I pumped a bunch of different frequencies down them - tried from 1 to 145 mhz (max on the eico). The secondary coax, acting as the antenna, I hooked to my scope.

One important thing, since I didnt have any 'leaky' coax, I just stripped two coax wires and wire them up so that the signal is output on the shield of the primary and the oscilloscope probe is hooked to the shield on the secondary. From my reading running a signal down the inner core wire would be useless as coax is designed specifically to shield those signals, by 'flipping' the wiring I assumed that the signal would propagate a lot more and be more easily picked up on the secondary. Oh and the secondary I soldered a resistor from the core wire to the shield. Ground from the scope probe is hooked to the ground on the signal output of the eico.

So far so good.

Wired it up, placed the parallel coax on the floor, fired up the eico, sent a bunch of signals thru it from 1 mhz to 145 mhz - and Voila ! when I walk over the secondary cable the signal on the scope literally hops off the screen and then back on. Some freqs seem to work a little better than others, at 36 mhz seems to work best but I think this is because the eico has range knobs and the high range signal is not as strong as the second from the last range.

So here are my main two questions :

1) When I say the signal hops off the scope, I mean that from my view of the scope screen (using human eyeballs) the entire waveform maintains its shape, it just appears to jump up off the screen then come back down - but again, to my view, the amplitude does not appear to change. As a complete newbie I don't understand what phenomenon I am seeing - can someone explain please ? And to add I'm not entirely sure what I am seeing is a real signal or not ?

and 2) Can I use a DAC to 'catch' this signal and process it ? Since from my view the amplitude is not changing, I don't see how the dac would catch this ?

Appreciate any help / advice anyone can give me.

[bigSteve]

Nice post.

Ok - so the wires are absolutely not acting as antennas in the traditional sense of antennas. To be even an extremely inefficient antenna, the length of the wires must be about 15% of a wavelength - so are your wires at least 1.25 meters long?

The human body acts as a lossy dielectric - this means it absorbs energy and also can "tune" the way energy spreads. Basically, your body alters the capacitance between the wires. Your body in no way acts as an antenna - an antenna must have a feed and a ground point (for instance, see the dipole antenna - it has two sides. A human body is just one nonconductive lump for rf energy: http://www.antenna-theory.com/antennas/dipole.php)

Now, what you are observing looks like a transient phenomena - it happens for a quick second. So definitely you are not changing the antenna efficiency of your system by having someone walk over there. If you were the effect would stay constant. It would be an incorrect assertion to say the body acts as an antenna "briefly".

This is more like how touch screens work - the finger (and its associated capacitance) alters a circuit behind a touch screen. This transient change in voltage then (caused by changed capacitance) can be picked up. In no way would a touch screen ever be considered as a bunch of small antennas (although possibly you could argue it).

Note also that NFC antennas are not antennas either, in case you wanted to read about that and further expand your antenna knowledge:
http://www.antenna-theory.com/definitions/nfc-antenna.php

[kochevnik]

Thanks for the reply - it explained some of the things I saw and did not see

l read the antenna sections on the site and spent a bunch more google time.

So to answer a few things - you were right, I don't think I was actually seeing a human body effect. First my Tx and Rx 'antennas' were basically just short sections of coax and they were maybe 1.5 feet long. The other thing I found after posting this question and experimenting some more is that the artifact I was seeing on the scope was actually coming from clothing not from the body. I found this when I had my kids jump over the wires - we found that certain pants and shoes would make the signal on the scope jump and others - including just bare legs would not. So I assume this is some kind of static electricity effect as we could take a pair of fleece pants and shake them with no body in them a see the same change on the scope.

I do have some baluns from another project so I am going to build a dipole antenna and a loop antenna matched to around 100 mhz and see if I can get some better / different results.

In your reply you said that the main effect of the body was that it changes the capacitance between the Rx and the Tx 'lines/antennas' - I know in the Wong paper above it seemed like they were not really sure what process was taking place in altering the received signal - he said that the human body at around 1.7 meters tall would act as a half-wave antenna for a 150 mhz signal.

So a dumb question - you mentioned that the human body 'alters the capacitance between the two wires (the Tx and Rx) - so what is the best way to measure this change ? On another forum someone mentioned using an rms rf detector to do this - is the change in capacitance going to change the amplitude of the rx signal and would I see this on a scope as a change in the peak voltage of the rx waveform ? Or is there a better way to do this ?

In the Wong paper the charts he shows looked like the amplitude/voltage of the signal changed when a person came near the wires - but he never had a picture from his scope, just some 2khz sampling waveforms, which would be good enough since the human is moving so slowly and the changes he saw lasted for a (relatively long time - 1/2 second +).

So what I am getting at is, if the human body affect is real, then I should see the peak voltage of the waveform on my scope either jump or drop, yes ?

Thanks again for you help.

- k