Bryan and Bob H:
I think there are lots of opinions out there about what these
magic items do (or don't do) - and this is just another. Nor do I intend
insulting the intelligence of those who already know more than I.
What complicates the task as compared to building/testing and
installing ground-based rigs is that first of all we restrict the
area/volume in which we paste 'em. Then we surround them with metal things
[and us] of various sorts and finally we take 'em away from Mother Earth and
can't figure out why they aren't perfect........
Fred has it OK - he mentioned nearby objects. When an antenna
(not a 'powered' one) does its job best is when it reverberates at the
frequency in use. This is arranged by shaping and cutting it until it
'rings' where we want. At that point, most of what we send up the pipe is
duplicated by the aerial and sent out over the ether. [Sort of like pushing
the swing at the right moment]. Getting the correct shape/size is a wizard's
work and outside our scope - however once we settle on a type, we must then
test to see how it reacts to nearby earth, objects of various sorts and how
well it matches the coaxial cable we attached it to.
One common standard is the 50ohm figure of impedance (resistance
to radio frequency). That's what most cable is designed to transfer to the
aerial. That requires careful assembly of the fittings with which we attach
them. It's also the figure most transceivers expect to see. If they
don'tsee it, modern units measure the power coming back down the pipe and
reduce the output power to save the transmitter burning out.
If the fittings or aerial aren't 'tuned' to these two data,
energy is wasted heating them with RF which doesn't get out to the ether,
and/or reflects them back to the transmitter. see above. By the way, the
coax fittings vary greatly in the qualities necessary for efficient
transfer. Be sure you have the best of materiel to assure top value.
Then there's the space. Often we have to compromise on the
volumn in which the antenna sits and this may reduce output at particular
freqs. Also the design sometimes is affected to achieve partial success. The
bigger the antenna/aerial, the greater the 'capture' area of received
signals - again space intervenes.
Here come the nearby objects - fuel lines, electric power,
signals for other uses, strobe flashers and structure. If a near item is
'tuned' to a freq in our range, it absorbs the signal and/or 'skews' the
planned signal. Let us not forget the engine (a fair block) and the
salt-treated water bags we call humans, who are able to absorb all sorts of
energy.
All of these things affect the total output, the azimuth
distribution of incoming signals, the power output across the band of
frequencies, and so forth - so it is not possible to achieve perfection
entirely. We can only approach it by compromise. The beauty of it is, there
are chaps who bounce VHF signals off the moon and get back one-millionth of
the power in return - we need only the tiniest fraction of a watt to receive
well and so do ground stations.
One thing I haven't seen discussed is the signal-strength
circle. One gets the aircraft in the clear and draws a circle around it of
about 100 feet radius. Then having picked a rational aero frequency, we walk
the circle measuring the signal strength at say 15-degree intervals - with
respect to the aircraft nose. Plot that circle and you have an appreciation
of any restriction to a full circle of radiated signals. The receive azimuth
will be a repeat of this. In the earlier days of aviation*, we had to wait
until we were well past the station site before passing a message because
both aircraft and ground aerials didn't work vertically. By the way, signals
are vertically polarised [don't ask] in aerofreqs and if you rotate one
antenna to the horizontal you can expect to lose 20db - a lot. The 1/4wave
monopole, the Europa vertical dipole and the groundplane 1/4wave are all
hung vertically for this reason. Ham VHF is vertical, TV is horizontal -
there are reasons.
Aviation is only aviators aviating - not airliners being built or
stewardesses at work, or half the magazines on the rack.
Like our friend said recently, I have to close now, - I hear Matron coming
down the hall.
Cheers, Ferg
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