Notes & Comments Antennas

Q: The ferrite toroids for the antennas have a green side and a white side. Is it important how they are installed?

A: No. These little things are simply ground up crystals that are pressed together with a glue, and then dipped in a green varnish. They are not directional.

From "Construction Notes" Falco Builders Letter, June 1991

Recently a number of builders have asked me about the notes in Chapter 14 of the construction manual about 'grounding other metal parts'. I seemed to recall that this was primarily an ADF/loran requirement only. But I wasn't sure, so I wrote Jim Weir, world-famous stealth-antenna designer. Jim replied:

"I bond metal parts for a whole bunch of reasons.

"1. If, for whatever reason, the metal part is in 'quasi' contact with another metal part and by moving the metals parts they make and break contact, there will be the goldurndest static display in the ADF that you can imagine, together with the loran breaking lock for no apparent reason. Now think about all the metal parts that are in "slide" contact with another-control cables on the horn, foot pedals in their bushings, and the like. Now think about that 160 hp vibrator up front sending out pulses every 40 Hz or so, and you have the ideal setup for mechanical resonances driving the ADF and loran just plain batty. (VOR/LOC/GS/VHF COM/MKR not so much.) Finding this problem on the ground or on the bench is well nigh impossible, since it is the aircraft in flight, bending and tweaking and vibrating that is the source of the noise.

"2. If you are near a fairly decent bolt of lightning, you can get field strengths of up to 10 kV/meter. What this means is that a wire one meter long will have 10,000 volts from end to end. This lightning-induced spike will pretty harmlessly go away from a grounded metal part, but an ungrounded part can hold that potential (like a big capacitor) for a while. Not only that, but the bigger the metal piece, the more charge it will hold-and I'd really rather not it build up to the point where it will arc to a fuel line. This isn't a problem in a metal ship where all the wires are electostatically shielded by the fuselage; a plastic or wood airplane is another matter entirely.

"3. The final point is that the more pieces of metal you can get for your ADF or loran ground plane, the better off you are. The wavelength for loran is on the order of two miles, and drops to 200 yards at the top end of the AM broadcast band (1600 kHz.). No way you will ever approach a fraction of the wavelength for the kinds of airplanes you and I fly, but you've gotta do the best you can."

So it's mainly an ADF/loran thing.

And while we're on the subject of antennas, Steve Wilkinson faxes, "We're getting an odd intermittent-transponder situation in which the Falco disappears from ATC's scope, then reappears just fine if you lift a wing-I don't remember if Mark said toward or away from the antenna, which in this case was Quonset Approach. It's happened several times. Some kind of antenna blanking?" I'll be jiggered if I know, but I'll ask Jim Weir at Oshkosh. Anybody else notice this phenomenon?

With a number of Falcos now flying with lorans, we could use some reports on what antennas work. I remember that Wendell Taylor and Dan Garn found the antenna up the tail didn't work for them and that they used the canopy frame which worked well. Terry Smith, on the other hand, has the antenna up the tail and says it works fine for him.

Jonas Dovydenas got tired of hearing all the conflicting advice about loran antennas and decided to build all of them and test them. He bought a Northstar M1. Northstar publishes instructions for making two antennas. One is made of coaxial wire and very specific lengths are required. A second antenna is a plain piece of wire, about 19" long and suspended from insulators, that is intended for use on the outside of the airplane. Jonas also built antenna of plain wire up the tail that RST suggested to us. The Northstar loran has a signal/noise indicator on it. Jonas tested the antennas in his shop and also out in the driveway. They all worked well in the driveway and less well in the shop. Jonas got the best results from the long wire up the tail, second best from the 19" plain wire and the worst results from the fancy coaxial antenna especially designed for use inside composite airplanes. That's technology for you.

Some years ago when we set up the antenna kits with Radio System Technology we made a mistake by creating two kits. Kit 861 Antenna Components is exactly the same thing as Kit 860 except that they don't put the ferrite torroids on the cable for you. Anyone who can rub a soldering gun against a piece of heat-shrink tubing can do that, so just stick with Kit 861.

From "Construction Notes" Falco Builders Letter, December 1986

Until this past year, only one loran, the Apollo II, was compact enough to fit in our instrument panel and still provide room for lots of other radios. There are now several; both Foster and Arnav have compact and desirable lorans on the market, and each new model has features you could previously only imagine.

Arnav now has three new models, the R-15, R-15B and R-30, all of which are variations on the same receiver. In addition to providing loran navigation, they can tie into a fuel computer so that you can know not only when you are going to run out of fuel but where. And if you crash in the boonies, the optional ELS gives your airplane's exact coordinates, ID number, and time since the crash in spoken English-in addition to transmitting a digitally coded signal to search and rescue satellites. Lord knows what features they'll add next year.

Arnav has put out a brochure for installing loran antennas in homebuilt aircraft. Some of you have had questions about their antennas, so I wrote Walter Dean, the author of the paper, for his recommendations on the Falco. He suggests installing the antenna coupler as shown in our plans and making the antenna of the "monopole" design, that is, of a single piece of RG-62/U coax. The antenna wire should run up the forward or aft face of the main fin spar as high as possible. The antenna should be as long as possible, and the last half of the coax wire should be stripped, thus if the overall length is 56", the last 28" should have the outer braid removed.

Although this antenna appears to be a monopole, it is electrically equivalent to a dipole since the lower half of the antenna is merely a coax cable and the shielding of the cable acts as the lower half of a dipole while the extended center conductor acts as the upper half. If the length of the shielded (unstripped) part of the antenna is two feet or less, use RG-58/U coax cable. If the antenna is longer than that, use RG-62/U cable. (There are some conflicting statements in his brochure, and Walter Dean confirms that the previous sentence is correct.)

From the antenna preamp-or "coupler"-to the loran you may use either RG-58/U or RG-62/U cable. It really doesn't matter, and RG-58/U is cheaper, lighter and more flexible.

As for the need to ground hinges and other small metal parts, Walter Dean says that he doesn't have enough experience with wood and plastic airplanes to say.

Which antenna do I recommend? I can't say, since I know so little about antennas. It seems to me that Jim Weir's antenna and Walter Dean's antenna are essentially the same thing. Jim Weir's antenna is also a dipole with a longer center conductor and with other parts of the airplane acting as the lower half of the dipole. Walter Dean's antenna has equal length of shielding and center conductor, and he specifies the impedance of the wire. My guess is that either will work. Following the same twisted logic that it's a good idea to use the same brand thinner as paint, even though all thinners are about the same, if you are going to use an Arnav loran, you should probably use Walter Dean's antenna.