Wednesday, February 29, 2012

Rasberry Pi - $35 credit card size computer

 Rasberry Pi was launched today with a big bang.   There are already 44,161 followers on Twitter and  the launch made to headline news in,  BBC and other news sites.

This credit card size little computer has been under development multiple years and is now available at $35 price point.

The price / performance ratio is pretty amazing  and since it is running Linux operating system it should be pretty straightforward to port / run  different ham radio applications on this tiny little computer.

I signed up on the waiting list - let's see how quickly the two distributors can ramp up. This initial batch of 10,000 units was sold in few minutes according to some websites.

73  de Mauri  AG1LE

Wednesday, February 1, 2012

Antenna Experiments - Human Body Resonance Frequencies

Inspired by "Ionic Fluid Antenna" article by  N9ZRT David Hatch I started wondering  whether  human body could be used as an antenna.  

Using human body as an antenna would have multiple obvious benefits such as 
Human body by Leonardo da Vinci
  • mobility  -  ham radio operator would have the antenna always with him / her
  • low cost  -  no need to buy expensive materials 
  • small size -  no additional wires or aluminum tubes to carry with you 
However, there are also a lot of risks involved so I don't recommend experiments as you may hurt yourself by causing RF burns.

It is a well known fact that human body contains ionic fluids and it both absorbs and emits electromagnetic signals.   In medical field these human body properties are used for many different purposes such as diagnostic imaging or diathermy treatment.  

For example MRI (magnetic resonance imaging) scanners  send and receive RF signals at Larmor frequency.  The frequency range depends on  magnetic field strength of the scanner  -  proton  ( 1) resonance frequency is 63 Mhz  at 1.5 Tesla field strength typically used in MRI scanners designed for hospitals.

RF coil for body imaging
In early part of my career I worked  in medical imaging field and thus have some hands on experience working with MRI  RF coils.  Designing RF coils for different body parts is a challenging job.   In order  to obtain best possible signal-to-noise ratio the RF coils need to be close to the target. On the other hand RF field homogeneity is also important to create clinically acceptable MRI images.  Human body couples  with  MRI RF coils so calibration and impedance tuning is important and is typically quick automated operation - many patients have difficult time staying still in the magnet for extended periods. Also,  SAR (specific absorption rate) level  (the RF energy absorbed by human body)  must to comply with FDA regulations.  

However,  most hams don't have superconductive 1.5 Tesla magnets or MRI RF coils in their ham schacks so I wanted to explore some other means how to couple RF energy to human body,  understand at what frequencies it would be resonant and how good of an antenna human body would be.        

After reading several articles on this topic it became clear to me that  human body resonates at multiple frequencies  and these frequencies also depend  on the environment. Human in free space  vs.  standing on the ground would have different RF characteristics like any normally used antenna. 

I wanted to study this a bit further  and  do some measurements myself.  I determined that by using the AIM UHF antenna analyzer (created by W5BIG Robert Clunn) I could safely measure my own resonance frequency. The power output of this excellent device is between 0.2 - 20 microwatts that is well below  SAR limits (for mobile devices typically 2W / kg for whole body).  Safety is obviously utmost important as I did not want to get RF burns or cause other tissue damage due to this experiment. 

Coupling a coax transmission line to my body was the next problem.  I created  a simple coupling device by attaching two electrodes designed for a transcutaneous electrical nerve stimulation (TENS) machine to a 50 Ohm coaxial cable.  The distance between electrodes was 16 inches and the coaxial feed point was in the middle.This distance allowed me to test different configurations in vertical and horizontal modes. Picture below shows the RF feeder device.
RF feeder

I analyzed first this RF feeder circuit itself. Hanging the feeder circuit in vertical position inside my ham shack I measured  SWR and complex impedance magnitude over  1 - 999 Mhz frequency range.  This gave me a baseline to compare how attaching the electrodes to my skin would change the resonance frequencies. The SWR was over 2.0 between 1 and 244 Mhz. The dip in SWR was centered around 252 Mhz corresponding to 46.8 inches wavelength. Given that the coupling circuit is essentially a half wave dipole with 16 inch length the velocity factor of TENS electrode material  is 16 / 23.4  = 0.68.  The SWR also dips below 2.0 around 860 Mhz.
RF feeder  measurement

The next step was to attach the electrodes on my skin while in supine position and run the same test again. 
I changed the SWR scale to get more detailed view.  This time there are 2 main dips in 65 Mhz (181.4 inches wavelength) and 112 Mhz (105.5 inches wavelength)  frequencies. Also, in 600 Mhz range the SWR dips below 2.0.  The prominent dip at 112 Mhz corresponds to 105.5 inches wavelength

Human body resonance
This result corresponds to literature.  Human-body resonances fall typically in the frequency range of 30 - 300 Mhz.

Looking at the SWR graph above  50 MHz ham band represents roughly 1:3 SWR ratio from impedance matching perspective.  I also noticed that the graph changed  as I moved my arms and legs.  With some practice  it is possible to shift the SWR minimum by about 2 MHz just by moving self.

The posting I did in has created a lot of discussion mostly focused on safety risks and RF burns. I want to emphasize that RF safety is utmost important and human body should not be exposed to RF signals.  Even at small power levels RF can cause diathermic heating in tissues causing painful tissue damage.

The fact the human body resonates at multiple frequencies presents also a problem if one is exposed to strong RF fields (like standing in front of directional antenna with RF transmission on).  The RF radiation may cause "hotspots" where induced currents are causing localized RF burns, even though the overall SAR limit is not exceeded.

Ionic fluid antennas may have some special applications but human body should not be used as an antenna.

So - it looks like traditional metal objects are far superior as antennas if the purpose is to effectively radiate the RF power and not to use it to generate heat.

73   Mauri  AG1LE

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