Antennas Demystified Scott Honaker N7WLO

Importance of Antennas Antennas are as important as the radio Antennas are as important as the radio A $5000 TV with rabbit ears will have a lousy picture A $5000 TV with rabbit ears will have a lousy picture Antennas are cheaper than amplifiers Antennas are cheaper than amplifiers Antennas are reciprocal – they hear as well as they talk Antennas are reciprocal – they hear as well as they talk

Choosing Antennas Frequency – Dictates size Frequency – Dictates size Mounting location – Base or mobile Mounting location – Base or mobile Omni or directional – Coverage or gain Omni or directional – Coverage or gain Polarization – Horizontal, vertical, circular Polarization – Horizontal, vertical, circular Resonant or non-resonant – Tuner required? Resonant or non-resonant – Tuner required? Power available Power available Feedline length and type Feedline length and type Cost Cost

dBi vs. dBd dBi - Gain vs. Isotropic Resonator dBi - Gain vs. Isotropic Resonator  Isotropic Resonator is infinitely small antenna with no feedline in free space radiating equally well in all directions (spherical pattern) dBd - Gain vs. Reference Dipole dBd - Gain vs. Reference Dipole  Gain referenced to a “real” dipole antenna with a donut-like pattern dBd = dBi dB dBd = dBi dB

Gain/Loss Calculations ERP (Effective Radiated Power) is the real number to consider ERP (Effective Radiated Power) is the real number to consider Gain uses a Log-10 scale Gain uses a Log-10 scale  3dB = 2-fold improvement  6dB = 4-fold improvement  10dB = 10-fold improvement  20dB = 100-fold improvement ERP=Power x (Gain - Feedline Loss) ERP=Power x (Gain - Feedline Loss)

Radiation Patterns Visual representation of gain, beamwidth, F/B ratio and F/S ratio in one plane Visual representation of gain, beamwidth, F/B ratio and F/S ratio in one plane E-Plane is cross- section that includes driven element E-Plane is cross- section that includes driven element H-Plane is perpendicular to driven element H-Plane is perpendicular to driven element

Dipole Patterns

Yagi Patterns E-PlaneH-Plane

Polarization SSB/CW is generally horizontal SSB/CW is generally horizontal FM is generally vertical FM is generally vertical Satellites can be circular - RHCP, LHCP Satellites can be circular - RHCP, LHCP Polarization loss can be significant at VHF/UHF and microwaves Polarization loss can be significant at VHF/UHF and microwaves Bounced signals can change polarization Bounced signals can change polarization Verticals are more susceptible to QRM Verticals are more susceptible to QRM

Antenna Design Considerations Gain, SWR, Bandwidth, Front/Back ratio are related and optimum values are not achieved simultaneously for each Gain, SWR, Bandwidth, Front/Back ratio are related and optimum values are not achieved simultaneously for each Does antenna have power going in desired direction? Gain/Beamwidth Does antenna have power going in desired direction? Gain/Beamwidth

SWR Power Losses All power fed into the line, minus the line attenuation, is absorbed into the load (antenna) regardless of the mismatch at the antenna terminals All power fed into the line, minus the line attenuation, is absorbed into the load (antenna) regardless of the mismatch at the antenna terminals Line attenuation (loss) is the key factor in determining losses due to mismatched antennas (high SWR) Line attenuation (loss) is the key factor in determining losses due to mismatched antennas (high SWR)

SWR Loss Examples SWR losses are added to line attenuation for total loss values SWR losses are added to line attenuation for total loss values 100’ 20 meters, 50’ 2 meters, 50’ Belden 70cm have nearly identical attenuation of 1.5dB 100’ 20 meters, 50’ 2 meters, 50’ Belden 70cm have nearly identical attenuation of 1.5dB SWR SWR Losses 1.0:10dB 1.5:10dB 2.0:1 0.2dB or 5% 3.0:1 0.6dB or 13% 5.0:1 1.5dB or 29% 10:1 3.0dB or 50%

Loading Inductive loads – base, center, top Inductive loads – base, center, top Screwdriver antennas (adjustable loading) Screwdriver antennas (adjustable loading) Hamstick-style antennas Hamstick-style antennas Hustler center-loaded whips Hustler center-loaded whips Rubber HT antennas Rubber HT antennas Capacitance “Hats” Capacitance “Hats” Texas Bugcatcher Texas Bugcatcher Cushcraft MA5B Cushcraft MA5B

Ground Plane Verticals ¼ wave is omnidirectional with unity (0dBd) gain when provided a proper ground plane ¼ wave is omnidirectional with unity (0dBd) gain when provided a proper ground plane ½ wave is unity gain with no ground plane and 3dBd with ground plane ½ wave is unity gain with no ground plane and 3dBd with ground plane 5/8 wave is 3.5dBd gain with nice omni pattern and low radiation angle 5/8 wave is 3.5dBd gain with nice omni pattern and low radiation angle Longer antennas have more omni patterns with asymmetric ground planes (vehicles) and lower radiation angles (see below) Longer antennas have more omni patterns with asymmetric ground planes (vehicles) and lower radiation angles (see below) ¼ wave½ wave5/8 wave

Ground Planes “Perfect” ground plane from 120 evenly spaced radials at least ½ wave in length “Perfect” ground plane from 120 evenly spaced radials at least ½ wave in length Wire mesh or wire from #12 to #28, above or a few inches below the ground work fine Wire mesh or wire from #12 to #28, above or a few inches below the ground work fine Elevated feeds (1/8λ or more above ground) can use four ¼-wave radials Elevated feeds (1/8λ or more above ground) can use four ¼-wave radials Vehicles provide poor ground planes at HF but elevating the feedpoint reduces loss Vehicles provide poor ground planes at HF but elevating the feedpoint reduces loss

Imperfect Ground Planes Number of radials Length of radials in wavelengths Total wire installed in wavelengths Power loss relative to “perfect” ground plane n/a Feedpoint impedance in ohms

Other Verticals Discone Discone Wide usable frequency range Wide usable frequency range SWR ~2:1 for fundamental through second harmonic SWR ~2:1 for fundamental through second harmonic SWR ~3:1 for remainder of coverage SWR ~3:1 for remainder of coverage Omnidirectional – Unity gain Omnidirectional – Unity gain Inverted-L Inverted-L 2-3 dBd gain with vertical and horizontal components 2-3 dBd gain with vertical and horizontal components Requires ground plane Requires ground plane

Balanced Feed Designs Dipole Dipole Simple and effective Simple and effective Vertical or horizontal polarization Vertical or horizontal polarization Loop Loop Full wave has 3dBd gain Full wave has 3dBd gain Circular, Quad (square) or Delta (triangular) design Circular, Quad (square) or Delta (triangular) design E and H-plane patterns vary with height above ground E and H-plane patterns vary with height above ground

Dipole Types Sloper Sloper Has 3dB to 6dB of directivity toward slope Has 3dB to 6dB of directivity toward slope Inverted-V Inverted-V Single high mount, internal angle should be >90 degrees Single high mount, internal angle should be >90 degrees Bent Bent Good attic antenna Good attic antenna Keep center section straight Keep center section straight Remainder of element can bend or curve to fit Remainder of element can bend or curve to fit

Dipole Types – Cont. Folded Folded High impedance needs open wire feed High impedance needs open wire feed Same overall size as ½ wave dipole but contains 1 wave of wire for nearly 3 dBd gain Same overall size as ½ wave dipole but contains 1 wave of wire for nearly 3 dBd gain Caged Caged Standard dipole with each leg made up of multiple wires around spacers forming a wire tube Standard dipole with each leg made up of multiple wires around spacers forming a wire tube Larger effective element diameter increases bandwidth Larger effective element diameter increases bandwidth Extended Double Zepp Extended Double Zepp Two 0.64λ elements provide 3dBd gain Two 0.64λ elements provide 3dBd gain

Multiband Dipoles Multiple Multiple Multiple dipoles/loops at a single feed Multiple dipoles/loops at a single feed Trap Trap Traps are tuned circuits used to generate multiple resonances on a single wire Traps are tuned circuits used to generate multiple resonances on a single wire Traps cause loss and decrease bandwidth Traps cause loss and decrease bandwidth G5RV G5RV Non-resonant – tuner required Non-resonant – tuner required Radiation patterns vary with frequency Radiation patterns vary with frequency

Off-Center Fed Dipoles Feedline attached 1/3 the length from the end Feedline attached 1/3 the length from the end Same ½ wave overall size Same ½ wave overall size Resonates at even harmonics, so 1 antenna can be used on 80m, 40m and 20m Resonates at even harmonics, so 1 antenna can be used on 80m, 40m and 20m 6 th harmonic (15m) has too high impedance 6 th harmonic (15m) has too high impedance Asymmetric impedance may cause current “in the shack” Asymmetric impedance may cause current “in the shack” Requires 4:1 or 6:1 current-type balun to match Requires 4:1 or 6:1 current-type balun to match

Other Multibanders Random wire Random wire Can be any length of wire Can be any length of wire Requires tuner Requires tuner Works against earth ground Works against earth ground Windom Windom “T” shape single wire feed attached 14% off center “T” shape single wire feed attached 14% off center Works against earth ground Works against earth ground “RF in the shack” is a potential problem “RF in the shack” is a potential problem

Wire Arrays Half Square Half Square Vertical polarization with up to 3.8dBd gain Vertical polarization with up to 3.8dBd gain Bi-square Bi-square Horizontal polarization with ~3.5dBd gain Horizontal polarization with ~3.5dBd gain Bobtail Curtain Bobtail Curtain Vertical polarization with bidirectional 5.8 dBd gain Vertical polarization with bidirectional 5.8 dBd gain Sterba Curtain Sterba Curtain Horizontal polarization from multiple phased loops Horizontal polarization from multiple phased loops Lazy “H” – Four element broadside array Lazy “H” – Four element broadside array Greater than 6dBd gain possible Greater than 6dBd gain possible

Yagis ½ wave dipole driven element ½ wave dipole driven element Reflectors are 5% larger Reflectors are 5% larger Directors are 5% smaller Directors are 5% smaller Number of elements and boom length determine gain Number of elements and boom length determine gain SWR, bandwidth, gain, boom length and front/back ratios all have to be considered SWR, bandwidth, gain, boom length and front/back ratios all have to be considered

Typical Yagi Gains 10m yagi with SWR 20dB 10m yagi with SWR 20dB Numbers are rounded to nearest 0.5 dB Numbers are rounded to nearest 0.5 dB Elements Gain dBi Gain dBd

Hybrid Yagis Quad Quad 1λ loop driven element, reflector and directors 1λ loop driven element, reflector and directors Up to 3dBd gain over standard yagi Up to 3dBd gain over standard yagi Wider bandwidth than standard yagi Wider bandwidth than standard yagi Quagi Quagi Loop reflector and driven element Loop reflector and driven element Simpler to feed and match at UHF Simpler to feed and match at UHF Looper Looper Entirely loop (generally circular) elements Entirely loop (generally circular) elements

Log Periodic Constant characteristics over wide band (2:1) Constant characteristics over wide band (2:1) Several varieties but hams generally use dipole array (LPDA) Several varieties but hams generally use dipole array (LPDA) All elements are driven All elements are driven Gain similar to 3 element yagi – 7dBi, 5dBd Gain similar to 3 element yagi – 7dBi, 5dBd Size similar to 3 element yagi at lowest frequency Size similar to 3 element yagi at lowest frequency

Reflecting Antennas Corner reflector Corner reflector Practical size at 222 MHz and up Practical size at 222 MHz and up Simple to construct, broadbanded, gains 10-15dBd Simple to construct, broadbanded, gains 10-15dBd Pyramidal Horn Pyramidal Horn Practical at 902 MHz and up Practical at 902 MHz and up Sides of horn are fed for up to 15 dBi, 13dBd gain Sides of horn are fed for up to 15 dBi, 13dBd gain Parabolic dish Parabolic dish Gain is a function of reflector diameter, surface accuracy and illumination Gain is a function of reflector diameter, surface accuracy and illumination

Parabolic Dish Gain MHz2’4’6’10’20’30’ dBi Ghz