Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Mon, 25 Aug 2003 08:27:50 -0400
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From: "Charles R. Patton" <charles.r.patton@ieee.org>
Reply-To:  charles.r.patton@ieee.org
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Subject: Re: [LML] Re: Antenna Performance Demo
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I received a personal email that went in part (I have removed the 
identifying info):

My name is xxxxx and I built and fly a non pressurized IV. The IV carbon 
fuselage/wings are all grounded with a maximum DC resistance less than 1 
ohm measured between any two points on the whole aircraft. I have 
established successful communication with Prescott radio over 300nm 
south of the US Mexican border using only the carbon of the airframe 
structure as the ground. The aircraft was at 19000ft during this 
communication. Best air to air communication distance is about 250nm 
usually to certificated aircraft only.

I would like to respond to the LM list to further explain some of my 
thought on this subject. I’m not saying things won’t work at all, but I 
do want to elaborate on some points
1) I have done antenna work, I just have not done antenna work with 
lossy ground planes, of which a carbon fiber composite is one, so this 
is a theoretical discussion only.
2) I do want to comment on the “1 ohm DC resistance” which seems low 
when compared to the typical 50 ohm antenna system and is approximately 
where I thought the carbon fiber conductively range was from a previous 
measurement that was done on Hal Woodruff’s IV. The real problem is it 
is just what you said, 1 ohm DC. At RF frequencies it is guaranteed to 
be higher. Just how high is difficult to predict, so I would make the 
measurement if it were me. However it is not inconceivable that it is 
many times the one ohm. The driving mechanism is called skin effect, and 
at the 100 MHz region, conduction is not through the bulk, but on the 
surface of the conductor, so resistance goes up by that ratio of 
effective volume at that frequency vs at DC. For copper, the skin depth 
at 100 MHz is about 0.26 milli-inches – i.e. its resistance has gone up 
by 1/(0.26e-3 * 2) ~= 1.9 thousand times at 100 MHz of that of a bulk 1” 
thick copper plate. So something in the 0.05” dia range might only be 50 
times as much. This is only the conductive loss. There are also 
dielectric losses that increase with frequency. The problem is that the 
skin depth formula is for isolated wire-like conductors many times 
thicker than the skin depth, not the amorphous mass of small diameter 
filaments of carbon cloth. But the point is, it only goes up, not down. 
It’s a complicated problem, and I’m trying to touch on some of the factors.
3) I’m not contesting that you have communicated over long distances, 
what I am saying is that the aviation community pays many dollars and 
sometimes many hundreds of dollars per watt of generated RF. It is a 
pity to waste that expensive RF in heat in the carbon fiber skin of an 
airplane when it has much more value if it is radiated as 
electromagnetic radiation. (And as this is a reciprocal process, the 
better the antenna is as a transmitter, the better it will be as a 
receiver.) Further to the point, it may work fine in good conditions, 
but what about the day there’s ice on the antenna, fuel is low, and you 
really want to talk to center – don’t you want the best margin you can 
get in your favor? I have used coat hangers for aerials in my day when I 
absolutely didn’t have anything else, but I’m not about to say that you 
should settle for the coat hanger.

I started responding to this thread to because I didn’t want people to 
think if they fixed VSWR, they necessarily had solved all the possible 
antenna gotcha’s. I really don’t want to turn it into an antenna course. 
It’s just that we all have some specialties and try to share the insight 
we’ve gleaned over the years. I think mine is in this neighborhood. I 
look to others when the discussion turns to ROP and LOP, prop 
efficiency, and the thousand and one other specialties that go into a 
homebuilt.

Charles Patton
LNC2 360JM