ANTENNAS 101 An Introduction to Antennas for Ham Radio. Lee KD4RE

Transcription

1 ANTENNAS 101 An Introduction to Antennas for Ham Radio Lee KD4RE Prepared for Presentation at the Vienna Wireless Society, 13 January 2017

2 So What is an Antenna Anyway? We are all familiar with wire antennas based on dipoles and loops including Uda-Yagi, Log Periodic, Inverted Vees, Marconi (vertical) antennas, etc. But antennas do not need wires Horn Antennas common at L band to millimeter waves. Dielectric Antennas that are made from plastic glass fiber. Sea Water or conductive fluids, Plasmas In general an antenna is a region or transducer between a guided wave (on a transmission system) and free-space according to Dr. John Kraus W8JK It couples energy from/to a transmission system (a bounded electromagnetic system) to free space (an unbounded electromagnetic system) -

3 Historical Antennas Replica of Karl Jansky s MHz directive antenna located at NRAO Green Bank, WVA Horn Antennas are typically are used above 1 GHz but can be made to work even at any Frequency is they are large enough Above is the Holmdale, NJ, Horn antenna at Bell Laboratories that was used to measure the background radiation (radio) noise by Penzias and Wilson in May 1964 and confirm theories of cosmic background noise at 4.08 GHz and the Big Bang Another famous Antenna at Holmdale was built in 1933 by Karl Jansky and detected noise at 20.5 MHz that was determined to come from the Milky Way

4 Terminology - Antenna Gain Antennas do not have gain like an amplifier that increase the output level compared to the input. They are passive devices. The total power out of antenna is input power. What we call Antenna Gain comes from the Directivity of the antenna. All practical antennas have some directivity They radiate or receive more power in one direction than in others. The difference in the maximum power radiated in a direction compared to a reference antenna is called Gain of the antenna Gain relative to an isotropic source (a perfect source that radiates power equally in all directions (over a sphere) in free space in expressed in dbi. Gain for communications is sometimes reference to the gain of a ½ λ dipole antenna (dbd) where 0 dbd = 2.15 dbi The difference between directivity and gain is the losses in the antenna. G = D - loss

5 Terminology - Resonance Resonance of antenna has nothing to do with the feed point impedance of an antenna being 50 ohms VSWR is NOT a measure of Resonance it is a measure of feed point impedance mismatch at the point measured. As an analogy to circuit theory, antennas are claimed to be resonant when the feed point impedance reactance = 0 ohms. this occurs at the middle of a ½ λ dipole and any antenna that is n/2 wavelengths long. Which leads to a broader definition that an antenna is resonant when its length is n/2 λ, (or n/4 λ for image antennas such as a vertical over a ground plane) regardless of the feed point impedance The current distribution on the wire is perfectly sinusoidal Frequencies at which the feed point Z = 0 will vary with feed point location even though the antenna has same current distribution on it for all feed points.

6 80 Meter Dipole Feed Point Z for different feed points Center Feed Dipole for 3.68 MHz Dipole Feed 1/3 distance from the end 1.00E E E E E E E E E E E End Feed Dipole Current Distribution on a Resonant Dipole

7 Resonance and Performance Does An Antenna Have to Be Resonant to Radiate Well? NO In many instances a non-resonant antennas provide the best performance 5/8 λ vertical produces the best pattern/maximum signal at low elevation angles of any simple vertical antenna above a ground plane My Lazy H is a non-resonant antenna. But a Resonant antenna can be easier to feed since its reactance is 0 ohms (in the middle) In general, reactance tends to change faster than the resistance so a broader VSWR bandwidth can be achieved with a resonant antenna.

8 Basic Math for Antennas as an Aperture In Free Space Power Density Produced by an antenna in Watts/meter 2 at distance d is: Power Radiated x Effective Gain of Antenna/ (4 * π * d 2 ) Effective Area of Antenna = Effective Gain * λ 2 / (4* π) Power Received = Power Density * Effective Area Power Transmitter * Effective Transmit Gain* Effective Receive Gain * λ 2 / [(4 * π) 2 * d 2 ] In db: Pr = Pt + Gt + Gr 20*log(F) - 20*log (d) (for F in MHz and d in meters) So say we transmit 100 watts into a dipole (2.1 dbi gain) power density at 100 meters distance = 1.29 mw/m 2 Power received by dipole at 10 MHz = 0.15 watts, Power received by dipole at 100 MHz = watts (but a dipole at 10 MHz is 10 times longer than one at 100 MHz)

9 A Little Physics From the early 1800s, it was recognized that there are Electrical (E) Fields and Magnetic (H) Fields both static and time varying. E Field are Electrical Intensity (Volts/meter) arising from 2 or more charges (different in Potential) in static case. H Fields are Magnetic Intensity (Amps/meter) arising between magnetic poles for the static case. Electromagnetic Radiation occurs when the E-Field and H- Field are time varying. Power is radiated in a direction that is orthogonal to the direction of the E and H Fields (called the Poynting Vector) Average P = ½ Re (E x H*)

10 Wire Antennas There are two fundamental types of wire radiators (antennas) a straight wire and a loop. Other various wire antennas are made from these radiating elements. A time varying current on a short wire generates a Magnetic (H) Field around the axis of the wire and two Electrical (E)Fields, one directed radially outward and one parallel to the direction of the wire. No Power is radiated from the Radial E Field, since it sums to zero - equal amounts going in opposite directions. Power is radiated from the parallel E Field component and outward which is orthogonal to the this E field and the H Field surrounding the wire. A small loop is the image of a dipole with one E Field and 2 H Fields

11 The Real World Antennas are impacted by their surroundings and the presence of the ground. Things to consider in your antenna design and installation Height of the antenna above ground will change the elevation pattern of the antenna. Height will affect the feed point impedance of the antenna The Optimum Height Above Ground for your Antenna will depend on what you want to accomplish If you want good short range communications via NVIS a dipole close to the ground will be ideal because most of the energy will be radiated at high elevation angles. If you want to work DX, then you want most of your energy radiated at low elevation angles.

12 Radiation Resistance for ½ λ Dipole As a Function of Height Above Ground From Practical Antenna Handbook by Joseph Carr.

13 Elevation Pattern vs. Antenna Height 80 Meter Dipole Antenna Height 5 Meters Antenna Height 10 Meters Antenna Height 40 meters Antenna Height 80 Meters Antenna Height 120 Meters Antenna Height 300 meters

14 So What Size Do I Need for My Antenna Any conductor with a time varying electric current will radiate energy. Is an antenna. How efficiently it radiates the energy and how easy is it to couple energy from the (bounder) transmission system to the antenna is however a design issue. For the wire antennas the maximum power radiated by the antenna = the antenna current squared at the feed point times the resistance (termed radiation resistance) at the feed point. For electrically small antennas (in terms of λ) the feed point resistance is usually very small (and quite often the feed point reactance is very high). This can create very high currents and voltages which further increase losses. Thus, ohmic losses in the antenna and ground can be a large percentage and exceed the radiation resistance of the antenna. Also matching networks to tune out the high reactance and match the antenna impedance also are very lossy compared to the antenna s radiation resistance. So most of the RF power you are generated will turn into heat in the ground, wires, coils, and capacitors in the antenna and matching network. Techniques like loading coils and capacitor hats are used to increase the electrical height (length) of physically small antennas to raise the radiation resistance. (e.g, the Buddipoles)

15 What about a Long Antenna Like the 80 meter dipole at 300 meters above the ground, antennas that are multiple wavelengths at some frequencies (e.g. the 80 meter windom on 10 meters, the G5RV at the upper end of the HF bands) will have an azimuth pattern with a lot of lobing (peaks and nulls). 80 Meter Dipole Az Pattern at 3.68 MHz 80 Meter Dipole Pattern at 28.1 MHz