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Re: Europa-List: Re: AAE Antennas

Subject: Re: Europa-List: Re: AAE Antennas
From: Tim Houlihan <houlihan@blueyonder.co.uk>
Date: Mon, 26 Jan 2009 22:53:05

TELEDYNMCS@aol.com wrote:
> With this in mind, and coupled with skin effect, which is the tendency 
> for weak signals to pass around your body rather than through 
> it because your body is made up primarily of water, you are in no 
> danger of ill effects from a cell phone's non-ionizing radiation, even 
> when held next to your head. Remember, it takes 600-1000 watts of 
> highly concentrated microwave energy to pop a bag of popcorn or 
> defrost tonight's roast for dinner. Your cell phone radiates 
> omni-directionally, so only a small portion of the weak signal being 
> transmitted is directed towards your head when in use. Skin effect 
> causes that weak signal to pass around, not into, your head.
>  
> Transponder antennae, on the other hand, in close proximity, 
> say closer than two wave lengths, do, IMHO, pose a significant RF 
> hazard when the exposure occurs repeatedly over a long period of time, 
> even with the short burst duration. This is particularly true for 
> those of us, like myself, who are routinely exposed to high levels of 
> RF. While the nominal power output of a typical transponder is around 
> 250 watts, peak power is usually rated in the two kilowatt range and 
> this does not take into account the gain of the antenna. Granted, 
> these are indeed short bursts, but why expose yourself to this power 
> level unnecessarily, even for short bursts? (unless of course, you are 
> beyond child bearing years or have no desire to father children) If 
> you keep the transponder antenna at least two wave lengths away from 
> your body roughly 96% of the energy is dissipated within this 
> distance. Skin effect takes care of the rest.
Hi John

Thanks for you very informative reply I was obviously wrong in assuming 
that cell phones are like real phones and carried two way conversations, 
but your description does explain those missing bits of the 
conversation. As I said before, my limited expertise was, 35 years ago, 
as an R.A.F. Airborne radar technician with a specialist radar 
countermeasures squadron. I do know how to change the SIM card and 
charge the battery of a Cell phone but that's the limit of my knowledge. 
Modern electronic devices are simply magic to me.

I feel I have to correct you when you state that the peak power is in 
the 2 Kilowatt range. I have checked a couple of spec sheets and to 
confirm things I phoned a major  transponder manufacturer just to make 
sure. Most transponders have a peak power of up to 250 watts not two 
Kilowatts.
For example the Trig TT21 has a mean power of less than two watts with a 
peak power of 140 watts at the unit connector. I am told that the duty 
cycle is less than 1 percent.
The worse one I saw had an input power of 27 watts total with a peak 
power of 250 watts. but none I saw had a peak power of more than 250 
watts. Remember this secondary not primary radar and the radiated power 
requirements are dramatically reduced
I think the comments about child bearing and fatherhood are difficult to 
justify at these low powers though at my age with children in their 
middle 30's this should not be a major issue for me !

The transponder aerial  is Omni directional so as you say only a small 
portion of the energy is directed at you, even less than with a cell 
phone held to your ear.
In a Europa, if the aerial is behind the baggage bay ( and the fuel 
tank) then it is more than likely greater than two wavelengths or 60cms 
(23") away from the crew.

Having said all that, I agree with you that we should try and reduce the 
risks its just that in the amateur built aircraft arena I think the 
risks from the transponder is well down the list of things to get 
excited about.

Tim Houlihan
G-BZTH



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