Radio Direction Finder Antenna Systems

Introduction

Servo Corporation of America has over fifty years of experience in developing and producing radio direction finding equipment. During that period the company has been involved with all types of DF systems; Adcock, Doppler, Quasi-Doppler and associated multiple-element, commutated (mechanical and electronic) arrays.

During the late 1940s Servo invented and patented the Quasi-Doppler concept for use in direction finding equipment and has been supplying various versions of this type of equipment to customers worldwide since that time. We have supplied these in numerous quantities to more than 30 countries while over 350 of our systems are owned and operated by the United States Federal Aviation Administration in airports throughout the United States.

DF Antenna Types

In response to the individual requirements of a variety of customers, Servo has developed a standard line of commutated array, Quasi-Doppler Antennas designed to cover the UHF and VHF air band as well as the marine band frequencies. Furthermore, DF antennas designed for the VHF air band are available in either a 16 element, wide aperture array or an 8 element, medium aperture array. Basically, the 16 element array features enhanced accuracy performance in a reflective environment as compared to the 8 element array, while the 8 element antenna is available at a lower cost than the larger 16 element antenna for those installations having limited space and/or less stringent site accuracy requirements.

A brief technical discussion of the site accuracy performance of these two antenna types is presented as follows.

DF ANTENNA PERFORMANCE

Description of a DF Site

At very high frequencies (VHF) line-of-sight conditions apply. This means that, except for occasional, unusual anomalies, a transmitter and receiver must be "visible" to each other for a path to exist. The VHF direction finder (VDF) uses the transmitted signal to determine the bearing to the target transmitter; the bearing is determined by evaluating the angle at which the RF wavefront passes over the VDF antenna site. Given a perfect site (with no obstacles or reflectors in the hemisphere from the VDF antenna to the horizon) the wave front from the transmitter will be spreading in a circle centered on the transmitter with its motion across the DF antenna site exactly on line to the transmitter. Under such a condition, the smallest, simplest, least sophisticated DF will be able to precisely locate the target transmitter and indicate an accurate bearing.

However, the utopian world of such a perfect site is never to be found on an operational site. On an operational site, there is a constant battle to reduce obstacles and reflections to a point where the impact on bearing accuracy is tolerable. Every obstacle and every reflection introduce aberrations in the wavefront crossing the VDF antenna site. The wavefront is no longer necessarily a simple plane. In fact there are several, some probably moving in directions different from the line to the transmitter. Under these conditions a VDF will evaluate the total RF field in accordance with its particular characteristics. Under such conditions, different systems will respond with very different overall accuracies. A Table of Guidelines for a "good" DF site issued by Transport Canada is contained in Attachment 1 to this paper.

Most VDF systems are required to demonstrate excellent instrumental accuracy on a perfect site. They are also required to demonstrate certain maximum peak and RMS bearing errors under conditions of a well defined reflection added to the main RF field. A typical definition of such a reflection is as follows: It has to have an intensity of 1/5^th of the primary field, that it is to radiate from a position at any angle from the VDF site, and that the phase angle of the reflection will be adjusted to yield the largest error at each position.

Theoretical Analysis of Reflection Caused Errors

As part of the development effort of direction finders and the several antenna alternatives, there has been some very useful mathematical analysis of the effects of reflections on various systems. A seminal paper on the subject was written in 1948 by Hopkins and Horner in England*. Mathematical analysis of the operational concepts of all standard DF antennas (including all those in use today) is reported together with the analysis of the effect of reflections on the bearing error of each type of antenna. Recognizing that every site is different, they focus on the utility of being able to compare DF systems (i.e. antenna types) for their sensitivity to a simple first-order reflection. Since the reflections at any site can be described as combinations of several simple reflections, the relative error sensitivity of DF types at any site can be correlated to the sensitivity for the first order reflection.

The mathematical analysis is detailed for both the maximum error for a single reflection at any angle, and the RMS error for the single reflection at all angles, just as described in the experimental test noted above. As might be expected, the analysis is quite complicated, particularly for the more sophisticated antenna types, but the formulas have stood the test of time and have proved to be a valuable tool in the selection of DF systems for particular requirements.

The antennas analyzed include:

• A pair of spaced antennas, each with omni-directional characteristic in the horizontal plane. This type is the basis for all small aperture systems (e.g. Adcock, time-of-arrival (TOA), crossed dipoles).

• Cyclical, differential measurement of phase. This is prominent in the Doppler and Quasi-Doppler antennas used today.

To summarize the information contained in the paper, it is shown that for all antenna types, the larger the antenna, the better performance in the presence of reflections. Since modern VHF DF systems relate very closely to only the two types of antennas noted above, only the performance of these types will be discussed.

As noted above, the larger the antenna, the better the DF performance. However, in the case of two spaced antennas, the spacing must be less than one-half wavelength to avoid bearing ambiguities; i.e., different bearings giving the same indications. Typical spacings are small compared to a wave length. In this condition, this type of DF system is most sensitive to reflected signals as should be expected from a small aperture antenna.

Specifically, the peak error for a narrow aperture Adcock type antenna with the standard reflection field is about 11.4° . The error-curve with position of the reflection around the DF site is described as a sine curve, with the result that the RMS error is close to the maximum; the RMS error is about 8.0°

*H.G. Hopkins and F. Horner: "Direction-Finding Site Errors at Very high Frequencies," Journal I.E.E. 1949, Vol. 96 Part III, p. 321.

In contrast, the Doppler or Quasi-Doppler DF system can be made much larger without encountering ambiguities. The multiplicity of antenna elements permits a large array diameter while maintaining a spacing between elements of less than ½ wavelength. It is precisely for the purpose of improving performance of the DF system in operational, less-than-perfect sites that a DF antenna be employed which covers a much large aperture than the simple two-spaced-antennas system. The performance of the Doppler antenna system is hence very much superior for this condition.

Furthermore, it then follows, that a larger diameter Doppler Antenna (e.g., 16 element) will provide greater site accuracy performance than a lesser diameter (e.g., 8 element) array under the same field conditions.

The peak error with the standard field is frequency dependent, since the antenna is larger (i.e., more wavelengths across) as the frequency increases. At the lowest frequency of interest, 118 MHz, the antenna now in use in all 16 element VDF systems has an aperture of about 1.2 wavelengths. For this frequency, this antenna yields a peak error of about 3.4° . The RMS error, because of the unusual advantages of the Doppler antenna in minimizing the effects of reflections over most of the circle from which reflections could come, is a much smaller percentage of the peak error than is the case with the Adcock or time-of-arrival type antenna. The RMS error, under the conditions above, is only about 1.5° for those installations utilizing the wide aperture, 16 element antenna.

In contrast, under the same conditions, an 8 element antenna having a lesser diameter (in order to maintain a similar ½ wave length spacing between dipoles) will yield a peak site error of about 5.2° and an RMS error of 2.8° .

For informational purposes only (carrying the calculation one step further) a 4 element, narrow aperture, Doppler Antenna will yield a peak site error of about 8.0° and an RMS error of 5.4° .

Antenna Type

Peak Error

RMS Error

Two-spaced (Adcock/TOA)

11.4°

8.0°

One-quarter size Doppler (4 arms)

8.0°

5.4°

One-half size Doppler

(8 arms)

5.2°

2.8°

Full size Doppler

(16 arms)

3.4°

1.5°

SUMMARY

Conclusions

Users have long recognized that there are some benefits of a low cost, small aperture, antenna system for a VDF. Its small size and weight are ideally suited to those installations with limited space or structural support such as found on shipboard or similar environments. As noted previously, the performance of this type of system will meet most customer requirements when installed in a near perfect operating site that is devoid of reflecting elements.

However, due consideration must be given to the enhanced performance of a large aperture antenna system when installed in the less than ideal sites that are normally encountered for most installations. Except where local conditions dictate otherwise, Servo has consistently recommended a system configuration utilizing our standard wide aperture, 16 element DF antenna in order to achieve maximum cost effective performance over a wide range of operating conditions. This antenna has been in continuous use at hundreds of domestic and international locations over a period of greater than 40 years thus attesting to not only its performance but also its reliability. Obviously, its slight increase in annualized cost over a lesser antenna is a small price to pay to achieve the primary, and indeed the only mission of a Radio Direction Finder, the timely and accurate location of a pilot and his aircraft in an emergency situation.