Antenna Installations L~Tronics offers a number of aircraft and vehicle antenna systems. They are available either for a single frequency range or band (single band) or for two different frequency ranges (dual band). Single band units consist of two whip antennas and a molded switchbox and cable assembly. Dual band units have a third whip mounted between the first two. Single band installations can be converted to dual band at any time by adding a suitable center antenna. Antenna kits for aircraft are offered using standard bent whips. Flexible whips can be used for some specialized aircraft installations but are intended for permanent mounting on vehicles. All antennas work with either LH or LL Series portable or LA Series aircraft DF receivers. The following describes aircraft installation, but the principles also apply to vehicles.
Following the section on aircraft antennas are instructions on installing and using magnetic, flexible and weatherproof antennas. This section can also be downloaded to make a printed copy. (download PDF, 2mb)
|Switchbox assemblies are NOT inter changeable between antennas with different whip styles. Antenna whip length and the length of the cables between the antennas and switchbox cannot be changed without degrading or destroying DF performance. The center antenna for dual band assemblies must not have any external electrical connection to it. A short tuning stub is provided with certain dual band models requiring it. This center antenna is NOT a "spare antenna" for use on other equipment.|
|Flight test ALL new aircraft installations.|
|Each receive frequency selected on the DF receiver may cause interference to one aircraft communications channel: for 121.5 the affected frequency is 132.2 MHz; for 121.6 it is 132.3; for 121.775 it is 132.475; and for 243 it is 126.85 MHz. If communications is required on these frequencies, TURN THE DF SET OFF.||
Antenna Test Procedures
All left-right DF antennas, including the wood frame crossbars used with the LH series DFs can be checked for shorts and diode damage without disassembly by using either an analog ohmmeter with a test voltage above 1 volt or a digital meter with a diode test function. See Antenna Testing for details.LAA Series Antennas The performance of any DF set is absolutely dependent on the antenna installation. For locating ELTs and other transmitters on the ground, THE DF ANTENNAS SHOULD BE PLACED ON THE BELLY OF THE AIRCRAFT wherever possible.
No single installation method will work best for all aircraft. The following principles and suggestions should ensure a good installation. The further your installation departs from the standard, the more flight testing will be required to verify proper performance.
Belly installation is preferred because the aircraft structure does not come between the ELT and the antennas when passing overhead. It also prevents possible interference to GPS from comm transmissions.and buzzing on comm signals that often occurs when comm and DF antennas are close together.
The whip antennas and the aircraft structure work together to form the directive antenna patterns necessary to the operation of the DF set. A doubler plate or other skin reinforcement is usually required around the antenna mounting holes. The switch box and antenna cabling must be adequately secured. Installation of the antennas and the permanently-mounted aircraft DF (LA Series) will usually require an FAA Form 337 signed off by a mechanic or radio repairman. The bent whip antennas are TSOed and the receiver is considered either portable or non-essential equipment by the FAA. Sample 337 forms can be found in the aircraft manual. Download PDF (20mb).The antenna rods are installed on a line at right angles to the direction of flight, as shown in Figure 7. The coaxial cable supplied with the antenna kit should have one of its two short leads marked "left." This lead is connected to the left antenna, the other lead to the right one for either top or bottom installation. The center antenna usually has no connection, although some models have a short, open-ended tuning stub supplied. If the leads are not marked, the leads can be identified by placing the switchbox as shown below. If the leads are put on the wrong antenna, the meter readings on the DF set will be reversed.
Three things are critical to successful installation of the DF antenna system: (1) the length of the antenna rods, (2) the length of the cables from antenna rods to the switchbox, and (3) the symmetry and freedom from blockage of the mounting location.
Antenna damage which shortens the whips by more than one half inch must be repaired by substituting new parts. The length of the cable from the switchbox to the receiver is not critical. Replacement parts are available from the factory.
Location and Clearance As noted above, BELLY MOUNTING IS STRONGLY RECOMMENDED except for seaplanes or for aircraft with special equipment like cargo pods. Spacing between the antennas can be varied somewhat to suit the aircraft structure. The nominal spacing of the outer antennas is 10 inches from either side of the aircraft centerline. This can be made as far as 12 inches and as little as 6 inches either side of center (12 to 24 inches total). If a choice exists, narrower spacing should be chosen. If the center antenna cannot be placed exactly on the aircraft centerline, all three antennas can be shifted left or right by up to 2 inches. The spacing of the outboard rods to the center rod (or center- line) must be identical. Mounting on a curved surface is OK as long as the outer antennas are not more than 40 degrees off vertical and are symmetrical. Mount the antennas to maintain spacing at a point 5 inches up the antenna from the base.
The DF antennas should be mounted well forward on the aircraft. A good spot on Cessna 172s and 182s is just below the rudder pedals. Inspection plates in the floor give ready access and DF antenna patterns are good. This also works for Piper Cherokees, except that covers will have to be made to protect the cables above the honeycomb panels. Mounting further back, between the gear, has given poor antenna patterns on several installations.Transponder, DME, and boat-type marker antennas commonly found on the belly do not cause interaction problems if they are one foot or more from the DF antennas. Sled type marker or ADF sense antennas, CB antennas, and 150MHz comm antennas should be on or very close to the centerline.
The antennas can be bent near the tip for ground clearance, shown as a dotted line in Figure 7, as long as all rods are bent equally. Leave the initial bend unchanged. Contact the factory if the total distance of the antenna from the fuselage is less than 10 inches. Hot bending is required; cold bending will radically reduce fatigue life, even if immediate breakage does not result. For helicopter or other installations where ground contact is probable, the flexible whip antennas can be used. They may also be bent as long as all are identical. Flexible whips are NOT TSO, so they may require more effort for installation approval.
If the antennas must be installed on top, they should still be very near to the centerline of the aircraft. Nearby VHF antennas and other objects of similar size must also be symmetrical if an accurate homing course with no false courses is to be obtained. Figures 8, 9, and 10 show some layouts that have worked.
Figure 8 shows the best layout for multiple antennas on the top of an aircraft. The arrangement of Figures 9 and 10 are also usable and have the advantage of retaining a commonly-used location for communications antennas, which MUST be of the same type as each other, but not necessarily the same type as the DF antennas. The comm antennas must be connected to a radio or otherwise terminated; any unconnected comm antenna near the DF antennas can ruin DF performance.
There is considerable interaction between DF and comm antennas when they are mounted close together on top. THE DF SWITCHING MAY PUT A STRONG TONE ON COMMUNICATIONS RECEIVER SIGNALS FROM SOME DIRECTIONS. The DF will have to be turned off or the aircraft heading changed for good comm intelligibility. There is also a potential interaction between comm, DF and GPS antennas that may cause loss of GPS track when transmitting on some aircraft comm frequencies. If the DF antennas must be installed on top, place the GPS antenna as far away as possible.
Things to Avoid
A few things to avoid are: mounting on the engine cowl due to excessive radio noise, vibra tion, and propeller modulation of the received signals; mounting within four feet of a non-retracting step more than one foot long (especially Cessna 310); mounting any further aft than one foot forward of the main gear legs on Cessna 172/182 aircraft and Yankees. Check other narrow tread fixed gear installations carefully.
Glide slope antennas should not be placed behind a three-antenna DF array if a forward location exists because of a small shield ing effect. Glide slope, Loran, ADF, and Stormscope antennas have no observable effect when mounted near the DF antennas. ADF sense, 150 MHz comm and Citizens Band antennas should be located on the aircraft centerline if possible.
Transmitting antennas in the 118-136 or 225-265 MHz bands should be separated from the DF antennas using the minimum safe distances shown in the following table. The switching diodes can be damaged if transmitting antennas are too close. If damage occurs, the DF meter will deflect to one side only or stay centered. Transmitters in other frequency ranges can use distances of half those listed.
The length of the lead-in cable from the switchbox to the DF receiver is not critical. See Figure 15 for connector assembly. Many problems with the DF set can be traced to poor connector installation, especially a center pin that is too far below the connector end. The switch box may be secured with cable clamps or ties. Be sure to attach the cables to the proper antenna, as shown in Figure 7a, or the DF needle sensing will be reversed. The threaded portion of the standard whip should be more than 1-1/2" long to accommodate all hardware. Occasionally the threaded ferrule supplied on the antenna must be tightened to give full thread length. The antenna mounting holes must be flat and burr-free and the fiber washers MUST be used to avoid breaking the ceramic insulators. We recommend coating the fiber washers with "RTV" rubber for improved cushioning and sealing. Also, the large, internal tooth lock washer MUST be between the skin and the large ground lug on the inside to assure long term ground contact. See Figure 14 for details. The inside of all antenna rods must clear surrounding structure by at least one inch and preferably two inches.
There is no electrical connection to the center rod of the LAA-16 antennas (121.5 MHz/243 MHz). The center rod is somewhat shorter than the outboard antennas. On some models, a short coaxial tuning stub is supplied for connection to the center antenna and all antenna rods are the same length. The free end of the stub should be tied down.Flexible Whip Antenna Installations
These antennas are not TSOed and should be considered only for specialized aircraft and helicopter application or for use on vehicles. The flexible whip installation differs from the standard bent whip installation both in whip type and in cable length from the whips to the switchbox. The harness comes with industry-standard NMO style bases which must be attached to the cables after the cables have been fed through the skin from the inside. The bases can only be installed from the outside. A doubler plate will be required in most light aircraft installations to take the stress at the antenna base clamp.
There are two identical antenna rods supplied for units operating on a single band (121.5,121.6, 121.775). A third, shorter rod is supplied for the center position on dual band (121.5, 121.6, 121.775 / 243) installations, similar to the bent whip antennas described above. Whip spacing and other mounting considerations are the same as for the bent whip antennas above. The antenna rods may be bent back 40 degrees on a gentle radius starting 5 inches from the fuselage or roof and then straight back near the tip to provide better ground clearance without affecting electrical performance significantly. Some instability may be noticed in the homing course at times due to antenna vibration. A small change in airspeed or engine RPM should reduce the effect.
If the antenna rods are to be removed when not in use, unscrew them from the base. When puttng the antennas on the plane, use a simple plastic strap wrench to tighten them. Just tightening by hand may result in loss from vibration. The setscrews should be kept very tight. A drop of Locktite in the whip socket would be a good idea for bottom mounted installations to prevent whip loss.Fabric and Composite Aircraft
Antenna installation on composite or fabric- covered airplanes presents a problem in obtaining an adequate, symmetrical ground plane for the antennas. The sheet metal fairings on the wing leading edges above the cabin or aft of the firewall on the belly of many aircraft have been successfully used. An adequate ground can be constructed of four or more 24" lengths of 22 ga. copper wire or one-inch-wide strips of adhesive- backed copper foil fastened under the fabric or composite skin, evenly spaced around the base of each antenna rod and connected to the braid of the antenna coax cable. If a choice exists, this constructed ground plane should be used for the communica tions antennas and the sheet metal ground used for the DF due to the disturbing effect of the normally unsymmetrical tubing fuselage structure. A metal area of at least 18" x 36' is required. If the DF antennas are installed over a wire or tape ground plane, one wire or tape should run directly between the bases of all antenna rods. A fully enclosed DF antenna is possible on some composite aircraft; however, this project will require an extensive engineering effort with substantial flight test followup.
Antennas for Little L-PerPortable DF sets on frequencies other than those listed in the catalog can be supplied on request to the factory. Internally mounted antennas, such as wires taped to windows of a metal aircraft, generally give unsatisfactory results. The major problems are ambiguity and false courses, particularly to the rear of the aircraft, and sensitivity to the presence and movement of cabin occupants. Thus, such an antenna may seem to work on a limited test but have major problems on a real search.
Some pilots and search orginizations have made workable temporary installations by fastening antennas to wing struts or replacement inspection plates located symmetrically on either side of the plane. The antennas are placed so that signal blockage by the fusulage causes them to receive better from the clear side. Results are variable so any such installation should be thoroughly tested before being used on an actual search. Because of the potential hazards, both physical and legal of such installations, L-Tronicswill not supply plans or material for them.
If more than occasional vehicle DF is anticipated, a flexible whip antenna kit can be installed on the car roof. Plastic roofs will require a ground screen like that described for fabric-covered aircraft. If a single pair of antennas is used, it will be most useful oriented fore and aft as shown. The diagrams and pictures below show the most common arrangements for either LVA (permanent mount) or LVA (magnet mount) sets. Permanent installations will have the connecting cables inside the vehicle. Magnet mount antennas should not be left outside for long time periods.
In this configuration, with an LH series receiver In the DF mode, a needle swing to the left of center Indicates that the signal is in front of the vehicle and a swing to the right indicates location to the rear. For LL series receivers, the display switches to up and down when the cable is plugged into the fore-aft jack. This method of operation gives less meter confusion than left-right operation when driving in the presence of many reflecting objects (which Is usually the case). A second pair may be mounted for left and right operation with a manual switch for pair selection for the LH series receivers or plugged into the left-right jack for automatic fore-aft-left-right indication with the LL series receivers. If the antennas are centered on the car roof, fairly good left-right discrimination can be obtained For vehicles with emergency light bars, place the left/front antenna in front of the bar and the right/rear antenna behind it.
LVA-10 Single band Fore-Aft
LVA-16 Dual band fore-Aft
LVA-100 Single band Fore-Aft-Left-Right
LVA-160 Dual Band Fore-Aft-Left-Right
LVA Series Magnetic Antennas
Magnetic antennas can make a temporary DF installation on any steel car roof. They will work electrically on vinyl covered roofs, but may blow off at modest speeds. The four most common styles are shown above.
The antennas are optimized for gain and SWR at the design frequency and will provide a usable pattern with less than 3 dB sensitivity loss over the listed useful range. Optimum spacing is also listed, but differences of ±3 inches produce negligible performance change. If a four (or five) antenna (fore/aft, left/right) installation is used, the antennas should be moved apart until the optimum spacing Is measured between adjacent corners. Dual band models have an additional center antenna, which determines the higher frequency range: the outside antennas determine the lower range. If operation Is desired on only the lower range at a given time, the center antenna may be left off.
The magnetic antennas require a ground surface to operate, normally supplied by a car roof. The antennas will work best over a ground surface of 40 x 40 inches or larger. Placing the array so the front antenna is close to the top of the windshield as shown in the pictures works well and provides maximum separation from transmit antennas at back of the vehicle. A wire screen can be used for a mounting surface if steel plates larger than the antenna bases are attached to the screen to hold the antennas.Transmitting antennas in the 118-136 or 225-265 MHz bands should be separated from the DF antennas using the minimum safe distances shown in the following table. The switching diodes can be damaged if transmitting antennas are too close, even if the receiver is turned off or disconnected. If damage occurs, the DF meter will deflect to one side only or stay centered. Transmitters in other frequency ranges can use distances of half those listed.
|5 watts 3 feet|
|10 watts 4.5 feet|
|25 watts 7 feet|
|40 watts 10 feet|
|80 watts 15 feet|
The antenna bases are covered with a plastic anti-scratch cover. Loss of this cover will degrade performance; however, tape or adhesive vinyl shelf paper can be used as a replacement. It is not recommended that these antennas be left on the vehicle for storage because both the bases and the coax cable can deteriorate and corrode from continued exposure to the elements.
Placement of antennas is described in the previous section. All comments apply to both magnetic and permanent installations.
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