I've been watching a thread on this subject. I think it started
when someone asked why that little stick of metal had to cost so
much money. There've been a number of responses most of which
contributed good and useable ideas but I'd like to summarize
some of what I've read and organized in no particlular way, add
a few thoughts of my own:
VHF comm antennas operate over a range of 118 to 135 MHz here
in the US . . . similar ranges elsewhere. The classic solution
to comm antennas on airplanes has been abasic, 1/4 wave, mono-
pole antenna using the aircraft skin as a ground-plane.The
antenna's radiating part has to be electrically connnected to
the coaxial feedline which carries signals between antenna and
transceiver.
The most durable materials for construction are the steels,
most popular of which is a stainless. Copper, brass and alluminum
have been suggested and used successfully for enclosed antennas
in composite structures but they are difficult to make live
on the outside. Non-ferrous materials work-harden and tend to
break off after some priod of time "waving in the breezes".
Electrical connections to copper and brass are easy . . solder
it. Aluminum and steels need soldered or solderless terminals
attached to the radiator with some sort of threaded fastener.
Many years ago, Cessna fabricated their own antennas by threading
the end of a 3/16" stainless rod, mounting the threaded end through
a two-piece, ceramic insulator in the cabin top and holding the
whole business in place with a nut inside and one outside. A third
nut attached the coax center conductor's crimped on terminal to the
base of the rod. Another terminal attached the coax shield to the
skin adjacent to the antenna base. The antenna would have worked
best if left straight but performance was only slightly degraded
by bending the rod aft to give it a rakish appearance.
This design suffered from the ususal problems. Things like this
stuck into the airstream like to shake rattle and roll. Skin
doublers were wont to crack around the hole where the insulator
came through. They leaked when it rained. They would also tend to
loosen up and if over tightened by a zealous mechanic, the ceramic
insulator would give it up with a sound like cracking ice.
None-the-less, the cost of fabrication and installation was
attractive and like most maufacturers then and since, cost
out-the-door was a MAJOR driver, not cost-of-ownership AFTER
it got out the door . . .
Aluminum and brass can be used for antenna rod material but
be prepared for periodic replacment. These materials work-harden
and break off regularly. Steel antennas can resonate and vibrate
in the wind and break too. I was riding shotgun with my sister
on a driving trip a few winters ago. We were getting some very
light precipitation and the OAT was about 29 degrees. I watched
in fasination as the am/fm antenna right out the window from me
gathered ice. As it's shape and mass changed, I watched it start
to "hum" . . it had a perfect full wave mode with a node right
in the middle that stood perfectly still. As the ice coating
grew, the amplitude of the hum grew too. My enjoyment of watching
the demonstration ended when the antenna just disappeared!
It broke cleanly from the fender mount right at the base.
People are generally unaware of the fact that MOST of an
antenna's good work happens in the first few inches off the
base. The design I just described had a substantial percentage
of it's working length INSIDE the cabin. The rod extended
through the cabin roof about an inch, an the coax cables I
saw had about another inch of exposed center conductor
hanging out from under the shield before a terminal was
installed. This material is PART of the antenna's overall
length at the base where the critter is trying to lauch
your signals into the ether . . . the part inside the
cabin was willing but crippled.
There was some mention of a radio shack automotive antenna
as a likely candidtate for vhf comm conversion. I bought a
12-1322 radio shack antenna for $10. First, the conducting
part is all one piece, not telescoping (good). The base is
designed for installation through a 7/8" hole with all the
work being accomplished from outside. All of the metals
appear to be stainless or plated except for the mounting
barrel which appears to be injection molded aluminum.
First, the coax feedline supplied with this antenna would
have to be discarded. If you cut it open you'll find a very
tiny center conductor designed to provide a low capacitance,
hi-impedance connection between antenna base and the input
stages of an automotive a.m. radio. This coax is unsuitable
for connecting the 50-ohm output of a transmitter to the
antenna. If you look inside the connector at the base of
the antenna, you'll see a stainless steel pin which is the
machined down butt of the antenna rod. The original connector
has a hard copper female sleeve that gets a fair grip on
this stub when the connector is engaged.
In am/fm radio service, this joint carries very tiny currents
in a receive-only mode. While you're transmitting, a 5 watt
transmitter will generate approx .3 amps in this joint. A
lot of degredation in this connection will go un-noticed in
am/fm applications while vhf comm transmission requires a
solid connection.
I think my approach would be to drill a piece of brass rod
for an interruption fit to the stub. Turn down the other
end for a solder post . . . .06 to .08" diameter. Or tap
it for 8-32 nut. Clamp the antenna rod in a vise and drive
the brass terminal down onto the stub.
When mounting this antenna, keep in mind that supporting
the whole thing on a skin area surrounding a 7/8" hole is
too much load on the material. A .06 or better doubler
should cover at least 4-6 square inches around the mounting
hole. Before installing the doubler on the skin, drill a
countersunk hole for a 6-32 or 8-32 flat head screw as close
to antenna base as possible and with head of screw against
the underside of the aircraft skin. Rivet the doubler to
the skin with generous sprinkling of flush rivets.
When the antenna is mounted, attach RG-58 coax center
conductor to antenna rod and shield braid to the ground
stud provided on the doubler. Use an SWR bridge or antenna
analyzer to measure the antenna's characteristics at
125 MHz. Use Dremmel with cut-off wheel to score the
antenna rod all around and then snap it off. When the
optimum length is achieved, use the hand grinder to
smooth and round the tip of the antenna rod.
Optimum performance dictates that the antenna be mounted
in a vertical plane. One respondant to this thread talked
about using "wingtip" antennas for comm . . . it was an
RV'er I think. I'm wondering if he is considering the
gamma-matched, monopoles offered by Bob Archer and described
in a variety of kit-type newsletters. Be advised that these
antennas are suitable only for vhf navigation (omni) because
they are horizontally polarized.
Another respondant suggested that amateur radio 2-meter
antennas may be cut down to a length compatable with VHF
comm operation. He did speak to the existance of "coils
and matching circuits" in the base but seemed to offer the
notion that an SWR meter doesn't lie when you trim the
aforementioned antenna to the length required to lower
the SWR at the frequency of interest.
Be advised that 2-meter (144-148 MHz) ham antennas are
generally 5/8 wavelength at the frequency of interest
and require a matching network in the base that really
clobbers the antenna's overall performance when trimmed
to operate in the 118-135 range. If you are considering
such a modification, take steps to bypass or remove any such
matching hardware before trimming. Just because the SWR can
be lowered to some satisfactory value doesn't make the antenna
work. A 50 ohm resistor has an SWR of 1:1 and radiates not
at all . . . it does get hot but we're not building coffee
warmers.
Without test equipment, it's impossible to optimize an antenna
in place. Without flying the antenna with test equipment -or-
taking the installation to an antenna test range, it's impossible
to put meaningul numbers on an antenna's performance. Be cautious
of suggested designs where individuals say, "it really works great!"
Consider that 95% of your communications occurs at within 10 miles
of the other station. A wet string hung out the window might be
said to "work great". The load it presents to your transmitter,
and the efficiency at which it covers the azimuth around your
ship with readable signals are all non-quantified in "works
great" evaluations. Further, you'll never know about the signals
you DIDN'T hear because of their position in some deep null of
the antenna pattern around your airplane.
If you enjoy the experimentation and have access to at least
rudimentary test equipment (I have an MFJ analyzer that I rent
out for such endeavors), then by all means go for it. If you're
trying to save money and wince at the idea of spending $50-100
for someone's production model but don't feel comfortable with
the experiment, then perhaps the purchased antenna is the better
deal . . . you can spend a LOT of time experimenting in the dark.
Be cautious also of no-name commercial offerings. Bob Archer's
antennas are well built and based on good engineering principals.
There are others with 'matching networks' and ferrite beads that
are questionable. Tiny, lumped matching networks may indeed
present a low SWR to the transmitter but they are very lossy.
Ferrite beads over the coaxial feedline to a dipole antenna
contribute so little to its performance that there's no value
in installing them.
Simply attaching the coax conductors to the center of a dipole
will produce useable peformance that is not improved by addition
of the ferrite beads.
For metal airplanes, a Dorn-Margolin or Antenna Specialist
monopole is about the best performing antenna you can build
or buy. It has a fairly generous base to spread loads, it's
been perfected over the years to provide reasonable service
life in 200 mph winds. It's electrical connection is made via
coax connector; ALL of the antenna's active length OUTSIDE
the airplane.
On composite ships, home-made or commercial monopole antennas
must have a metalic "ground plane" under them. Bob Archer has
some half-wave dipole offerings that do not require a ground plane.
My objective here is NOT to discourage experimentation -or-
to throw water on the do-it-yourself endeavor. Irrespective
of your reasons for deciding to buy or build, the outcome
can only improve if you arm yourself with knowlege. With
antennas, the devil is in the details.
Regards,
Bob . . .
AeroElectric Connection
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