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HARC Technician License Prep February 18, 2012 Homework • Before passing the test – Read the HamElmer.com Technician Test Self Study Guide – Take several online practice exams (see wiki) • Take test at the Vienna Wireless Winterfest, – Northern Virginia Community College, Annandale – February 26 at 9 AM sharp – Bring $15, Photo ID, SSN or FRN, 2 #2 pencils • After passing the test – Read Ethics and Operating procedures for the Radio Amateur by ON4UN, John Delvodere Circuits Formulae • Ohms Law: E = IR, I = E/R, R = E/I • Power: P = IE, P = I2R, P = E2/R • Decibels: DBP = 10log(P1/P0), DBv = 20log (V1/V0), dbm referenced to 1 mW • Frequency and Wavelength: λmeters = ν nominally c/ Fhertz • Speed of light (c) in Free Space is 299,792,458 meters/second – We round to 300,000,00 or 186, 000 miles/sec – slower in other media • Dipole in Free Space: Lfeet = 468/FMHz • Metric System, etc.: Mega, Kilo, Deci, Milli, Micro, Nano, Pico Combined Formula Diagram From the center out is the Summary of Ohm’s Law, Angle in degrees and the sine of the angle, Resistor color codes, 24 hour clock face and finally a compass rose. Devices • Two terminal devices – – – – Resistor (ohms) reduces current flow Capacitor (farads) blocks DC, stores energy electro-statically, differentiates Inductor (henrys), passes DC, stores energy magnetically, integrates Rectifier Diode, Small signal Diode, Light Emitting Diode (LED), Zener Diode. All pass current one way only., cathode has band marking – Fuse • Three terminal devices – Transistor, Bi-polar (emitter, base, collector)and Field Effect (source, gate, drain). Capable of gain (amplification) – Potentiometers • Other – Transformers (power, balun/unun) – Vacuum tubes (capable of gain and amplification to very large powers) – Relays (switches operated by an electromagnet) Schematic Symbols This page gives an extensive overview of schematic symbols: http://www.faqs.org/docs/electric/Ref/REF_9.html We will read this page now. Functional Components • Oscillators • Amplifiers: small signal and power, audio, IF and RF, – limiters: used on FM and PM receivers to remove amplitude variations • Mixers: Fa and Fb combine to produce (Fa – Fb) and (Fa + Fb )at output. One is desired and the other is an image. • Modulators • Detectors: diode, discriminators, product detectors • Filters • Power supplies and regulators (voltage and current) Modulation • Conveying information over radio • Continuous Wave (CW) – Morse code • Amplitude Modulation AM – Single Sideband (SSB): USB, LSB – Voice and Sound Card digital modes, SSTV on HF • Frequency Modulation, commonly used at VHF and higher in mobiles and handie talkies (HT). • Phase Modulation, ex: PSK-31 • Frequency Shift Keying (FSK) – RTTY (Radio Teletype) Examining a modern transceiver K2 Block Diagram K2 Schematic - 1 K2 Schematic -2 K2 Schematic -3 K2 Schematic - 4 K2 Schematic – 5 K2 Schematic - 6 K2 Schematic - 7 Some antennas http://www.w8ji.com/curtain%20sterba%20USIA%20array.htm VSWR (SWR) WHY VSWR EXISTS To get maximum power into a load required that the load impedance match the generator impedance. Any difference, or mismatching, of these impedance would not produce maximum power transfer. This is true of antennas and transmitters as well but, except for handie-talkies, most antennas are not connected directly to a transmitter. The antenna is usually located some distance from the transmitter and requires a feedline to transfer power between the two. If the feedline has no loss, and matches BOTH the transmitter output impedance AND the antenna input impedance, then - and only - then will maximum power be delivered to the antenna. In this case the VSWR will be 1:1 and the voltage and current will be constant over the whole length of the feedline. Any deviation from this situation will cause a "standing wave" of voltage and current to exist on the line. There are a number of ways VSWR or its effects can be described and measured. Different terms such as reflection coefficient, return loss, reflected power, and transmitted power loss are but a few. They are not difficult concepts to understand, since in most instances they are different ways of saying the same thing. The proportion of incident (or forward) power which is reflected back toward the transmitter by a mismatched antenna is called reflected power and is determined by the reflection coefficient at the antenna. The reflection coefficient "p" is simply a measure of this mismatch seen at the antenna by the feedline and is equal to: P =(Z1-Zo)/(Z1+Zo) Here Z1 is the antenna impedance and Zo is the feedline impedance. Both Z1 and Zo are complex numbers so "p" is also a complex number. From elementary AC mathematics a complex number has a "phase angle" associated with it. The phase of the reflected signal will be advanced or delayed depending upon whether the antenna appears inductive or capacitive to the feedline. If the antenna appears inductive the voltage will be advanced in phase, and if the antenna is capacitive, the voltage will be retarded. The reflective signal travels back to the transmitter and adds to the incident signal at that point. Thus, any mismatch at the antenna gives rise to a second 'travelling wave' which goes in the opposite direction from the incident wave. When Z1 = Zo the reflection coefficient is zero and there is no reflected signal. IN this case all power is accepted by the antenna and this is the ideal situation where VSWR is concerned. The problem is that this condition is rarely, if ever, achieved and so "p" will have a value different from zero. Note that "p" can have negative values, but in calculating VSWR from the reflection coefficient, only the "absolute value" is used - which is a positive value lying between 0 and 1. As the two travelling waves pass each other in opposite directions, they set up an interference pattern called a "standing wave". At certain places on the feedline the voltages will add producing a voltage maximum, and at others their relative phase difference will cause a voltage minimum to exist on the feedline. These maximum and minimum points occur 1/4 wavelength apart. In the days when open-wire feedlines were used these points could easily be measured with simple indicators. Coax cable however presents another problem since the "inside" of the cable is not readily available for measurements. Consequently, VSWR measurements on coax are usually made at the transmitter end of the feedline. Therefore you are presented with the VSWR of the entire system which includes all losses associated with the entire system. VSWR (continued) INTERPRETING WHAT YOU HAVE READ Many VSWR meters are calibrated to read FORWARD power as well as REFLECTED power. They may actually be measuring voltage, and simply have the scales calibrated in power. The important point is to understand what the meter is actually telling you. Assuming for the moment that the VSWR meter contributes no errors, the FORWARD reading is the SUM of the forward power and the reflected power. As a result, it is greater than your actual power output. The REFLECTED power reading is that amount of power which was not initially absorbed by the antenna and has been sent back down the feedline. At the transmitter end it encounters the transmitter output circuitry and is re-reflected back towards the antenna. This happens because you do, in fact, have a VSWR greater than 1:1 as seen by the transmitter. When the re-reflected power encounters the antenna, a portion of it is absorbed and the whole process starts over again. Ultimately then, most of your signal is eventually absorbed by the antenna. You might be tempted to think that all of this bouncing back and forth would cause "smearing or blurring " of your signal but this is not so. The average transmitted signal appears as a "steady-state" signal to the feedline and antenna. Remember your signal is travelling at a significant fraction of the speed of light. For instance, the velocity of propagation of RG-8/A is 0.66 or 2/3 the speed of light. The speed of light is close to 1000 feet per microsecond, and a dot or voice peak takes milliseconds to complete. If the speed of light were 20 miles-per-hour then the situation would be completely different and we probably wouldn't have radio transmission at all. (Ed. Note, it would be as fast as the mail then.) Given the reality then that almost all power launched down a feedline reaches and absorbed by the antenna, one has to wonder why VSWR is all that important. The importance is due to the fact that feedlines have losses and, antennas have something called radiation efficiency. They are what make proper interpretation of VSWR important. Power is lost due to feedline attenuation and this loss goes up as the VSWR goes up. The efficiency of an antenna is determined by the ratio of its "radiation resistance" to its "loss resistance". Antenna efficiency can simply described by the following equation: % Efficiency=[Ra/(Ra+Rloss)] X 100 The radiation resistance is Ra, and Rloss is made up of any associated losses of the antenna such as loading coils and ground systems. How well you "get out" therefore depends more on low losses and efficient antennas than on what your actual VSWR is. Retrieved from: http://www.antennex.com/preview/vswr.htm on February 12, 2012 at 7:20 PM EST and lightly edited. Radio Propagation Electromagnetic Wave in Free Space Electric fields (blue) and magnetic fields(red) radiated by a dipole antenna Knife Edge Diffraction (A Fresnel effect) Ionospheric Propagation Propagation by Frequency Band ELF Radio frequencies and their primary mode of propagation Frequency Wavelength Extremely Low Frequency 3–300 Hz 1000-100,000 km Propagation via VLF Very Low Frequency 3–30 kHz 100–10 km Guided between the earth and the ionosphere. LF Low Frequency 30–300kHz 10–1 km Guided between the earth and the D layer of the ionosphere.Surface waves. Surface waves.E, F layer ionospheric refraction at night, when D layer absorption weakens. MF Medium Frequency 300–3000kHz 1000–100 m HF High Frequency (Short Wave) 3–30 MHz 100–10 m VHF Very High Frequency 30–300MHz 10–1 m UHF Ultra High Frequency 300–3000MHz 100–10 cm SHF Super High Frequency 3–30 GHz 10–1 cm EHF Extremely High Frequency 30–300GHz 10–1 mm E layer ionospheric refraction.F1, F2 layer ionospheric refraction. Infrequent E ionospheric refraction. Extremely rare F1,F2 layer ionospheric refraction during high sunspot activity up to 80 MHz. Generally direct wave. Sometimestropospheric ducting. Direct wave. Sometimes tropospheric ducting. Direct wave. Direct wave limited by absorption. USA Frequency Allocations Maidenhead Grid Square Map - NA 299,792,458 Question Review Question and Answers • We go to this web page and look at a list of all 200 questions you may be asked, along with the correct answer. http://hamelmer.com/Assets/Docs/Tech/2010_Element_2_QandA.pdf Join HARC mailing list Go to: http://groups.google.com/a/hacdc.org/groups/dir Sign up for: HacDC Amateur Radio Club Archive is here: https://groups.google.com/a/hacdc.org/group/harclist/topics Good Luck! Use the mailing list to ask any questions or clarify any issues. We’re here to help!