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1. 04:31 AM - Re: Hand-made Glide Slope Antenna (Robert L. Nuckolls, III)
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| Subject: | Re: Hand-made Glide Slope Antenna |
At 01:57 2015-03-08, you wrote:
>
>Bob,
>
>Are you still a believer in ferrite beads for transmit antennas, if
>not for receivers? I was under the impression they helped form a sort
>of crude balun, preventing RF current from flowing back through the
>coax sleeve rather than into the arm of the dipole.
>
>Cheers, Henry
Sorta . . . but there's a lot more to the
story than being a 'believer' in the virtues
of any particular 'technology'. Many common
beliefs ARE based on a modicum of physics
but blown all out of proportion for demonstrating
a return on investment for the practice.
It's like an admonition handed down from
somewhere that keeping your tires properly
inflated is going to save a boat load of
oil. Yes, if you fitted a vehicle with
instrumentation of sufficient sensitivity
and accuracy, you can detect and quantify
variability in rolling coefficient of friction
of a tire . . . but in the grand scheme of
things, getting the planet's population to
march in lock step with the Tire Pressure
Gods is probably not going to put a measurable
dent in the demand for liquid energy.
The BALUN does indeed serve good and useful
purposes for improving the efficiency of
conducting energy captured in the wild by
your antenna into a feed line and ultimately
to the receiver. The benefits are perfectly
reversible. What 'good' for receiving is equally
'good' for transmitting.
The question then is how big is the goodness,
does it cost and what is the observable benefit?
In the case of a g.s. antenna, signals
in the wild are measured in microwatts
captured by an antenna looking right down
the radio frequency gun barrel of an antenna
from a distance of not more than 10 miles.
A wet noodle with 'terrible SWR' would
probably work just fine. Many G.S. installaitons
use a coupler in the vor antenna feed line
to split out a g.s. component of rf from
the vor antenna . . . even tho it's not
very efficient. So, fussing around with
anything expected to lower SWR on the g.s.
antenna is not going to produce any benefit
you can observe in the cockpit . . . and
requires some pretty good equipment to
detect in the lab.
Then let's consider the physics of slipping
those beads over the coax. Some months ago
I cited a product offered by Miracle Antenna
Company out of Canada called the "Air Whip".
Aircraft Spruce offered this product for a time
and it's still listed in their European catalog
http://tinyurl.com/mgm26yh
. . . but I understand the gentleman who built
and ran the company died in 2012. I'm not sure
the company is still in business. But the Air
Whip was an EXCELLENT example for effectiveness
and value of a 'balun'. That red heat shrink
covered lump in the coax feeder was identified
for us by a writer who disassembled an earlier
version of the Air Whip
Emacs!
Yup, there it is. A ferrite torroid. Not a string
of torroids but a single device with 7 passes of
coax through the center. The inductance offered by
such a technique varies with the SQUARE of the
turns on the core. By passing the coax through the
core 7 times, the net effect is 49 times as effective
as a single pass thorough one core.
Further, this antenna's design DEMANDS the inclusion of
this de-coupling technique . . . which is not a BALUN in
the purest sense although it has the appearance of one.
Coax feed lines, antennas, radiation efficiencies,
radiation patterns, SWR and interaction between
radios and other systems on board the airplane form
a complex stew that yields to an analysis of the
physics. Analysis of single beads on the g.s. antenna,
reveals usefulness on the same order as carrying a
couple more pounds of pressure in your tires.
Then, as you've discovered, not all torroids are equal.
They are fabricated from another book of recipes to
meed design goals for the transfer of energy from one
place to anoth4er in the system (BALUN or decoupling)
or the conversion of energy from RF to heat (noise
suppression). And to make the stew more interesting,
high efficiency energy transfer ferrites are often
used in noise suppression roles. So running down to
the local electro-whizzy store to buy beads assumes
that the sell of such beads can answer questions
their suitability to any given task.
I bought a network analyzer last fall and I've expanded
my box of RF measurement tools, There are a couple
of antenna projects I'd like to explore and then
describe to the community along with numbers to support
those descriptions. One goal will be to quantify the
value of adding any sort of BALUN to the installation
of DIY VOR and GS antennas.
Bob . . .
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