Admin: /* Seciton 7.5: Feed lines */

2016-06-30T22:16:30Z

Seciton 7.5: Feed lines

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	* Disadvantage of multiband antennas is poor rejection of harmonics/spurious signal generation		* Disadvantage of multiband antennas is poor rejection of harmonics/spurious signal generation

	==~~Seciton~~ 7.5: Feed lines==		==Section 7.6: Feed lines==

	* Feed lines have 2 conductors		* Feed lines have 2 conductors

Admin: /* Section 7.4: Summary */

2016-06-27T07:22:48Z

Section 7.4: Summary

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Admin: /* Section 7.2 Summary */

2016-06-27T06:43:35Z

Section 7.2 Summary

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Admin: /* Section 7.2 Summary */

2016-06-25T22:54:44Z

Section 7.2 Summary

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			=Flags=

	~~[[Category:General]]~~		{{GeneralFlag}}
	~~[[Category:Radio]]~~

Admin: /* Section 7.2 Summary */

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Section 7.2 Summary

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	* For a dipole antenna in free space, the radiation pattern is a figure-eight at right angles to the antenna		* For a dipole antenna in free space, the radiation pattern is a figure-eight at right angles to the antenna
	* Antenna height affects horizontal (azimuthal) radiation pattern of a dipole by making antenna omnidirectional if low (less than 1/2 wavelength high)		* Antenna height affects horizontal (azimuthal) radiation pattern of a dipole by making antenna omnidirectional if low (less than 1/2 wavelength high)



			[[Category:General]]
			[[Category:Radio]]

Admin: Created page with "=Chapter 7: Antennas= ==Section 7.1: Antenna Basics== Review of terminology: * Elements - conducting portions of an antenna that radiate or receive signals * Polarization -..."

2016-06-25T22:04:55Z

Created page with "=Chapter 7: Antennas= ==Section 7.1: Antenna Basics== Review of terminology: * Elements - conducting portions of an antenna that radiate or receive signals * Polarization -..."

New page

=Chapter 7: Antennas=

==Section 7.1: Antenna Basics==

Review of terminology:
* Elements - conducting portions of an antenna that radiate or receive signals
* Polarization - orientation of electric field radiated by antenna
* Feedpoint impedance - ratio of RF voltage to current at antenna feedpoint
* Resonance - when feed point impedance is purely resistive, with no reactance
* Radiation pattern - graph of signal strength in every direction
* Azimuthal pattern - shows signal strength in horizontal directions (birds eye view)
* Elevation pattern - shows signal strength in vertical directions (side view)
* Lobes - regions in radiation pattern where signal is being radiated
* Nulls - points at which radiation pattern is at a minimum
* Isotropic antenna - radiates equally in every possible direction 9horizontal or vertical)
* Omnidirectional antenna - radiates a signal of equal strength in every horizontal direction
* Directional antenna - radiates preferentially in one or more directions
* Gain - concentrating transmitted or received signals in a specific direction
* dB - decibels, units of gain
** dBi = gain with respect to isotropic antenna
** dBd = gain with respect to dipole antenna

==Section 7.2: Dipoles, Ground Waves, Random Wires==

Dipoles
* Dipole = 2 electrical polarities
* Dipoles have a figure-8 radiation pattern, signal is strongest broadside to antenna
* Actual ground installation changes radiation pattern
* Half wave dipole - 1/2 wavelength total, feedpoint in center, each half of the dipole is 1/4 wavelength
* Voltage: lowest at feedpoint, highest at ends
* Current: highest at feedpoint, lowest at ends
* Length of a dipole in feet: 492/f
* Feedpoint impedance of center-fed, free-radiating dipole should be 72 Ohms
* Becomes several THOUSAND Ohms when feedpoint is connected at the ends
* Physical thickness of wire can '''electrically''' lengthen wire, meaning you need a shorter antenna
* Height above ground affects resonant frequency
* Insulation, nearby conductors, and method of insulation all affect resonant frequency and length of dipole
* Start near free-space length, and use SWR/antenna tuner to trim dipole
* Exam asks to approximate resonant length for dipole. Use 492/f - pick the closest value to that

Ground plane vertical antena:
* Dipoleantenna, with missing half: ground plane acts as electric mirror
* Ground plane consists of sheet of metal or ground radial wires
* Often installed vertically, making them omnidirectional (good for VHF/UHF)
* Base impedance is 35 Ohms (half of dipole)
* Sloping ground radials down will increase impedance
* Downward angle of 30-45 degrees will match 50 Ohm impedance of coax cable
* Impedance increases as feed point moved further from the "center"
* Start with half of the free-space 1/2 wavelength length: length in feet is 246/f
* Example calculation: approximate length of 1/4 wave ground vertical antenna that is resonant at 28.5 MHz is:
** Free space length = fsl = 246/f = 246/28.5 = 8.6 ft

Mobile HF antennas;
* Mobile HF antennas are ground-plane antennas
* Vertically oriented whip antennas - thin steel rod mounted atop vehicle surface
* Full-size 1/4 wavelength whip not feasible below 10 m
* Loading techniques used to physically shorten/electrically lengthen an antenna
* Loaded antennas have reasonable impedanc, but inefficient
* Screwdriver antenna: whip with adjustable loading coil at base

Random wires:
* Feed point impedance and radiation patterns are totally unpredictable
* May have lobes at multiple angles and multiple locations
* Connected to transmitter output or to antenna tuner without feedline
* Antenna, radio, station equipment are all part of the antenna system
* May result in significant RF currents/voltages on station equipment
* Can result in RF burns
* If impedance can be matched, can give excellent results

Effect of height:
* Feed point impedance and radiation pattern affected by antenna's physical height above ground
* Presence of electrical image created in conducting ground below antenna affects performance
* Image is electrically reversed
* As antenna and image get closer, they start to short each other out
* Close to ground level, feed point impedance is near zero
* Above half wavelength, impedance varies. Goes through maxima and minima
* Maxima at 1/4 wavelength, then 1 wavelength
* Ground also affects radiation patterns: real radiation patterns composed of energy received by antenna, and energy reflected from ground
* Direct and reflected signals take different times to travel, and may combine, cancel out, or anywhere in between
* This leads to new lobes/nulls pattern
* Depending on height, you can have most radiation going UP, or most radiation going out via side lobes

Polarization:
* Polarization can affect amount of signal lost due to ground resistance
* Radio waves reflected from ground: lower losses when polarization is parallel to ground
* Radiation pattern consists of reflected waves combined with direct waves not reflected
* Lower reflection loss means higher maximum signal strength
* HF DX done with vertical antennas - lower losses at low angles (but at higher angles, still more effective than horizontally polarized)
* Ground-mounted vertical: not polarized efficiently, but still generates stronger signals at lower angles of radiation from horizontally polarized antennas at lower heights
* DX or HF bands use verticals, because horizontally polarized antennas require unreasonable hieghts

===Section 7.2 Summary===

* A capacitance hat on a mobile antenna is used to electrically lengthen/physically shorten an antenna
* A corona ball on an HF mobile antenna reduces high voltage discharge from the tip of the antenna
* A disadvantage of a shortened mobile antenna is a '''smaller bandwidth'''
* A disadvantage of a directly-fed random-wire antenna is that you may experience RF burns if you touch it
* To adjust the feedpoint impedance of a quarter wave ground plane vertical antenna, slope the radials downward
* If a ground plane antenna's radials are changed from horizontal to downward, the feed point impedance '''increases''' (maximum angle makes them into a dipole, which has double the feed point impedance of a ground wave vertical antenna)
* For a dipole antenna in free space, the radiation pattern is a figure-eight at right angles to the antenna
* Antenna height affects horizontal (azimuthal) radiation pattern of a dipole by making antenna omnidirectional if low (less than 1/2 wavelength high)

General/Chapter 7 Study Guide - Revision history

Admin: /* Seciton 7.5: Feed lines */

Admin: /* Section 7.4: Summary */

Admin: /* Section 7.2 Summary */

Admin: /* Section 7.2 Summary */

Admin: /* Section 7.2 Summary */

Admin: Created page with "=Chapter 7: Antennas= ==Section 7.1: Antenna Basics== Review of terminology: * Elements - conducting portions of an antenna that radiate or receive signals * Polarization -..."