Hi Frans,
Thanks for the input. Antennas are quite the black art, I think.
Just 4 extra remarks to add to some of your statements:
(responses left long for clarity)
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>> -From my reading, it appears that the dipole antenna does not HAVE to be
>> straight! The tips can be bent slightly with very little effect, since
most
>> of the radiated energy is closer to the feedline.
>Yes, but bending it will change the impedance. There are two properties
>here: a too long antenna behaves like a coil, too short as a
>capacitance, but if correct, it will present itself as a pure
>resistance. Now, the resistance of a tuned antenna will be around 50
>ohms. But only if it is a normal dipole. Mess with it (bending it, put
>other conductors nearby) and the impedance (resistance) will stray away
>from 50 ohms. Even if the antenna is tuned properly, you will still get
>a bad VSWR because the impedance of the antenna doesn't match the
>impedance of the coax (more correctly, the output/input of the
>transmitter/receiver).
Of course Frans, you are completely right. Bending an antenna will change
its impedance. Looking at the impedance curve diagram for a half-wave
antenna, it shows that the impedance is very high out at the tips, and very
low at the feedline. So I would expect that if most of the antenna is
straight, and only the tips are bent slightly, the impedance change will be
minimal. I am sure there are calculations to be done for figuring that out.
I have seen good reports with tip bended antennas.
Also, it appears that many of the antenna and feedlines on aircraft are not
matched anyway by up to 23 ohms (see below), unless the antenna has a
matching network. It seems to not matter too much though, and this is
considered 'good enough' for aircraft (and line-of-site) usage in much of
the literature.
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>> -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?
>I'm not sure what you mean. The best VSWR is 1:1, although it makes
>little difference if it remains below 1:2.
Let me explain better. I think it was a mismatch calculation between the 50
ohm coaxial cable (that we use) and the Hertz dipole antennas nominal
resistance (digging up my papers, now).
For example:
A transmission line reflection coefficient (lets call it RefC)
is defined as:
( Z(L)-Z(o) ) / ( (Z(L) + Z(o) )
Where: Z(L) is the load, or the antenna
Z(o) is the transmission line impedance
Then VSWR can then be directly calculated, and is defined by:
( 1 + RefC ) / (1 - RefC)
A well made Hertz dipole antenna has a pure resistance of 73 ohms, nominally
(the kind with two copper strips connected to the center conductor and
shield of the coax). The coax cable is 50 ohms impedance.
Thus, RefC = (73 - 50) / (73 + 50 ) =.18699...
Then the VSWR is ( 1 + .18699) / (1 - .18699) = 1.46, or about 1.5
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>> -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.
>Yes, but you can't use both together at the same time.
Well, according to this book you can:
In MODERN ELECTRONIC COMMUNICATION by Gary M. Miller, circa 1978,
Go to page 422, and it shows a tank circuit in between the two dipoles, at
the feedline.
To quote:
"With the tank circuit resonant at the frequency corresponding to the
antenna's (half wavelength), the tank presents a very high resistance in
parallel with the antenna's 73 ohms,and has no effect. However, as the
frequency goes down, the antenna becomes capacitive, while the tank circuit
becomes inductive. The net result is a resistive overall input impedance
over a relatively wide frequency range."
It is called an 'Impedance-compensated dipole'
Now, just because it is in a book, does not mean it is right. Being that it
is a highly tuned tank circuit, you may be right-O-right-on. It may be much
too rejective of frequencies around the resonant frequency the tank is set
to. Band spreading resistors in parallel will only progressively short out
the antenna. Possibly, it may be for a more limited frequency range. I think
the burden of proof will be on why the compensation will work (for aircraft
bandwidth), rather than why it won't.
I will try to look into this further...
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>>Nope. If you use a ground plane, the antenna is 1/4 wavelength. A dipole
>>is two 1/4 wavelength stacked on top. The bottom one acts as a ground
>>plane. There is no advantage in using a "real" groundplane or 1/4
>>wavelength element. In composite aircraft you have the choice, in metal
>>airplanes you are bound to the groundplane model.
Yes, of course that is right. In a metal airplane, most people would use a
1/4 wavelength antenna to keep the wind drag down due to the external
antenna, and their airplane skin would be used as the ground plane (Like a
Marconi Antenna).
The point I was trying to make here, but did not quite describe it well
enough, is that the overall height of the Marconi antenna is only half the
equivalent overall height of the Hertz antenna. That might be where an
advantage is.
Since 1/2 wavelength is better than 1/4 wavelength, of course 1 wavelength
is better than 1/2 wavelength, for antenna heights.
In a composite aircraft, a half wavelength Marconi actually might be able to
fit, since it is the same height as a 1/4 wavelength Hertz-style Dipole.
In the Europa, the only place a 1/2 wavelength 'straight' Marconi antenna
would fit easily, would be in the tail fin (same place where we put the
Hertz dipole). Unfortunately, the only ground plane for this antenna would
be forward into the fuselage, so this would be far from omni-directional,
and would not work. Now, if the tip could be slightly bent (and with proper
adjustment), one might be able to put one in the mid-section of the
aircraft, with a ground plane on the bottom of the fuselage. I 'might' try
this at some point, but will probably stick with the Hertz dipole we all are
accustomed to, maybe with a few minor adjustments.
A side note: I believe that one of the books I was reading stated that a
1/4 wavelength Marconi acts as a 1/2 wavelength Antenna with the earth as
the ground plane. Something about an 'image' wave coming from the earth
itself, equivalent to another 1/4 wavelength of antenna height. That is, it
has the same efficiency in delivering the signal to the receiver, as a half
wavelength antenna. This would probably work just as well with a 1/2
wavelength vertical antenna (with earth as ground plane) being the same as a
full wavelength vertical antenna. Now, talk about an efficiency increase
---From good old mother earth! This might be only when using the earth as a
ground plane, and may only apply at the 'bouncy' lower frequency bands
(being a Marconi and all), or it might depend on how far the antennas
artificial ground plane is from earth, so that it can interact with it as a
'counterpoise'. I will have to research more, to be sure.
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Best Regards!
Greg Fuchs
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