Least understood topics by most HAMs • RF Safety
Least understood topics by most HAMs RF Safety Ground Antennas Matching & Feed Lines Remember this question from the General License Exam? G0A03 | (D) How can you determine that your station complies with FCC RF exposure regulations? A. By calculation based on FCC OET Bulletin 65 B. By calculation based on computer modeling C. By measurement of field strength using calibrated equipment D. All of these choices are correct Grounding the least understood radio topic Amateur radio stations have three unique and separate grounding functions; Electrical (Safety) Ground, Antenna (Counterpoise) Ground & Station (RFI) Ground Electrical Safety Ground is a connection to chassis to prevent operator contact with dangerous voltage if the electrical power source insulation fails. This connection provides a return path to trip the circuit breaker. Antenna (Counterpoise) Ground is the other half of the antenna, normally a wire one-quarter wavelength long. It does not need to be straight. Before you dig remember placing radials in the sandy soil DEGRADEs effectiveness. A BALUN is needed to isolate to antenna ground from the station ground. Station RFI Ground is a single point at which shack equipment ground lugs are connected together through a very short, low impedance lead. so any RF that reach the station will be largely shunted to Earth. RF Bands > approx equal the Speed of Light / Frequency = (300 Meters / F MHz) Physical Length (Shorter is Higher Frequency) 1/4 Wave Antenna = Physical Length of 234 Ft. / F MHz) 1/2 Wave Antenna = Physical Length of 468 Ft. / F MHz) Full Wave Antenna = Physical Length of 936 Ft. / F MHz) Polarization Vertical Antenna > Electric Field is perpendicular to the earth (Flag Pole) Horizontal Antenna > Electric Field is parallel to the earth (Cloths Line) Common Antennas 1/4 Wave > Common Mobile (whip, rubber duckie, flag pole) 5/8 Wave > Used as mobile for more Horz gain 3/4 Wave > Base for more Horz gain Least common due to size J-Pole > Mostly VHF & UHF due to size but most gain for omni Slim Jim > Like J Pole but even more gain for omni Dipole > Most common of all has small gain Dipole most common Multi-Band > Windom, OCF, G5RV Loop > Provides high Gain & Low Noise less common due to size Yagi > Most gain (Beam, Quad Beam, Array) Load Coils, Matching Coils, Traps Load Coils make an antenna “look” longer to the RF Matching Coils do same as load coils plus change input impedance Traps in multi-band antennas make it “look” shorter as freq goes up Multi-wire Dipoles (FAN) let the RF “select” best pair without loss 1 / 4 Wave Antenna > Commonly used antenna using ground plane for a counterpoise Antenna Matching A tuner should be called matcher because it matches the impedance presented to the transceiver to the impedance on the feedline. You could say the tuner takes whatever you have on your feedline and makes it look like 50 Ohms resistive to the transceiver. This is a simplified answer that is not all true, but it is the general idea. Today automatic tuners are the standard; twenty years back the standard was an external manual tuner. Prior to 1980 transmitters normally had internal tuning and loading controls. Think about the controls, controls for matching, this was what you pay extra today is called a built-in tuner! Typical Automatic Antenna Tuner Block Diagram Looking at the schematic above you see many components but few are actually the key to tuning. Most are for the SWR, measuring forward and reverse power. So looking at the schematic above and the simplified version below you can start to see manufactures use switches to vary the inductance values. Other models use switches for both capacitance and inductance control. Most use series capacitors as this is generally a more efficient design. Capacitors do not contribute any significant loss in the circuit as they have very high Q. Q is the ratio of electrical resistance and reactance at resonance. Capacitor Q is much larger than the Q exhibited by most coils. Simplified Schematic Antenna Tuner In this design you will see a switch that moves the capacitor(s) from the transmitter side to the antenna side saving in hardware costs as well as practical design. It includes other features common to many commercial tuners with an antenna selector and current BALUN built-in. Antenna Feedlines Feedline is required to transfer the RF output from the tuner to the antenna and is normally 50 ohm coaxial cable because the transmitters made today are designed for 50 Ohm output. Notice I said the tuner is connected to the feedline not the transmitter. The tuner’s job to match the impedance from the feed input to the transmitter’s 50 Ohm output has just been covered by the previous section. A 50 Ohm coaxial feedline terminated into a 50 ohm antenna at a distance of an exact half wave length are 1 to 1 transformers with very minor resistive loss. Use any other length line and you are no longer terminated in 50 Ohm impedance this also changes the impedance into something else at the transmitter end. The changed value is a function of antenna impedance, line loss, and the length and characteristic impedance of the feedline. There are three feedline characteristics to consider in your antenna system; Resistive Line Loss Antenna Impedance Length Relative to Wavelength Coaxial cable resistive loss is a one factor in an antenna system that most HAMs consider but not always. In every run, there is some loss of signal strength as the signal travels between the antenna and the tuner. The longer the run of cable, the greater the loss will be is understood. At higher frequencies, the losses in the cable are much greater. We all know not kink the cable, or fit tightly around corners can lead to splits in the jacket and shield, which will lead to a downgrade in performance over time with water intrusion. So we are now good for losses when connecting a 50 Ohm coaxial cable to a resistive 50 Ohm antenna. These are the loss numbers published by the cable manufactures. These are not the values for mismatched loads! Antenna impedance must be considered to prevent reflections at the antenna from causing standing waves. The feedline must match to the characteristic impedance of the antenna to prevent reflections. It is not difficult to select a feedline to match the antenna but it is often not even considered. Coaxial cable impedance is determined its dimensions and materials used and is a purchased item. Common coax values are 50 and 75 Ohms but an alternative using parallel conductors provides values of 300, 450 and 600 Ohms determined by spacing and materials. An example of the right way is the G5RV design that matches the higher impedance dipole to the 50 Ohm coax with a tuned length of parallel feedline. Another example of the right way is matching a Windom Dipole (OCF) design high impedance to the 50 Ohm coax with a 4:1 BALUM. Looking at a typical case of a 100W transceiver connected to an 40M antenna with 100 Feet of RG-8U you can see a dramatic difference in antenna RF radiated power by matching at the antenna feed point to the feedline. The dramatic effects of matching are shown in the chart below. Relative RF Power Radiated vs. Antenna to Feedline Matching Any length of feedline can be used if you understand feedline SWR. A general rule is to use a feed length or ½ WL and measure the VSWR at the tuner-feedline connection prior to use in all bands. You can go from perfection to dead short by selecting a frequency that yields a wavelength equal to a ¼ WL multiple of the feedline length. Here is the kicker but not always, ¼ WL multiple feedline can also transform a high impedance to a low impedance. The contributions of a ¼ WL can be tricky on a multiband antenna. Sometimes HAMs blame their tuner for not working, some understand enough to add or subtract 1/8 WL from the feedline there is no magic in a ¼ WL just be aware of how it works. The feed impedance at one end is purely resistive, the impedance at the other end will also be resistive, and a random length section can be resistive at one end and yet have a complex impedance at the other end. Remember to measure your multiband antenna in all bands prior to use.
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