Practical Antenna Theory - Part 9
In this series of articles for TARC News we have learned some basics, some theory, and dispelled a few myths that are generally perpetuated in the Ham Radio community about antennas and feedline.
For this discussion we will review the information supplied previously in our descriptions of the complex nature of antennae and apply that information when evaluating antenna measurements.
There is a very real myth that persists in ham radio that the average ham cannot design, build and make meaningful measurements of antennas. This is completely erroneous. This discussion is all about how to bust that myth wide open without going bankrupt.
We have discussed several measurable aspects of antennae and feedline. We will take each and demonstrate affordable ways to measure each aspect.
The first is SWR. This is the most common aspect of antenna/feedline performance measurements. SWR is measured by an instrument appropriately named an SWR meter. The pictorial below is of a widely used commercial SWR meter. The Bird 43 thru-line watt meter has been around for more than 50 years and proven to be extremely durable, reliable, and accurate.
The detecting elements are selectable and capable of detecting RF current in the forward (transmitter to ant) and reverse direction (ant to transmitter). The detecting element can be seen as a small disc shaped object just below the meter indicator.
Professional instruments of this type can run from several hundred dollars new to a couple hundred dollars used in mint tested condition. Great values can be obtained at local hamfests and online.
A more affordable alternative is shown in the next pictorial. It is commonly available from many ham equipment suppliers for well under $100. The obvious differences (other than appearance) are that there are no selectable detector elements and both forward and reflected power are indicated by opposing needles that cross in the middle of the meter face. This “cross needle” design is quite popular and appears in many SWR meters on the market in all price ranges. The detector circuit in this type of meter is designed to operate over a wide range of frequencies. The accuracy is not linear over the entire range. Some time should be spent verifying the accuracy with a known good SWR wattmeter.
When selecting a SWR meter, be sure you understand the frequency limitations of the model selected. The unit shown is accurate for 1.8 to 300 mHz. Many do not have that wide an operable accuracy range.
The illustration below shows the percent of power as a function of SWR. You must recall our discussions of SWR, power loss and coax loss factors for this chart to be meaningful.
This chart is included to demonstrate how much power can be lost with SWR that is uncontrolled.
However, we have seen over the last several articles that SWR is only part of the picture. We need a means of measuring the antenna impedance. This is a much more complex aspect to measure reliably and accurately. To do this an instrument known as an antenna analyzer may be used. Articles have been published in 2006 QST issues comparing commonly available antenna analyzers for accuracy. Some did not make the grade for the cost given. Those that did, are in the hundreds of dollars.
The pictorial below is of one such analyzer, the MFJ-269. The retail price on this instrument is several hundred dollars and still has some shortcomings that cannot be ignored. If your resources allow the purchase, it is quite capable and handy when doing serious antenna work.
This instrument contains an RF source for a range from HF to UHF and impedance detection circuits. It displays SWR and general impedance reactive values on two analog meters and a digital display (it indicates the magnitude only - not the + or – value of the reactance). Quite often the “average” ham does not have the disposable income to fund purchases of this type. Stay with me here, all is not lost.
Accessories are available that will aid in determining impedance values for passive components as well. This could be a valuable tool on the workbench for any serious experimenter.
Another valuable tool that can be used for antenna design and test, is far less expensive and moderately accurate. It is called a noise bridge (sometimes called a noise phase bridge). Ordinarily, the ardent ham avoids noise of most types. But in this case we want noise.
The theory behind the noise bridge is similar to other types of bridge networks. One of the most familiar would be the Wheatstone Bridge circuit for DC measurements. In this circuit, four resistances are placed in a configuration that allows a meter to display when the current in all parts of the bridge are equal by indicating a zero-center reading. The diagram below is of the common Wheatstone Bridge for DC meters.
The noise bridge operates in a functionally similar fashion. Instead of using a meter in the bridge, the zero balance indicator is your receiver.
While the Wheatstone Bridge measures resistances, the noise bridge measures complex impedances. This means that the relatively simple resistive components of the Wheatstone Bridge will become reactive components in a noise bridge. We have learned in past lessons that resonance occurs where reactive values become a zero sum and only resistance remains. Therefore the noise bridge can indicate not only the resistance (the R of our impedance expression R+jX) of an antenna, but the reactive components as well (the +jX part of the impedance).
The next pictorial displays a commonly available noise bridge. It has a retail price well under $100. Quite often you can come across an older one at the local hamfest for $10 to $20. It should be obvious from the labels on the adjustments of the front panel, that both resistance and reactance can be measured and read directly from the indicator knobs of the front panel (older models may have different but similar labeling).
What is not shown in this pictorial is the connections on the back panel. One is for your receiver and one is for the unknown (antenna, load, or coax). When the bridge is connected to your receiver and antenna in this fashion, it is possible to measure the antenna impedance directly (or coax impedance if terminated at the far end with the appropriate impedance).
To make this measurement, the noise bridge and receiver are powered on with the receiver attached to the appropriate connection and an antenna or load (antenna, terminated coax, or even a tuned circuit).NOTE: It is important to lock out the transmit function of a transceiver. Do not under any circumstances transmit while the noise bridge is connected or you risk permanent damage to the bridge.
The first paradoxical thing to notice is that noise level indications on the receiver are very high. This is why it is called a NOISE bridge. Noise will be high for every frequency on the dial EXCEPT the resonant frequency of the load (aka. your antenna).NOTE: When using a purely resistive load, the resonant frequency is anywhere the receiver is tuned, as pure resistance is resonant by definition. The reactance is by definition zero. Resistive loads are used to calibrate an instrument of this sort or provide a non-reactive load to coax. In which case the noise will null at the resonant frequency that is the fractional wavelength of the coax.
For reactive loads like an antenna or coax, near the resonance point the noise becomes very low. The receiver frequency should be tuned over a very wide frequency range in the same band to find the lowest noise indication using the speaker and/or S-meter indication. On some bands and with some antennas, this noise “null” could be very narrow, especially on 80 and 40 meters. It is helpful to know the design frequency of the test object so finding the null is not as difficult. You could easily hunt for the null point for a very long time before giving up unnecessarily.
Start by setting the front panel controls of the noise bridge to the expected resistance and zero reactance. Next set your receiver to the frequency band that the antenna or load is expected to handle. We will assume an antenna at 40 meters for the sake of our discussion. While listening to the speaker noise on the receiver, tune across the 40 meter band very slowly (actually begin below the band limits and tune well above to make sure you don't miss the resonant point), listening for (and/or observing the S-meter on the receiver) a dip in the noise level. The noise may not go all the way to zero, but there should be a pronounced drop near the measurable resonance.
When this point is found, rotate the resistance knob in each direction for the same kind of null. Then the reactance knob for an even lower noise level. When no more noise can be eliminated from the receiver by tuning the frequency on the receiver, the resistance control, and the reactance control of the bridge, you can assume that the reading on the front of the noise bridge indicates the antenna impedance. The limitation using this method is the range of the noise bridge controls and the accuracy of the markings for each control. The nominal frequency range of the noise bridge is 2 to 28 mHz for most consumer grade products commonly available to hams. It is not appropriate for VHF and above nor 160 meters. The resistance control may measure from 10 to 500 ohms and the reactance control may measure +/- 150 ohms. Some newer models such as the one shown extend this range, but results become less accurate in the extended range. Your antenna could very well exceed the measurement ability of this bridge. You will, however, get a very good indication as to the possible solutions available. From the material presented so far, you should be able to take the impedance readings and null point frequency to interpret whether the antenna load should be shortened or lengthened (or values changed in the case of a tuned circuit).
If your measurements are close enough, the indications could show what the possible working bandwidth will be. The noise bridge does not however, indicate SWR for antenna loads directly. Our discussions previously indicated SWR is a result of antenna to feedline mismatch. SWR then, can be inferred by calculating the measured impedance and dividing by the feedline impedance or measured directly with the SWR meter mentioned earlier after disconnecting the bridge and substituting the SWR meter in-line. As an example, let's say we measured the impedance of our antenna under test at the shack to be 55-j102 on 7.245 mHz. Our feedline is RG8U coax at 50 ohms, cut to a ½ wavelength for the center of the 40 meter band. Using some simple math we can infer a SWR some where near 1.1 to 1. This is not a precise measurement because there is a reactive component that may influence SWR to some extent (more influence the farther away from resonance and less closer to the resonant frequency), and the length of the coax may not be exactly ½ wavelength for our test frequency. The instruction sheet that comes with the noise bridge has formulas that will render a more precise impedance, taking into account the reactive values and feedline length. A few minutes with a calculator and you have an accurate, fully compensated, impedance value for either the transmitter end or antenna end. This method is for the most particular and mathematically capable of users only.
Current editions as well as older editions of the ARRL Antenna Book contain homebrew projects for a well-capable noise bridge as well as other antenna measuring instruments that can be easily built for reasonable costs. Even vintage issues of the Antenna Book and ARRL Handbook have valuable projects for the antenna experimenter that can be constructed for a minimum of cost with accurate results.
One such project is the homebrew antenna or coax impedance meter found in the © ARRL Handbook 1986 (Chap. 25 p. 33 R.F. Impedance Bridge for Coax Lines). This is a direct reading impedance meter using a single frequency RF source - not wideband noise as discussed before. However, operation of this type test instrument is very similar in operation yielding similar but more accurate results when compared to a currently available noise bridge (albeit at a possibly higher cost).
The serious VHF and UHF antenna experimenter may also wish to construct a standard antenna for measurement and test of antenna gain. This antenna is built from commonly available copper or brass components from your local hardware or home improvement supplier. The standard antenna for VHF is easy to construct and is very useful when making gain and pattern measurements of VHF or UHF designs (© ARRL Antenna Book 20th edition p. 25-51). The dimensions given are in wavelengths, but may be converted to metric or English measurements using the appropriate formulas for frequency to wavelength conversion. Pay particular attention to construction tolerances. Accuracy in construction is necessary to insure proper operation. Wavelength at VHF and UHF is quite short compared to HF frequencies. An error of a half inch (12.5 mm) in VHF frequencies and one sixteenth (1.5625 mm) in UHF frequencies could be fatal to the intended design. The higher the frequencies, the closer the construction tolerances must be. This is true for operating antenna measurements as well.
In this discussion we have demonstrated that with inexpensive instruments we can learn a lot about the characteristics of antenna and feedline systems and obtain acceptable results. With an inexpensive noise bridge and SWR meter or homebuilt impedance bridge, the experienced antenna builder can obtain accurate and reliable results. Antenna efficiency (i.e. The amount of signal actually radiated versus being lost to coax attenuation or ground loss absorption) can be measured or calculated, and increased through such measurements with little effort.
For the beginner, much can be learned about antennas, feedlines, impedance, and SWR without breaking your pocketbook.