Greg Fuchs wrote:
> 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::
Ok, I will give a few corrections:
> 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.
It is indeed a starting point. There are many things that can influence
the final length. Especially if you bend the antenna in some way.
> 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.
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.
> 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
It doesn't need to interact with the real earth ground plane.
> -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.
Correct.
> -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.
> -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.
> 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.
Incorrect. If you look carefully at the electrical diagram of the
antenna, you will see that the outside of the coax is connected to one
of the 1/4 wavelength elements. The problem is, at the end of the coax,
the electrical current is not sure which way it should go: it has two
choices, the 1/4 wavelength element, or the outside of the coax. Part of
the current will choose the outside of the coax, and then discover that
it doesn't end after 1/4 wavelength. It continues to travel over the
outside of the coax, creating havoc with other conductors nearby, and
creaping altogether into your electrical system instead of being
radiated away.The ferrite cores are there to block this route. True
enough, if you have current on the outside of the coax, the VSWR will
get worse. So ferrite cores seem to improve VSWR, but only bad VSWR due
to currents on the outside of the coax. Ferrite cores won't do anything
if the VSWR is bad due to a too long or too short antenna.
> -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.
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).
Hope this is of some help.
Best regards,
Frans Veldman
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