How do I tune a helipad antenna?

The Helipad antenna (H-Antenna) consists of approximately 300 feet of antenna wire that is mounted to the extreme perimeter of the Heli-deck safety rigging on the platform or other structure by as many as eight support poles. The transmitter / antenna coupler feeds directly into the “capacitive top hat” portion of the antenna.

The shape, location, surrounding superstructure, and height AGL of the Helideck are always variable with respect to the various rig structures and therefore defy prediction as to the operating parameters and performance of the antenna. However, basic series resonant RF circuit theory will prevail in most every case, and the antenna will eventually be made to resonate, which is considered to be “tuned”. After the antenna is tuned, it can then be “matched” to the proper impedance (Z) of the transmitter output (50).

As you know, most communication antenna radiator elements are ¼ ?. The addition of a ground plane of sorts results in a total electrical length of ½ ?. These antennae are generally resonant slightly below (i.e. physically long) the operating frequency such that if any “tuning” is necessary, it is readily accomplished with the use of capacitive tuning elements, or else you trim the antenna. The input impedance is a primary result of the radiation resistance of a naturally resonant length.

At 300kHz, ½ ? is approximately 500 Meters in length. This would be impractical at any installation. In lieu, most practical LF antennas are extremely short in regard to actual wavelength or operating fractions there of. Because the antenna is physically, as well as, electrically short at the general operating frequencies, it requires an inductance element to “tune” the antenna. It is on this premise that Southern Avionics Company designs the tuning element of its’ antennas. This will also be the key in the installation of the equipment you have in hand.

As we begin to discuss the operation of the H-Ant and in particular, its’ operation in the 1500-1800kHz range, the above background knowledge must be kept in mind. As a rule, you should already have an oscilloscope, a frequency counter and a voltmeter in hand during installation. As it is currently impossible to guess the possible length you will be able to string the antenna wire, I would highly recommend, if not declare that you will require an RF signal generator capable of sweeping 400-3000kHz. It does not have to be sophisticated, just functional. If you will not get one, then I suggest you follow the procedure in the manual, keeping in mind you are simply tuning a series resonant circuit with three variables to deal with. These three variables are: 1.)Capacitance (antenna wire), 2.) inductance (PC1000H) and 3.) the frequency. The transmitter can be used to sweep 1500-1800kHz, but your chances of damaging the transmitter are greatly increased unless you well understand the transmitter and it’s limitations.

Three tuning procedures come to mind.

1. The antenna appears to be a capacitor of unknown value. Shortening the length of the wire decreases its capacitance. Increasing the length increases the capacitance. Decreasing the capacitance increases the capacitive reactance for a constant frequency (e.g. 1644kHz) and causes the “Q” (ratio of reactance to system resistance) of the antenna to be very high resulting in an unstable antenna system. This unstable antenna system ultimately will appear as if you are having constant problems with the transmitter / antenna coupler. Additionally you will have inordinately high values of RF voltage on the antenna. It is therefore desirable to have the antenna wire strung as long as possible, yet still be in the range of coupler inductance and operating frequency. As you can imagine, changing the length of the antenna wire to tune the system is cumbersome, as well as impractical. It may have to be done though once, maybe even twice, depending upon your understanding and chosen tuning method.
2. The PC1000H is a variable inductor that will tune any reasonable antenna in the 1500-1800 kHz range. For a given antenna length and constant frequency, it is possible to select, 1st, a coarse tap (CT), then 2nd, set one of eleven fine taps (FT), then noting on an oscilloscope a peak indication in radiated voltage “off the air” , while 3rd sweeping through the 90 degree range of the vernier control. This procedure is called bracketing and a similar procedure for an antenna of fixed or known capacitance is outlined in the manual. The problem that one may encounter with this procedure in tuning the H-Ant, is that the antenna length may actually be longer than the resonating capabilities of the coupler, i.e. require capacitance to tune the antenna instead of inductance. If, as the procedure progresses, the CT number becomes smaller in value as the “off the air” signal gets larger, yet without a peak, there is a high probability that the antenna length is too long. At this point it becomes necessary to shorten the antenna by one support.
3. 3.) With the antenna at fixed length and coupler at fixed CT and FT’s, it is possible to frequency sweep the system with an RF generator to determine at what frequency the system is resonant. The indication of resonance is an oscilloscope peak indication in radiated voltage “off the air”, as above. If the resonant frequency is above or below the operating frequency it is then possible to intelligently adjust the antenna length or coupler settings before actually loading the transmitter into the antenna system. An initial CT selection of 15, and FT of position 6 is suggested. If the resonant frequency is above the operating frequency, it will be necessary to lengthen the antenna or decrease the number of CT and/or FT’s or both. On the other hand, if the resonant frequency is below the operating frequency, it will be necessary to shorten the antenna wire or increase the number of CT and/or FT or both. The transmitter may be used to sweep, but practically speaking, the frequency range of 1500-1800 is quite small compared to the possibilities in initial resonant points.
4. After the system is resonant, proceed to the Impedance matching section of the manual. REMEMBER, the system MUST be tuned before the system can be matched.
5. Above all, the set up and tuning of this system will require patience and a desire to set it up for trouble free service. Of the three steps outlined, I personally prefer number three for your particular situation.