Why Use a Tuned & Matched Antenna?
I believe the above represents the most often asked questions in Ham Radio relative to antennas. Because it represents a lack of understanding of the overall concepts relative to antennas, it deserves an in-depth answer.
The basic reason for selecting and constructing a particular antenna is to provide the desired radiation pattern. The antenna could be a simple vertical or a complex multi-element beam, or hundreds of other variations. It is the configuration of the antenna (the arrangement of the wires or pipes), and the location of the antenna relative to ground, that defines the radiation pattern. Yes, a simple random wire will radiate, but does it radiate the signal in the desired direction to optimize the type of communications you desire?—not likely.
The physical size of the antenna and proximity to ground (and loading coils or other reactive networks, if used) control the resonant frequency of the antenna. If the antenna is not resonant at the operating frequency, currents flowing in the antenna will not be the same as the antenna was designed for. Therefore, the antenna will not produce the intended radiation pattern. Fortunately, the pattern shape of simple antennas does not change much over the narrow frequency range of a ham band, so if the antenna is tuned to the center of the operating range, the radiation pattern will be about the same over the band. The exception is multi-element beams. The front-to-back ratio on a beam changes rapidly with frequency, and will produce the desired results only over a very narrow frequency range.
Now, your antenna has been constructed and tuned to the center of the band, What about the matching network? The impedance of an antenna changes rapidly over a narrow frequency range of a ham band. Unless we start with it well-controlled near the center of the operating range, it will be far into left field at one end of the band. So what?—if the tuner will keep the transmitter happy?
That leads us directly into a consideration of the effects of the transmission line. Unless the transmission line is a multiple of 1/2 electrical wavelength, there can be a wide difference in the impedance at the two ends of the transmission line. Even if the tuner makes the transmitter happy, it may not make the antenna happy. Except near resonance, there will be a high impedance at the antenna. To transfer the transmitter power into a high impedance may result in very high voltages at each end of the coax. This could blow the balun, and may even damage the coax. A very important factor to consider is the loss in the coax line due to mismatch loss. This loss is in addition to the normal loss in a matched coax line.
To illustrate these effects, we will examine a simple dipole on 80 meter, using a 1 kW linear. Our example antenna has been constructed and tuned to resonate close to 3.9 MHz, because we primarily work 75 meter. If we choose to load it with a tuner and work other frequencies, the associated parameters of the antenna will be as listed above.
The antenna values (R +/- jX) are derived from MININEC. The antenna impedance (Z) is the vector sum of R +/- jX. The current is that value required to achieve 1000 watts power in the radiation resistance. The voltage is based on the required voltage across the feedpoint impedance (Z) of the antenna to cause that current to flow. The loss in the coax is due to the mismatch loss, assuming a low VSWR at the transmit end of the coax. (This would be a worst case and is used as an example.)
Manufacturer’s specifications for coax cable allow 1900 volts rms for RG-58, and 5000 volts rms for RG-8 and similar size cables.
Summary
If you use an antenna tuner, all of the above says that if the antenna is constructed properly to achieve the desired radiation pattern, tuned to resonance in the band and properly matched, the major concern will be the voltage across the feedpoint of the antenna. This may be more than the coax can handle, with resulting damage if a high power linear is used.
The loss in the coax can become very significant, when very far from the resonant frequency. If, for whatever reason, you prefer a non-resonant antenna, you should feed it with open wire line to avoid the mismatch loss in the coax. If you want to cover an entire Ham band with a single antenna, tune the antenna at the center of the operating range.
In a multi-element antenna (beam), the impedance changes more rapidly than a simple dipole, and the radiation pattern must be considered over the frequency range.
Unless the above is considered and applied to a ham antenna, there can be significant loss in the system, either due to mismatch loss or variation in the radiation pattern. Seems to me there are a lot of linear amplifiers being used that don’t need to be. Listen very carefully to the big signals when they describe their antenna. Then compare the weaker signals and listen to what they have to say about their antenna.
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| Frequency (MHz) | R (Ohms) | +/- jX (Ohms) | #NAME? | I (Amps) | E (rms) Volts | E (pk-pk) Volts | VSWR | Loss (dB) |
|---|---|---|---|---|---|---|---|---|
| 4.0 | 59.25 | 042.57 | 072.96 | 4.11 | 299.73 | 834.24 | 2.28 | 00.71 |
| 3.9 | 52.02 | 008.16 | 052.64 | 4.39 | 230.83 | 646.32 | 1.17 | 00.03 |
| 3.8 | 46.61 | 026.24 | 053.46 | 4.63 | 247.65 | 693.42 | 1.71 | 00.31 |
| 3.7 | 41.27 | 060.66 | 073.37 | 4.93 | 361.15 | 1011.22 | 3.53 | 01.62 |
| 3.6 | 36.25 | 095.26 | 101.99 | 5.24 | 534.23 | 1495.84 | 6.93 | 03.56 |
| 3.5 | 32.12 | 130.09 | 133.99 | 5.58 | 747.66 | 2093.45 | 12.65 | 05.66 |
| 3.0 | 16.42 | 311.98 | 312.41 | 7.80 | 2438.04 | 6826.51 | 121.91 | 14.91 |
Appendix
To derive the values for the antenna, MININEC was used to calculate the impedance value of the antenna. Figure 1 presents the equivalent circuit. The following equations were used to calculate the voltage across the components:
P = I1 ^2R, Therefore, I1 = (P/R)^.5.
For 1000 watts, I1 = (1000/R)^.5
Now that the current is known, E= I1 *Z
Since Z = (R^2 + X^2)*.5, then E = I1 *(R^2 + X^2)^.5
Peak-peak voltage = RMS*2.8
VSWR = (1 + p)/(1-p), where p = (Z-50)/(Z+50)
Mismatch loss in coax = 10*log((S+1)^2/(4*S)), where S is the VSWR.
()^2 means the value squared
()^.5 means the square root of the value
Originally posted on the AntennaX Online Magazine by Ted Hart, W5QJR
Last Updated : 16th March 2024