Date: Sat, 04 May 1996 23:21 +0000 (GMT)
From: dbosomworth@ccmail.meto.govt.uk
Subject: RST-Plastic airplane antennas
Date: Sat, 04 May 1996 23:08 +0000 (GMT)
From: dbosomworth@ccmail.meto.govt.uk
Subject:
Hello all,
Found RST, did'nt get the url but probably something like
http://www.rst-engineer.com. If it does'nt work browse YAHOO for
'homebuilt'. email is: rst-engr@oro.net .
chus, dave
kit67
>> PLASTIC AIRPLANE ANTENNAS <<
Away back in 1978 the homebuilt airplane world was a-changin'. Wood
and
metal were giving way to glass and foam. The Rutans, the Jewetts,
the
Rands, and the Monetts of the world were starting to pour airplanes
out of
plastic bottles. A young engineer fresh from the world of
hidden
antennas on "spook" stealth airplanes and Lunar Landing Modules
convinced
the new designers that the old world of porcupine antenna spines out
in the
slipstream was as out of date as silk scarves and grade-A cotton.
You see,
fiberglass and resin are as transparent to radio waves as air, so
there was
no reason to put a drag-stick outside the airplane skin when the
antenna
elements could just as easily and as efficiently be put underneath
the
skin.
The question has always been asked -- how MUCH do you save in drag
by
hiding the antennas? I can't give you a hard answer for your
airplane
without doing a lot of calculations, but we DID do the work on
Voyager and
came up with an answer of somewhere around 60 gallons of fuel saved
during
the round-the-world flight. I won't tell you that your airplane
will save
this 5% in drag, but it will be somewhere around this number --
depending
on the cruise speed and the number of antennas your savings will be
somewhere between 2% and 8% of the total drag of the airplane.
As with all good things, after I published a few articles to show
you all
how to do antennas yourself, the suede shoe gang decided to get in
on the
act. You can buy "plastic plane antennas" anywhere from the $5 or
so I
charge up to hundreds of dollars for antennas with "magic
properties" that
are "hidden in epoxy" and "tuned to the airframe". Horsefeathers.
An old
antenna teacher of mine once said that antennas start with a
spaghetti
noodle in a copper septic tank and things only get better from
there.
Anybody that tells you that they have a better airplane antenna than
a
copper tape dipole is just not tellin' you straight.
One of the people that want to sell you hundred dollar antennas
poo-poohs
my "ferrite donuts" or toroids (TOW-roids) as useless. "He tested
them"
and
found that "they don't do anything". When further pressed at
Oshkosh a few
years ago, he admitted that his "test equipment" consisted of
sliding a
piece of tinfoil up and down the feedline and watching for a change
on his
"power meter". Sort of sounds like the self-proclaimed "CB" expert,
doesn't
it? Do you know the funny part? He tested it at the center
frequency of
the antenna where the toroids actually DO nothing. They are only
active as
you move away from band center and compensate for reflected power at
the
band edges. They BROADBAND the antenna, but do nothing in the
center of
the band.
OK, here's the truth of it. The ferrite donuts act as nothing more
or less
than a very efficient low-loss "balun". A balun does nothing more
than
matching a BALanced antenna to an UNbalanced feedline (hence the
name bal-
un). In the direct-connected dipole that we use, the balun does
nothing
more than keeping reflected power from travelling down the outside
of the
coax back to your radio -- or in the case of a receiver, from the
input
stage of your radio back out to the antenna. In either case, the
little
chunks of powdered iron prevent unwanted radiation from the outer
surface
of the coaxial cable braid. Think of it like the iron core noise
filter
you put on your alternator line. It lets the direct power through
and
strips off the noise. All we are doing is that same strip-the-noise
routine at a much higher frequency.
To those who say that iron machine nuts will do the same thing as
toroids
just doesn't understand much about the behavior of ferrite material
at VHF.
It took us the better part of three months of design and experiment
to
select the size, ferrite recipe, and number of toroids that
optimizes match
and minimizes loss. After all this, we sell it to you for $5 an
antenna --
hardly an amount calculated to keep us in champagne and caviar.
Unless you have a good reason to do otherwise, let me make a
suggestion.
If you are doing a homebuilt airplane from scratch, buy the antenna
reference text, a 100' roll of tape, and a bag of 20 toroids. For
less
than $40 (including shipping) you have enough antenna material for
your
entire airplane if you covered every available surface with
antennas. Or,
when you get done, sell the remaining part of the copper tape roll
and the
text to a friend who, for a $7.50 bag of toroids, will have enough
material
to install another complete airplane antenna system.
**************************************************
ANTENNA REFERENCE TEXT
RST-802
Price $5.00
**************************************************
Over the years, we've written some two hundred articles on all
aspects of
aviation electronics. Seven of them are directly concerned with the
design
and construction of "plastic plane antennas". We've collected these
articles into a small booklet which we have reprinted for your
convenience.
There is nothing that we know about plastic airplane antennas that
is not
in this booklet.
**************************************************
COPPER TAPE ANTENNA MATERIAL
RST-2800 (100' roll)
Kit Price $20.00
RST-2801 (10' strip)
Kit Price $5
**************************************************
The antenna reference text shows the use of copper tape for the
aircraft
antenna elements. We have chosen this tape because it (a) solders
easily,
(b) is "stickyback" and adheres well to foam, wood, and fiberglass,
and (c)
is relatively inexpensive. Not only that, but copper is by far the
best
possible antenna element material with the exception of sterling
silver --
and if you can afford silver antennas, you are sort of out of our
league.
It takes about 4' of tape for a VHF NAV or COM antenna, 1.5' for a
glideslope, and 7' for a marker beacon antenna.
**************************************************
FERRITE TOROIDS
RST-2801 (bag of 20)
Kit Price $7.50
**************************************************
It takes 3 toroids to make a single antenna, so a bag of 20 lets you
make 6
antennas with 2 toroids left over. Y'know? An AM-FM "music radio"
antenna
is such a noncritical antenna I'll bet that those leftover 2 toroids
will
do just fine to make one of these antennas. How long to make an
AM-FM
antenna? Whatever you have left over on the airframe AFTER the
required
aircraft antennas are installed -- anywhere from 2 inches to 2 yards
long
(the longer the better for AM reception).
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