THE APT "STEALTH" ANTENNA john L DuBois, WlHDX 873 Hill Road, Boxborough, MA 01719 USA Background The patch, or microstrip, antenna has become a common workhorse for the microwave spectrum from about 1 GHz to tens of GHz. This type of antenna consists of two conducting layers separated by a dielectric to form an extended capacitor. Often the the structure is planar, the sides about 1/2 wavelength long and the separation a few percent of the wavelength. It performs as a resonant cavity which radiates from the electromagnetic fields at the edges. The great utility of this form comes from the possibility of conforming its shape to just about any application: missiles, airplanes, hand-held GPS, etc. and from the ease of fabric- ation which makes arrays of patch elements both accurate and low in cost. At microwave wavelengths, patch elements are usually made by "printed circuit" or photolithographic techniques. The radiation pattern of patches depends on the final shape of of their radiating edges but in the planar form is a smooth, hemispherical pattern without nulls. In effect, gain is traded for uniform coverage over half a sphere. The radiation can easily be made to have linear or circular polarization. Finally, an identifying property of patch antennas is narrow bandwidth, up to only a few percent of the design center frequency. The properties described are not ideal for every application. They do, however, seem to answer the problems unique to reception of polar- orbiting APT weather satellites. Here, we want a hemispherical shape to the pattern particularly without an overhead null which is characteristic of dipole or monopole elements. We also want circular polarization and narrow bandwidth to reject nearby (in frequency) sources of VHF interference. Existing APT antennas which answer these desires are available in the quadrifilar helix and Lindenblad designs. These are mechanically intricate, however, and it seems worthwhile to investigate weather the patch design could offer a third alternative. Design A quick calculation shows that a 1/2 wavelength patch for 137.5 MHz would be about 43 inches on each side assuming free space wavelength. Since the thiclcness will be only a few inches this may not be too large. How will we build it? Printed circuit board material this large is heavy, not easy to obtain and, even at 1/8 inch thickness, would vield a very narrow bandwiidth in the finished antenna. Happily, there is an ideal material available from the home construction industry. Foam insulating board is available with aluminum foil facing pre-applied to both sides. It comes in 4 ft. by 8 ft. sizes, is 2 inches thick and, at first glance, seems like it was made for building patch antennas at VHF wavelengths! The only minor unknown is the dielectic constant of the foam. The double foil faced foam board material is available from larger building supply stores and from insulation specialty supply stores. The foam material is polyisocyanurate and there are at least two manufacturers: Celotex and Owens Corning. The Celotex brand name is "Tuff-R" and the Owens Corning brand name is "Energy Shield". In the Boston area in early 1994 the price was about $23.00 per sheet. A test patch was made from scrap material with active patch dimensions of 21 x 14 inches to determine the the foam dielectric constant. For an assumed value between 1.0 and 1.20, the calculated resonant frequency of this patch would range from 256.7 MHz to 235.9 MHz. The measured figure was 246.4 MHz, indicating a constant of just under 1.1. A simple linear polarized patch design proceeds by using the radiation from only one pair of sides. (A general design reference is: "Microstrip Antenna Design", K.C. Gupta A. Benalla, Artech House, Norwood, MA, 1988, ISBN 0-89006-331-l.) Resonant frequency is determined by the entire area between faces but the principal radiation mode is determined by the longest side. Circular polarization, on the other hand, is achieved by making the patch almost square so that there are two resonant modes from adjacent sides, separated in frequencv bv approximatelv the bandwidth desired. Both modes are excited by feeding theJantenna along a diagonal. Radiation from the two modes has a relative phase shift because of the difference in resonant frequencies and by careful calculation this phase shift can be made 90 degrees, producing an "automatic" circular polarization. (There are other feed methods to obtain circular polarization but this one is simple and produces an acceptable bandwidth.) A calculation fo 137.5MHz center frequencv produces patch dimensions of 37.75 x 39.25 inches for the 2 inch thick foam board. The feed point for right hand circular polarization and approximatelv, 50R impedance lied 18.5 inches from the outside corner along a diagonal running according to Figure 1. It is important to get this right as using the diagonal displaced bv 90 degrees will produce left hand circular polarization. A patch is usually assumed, for calculation, to lie above an infinite ground plane. However, experience shows thad a ground plane about 10% larger than the patch on all sides is adequate for most applications. Here, for simplicity, we will cut the 4 ft by 8 ft raw sheet in half and use the resulting 48 inch square as the basic ground plane and cut the foil on the other side to be the patch. Feeding the patch is also simple. Since the feed point impedance has been designed for 50 ohms, a coaxial cable shield can be attached to the ground plane side and the center conductor run straight through the foam and attached to the patch on top. Since it is almost impossible to solder to aluminum film, we use small pieces of copper foil with conductive adhesive to form solderable regions around the loations for attaching coax. Such foil with conductive adhesive is not common but one source is: McMaster Carr, PO Box 440, New Brunswick, NJ, 08903-0440, (908) 329-3200, catalog #76555A644 for a 36 yard roll of 1" tape. Since this is a lot of tape to buy for one antenna, the author will be happy to supply two 1" squares for a self addressed and stamped envelope (USA only). Construction To make the antenna: Cut the 4 ft x 8 ft foam board in half to get a 4 ft x 4 ft piece. Then, on one side, mark out a rectangle on the foil 39-1/4 inches by 37-3/4 inches centered on the board. Using an Exacto knife or scalpel, cut thru the foil along the lines you have laid out. It's OK to cut a little ways into the foam when you do this as the foam is pretty strong stuff. Now peel off the outer strips of foil to leave the 39-1/4 by 37-3/4 inch rectangle of foil in the center of the board. This peeling may be a little tedious in places where the foil-paper stuff wants to stick but be patient and tease it up. Don't worry about chipping away some of the foam in the process as it won't have any affect on the antenna. Now orient the board with the 39-1/4 inch dimension along the top in front of you and the 37-3/4 inch dimension toward you. Mark a diagonal from the top left corner to the lower right corner. Then mark a point 18-1/2 inches along and on this diagonal from the upper left corner. This is where the feed point will be. Punch a 1 / 8 inch hole all the way thru the board at this point with an awl or drill bit and trim the foil edges of this hole clean. Take two pieces of copper foil about 1 inch x 1 inch with conductive adhesive. Cut a 1/16 inch hole in the center of one and a 1/8 inch hole in the center of the other one. Stick the one with the 1/8 inch hole over the punched hole in the foam board on the side which does NOT have the cut rectangle. Take a convenient length of RG58 and separate the shield and center conductor for a distance of 2-1/4 inches. Strip off the center insulation for 1/4 inch and tin the center conductor. Cut the shield to a length of about 3/8 inch and mash out flat at a right angle to the cable. Push the center conductor thru the copper foil you have stuck to the foam board so that the shield fits up flat against the copper foil. The tinned center conductor should be sticking out the other side of the board. Solder the shield to the copper foil using a minimum of heat. Now fit the copper foil piece with the 1/16 inch hole over the tinned RG58 center conductor on the other side of the board and stick it to the foil on the board. Make sure you have trimmed the center insulation back far enough for the foil to fit flat against the board. Bend the center conductor over and solder it to the copper foil using a minimum of heat. Note that you will have to remove the blue protective film from the sticky side of the adhesive copper foil before using it. The front of the antenna is the side with the cut foil square. It can be mounted by nailing, clamping or screwing down through the exposed foam sides around the cut pattern but keep the fasteners at least an inch away from the front foil pattern. The final result will weigh about 3 pounds. As with any APT anterma, a good, low noise preamp should be located close to the antenna itself. The weather resistance of this antenna has yet to be seriously tested. The polyisocyanurate foam's cell size is very small and we would not expect it to absorb much moisture. However, a light spray of clear or colored acrylic has very little effect on the resonant frequency and will effectively seal any exposed foam. The foil surfaces can be likewise coated with no effect. It is probable that extended exposure to the ultraviolet components of sunlight will weaken the exposed foam but there should be little practical problem from this effect. Results The calculated pattern of this patch is shown in Figure 2. The E and H plane patterns are almost identical above 20 degrees elevation. Under 20 degrees, there is a null in the H plane but the E plane pattern goes "below the horizon". We would expect from this to see reduced or poor performance below about 10 degrees for the circular polarized configuration used. The swept return loss (in a 50R system) measured on the prototype to evaluate impedance matching is shown in Figure 3. The maximum value of - 18 dB at 136.6MHz corresponds to a VSWR of 1.3:1 cvhich is entirely acceptable. This curve also shows the bandwidth of the antenna to be about 8 MHz which is also acceptable. Live reception was tested using the Quorum Communications "Explorer' PC board and "QFAX" software. Signals and image were clear and strong over satellite elevation angles between 10 and 170 degrees. Under about 10 degrees, as predicted, reception fell off and picture noise observed. The pattern of noise bands suggested that the antenna had several minor maxima and nulls in this region. Since this patch has only a 2 inch profile on a roof and can be painted in a variety of colors, it will be very hard to detect from a distance. For situations where zoning, neighbors or spouses present objections to "unsightly" antennas, this construction may well solve the problem and still permit APT reception. As a result, it seems appropriate to call it a Stealth Antenna. [Editor's note: The foam board is available in the UK from insulation specialists and some builder's merchants in similar sheet sizes; 1200 x 2400 x 50mm. Warren Insulation on the Poyle Industrial Estate near Heathrow quote a price of 21 + VAT. Copper tape with a conductive adhesive is available from RS. As the minimum quantity costs 22 + VAT and is sufficient for about 300 antennas, I will supply a quantity sufficient for one antenna upon receipt of a small selfaddressed envelope. [Metric conversions: 1 inch = 25.4mm. 1 foot = 12 inches = 304.8mm]