TELEDYNMCS(at)aol.com
Guest
|
 Posted: Mon Jul 16, 2007 7:24 am Post subject: RF skin effect and wave length as it pertains to transponder |
 |
|
In a message dated 7/16/2007 3:01:58 AM Eastern Daylight Time, europa-list(at)matronics.com writes:
| Quote: | So, we're talking about an average handful of milliwatts here rec'd by our body.
Like trying to heat food in the microwave by turning it on for just 2 seconds
each hour. In a week, it will still be room temp. |
Hello Fred,
You are absolutely correct here. It should also be noted that microwave ovens use a specifically designed antenna in the roof of the oven to concentrate and focus their energy directly into the center of the oven in order to heat whatever is being heated. The bottom of the oven is a reflector of sorts that reflects any energy not absorbed initially by the object being heated back into that object. This focusing of energy and reflection of unabsorbed energy overcomes "skin effect" of the object being heated. This heating effect by direct RF exposure was first noted by engineers at Ratheon back in the late 1930's when RADAR was being developed for the military and the first commercial microwave ovens that were sold by Ratheon in the 1950's were called "Radar Ranges".
Skin effect, in the case of, but not limited to, living or once living tissue, is a phenomena that occurs primarily because of the water content in whatever is being subjected to the radio frequency (RF) radiation. Lenz's Law states that the density of current in a conductor (including your body) is least at the center and increases towards the outer circumference of the conductor at radio frequencies. RF energy tends to go around bodies with large amounts of water, iow, the path of least resistance, particularly at higher frequencies. Hence, the term skin effect. Above about 30 Ghz the rules change some, but I won't go into that here. This skin effect is also the reason why no one has been able to consistently show, and never will show, a link between non-ionizing radiation and the formation of cancer, despite the trial lawyer's best, most tireless efforts to prove a link so they can make lots of money.
The amount of RF penetration into organic tissue is primarily dependent of the power level transmitted, the distance from the antenna and the focal point, in any, of the antenna. In the case of a microwave oven heating a piece of meat, what actually happens is the focused energy heats the water in the meat tissue, which in turn heats the surrounding tissues, That's why it takes 1000 watts of concentrated microwave energy centered in the focal point of the antenna to defrost a turkey for dinner or pop a bag of popcorn. It's also why you can turn a good steak into shoe leather if you let it go too long in the microwave.
By contrast, your cell phone operates at 6/10ths of a watt at it's highest power setting (when located farthest from the cell) and radiates omnidirectionally, and thus there is very little penetration, if any, into your body. Similarly, transponders, which typically pulse in "bursts" of around 250-300 watts, broadcast their energy from the antenna, usually a vertically polarized dipole, in all directions. The pattern of radiated energy is determined by the type of antenna used. In the case of a vertically polarized dipole the pattern resembles a bow tie stood on end. Skin effect, coupled by the short duration of the burst of RF, plus the pattern of radiation, results in very little, if any, tissue penetration of RF from your transponder antenna unless you are sitting on it or happen to inadvertently swallow it.
Without loosing everybody in the math, it's also notable that, due to the physics of radio frequency transmission in the free space of our atmosphere, roughly 96% of the radiated energy is dissipated in the first two wave lengths from the source of the radiation, i.e. the antenna. The wave length is roughly determined by the speed of light divided by the frequency of operation. There are other minor factors that determine specific wave length and before anybody jumps up and down, please note, I am ignoring the effects of modulation on wave length and field strength here for simplicity.
In the case of a 1090 Mhz transponder, the peak to peak wave length is about 11 inches (10.83"). So, if you have your transponder antenna more than 11" away from your body, you aren't gong to receive enough RF radiation to make any difference in your reproductive efforts, so don't count on it as a method of birth control. Similarly, to minimize shadowing, you should locate your transponder antenna at least 11" away from reflective (metallic) objects for best results. 3 wave lengths is even better.
FWIW, I put my transponder antenna on the right side of the fuselage, just in front of the joggle at the tail section. Fiberglass is invisible to RF. In fact, most commercially available microwave radomes (covers that shield feed horns from bird poop and ice falling off the tower) and most antenna masts are made of fiberglass for that reason. Therefore, there is no reason to locate any RF antenna on the outside of a fiberglass airplane unless directed (coerced) by your gubmint to do so.
I used an Antenna Specialists dipole antenna. I shaped a piece of balsa wood to fill in the curve of the inside of the fuselage, bonded the balsa to the fuselage side with Redux, then bonded the antenna to the balsa with Redux. I have not had any ATC report any shadowing effects whatsoever and, at altitude, I've been "seen" as far away as 80 nm from the radar transceiver. It did take a rather expensive ($225) 14' piece of precisely tuned, mil spec, precision coax, with precision connectors, to achieve the maximum of 1 dB of cable/connector loss (as required by Garmin) to locate the antenna where I did, though. My primary reason for locating my transponder antenna where I did was more to get it away from other devices in the cockpit that it might interfere with, and several wave lengths away from the COM antenna, not to prevent the pilot and copilot from receiving RF radiation from the transponder antenna.
After commissioning over 7,000 cellular telephone sites in 16 countries, along with about 1800 co-located terrestrial microwave sites, and about 25 years as an amateur radio operator, if RF radiation were going to make anybody glow in the dark it'd be me........and I still have to use a flashlight (torch) to find my way in the dark......
Regards,
John Lawton
Whitwell, TN (TN89)
N245E - Down for annual inspection after finally receiving my Repairman's certificate 9 months after application to the FAA!
Get a sneak peak of the all-new AOL.com.
| | - The Matronics Europa-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
http://www.matronics.com/Navigator?Europa-List |
|
|
|
Get Email Distribution Too!
FAQ
Search
Memberlist
Usergroups
Register
Profile
Log in to check your private messages
Log in
