February 2003 (Volume 2, Issue 2)
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.
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
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/).
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: firstname.lastname@example.org 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:
If you still have questions, send me a message at email@example.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!!!)