Antennas Demystified

Antennas Demystified: 

Antennas Demystified Scott Honaker N7WLO

Importance of Antennas: 

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

Choosing Antennas: 

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

dBi vs. dBd: 

dBi vs. dBd 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 Gain referenced to a “real” dipole antenna with a donut-like pattern dBd = dBi + 2.15 dB

Gain/Loss Calculations: 

Gain/Loss Calculations ERP (Effective Radiated Power) is the real number to consider 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)

Radiation Patterns: 

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

Dipole Patterns: 

Dipole Patterns

Yagi Patterns: 

Yagi Patterns E-Plane H-Plane

Polarization: 

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

Antenna Design Considerations: 

Antenna Design Considerations 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

SWR Power Losses: 

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 Line attenuation (loss) is the key factor in determining losses due to mismatched antennas (high SWR)

SWR Loss Examples: 

SWR Loss Examples SWR losses are added to line attenuation for total loss values 100’ RG-58 @ 20 meters, 50’ RG-8x @ 2 meters, 50’ Belden 9913 @ 70cm have nearly identical attenuation of 1.5dB SWR SWR Losses 1.0:1 0dB 1.5:1 0dB 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: 

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

Ground Plane Verticals: 

Ground Plane Verticals 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 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) ¼ wave ½ wave 5/8 wave

Ground Planes: 

Ground Planes “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 Elevated feeds (1/8 λ or more above ground) can use four ¼-wave radials Vehicles provide poor ground planes at HF but elevating the feedpoint reduces loss

Imperfect Ground Planes: 

Imperfect Ground Planes Number of radials 16 24 36 60 90 120 Length of radials in wavelengths 0.1 0.125 0.15 0.2 0.25 0.4 Total wire installed in wavelengths 1.6 3 5.4 12 22.5 48 Power loss relative to “perfect” ground plane 3 2 1.5 1 0.5 n/a Feedpoint impedance in ohms 52 46 43 40 37 35

Other Verticals: 

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

Balanced Feed Designs: 

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

Dipole Types: 

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

Dipole Types – Cont.: 

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

Multiband Dipoles: 

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

Off-Center Fed Dipoles: 

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

Other Multibanders: 

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

Wire Arrays: 

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

Yagis: 

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

Typical Yagi Gains: 

Typical Yagi Gains 10m yagi with SWR <2:1 and Front/Back >20dB Numbers are rounded to nearest 0.5 dB Elements Gain dBi Gain dBd 3 7.5 5.5 4 8.5 6.5 5 10 8 6 11.5 9.5 7 12.5 10.5 8 13.5 11.5

Hybrid Yagis: 

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

Log Periodic: 

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

Reflecting Antennas: 

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

Parabolic Dish Gain: 

Parabolic Dish Gain MHz 2’ 4’ 6’ 10’ 20’ 30’ 420 6.0dBi 12.0 15.5 20.0 26.0 29.5 902 12.5 18.5 22.0 26.5 32.5 36.0 1215 15.0 21.0 24.5 29.0 35.0 38.5 2300 20.5 26.5 30.0 34.5 40.5 44.0 3300 24.0 30.0 33.5 37.5 41.5 47.5 5650 28.5 34.5 38.0 42.5 46.0 52.0 10Ghz 33.5 39.5 43.0 47.5 51.0 57.0