General License Class Chapter 6 - Antennas

General License ClassChapter 6 - Antennas

Bob – KA9BHD

Eric – K9VIC

Learning ObjectivesLearning 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

TerminologyTerminology

• Patterns

• Gain

• Front-to-Back Ratio

• Main Lobe

• Take-off Angle


• 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


• Gain– Comparative power

– Typically expressed in dB• Logarithmic power ratio

• dBi, dBd

– Only one aspect of performance to consider (not necessarily the only consideration)

Terminology

• Gain (where does it come from?)

Terminology

• Front-to-back ratio

Terminology

• Main Lobe

Terminology

• Take-off Angle


• 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


Antenna A Antenna B

Gain (dBi) 11.10 11.87

F/B (dB) 11.47 17.17

Beam Width 72.6° 68.8°

Basic AntennasBasic Antennas

• Random wire (not long wire)

• Ground plane (vertical monopole)

• Ground mounted vertical

• Dipole (vs. doublet)


• 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


• 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

Basic Antennas

• Ground Plane


• Ground Plane– What do the radials actually do?

– Are they passive?

– Do they radiate?

Basic Antennas

• Ground Plane

showing

currents

Basic Antennas

• Ground Plane

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


• 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(Uni) Directional Antennas

• Yagi-Uda

• Cubical Quad

• Delta Loop

Directional Antennas

• Yagi-Uda

Directional AntennasDirectional 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


• Yagi-Uda– Adjustments to boom length, number of

elements, spacing of elements can be used to change:

• Gain (3-element has theoretical 9.7 dBi)

• F/B Ratio

• SWR Bandwidth– Can also be increased with fatter elements


• 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)


• Yagi-Uda, Stacked pattern


• Cubical Quad


• 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


• 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 AntennasSpecialized 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)


• 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)


• 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)


• Multiband Antennas– Advantages

• More than one band with single feed line

– Disadvantages• Low suppression of harmonics

• Can be higher maintenance

• Price

Case StudyCase 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 IndicatorSWR 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


• 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)


• 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)


• 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?


• 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


• Antenna pattern with and without feedline losses


• Reasonable SWR does notindicate your antenna is

working well!

• Total power into the antenna = 0.2 Watts


• 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


• 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


• Antenna pattern with and without feedline losses


• Very High SWR does notindicate your antenna won't

work well!

• Total power into the antenna = 9.1 Watts

• Difference from Joe's Antenna ~ 25 dB

Questions?Questions?

If you have any questions while reviewing the

material or the CD, please e-mail me at:

[email protected]

or

[email protected]

See: www.k9vic.info

General License Class Chapter 6 - Antennas

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