General License Class Chapter 6 - Antennas Bob KA9BHD Eric K9VIC
Learning Objectives Teach you enough to get all the antenna questions right during the VE Session Learn a few things from you about antennas and your experiences Un-teach you a few things you might have picked up from friendly sources on the repeater Have fun (it s a hobby, right?) Finish before noon
VE Session Total of 4 questions from this chapter One each in these areas: Feedlines, Characteristic Impedance and Attenuation, SWR Calculation and Measurement and effects, Matching Networks Basic Antennas Directional Antennas Specialized Antennas
Feedlines Coax Twin Lead Ladder Line
Feedlines Coax For amateur use, 50 and 75 Ohm typical Impedance determined by size and spacing of conductors, and the type of dielectric Unbalanced Convenient Impedance matches most modern radios Physical routing easy Losses (db/100 ft.) increase with increased SWR and frequency
Feedlines Balanced Types Twin lead (300 Ohm) Ladder Line (~450 Ohm) Impedance determined by the distance between conductors, and the radius of the conductors Balanced Low loss even with high SWR Not always convenient Matching network usually required Physical installation can be tricky Not so good at VHF/UHF
Feedlines SWR (Standing Wave Ratio) Caused by reflection of RF at impedance mismatch Expressed as a ratio, always 1:1 or greater Cannot be eliminated by using a tuner at the radio end of the line Calculated by taking a ratio of impedances
Terminology Patterns Gain Front-to-Back Ratio Main Lobe Take-off Angle
Terminology Patterns Elevation What the radiation pattern would look like in the air from the side Varies from different side angles Azimuthal What the radiation pattern would look like if you could look down on it on the ground Varies at different elevations
Terminology Two Plots of the same antenna: Elevation Azimuth
Terminology Gain Comparative power Typically expressed in db Logarithmic power ratio db i, db d Only one aspect of performance to consider (not necessarily the only consideration)
Terminology Gain (where does it come from?)
Front-to-back ratio Terminology
Main Lobe Terminology
Take-off Angle Terminology
Terminology What s as important as gain figures? F/B Ratio Source (feedpoint) impedance Take-off angle (usually varies with height, not antenna design) Performance over what frequencies? Example
A Tale of Two Antennas
A Tale of Two Antennas Antenna A Antenna B Gain (db i ) 11.10 11.87 F/B (db) 11.47 17.17 Beam Width 72.6 68.8
A Tale of Two Antennas
A Tale of Two Antennas
Basic Antennas Random wire (not long wire) Ground plane (vertical monopole) Ground mounted vertical Dipole (vs. doublet)
Basic Antennas Random wire Advantages Cheap Can be put just about anywhere Disadvantages Radiation pattern unpredictable Requires remote match or leaves RF in the shack
Basic Antennas Random Length Wire
Basic Antennas Ground Plane Length roughly 234/f Omnidirectional pattern Characteristic impedance varies with radial angle (~35 Ohms with straight radials) Impedance Increases when bending radials (typically 4) downward
Ground Plane Basic Antennas
Basic Antennas Ground Plane What do the radials actually do? Are they passive? Do they radiate?
Basic Antennas Ground Plane showing currents
Ground Plane Basic Antennas
Basic Antennas Ground mounted vertical Requires radials (on or below ground) Radials ~1/4 wavelength, need not be tuned At least 16 or more radials for better performance Take-off angle relates to ground (not radial) type Some ground reflection loss due to vertical polarization
Basic Antennas Dipole Length roughly calculated as 468/f Free space impedance = 73 ohms when center fed Impedance increases if not center fed Real Life impedance from 65 90 Ohms (goes down under ¼ wavelength)
Basic Antennas
Basic Antennas Dipole at 1/2 wavelength
Basic Antennas Dipole at 0.1 0.25 wavelengths (NVIS)
(Uni) Directional Antennas Yagi-Uda Cubical Quad Delta Loop
Yagi-Uda Directional Antennas
Directional Antennas Yagi-Uda Driven element (DE) and some combination of Reflector and Director(s) Reflector slightly longer than the DE Driven Element about ½ Wavelength Director Shorter than the DE More Directors (on longer boom) = more gain DE impedance typically 35 Ohms Gamma match typically used to match 50 Ohm line
Directional Antennas Yagi-Uda Adjustments to boom length, number of elements, spacing of elements can be used to change: Gain (3-element has theoretical 9.7 db i ) F/B Ratio SWR Bandwidth Can also be increased with fatter elements
Directional Antennas Yagi-Uda Can be stacked Two 3-element Yagis ½ wavelength apart vertically increase gain by 3 db Elevation of major lobes gets tighter (lower overall elevation pattern)
Directional Antennas Yagi-Uda, Stacked pattern
Cubical Quad Directional Antennas
Directional Antennas Cubical Quad Driven Element (DE) and Reflector DE is about 1 Wavelength long Reflector is about 5% longer than the DE Spacing between elements is about 0.2 wavelengths Gain roughly the same as 3-element Yagi Feed at bottom or side
Delta Loop Directional Antennas
Directional Antennas Delta Loop Driven Element and Reflector (triangles) DE about 1 Wavelength long Reflector about 5% longer than the DE Gain about the same as the quad
Specialized Antennas Log Periodic Dipole Array (LPDA) Beverage Multiband Antennas
Specialized Antennas LPDA Logarithmic arrangement on boom Yagi-like gain Frequency range of 2:1, even 3:1 practical (theoretically, range could be infinite)
Specialized Antennas Beverage Antenna Several wavelengths of wire <0.05 wavelengths in height Terminated to ground through a resistor Directional For receive only due to ground losses (used on low HF bands)
Specialized Antennas Multiband Antennas Use traps/stubs to radiate on more than one band Trap is a tuned circuit At resonant frequency it presents high impedance Below resonant frequency it s typically inductive (makes antenna shorter)
Specialized Antennas Multiband Antennas Advantages More than one band with single feed line Disadvantages Low suppression of harmonics Can be higher maintenance Price
Case Study Not on the exam, but good practical knowledge. One reason it s good to have some knowledge of what we ve covered here this morning.
SWR as an Indicator Joe's been checking into the DARC 2- meter AM enthusiasts net on 144.144 MHz using a 5/8 wavelength J-pole antenna. Someone asks him why he never checks in on 6-meters, and he says he doesn't have an antenna Joe's figures he'll try his 2-meter antenna on 6-meters
SWR as an Indicator Joe's antenna is 61.75 (0.26 λ on 50.4 MHz) long (with 13.625 matching section) Antenna is at 30 feet Antenna is fed with 75 feet Belden 9258 (RG-8X) Joe's antenna analyzer shows the impedance in the shack on 2-meters is 53 + j7 Ohms (SWR = 1.15:1)
SWR as an Indicator Joe measures the impedance in the shack at 50.4 MHz and finds that it's 14 +j23 Ohms (SWR = 4.4:1) Joe looks up loss data for his coax, and finds that 4.5:1 SWR will cause an additional 1.5 db loss Joe models his antenna and finds it has 4 dbi gain at 50.4 Joe runs 20 Watts on 6-meters; he calculates his ERP at 35.5 Watts (4 1.5 = 2.5 db gain)
SWR as an Indicator Joe puts a simple L-network (loss <<1dB) in his shack, and measures the SWR at 1:1 on 50.4 MHz. He calls and calls, but he can barely hear the net, and no one can hear him trying to check in. What happened to poor Joe?
SWR as an Indicator While at 50.4 MHz the SWR in the shack was 4.4:1, at the antenna it was 263:1 (impedance = 11.6 j440.6 Ohms) Line losses from SWR are 17 db Matched line loss is 2 db Total losses = 19 db
SWR as an Indicator Antenna pattern with and without feedline losses
SWR as an Indicator Reasonable SWR does not indicate your antenna is working well! Total power into the antenna = 0.2 Watts
SWR as an Indicator Mary has a dipole 65.5 long cut for the bottom of the FM broadcast band She knows the SWR at the antenna can't be measured with her SWR meter (>300:1) as the impedance is 16.6 -j542 Ohms She's determined to get on the 6-meter net
SWR as an Indicator Mary feeds her antenna with ladder line, and uses a tuner at the point it enters the house Mary's feedline loss is about 3 db, and tuner losses are negligible Mary's antenna has ~ 7 dbi gain at 50.4 MHz Total gain with feedline is about 4 dbi
SWR as an Indicator Antenna pattern with and without feedline losses
SWR as an Indicator Very High SWR does not indicate your antenna won't work well! Total power into the antenna = 9.1 Watts Difference from Joe's Antenna ~ 25 db
Questions? If you have any questions while reviewing the material or the CD, please e-mail me at: K9VIC@arrl.net or KA9BHD@arrl.net See: www.k9vic.info