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From: "Charles R. Patton" <charles.r.patton@ieee.org>
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Subject: Re: [LML] Re: Antenna Performance Demo
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This is going to be a long reply so I’ll quote directly inline using CP 
for my previous comments and BR for Brent Regan’s comments then I’ll 
follow with CPN for my latest comments.
CP<<<Burying the radials in the carbon fiber or such that the radials 
were more to the inside would go a long way toward removing the 
beneficial effects of the radials.>>>

BR-<Really? That is a very interesting point but you completely lost me 
on your aluminum airplane analogy. Expanding your logic, in the case of 
an aluminum airplane, an antenna will have poor performance if mounted 
inside the airplane but we know (believe) it will behave much better if 
mounted outside. Shouldn't the aluminum damp the signal as you say the 
carbon will? We know this is not the case so it seems you were right 
about the "dumb thought". It would seem that, as you implied, that the 
AC impedance of the carbon structure is the critical, and unknown, 
element here. I wonder what is the impedance of the carbon skin at COM 
frequencies. DC, low current resistance is on the order of a few 
milliohms per square inch per inch. I doubt that we are so "lucky" as to 
have the skin impedance at the frequencies of interest to be anywhere 
near the critical damping impedance. >

CPN -- There’s a lot to cover here. First the aluminum can analogy. 
Suppose I put a radiating source inside a can with no holes (i.e., it’s 
battery operated.) The loss is very low, but real. The standing waves 
build up until the circulating currents times the real component of the 
loss (the IR loss) equal the output of the emitter and the container 
heats up. Now we puncture the wall. Depending on the size and 
relationship of the hole to the circulating current, the hole becomes a 
re-radiating antenna. For instance, a slot ½ wavelength long and 
perpendicular to the current is just as good as if it were a ½ 
wavelength wire antenna fed by an equal amount of current. This is one 
of the primary mechanisms in EMI problems with electronic equipment. The 
electronics inside generates currents on the box that typically has a 
cover. Said cover forms slots with its mating box (look at your typical 
PC for instance), and said slots re-radiate the problem EMI. At this 
point the primary loss mechanism is the loss through the slot and the 
wall losses fade into insignificance. So your comment about “critical 
damping resistance” is right in that it is unlikely to be right at 50 
ohms, but that still doesn’t mean it can’t be adsorbing some significant 
portion of the energy.


BR <While this is "outside my area of expertise", I had understood that 
damping became less effective as you moved from the high (EMF) potential 
end of the antenna (tip) to the low potential end (ground plane). So, 
how do you bleed off energy that is at ground potential? The current is 
highest at the base but the resistance to the feedline is lowest>

CPN -- Here is the crux of my concern. You’re right about the EMF on the 
tips of the antenna. But an additional way to think about it is that you 
have a continuous transformer going from approx. 50 ohms at the 
feedpoint to hundreds of ohms at the tip. But to me, the important point 
to remember is that you can’t radiate from a monopole (a nice 
theoretical concept) you require a counterpoise in some form. (We’ll 
come back to this in your question below.) Generally this counterpoise 
is the groundplane formed by the skin in an airplane. But it’s important 
to realize an equal current also flows in this groundplane (i.e., the 
current you measure in the center of the coax – the feed – goes out the 
¼ wave whip, through the air, back to the ground plane and returns on 
the shield If the plane is resistive, then electrical current is used to 
heat the resistance resulting in heat. One way to view this is that at 
these frequencies, the capacitive coupling between the tips of the 
antenna and the ground plane form a closed loop and the RF current 
circulates. The residual inductance of the wire is balanced by this 
capacitance, and the leftover is a real resistance which a different way 
of stating that the antenna is launching energy into space as 
electromagnetic radiation. But another thing about this field around the 
wires/ground plane is that it wants to minimize the impedance, and part 
of that minimization process is that the field wants to minimize the 
enclosed loop area if possible. So this means that it will try and 
occupy the surfaces closest to each other commensurate with conductivity 
which means some of the current will go onto the surface of the carbon 
fiber if it is in the way. Or another way to look at it is that imagine 
the ¼ whip standing on a groundplane. There are circulating current and 
voltage fields. If you place a wire in the field, it will intercept 
these fields and as a consequence will have a circulating current. This 
will reach a maximum if the wire is ½ wavelength long. This element will 
now re-radiate the energy. This is the principle behind the Yagi 
antenna. But what if this element is resistive? It will adsorb energy 
from the field and dissipate it as heat. This is the condition of a 
resistive ground plane in the energy field, i.e., the ground plane 
radials are inside the airplane. How bad this is depends on what the 
ground plane resistance is in this frequency range.


BR<And just when you thought it couldn't get better, the airplane skin 
is actually two skins separated by the core. What is the effect of the 
capacitive and inductive coupling between the skins?>

CPN -- Generally my rule of thumb is that if the spacing is 
significantly less than a wavelength, the capacitance will dominate and 
it can be treated as one plane (where I’m assuming the planar dimensions 
are significant portions of a wavelength.


BR<Even so, I always thought of a ground plane as an electrical "mirror" 
that makes a 1/4 wave antenna "look" like a half wave. It is not part of 
the system that receives the signal other than "holding" one end of the 
antenna so it can resonate. As such, it shouldn't matter if it is 
wrapped in air, carbon or wet noodles. For transmitting, the ground 
plane soaks up the antenna's "reflection" like a sponge. The earth makes 
a good ground plane, so why wouldn't copper/carbon? >

CPN -- So, first thinking of the ground as a reflector is good, 
generally thinking of it as a “sponge” is bad. Good ground planes are 
good conductors at the frequency of interest and have equal currents 
flowing in them. The earth is not as good, but you take advantage of the 
area principal. The area times the skin depth is much higher for the 
typical installation, such as a typical 550 KHz to 1.5 MHz AM 
transmitter. And in point of fact, many of these installations include 
buried radials to improve the efficiency of the installation. (Also to 
complicate matters, not all installations are ¼ wave resonant systems 
especially the AM just mentioned. A ¼ wave 550 KHz would be about 446 
feet tall – that’s a tall antenna.) But as frequencies go up to FM and 
TV range, the antennas are either dipole or ¼ wave on ground planes, but 
in either case, the counterpoise conductivity is satisfied with actual 
conductors of metal.

BR<One could reason that either the carbon is a good conductor of RF or 
it isn't. In any case, adding copper radials wont hurt and may help.>

CPN -- I absolutely agree. The copper radials can only help. I’m just 
suggesting for maximum benefit they should be on the outside with the ¼ 
wave whip, especially on lossy medium such as carbon fiber construction.

BR<Sounds like it is time for more tests! >

Yep, you can’t beat an antenna range!

BR<In the meantime, theories aside, we do have empirical evidence from 
the lab and the field that ground radials on the inside of the carbon 
skin produce satisfactory results. Not being an expert, I tend to rely 
on experience.>

CPN -- I agree, I’m only quibbling about the word “satisfactory” and how 
that should be defined.

BR<PS: And what about polarization? Two thirds of the bent whip antenna 
on my airplane is horizontal and therefore 90 degrees out of 
polarization phase with COM ground transmissions. Wouldn't it work 
better if I took the kink out? ?

CPN -- Man, you don’t want much do you?<grin> This is a big area. I’ll 
just say I don’t know, the bend will introduce distortions to the 
antenna radiation pattern and is likely to reduce its effective height 
(i.e. it’s not quite as efficient as a ¼ wave whip. ¼ wave whips can be 
made much shorter with a technique called a “top hat” where a plate or 
disk is added to the top of the antenna to increase its capacity. The 
antenna length is shortened considerably, but the end result is that 
although it VSWR may be wonderful, it radiation efficiency is terrible 
because of the reduced physical dimensions have reduced its effective 
height. This is another aspect of the point I was originally trying to 
make, to wit, just because you have a good match – low VSWR – doesn’t 
mean you have an effective antenna.) To imagine the field from a ¼ wave 
vertical whip on a ground plane, drop a doughnut over the whip. The 
doughnut represents the field in space. The length of the vector from 
the junction of the whip and the grounplane to the outer surface of the 
doughnut represents the gain of the gain of the antenna in that 
direction. So one direction the antenna is “blind” is along the axis of 
the whip. When you bend the whip down, it would be a bit like squishing 
the doughnut under the narrowed area and expanding it directly opposite. 
In other words, you started making a directional antenna out of it and 
the lobe away from the bend starts lifting off the ground plane. It’s 
not necessarily less efficient (but keeping in mind the “top hat” 
example above it probably is a bit less efficient than a vertical ¼ wave 
whip), it’s just not even in all directions. Which, while I’m on the 
subject, suggests that ¼ wave whip on the belly is better than a ¼ wave 
whip on the top of the airplane. Look at the doughnut, the angle of 
radiation is about 45 degrees from the ground plane horizontal – now 
imagine which direction you want to transmit to/receive from when you’re 
at altitude. You’re above the radio stations (I hope, because any other 
concept scares me) so you need your antennas to angle the radiation 
down. The post from Bob Jude today is correct, the cone will start to 
lower the radiation angle. But the ultimate end point of that process is 
a vertical dipole whose pattern is perpendicular to the line of the 
dipole and it becomes a variation that is called a “bazooka.” So I hope 
that Bob’s picture is of the roof of his plane, not the belly, as I 
would think that would provide a better radiation angle in flight.

BR<Oh, the things we do for vanity and speed.>

CPN -- But occasionally I think we make allowances for safety and 
comfort else why carry around all that surplus weight in radios and 
navigation gear, and extra seats! <grin>

I hope this made sense – if not, ask questions and I’ll try again.
Regards,
Charles Patton
LNC2 360JM