Evaluation of a Coaxial Resonator Match for a Near Vertical Incidence Skywave Antenna

 

By James A. Lacy, P.E.

 

August, 2008

 

Abstract

 

The purpose of this paper is to report the feasibility of a coaxial resonator match incorporated into a near vertical incidence skywave antenna. Using primary and secondary research sources, an antenna to transmitter matching system is built and measured. The result is that, within limitations, the coaxial resonator match is feasible for near vertical incidence skywave antennas.

 

 

Background

 

Near vertical incidence skywave (NVIS) antennas provide short-range communications between high frequency radio stations. The high takeoff angles of an NVIS antenna allow radio waves to be refracted in the ionosphere back to earth. Takeoff angles of 40 to 90 degrees allow communications from 0 to 350 or to 500 miles.

 

Short-range communication is intentional. An NVIS antenna will attenuate outside interference, such as electrical noise from lightning. Thus, signal to noise is maximized for the desired zone of communications.

 

For most antenna design, matching the generator or transmitter impedance to the feedline and antenna impedance is important. Solid-state transmitters do not tolerate mismatches well. This is because a mismatch will generate high voltages destructive to the transmitter components. What is considered excessive Voltage Standing Wave Ratio (VSWR) depends on the transmitter, but 2:1 is a typical limit. An antenna tuner may be used to match a transmitter to an antenna. That entails additional expense for extra equipment. Even though an antenna tuner may be employed, if it is positioned close to the transmitter and away from the antenna, large VSWR may remain on the feedline and antenna. Large VSWR on the feedline and antenna require components capable of handling the increased voltage.

 

Baseline

 

The evaluation approach is to begin with a baseline antenna, and then to add a coaxial resonator match.

 

The baseline antenna is a 1/2-wavelength dipole constructed of 14 AWG copper wire for the daytime 7 MHz band. 7 to 7.3 MHz includes voice emergency channels, health and welfare voice channels, radio teletype circuits, email over radio, and Morse code message traffic circuits. Unfortunately, the baseline dipole by itself has only a 79 kHz bandwidth, not the desired 300 kHz.

 

Due to site physical limitations, the baseline antenna is under elevated. An optimal height for an NVIS antenna would be about 1/8 wavelength, or about 5 meters (16 feet) above ground. One result of such a low height is that the antenna resistance is much lower than the transmitter output resistance of 50 ohms.

 

The site of the baseline antenna is such that the antenna must be physically folded into a horizontal vee shape with about a 60 degree angle. The apex height is 2.7 m high, with the legs sloping to a height of 1.8m. The antenna is within a fenced area to prevent radio frequency exposure to people.

 

Radiation patterns for the baseline antenna are calculated using Numerical Electromagnetics Code (NEC2), with a ground dielectric constant of 5 and conductivity of  2 milliSiemens per meter. Despite the folded configuration, the azimuth pattern is almost uniform. Takeoff angles are from 40 degrees to 90 degrees.

 

Resistance Matching

 

The feedpoint of the baseline antenna is at the apex of the vee. Resistance measures 25 ohms at 7.15 MHz. If resistance were the only consideration, matching is simple. To match a 50 ohm transmitter to a 25 ohm antenna, all that is needed is a quarter-wave tapped transformer. An electrical quarter-wave transmission line, here a coax, has one end open and the other closed. The electrical angle is pi/2 radians. The transformation ratio is close to:[i][ii]

 

 

Where èS and èP are the electrical angles of the secondary and primary taps, respectively, taken from the closed end of the quarter-wave transmission line. In other words, the antenna is tapped into the quarter-wave resonator, and the transmitter is tapped into the same resonator at a different point to satisfy the transformation equation. This is a useful attribute and will be used in an overall design.

 

Matched Bandwidth

 

United States Patent no. 4,479,130, issued Oct 23, 1984, to Richard D. Snyder for "Broadband Antennae Employing Coaxial Transmission Line Sections" contains three pertinent ideas for the application under evaluation. One was the fact that a coax cable could carry signal between the center conductor and inner surface of the shield, but at the same time carry currents on the outside surface of the shield and serve as a radiator. Two was the use of compensation stubs fashioned from coax cable. Three was the use of two stubs to broaden the antenna bandwidth. This was accomplished by tuning one stub to a frequency halfway between the antenna desired low and mid-frequency, and the second stub halfway between the antenna desired high frequency and mid-frequency.

 

Use of paired stubs in this manner fits bandpass filter theory. The general equations for bandpass response are found in Reference Data for Radio Engineers, 6th Ed., pp. 9-5 through 9-9, along with the conditions assumed. For double-tuned circuits, Q of both circuits must be known.

 

Apparently bandpass filter theory is the underlying basis for the mathematical derivation of Frank Witt's equations described in "The Coaxial Resonator Match," The ARRL Antenna Compendium, Vol. 2.  His approach is to treat a conventional dipole as a tuned circuit, use a 1/4-wave coax stub as a second tuned circuit, simultaneously use the coax stub as an impedance transformer, use the paired circuits as a bandpass filter, and let the outer surface of the coax stub act as a radiator. Additionally, not constrain the antenna feedpoint to the center of the dipole, but allow the feedpoint to be offset to vary the feedpoint impedance as needed for best bandwidth results.

 

 

Coaxial Resonator Match Methodology

 

Witt provides a complete description of design equations and physical meaning of the coaxial resonator match.[iii]

 

The baseline wire dipole QA and RA are taken by measurement with an MFJ HF/VHF SWR Analyzer Model MFJ-259B and MFJ-731 SWR Analyzer Filter. RA is read directly. QA is calculated by:

 

QA =

 

The coax line selected is RG-58, both for the resonator and for the feedline. This is sufficient for 100W transmitter power. ZT, ZS, VF, and A are the manufacturers' values for RG-58. These values are probably not exact. A computer spreadsheet holds the equations and performs the calculations. Data and terms as described by Witt are:

 

 

	Input data
7	FL	lower band edge frequency, MHz
7.3	FH	upper band edge frequency, MHz
23.88	QA	dipole Q
25	RA	dipole radiation resistance at center, ohms
1.3	SM	maximum SWR over band, achieved at band edges and center
53.5	ZT	transmission line characteristic impedance, ohms
53.5	ZS	resonator characteristic impedance, ohms
0.66	VF	velocity factor
1	A	resonator attenuation per 100 feet at frequency FO, dB
25.1	RAF	dipole radiation resistance at off-center feed point, ohms
	Unknown Parameters
7.15	FO	Dipole resonant frequency, MHz
0.30	BW	SM:1 SWR bandwidth of compensated dipole, MHz
0.079	BWD	SM:1 SWR bandwidth of uncompensated dipole, MHz
30.05	QN	resonator Q
0.339	BWMAX	maximum SM:1 SWR bandwidth of compensated dipole, MHz
1.23	SMMIN	minimum allowable value SM for a given bandwidth
2.79	ZN	matching-network impedance level, ohms
0.47	NZ	transformer impedance ratio
1.14	LOC	matching-network loss at band edges, dB
2.04	LOE	matching-network loss at band center, dB
0.204	thetaS	electrical angle of primary stub measured from shorted end, radians
0.300	thetaP	electrical angle of secondary stub measured from shorted end, radians
-0.063	thetaD	electrical angle of feed measured from center of antenna, radians
19.77	LO	length of open stub, feet
4.33	LS	length of shorted stub, feet
-1.39	LL	length of link, feet
25.10	next RAF
65.47		approximate total antenna length, feet

 

LS is the primary, LL is the secondary less primary, and LO is 1/4-wavelength less primary and less link. The negative value of LL signifies that the crossover, instead of being above the feedpoint in the open stub, is below the feedpoint in the shorted stub.

 

Results

 

The coaxial resonator match antenna is constructed with 14 AWG insulated copper wire and RG-58 coax cable. All connections are sealed and protected against water infiltration. An MFJ HF/VHF SWR Analyzer Model MFJ-259B and MFJ-731 SWR Analyzer Filter provide the VSWR results as shown in Figure 1.

 

 

                        As-Built Coaxial Resonator Match Antenna VSWR

                                                Figure 1

 

 

VSWR at the lower edge frequency of 7.0 MHz is 1.7, and at the higher edge frequency of 7.3 MHz is 1.3. A maximum 1.3 VSWR is the design goal over the 7.0 to 7.3 band. That goal is met only from 7.2 to 7.3 MHz. But on a practical side, VSWR is 2.0 or less from 6.9 to 7.37 MHz. Since the final tuning is accomplished by trimming the ends of the antenna and the 1/4-wave resonator, it is possible that better trimming could improve the results. Another possible side effect is that the loss due to low ground elevation could be distorting the VSWR measurements, making them look better than they actually are.

 

The major advantage of the coaxial resonator match antenna is that no antenna tuner is used with a 100W solid-state transmitter. The baseline wire dipole is unusable due to its low resistance. The coaxial resonator match antenna performs as expected for an NVIS antenna. Communications are carried across the band in voice, Morse code, and radio teletype.

 

Adding a coaxial resonator match is essentially a special case of tuned stubs separated by a length of transmission line. Operating the antenna at other than the desired primary band will give unpredictable results. This makes the antenna a one-band antenna. But since NVIS antennas are frequency and height dependent, this may not be that big of an issue.

 

A minor advantage of the antenna is that it is DC grounded due to the short in the 1/4-wave coaxial resonator. The benefit of this is that the antenna self-discharges any charge buildup.

 

 

About the Author

 

James A. Lacy is a registered professional engineer in Texas. His publications include Systems Engineering Management: Achieving Total Quality, McGraw-Hill, 1992.

 

 

List of Works Consulted

 

Edward C. Jordan, Keith G. Balmain. Electromagnetic Waves and Radiating Systems, 2nd Ed. Englewood Cliffs, NJ. Prentice-Hall Inc. 1968.

 

Richard D. Snyder. "Broadband Antennae Employing Coaxial Transmission Line Sections." United States Patent no. 4,479,130, issued Oct 23, 1984.

 

Frank Witt. "The Coaxial Resonator Match." The ARRL Antenna Compendium, Vol. 2. Newington, CT. American Radio Relay League, Inc. 1989. pp.110-118.

 

Reference Data for Radio Engineers, 6th Ed. Indianapolis, IN. Howard Sams and Co. Inc., 1982.