I did this and a little bit of research not long ago, but don't have the
paperwork next to me, so from memory, but if I make a mistake for Gods sake,
correct me rather than pass on bad information::
Note that this is for the communications band, but VOR band is similar in
concept, except that it is is horizontally polarized, instead of vertically
polarized. I believe the glideslope is horizontally polarized, as well.
wavelength = v/f, where v=velocity of light and f=frequency.
V=3*10^8 (m/s), and f=127Mhz (cycles/sec)
Hence wavelength = 3*10^8/127*10^6 (127Mhz is center of the band for audio
communications) = 2.3622 m/cycle
The dipole is set for half wavelength:
Half wavelength would be 1.181m, so each leg of the dipole is 0.59055m, or
23.25 inches (for dipole in free air). Actual length will be somewhat
smaller (15% plus?), to be adjusted with a VSWR meter, but this is a
starting point.
This dipole is oriented vertically: center conductor of coaxial cable
connected to the copper strip oriented vertically up, and coax ground
connected to copper strip oriented down. It does not need a ground plane, as
in a Marconi style of antenna. Metal covered planes will use the Marconi
style. Composite aircraft can use them too, if they have ground plane or
ground plane strips laid down that are roughly the length of the antenna
element. One advantage of a Marconi style antenna is that it (along with a
proper ground plane) simulates a full wavelength antenna while using only a
half-wavelength physical height. The theory goes deeper, since this is a
counterpoise method, which either creates its own ground plane, or is
supposed to interact with the real ground plane. I am not sure which is
true. If it needs to interact with the real earth ground plane, then
planes(airplanes) may loose some of the Marconi advantage, since they are so
far away from the real ground. These antennas are used in the tops of
skyscrapers, effectively, though.
Other notes for those with enquiry minds (like me):
-The bigger the diameter of the antenna conductor, the broader the bandwidth
of response (and a slight decrease in gain, most likely). A small diameter
wire will tend to be highly tuned to one frequency. I think that the copper
tape will simulate a bigger diameter, and that is good, I suppose.
-Lowest VSWR should be at 127Mhz, however it will increase at both ends of
the band, 118Mhz and 136Mhz. One of my computations put best theoretical
VSWR for a dipole at 1.5. I am not sure if this is correct. Anyone?
-Antenna can be impedance compensated, ie a resonant tank circuit between
the two antenna elements, set to resonate at the center frequency of 127Mhz.
At 127Mhz, the signal will see the added components as an open circuit.
Since an antenna will look capacitive if it is short, it will require an
inductive reactance to compensate. It will look inductive if the antenna is
long, requiring a capacitive reactance to compensate.
Hence at the lower end of the band (118Mhz), the antenna will look short to
the signal, and the inductance in the tank circuit will compensate somewhat
for the VSWR issue, since its reactance lowers, while capacitive reactance
increases.
At the high end of the band (136Mhz), the physical antenna will look long to
the electrical signal, and the capacitance in the tank circuit will
compensate for the frequencies higher than the center, since its reactance
will lower, while the inductive reactance increases.
I would try setting Xc (capacitive reactance) equal to Xl (inductive
reactance) at both ends of the band, when calculating the tank circuit.
I should probably leave the above topic out, since for all practical
purposes, the high frequency ferrite cores placed on the end of the coax
(antenna side) will reduce the VSWR, since it presents a lossy environment
for energy lost in the bouncing signals in the coax, and then there is no
need for impedance compensation. This is how the Europa style Antenna is,
and is perfectly adequate, especially for line of site signals, and is the
easiest to implement.
-From my reading, it appears that the dipole antenna does not HAVE to be
straight! The tips can be bend slightly with very little effect, since most
of the radiated energy is closer to the feedline.
This knowledge could be handy for tight spaces. To use this info to
advantage, I am planning on installing a glideslope made out of copper tape,
following the curvature of the Europa fuselage top (inside the fuselage, of
course), and located just behind the front main window. This should take out
some of the 'parasitics', such as ME as an antenna parasite (hmmm.). Mostly
it is to keep the antennas away from each other.
Hopefully this information will be useful for someone. It is the type of
info I have been craving of late.
Remember, I have never taken antenna theory before now, and have never built
an aircraft antenna yet, so I am only just learning (work in progress), so
let me know of any additions or better explanations, or if I have made a
mistake here. From reading the responses on the server, I know there are
other technical types out there, so If there is a frown, let us know :-)
Also, Although the copper tape antenna is omnidirectional, I assume it has
slightly more radiant power either orthogonal to the plane of the tape, or
does it have slightly more radiant energy parallel to the plane of the tape?
(if anyone knows)
Greg
________
_____
From: owner-europa-list-server@matronics.com
[mailto:owner-europa-list-server@matronics.com] On Behalf Of Carl Pattinson
Sent: Monday, August 31, 2009 3:17 PM
My Physics is a bit rusty but i am sure there are those that would know the
formula for aerial length. v = f X wavelength (I think) though im not sure
what v should equal ?
Hope that helps.
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