rparigoris wrote:
> If you don't mind I have a few questions: *I have a Bob Archer big
> "E" model SA-006 in the vertical fin for Becker transceiver, will
> adding torroids decrease performance of this antenna? Will adding 10
> turns to this antenna decrease it's performance?
This works with all antenna's. In a good design, the antenna-cable is
passive, is just to transfer the RF energy, and is not used to radiate
it. However, the electrons do not always understand our intentions, and
will happily follow the outside of the coax cable, if the antenna is not
pure symmetrical. Torroids/coils block the radiation of the outside of
the coax cable.
I would toss the Bob Archer antenna away, for the following reasons:
1) A half-wave dipole antenna is the best radiator. The Bob Archer
E-antenna is nice for airplanes where there is no room for a dipole
antenna. But in the vertical fin of the Europa, there is enough space
for a half-wave dipole antenna. Despite its simplicity, nothing really
beats a half-wave dipole antenna. Anything with a different shape is
always a compromise. It is just a matter of physics, despite all kind of
marketroids trying to tell you into something different. The Bob Archer
antenna can be a fine antenna, if you lack the size for a normal dipole
antenna, but this does not apply to the Europa.
2) The best place in the Europa for a VHF antenna is in the vertical
fin. This is the location which has the best separation from all kind of
noise sources, keeps the radiation as far as possible away from
sensitive electronics in the instrument panel, and keeps the antenna as
far away as possible from variable influences (crew, luggage, fuel)
which can dynamically alter the VSWR and radiation pattern. Also, there
are not many (none!) cables which must pass the antenna, as all other
electronics is located before the fin. The Bob Archer antenna does not
fit into the fin, forcing you to use a less optimal antenna location.
3) The standard polarization of aviation communications is vertical
(hence the vertical orientation of the antenna). Any antenna with bend
shapes in the "dipole-section" introduces some horizontal component in
the polarization. This energy is useless, as the station on the other
end will only receive/transmit the vertical component. Worse, most of
the electrical noise in our little airplanes comes from horizontal
orientated cables, and are transmitting horizontal polarised noise. By
keeping the antenna as vertical as possible, you null out most of the
noise sources. And during transmissions, a pure vertical antenna does
not induce much RF in horizontal orientated cables.
I suspect that a Bob Archer antenna has a strong horizontal element, so
stay away from it, unless you have no room for a pure vertical antenna.
A folded dipole has about a similar performance as a normal dipole, but
with just a little bit more bandwidth. But that comes with the cost of a
spoiled impedance, making it necessary to use some kind of transformer
with its own penalties (weight, costs, loss).
And any trick to broaden the bandwidth will negatively affect the
radiation efficiency of the antenna.
In my setup, I made a dipole antenna from aluminium parts, with a small
T-section on top and bottom. This broadens the bandwidth just enough to
get a good overall SWR over the entire communications band. It fits
nicely into the fin. This could also be achieved with a folded dipole,
but "my" setup is lighter, easier, and more economical. ;-) If you can't
imagine what I did, I can make a picture.
Just don't forget there aint anything like a free lunch. Any antenna
with a claimed "gain", will suffer somewhere in its radiation pattern.
The only way to create gain, is to take energy from somewhere else and
focus it in a certain direction. Antenna's with gain are very usefull...
provided that you aim the lobe with the gain to the destination. In our
airplanes however we have no use for gain, as our communication should
be received/transmitted in all directions equally.
Compare it with a light bulb. You can't get more light from the
lightbulb with lenses and reflectors, but you can certainly create
"gain". This "gain" however always create dark spots in the radiation
pattern.
A vertical dipole has a very uniform circular radiation pattern and this
is again a reason why a dipole is the best antenna for aircrafts.
If you manage to find something that has more gain in all directions
compared to a dipole, you have found something that will change the laws
of physics forever. ;-)
--
Frans Veldman
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