February 2003 (Volume 2,
Issue 2)
Antenna Installation
By Jim Vance
The shortest antenna that will resonate at a given
frequency and be efficient at accepting the signal from the transmitter is
one-half of a wavelength long. If we
plot the voltage and current along this half-wave antenna, it would look like
this:
As you can see, the voltage
is at a maximum at the ends of the antenna, and the current is at a maximum at
the center. Using Ohm’s law, the
impedance of the antenna is R (impedance) = Voltage divided by the Current. Since the current is zero at the ends, and
the voltage is at a maximum, the ends of the antenna would have infinite
impedance. In the center, the voltage
is at zero and the current is at a maximum.
This would make the impedance zero.
Since it would be
impractical to try to feed the antenna with an infinite current, the designers
have decided to offset to the side of the center to get a practical
voltage/current/impedance. They have
standardized on 50 ohms nominal impedance, and the coaxial cable and radios for
aircraft are designed to that standard.
How do we shift to get the
50 ohm impedance on the antenna? The
trick is to make the radiator half of the halfwave antenna the length required
to resonate at the desired frequency.
The counterpoise—the other half of the antenna--is made 5% longer.
A quarterwave antenna consists of the radiator (the powered
part of the antenna) and some sort of counterpoise. In the case of a metal skinned aircraft, the surface of the
aircraft is this counterpoise (sometimes called a ground plane). To be an effective antenna system, the area
of metal surrounding the base of the quarterwave radiator must have a radius at
least 105% that of the length of the resonant quarterwave radiator.
In composite and fabric
covered aircraft, some other means of providing the counterpoise must be
fabricated. One approach is to have at
least four radials that are horizontal when the radiator is vertical. They are normally spaced 90 degrees apart,
and the shield of the coaxial feed cable is attached to them. They can be made of the same material as the
radiator.
The wider the radiator is, the broader the resonant
frequency is. Therefore, an antenna
made with a copper foil that is an inch wide would have a very broad frequency pattern. However, the flip side of this is that as
the radiator becomes wider, the ability of the antenna to accept the signal
becomes less. At aircraft radio
frequencies, this ability (called Q, as in quality) is acceptable when the
antenna radiator is no more than ½ inches wide. The width of the antenna is usually more a function of the
mechanical design to effect a permanent mount that it is to obtain a certain
bandwidth.
Part of the problem with a
wide antenna radiator is that it functions like one plate of a capacitor. The larger the area, the more of the signal
is absorbed by the surrounding structure and is dissipated as heat (the same
principle as a microwave oven).
Luckily, the foam in a composite aircraft has a lower dielectric
constant than the insulation around the center wire of the coaxial cable. Electronically, the antenna functions almost
as well as it would out in free space.
Ribbon type antennas should
be made of copper or brass. The
connection to the coaxial cable should be soldered. If aluminum is used, the cable must be connected with bolts or
screws. Over a period of time, the
dissimilar metals in the aluminum, copper of the coaxial cable, and the fasteners
will corrode, resulting in intermittent connections.
At the aircraft radio frequencies, the orientation of the
antenna is important. If the antenna
were mounted horizontally, the signal would be radiated perpendicular to the
antenna. There would be two dead zones
with little signal off the ends of the antenna. This is why the FAA ground antennas are mounted vertically so
they will perform equally well in all directions. Therefore, the optimum orientation for the aircraft antenna is to
be vertical also.
Some radio installers will place ferrite beads around the
coaxial cable near its junction with the antenna. The beads WILL NOT compensate for a poorly tuned antenna. The purpose of the beads is to act as a
choke to keep stray radio signals from traveling over the outside surface of
the cable shield. This is important to
minimize interference with sensitive integrated circuits in engine instruments
and control computers.
Not all ferrite beads are
created equal. Most beads are used in
electronics on power wires going into a printed circuit board. They allow the direct current from the power
supply to pass uninhibited, while greatly reducing the radio frequency noise
that may be on the wire. These beads
rarely have a maximum frequency limit greater than 20 megahertz. The Radio Shack ferrite beads I have
purchased had no color coding.
Ferrite beads are color
coded with spots of paint. The only
units that are designed to perform at aircraft frequencies (90 to 150
megahertz) are colored tan. Green and
white markings have a range of 40 to 90 megahertz. The Radio Shack ferrite beads I have purchased had no
color-coding. I have to assume that
their performance at 120 megahertz will be about the same as painting Cheerios
black and installing them. The only way
to get beads that are of the right design is to buy from an electronics supply
outlet.
On coaxial cables for
aircraft antennas, the ferrite beads must be tan in color. All others will provide no electrical
advantage and only add to the weight of the aircraft.
The ferrite beads must be about
¼” inside diameter so they will fit over the shield and cover of the coaxial
cable. If a smaller bead is used that
mounts over the center wire and dielectric, it will function to PREVENT the
radio signal from getting through to the antenna. This would be much worse than not having any beads at all.
Some older radios will have
a PL-259 connector. It is over ½” in
diameter on the outside, and the center pin is about 1/8” in diameter by ½”
long. They will not maintain a constant
50 ohm impedance through the connection, and usually result in about 0.5 db of
signal loss at aircraft frequencies.
These connectors were developed during World War II when the maximum
frequency in use was about 10 megahertz.
The smaller BNC connectors
do maintain a constant impedance through the connector. Consequently, there is almost no signal loss
through them. They are difficult to
assemble, and special care must be given to soldering techniques. The Amateur Radio Relay League publishes
several good books that include details on how to properly assemble them. A local ham radio operator—especially one
who builds his own equipment—would know how to do it right. Or, check with your local library for a
copy.
The only good way to properly tune an antenna to resonance
is to use a standing wave ratio meter.
It is simple to use, but the analysis of what it tells you is somewhat
more complicated. I will outline the
procedures in a subsequent article.
You can reach me at Vance@ClaflinWildcats.com if you
have questions.
Jim Vance
Alternative KR Engines
The
Rand Robinson KR series homebuilt aircraft began life as a VW powered aircraft. There are a lot of them flying and they
comprise a group of builders/flyers that believe the only true KR is a VW
powered KR. In reality, however, there
are different engines being applied as KR power plants within the KR homebuilt
world as we speak and we present several of them in this article. There may be more out there, which I am
unaware of. (If so, I’d like to hear
from you. Perhaps you would take the
time to write an article about your firewall forward package). One of the main differences is the
impression that a reduction drive is needed.
Suffice it to say that almost all of these power plants can be run as
a direct drive power plant and have done so very
successfully. There are many
reduction drives on the market today and they vary from around $900 to over
$4,000. It can easily become one of the
most expensive components on your aircraft.
I’ll talk more on this in another article on the PSRU I designed and
built using a planetary gear
system.
VW 2275cc Great Plains VW Engine Centre
Volkswagen
The
KR was originally designed for the VW engine, although few realize that it
requires a reduction drive to provide its full potential! It is primarily in use as a direct drive
power plant and it is perhaps the cheapest of all engine options. VW technology is as old as the Lycoming and
Continental engines and, like them it is also air-cooled. If you have ever driven a VW Beetle up over
the Continental Divide, as I have in my early years, you would find that this
power plant does not handle altitudes well in its stock configuration. This same engine blew up on me just as I got
back into South Florida. Years later, I
had to replace my Piper’s engine at a cost of about $15,000. Having had these experiences, I immediately
began investigating inexpensive alternatives.
The VW has been around for a long time, with no changes in
technology. It runs today on the same
technology from the 1950s. (See Great
Plains at: http://www.greatplainsas.com/ and VW
Engine Centre at: http://www.vw-engines.com/).
Engine mm/mm
Size Bore mm/Stroke mm Comments HP - Fuel Wt Price Vendor
1600cc 69mm x
85.5mm Kit 50 - 116 $1,695 Great
Plains
1835cc 69mm x
92 Kit 60 - 116 $1,895 Great
Plains
1915cc 69mm x
94 Kit 64 - 117 $1,895 Great
Plains
2180cc 82mm x
92 Kit 70 121 $2,895 Great
Plains
2275cc 94mm x
82 Assembled 105 – 4.0 gph 175 $6,495 VW Engine Ctr
2275cc 94mm x
82 Kit 120 – 4.7 gph 175 $5,995 VW Engine Ctr
2500cc 94mm x
90 Assembled 100 – 4.1 gph 160 $6,995 VW Engine Ctr
2500cc 94mm x
90 Kit 85 – 3.9 gph 160 $6,495 VW
Engine Ctr
The Corvair engine is being
adapted in both the direct and reduction modes. While a little weightier than the VW engine, it also provides
more horsepower availability.
Like the VW it is
horizontally opposed and air-cooled.
For those interested in this power plant, there is a Corvair School and
a number of KR builders available
to discuss this power
plant. I declined to use this engine
because it did not contain state-of-the-art technology and was basically
another 1950s/60s era or earlier power plant.
(I also owned one of these and thoroughly enjoyed the little car. It was a peppy little car but leaked oil all
over the driveway). (P.S. Mark Jones and Brad Glasco, hope I haven’t
stepped on your Corvair toes …each to his/her own right!)
Eggenfellner Aircraft’s EJ-25 NSI
Aerospace
Having owned a Subaru in the
80s, I knew that this power plant was as solid as a rock, had been tested on
the salt flats non-stop for over 100,000 miles, had all the modern technology
such as electronic fuel injection and electronic ignition system, was the most
economical engine I could find, and would go-the-distance with me. Investigating further, I found the Subaru
engines to be the new rising star in alternate homebuilt aircraft power
plants. The rotary folks had been using
them successfully for years, but the fixed wing crowd has been slow to catch
on! Eggenfellner Aircraft just
reported over 100 RV conversions!!!
(Times are a changin’).
Research reviewed the cost of a good engine wasn’t too expensive. It cost around $900 for an EJ-22 and the conversion
didn’t seem too difficult to accomplish.
The Subaru engine series includes the:
Within
3600 Est.
Model Stock @RPM Prop Speed Cost Vendor
EA-81 73-85 hp @ 2400 Yes $ 850
EJ-22 180 hp @ 5900 $12,695 Stratus Inc
EJ-25 165 hp $15,995 Eggenfellner
EJ-25 205 hp $15,400 NSI
EJ-27 145-156hp@ 4000 Req reduction $ 1,400
EG-33 228-230hp@ 4400 Req reduction $ 1,800
STRATUS INC.’s EA-81
The first question that pops into mind when considering an engine
conversion is what is required to convert the engine and just how difficult is
it? We know the VW requires removal of
all excess weight materials, a change in carburetion and a firing system to be
operational. So what does the Subaru
engine need? This is what I went in
search of before deciding to go with the Subaru EJ-22 engine and here are the
answers:
http://www.geocities.com/subaru_builders/index.html
Subaru Aircraft Builders List. Email: rfi@galstar.com or http://www.synchrolite.com/homebuiltrotorcraft.html for Don Parham’s Rotary Flight
International, http://www.flash.net/~contact1/index/
CONTACT! Magazine
As with the VW, all of the
accessories and excess weight must first be removed from the engine. If the engine is an EA series engine, it requires
a carburetion system and a firing system (same as the VW). The Ford Escort distributor and the Holley
5200 series carburetor works well on this engine. If the engine is an EJ series or an EG series, they simply
require a modified wiring harness; the EFI and stock firing system work
extremely well.
Because the EFIS includes a
computer and electronic sensors on the engine, this is where most people become
confused. It isn’t really that hard to
take an automotive repair manual on the engine and figure out which sensors and
wires are necessary for the engine and fuel system to run and which
aren’t. For those that either can’t
figure this out or lack the desire to attempt it, you can buy a pre-wired harness
from Don Parham for about $300. By the
way, I forgot to mention that almost all of the gyrocopter folks are using
these Subaru engines so there is a lot of support already out there for the
engine! If you have a lot of money to
waste, you can even buy a complete firewall forward package. Here are some points of contact:
www.subaruaircraft.com Eggenfellner
Aircraft (commercial EJ-25 RV & Glastar Engine Builder)
www.stratus2000.homestead.com/
Stratus Inc. (commercial EA-81 Avid Engine Builder)
http://avidair.com/stratus.html Avid
Aircraft Subaru engine conversions
http://www.nsiaero.com/aero_home.html
NSI Aircraft (Subaru engine conversions)
http://www.zenithair.com/zodiac/xl/subaru.html
Zenithair Subaru Engine Conversions
http://www.sportflight.com/kfb/engines.htm
KitFox Subaru Engine Conversions
There are many questions
builders have before choosing to go with a Subaru installation (commonly called
SOOB). Frequently asked questions
achieve located at: http://www.interstice.com/~kevinh/soobfaq.html
that will answer most of these questions for you. Data regarding engine horsepower output varies from between
commercial engine builders. A good rule
of thumb on horsepower can be found at:
http://www.ravensblade-impreza.com/modifications/drivetrain/enginespecs/enginespecs.html
If you still have questions,
send me a message at planecrafter76262@yahoo.com and I will either answer them for you or try
my best to get you an answer. Attached
are some of the Subaru engine builders and installations.
KR aircraft have formed the
basis of many other aircraft models on the market today: the Glasairs, the Lancairs, and the WWII
replias, just to mention a few. The KR
series homebuilt is again evolving, this time in the power plant department,
with tremendous results! Besides the
advantages listed, Subaru engines are water-cooled which reduces the cooling
shock to the engine at altitudes and they weigh-in at about the same weight as
the Lycoming and Continental engine packages.
No small wonder builders are choosing these engines ranging from $3,200
to $6,500 over their $15-20,000 counterparts!!
The homebuilder who does his own engine work gets off even cheaper.
New builders are beginning
to take advantage of the newer engine technology, economy, and greater
durability available on automotive engine conversions. Over 50 Subaru EJ-25 units have been sold to
Glastar builders last year and a similar number to RV builders. Countless Avid Aircraft uses now have the
EA-81 firewall package and Avid uses the EA-81 as its standard installation
package. All these builders can’t be
wrong!
If you check back through
the KR Newsletter issues, you will find that someone even adapted a turbine
engine to a KR2. I saw this one running
at the KR Fly-in in Perry, Oklahoma several years ago. The line at the bottom seems to be that the
choice of power plants belongs to the builder alone, and his/her pocket
book. (Comments herein are
informational in natural and not an endorsement for any vendor’s products. NSI is the trademark of NSI Aerospace,
Subaruaircraft is the trademark of Eggenfellner Aircraft, and Stratus Inc. is
the trademark of Stratus Aircraft Inc. You should check thoroughly into the background
of any vendor before giving him your money!!!)