Session 4 Operating a Ham Station Chapter 5 Transmitters and Receivers Antennas & Feedlines Figures in this course book are reproduced with the permission of the American Radio Relay League. This booklet was compiled by John P. Cross AB5OX
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Typical Amateur Station Layout
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Typical Amateur Packet Station
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CW Transmitters are the Simplest
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Block Diagram of FM Transmitter
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Schematic of FM transmitter
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Receivers • Radio receivers demodulate the signal - they retrieve the information from the RF wave. • Receivers convert radio signals into audio signals. • The heart of the receiver is the detector. • Modern receivers are very sensitive and very complex - use feedback to increase signal strength
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Simple AM Crystal Set
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Superheterodyne Receiver
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Low Pass Filter
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High Pass Filter
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Band Pass Filter = low + high in series
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CW Receiver
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AM Receiver
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FM Receiver
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Data Modes
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Use of Phonetic Alphabet Improves Understanding
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Antennas & Feed lines (Chapter 4)
Figures in this course book are reproduced with the permission of the American Radio Relay League. This booklet was compiled by John P. Cross AB5OX
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Antennas - General Information • We convert electrical current into radio waves with an antenna. • The purpose of the antenna is to radiate the energy, propagate the radio wave. • When receiving, the antenna converts a radio wave into an electrical current. • A good antenna is worth more than a big amplifier! • It pays also to have good, clean connections to prevent power losses.
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Transmission or Feed Lines • Special cables or wires that connect the transceiver to the antenna. • Feed lines, like antennas, have a characteristic impedance which needs to be matched to the transceiver and antenna. • Matching devices are used to adapt one impedance to another. • Coaxial cable and parallel conductor (twin lead) are the two most common feed lines. PHYS 401 Physics of Ham Radio
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Coaxial Cable • Impedance 50 Ω: RG-58, RG-8 RG-213 • Impedance 72 Ω: RG-59 RG-11 • Thick cable (RG-8, RG-11) and good shielding reduces losses. • Advantages are: – weather resistance – it can be buried – it can be bent or coiled – it can be next to metal – impedance matches most antennas. • Disadvantage is cost.
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Twin Lead (ladder line) • Impedance is 300-450 Ω • Major advantage is low losses (can have a long run). • Disadvantages are: – cannot be coiled. – cannot be run near metal. – impedance doesn’t match modern transmitters. – Limited to less than 30 MHz
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Cable Attenuation
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Connectors are Important • BNC connectors are compact, often used for hand held radios – designed for use with RG-58 – low loss, quick connect. • PL-259/SO-259 commonly used for HF and VHF applications. • N-Type, designed for RG-213 and RG-8, – low loss – used for UHF applications • Good soldering technique and careful construction are critical to making good connections of cable to connectors. If solder joint is dull, not a good connection (“cold solder”) Best if it’s shiny
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Impedance Matching • Devices are networks of capacitors and inductors. • Transmatch is a device that has adjustable characteristics so it can be used on several bands. • SWR (standing wave ratio) meter is used to measure impedance matching. It is connected between the transmitter and the transmatch. • A balun (balanced to unbalanced) is a device to couple a balanced load to an unbalanced load. • Balanced: e.g.twin lead, dipoles, neither side to ground. • Unbalanced: e.g.coax and verticals, one side to ground. PHYS 401 Physics of Ham Radio
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Impedance Matching Hookup
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Standing Wave Ratio (SWR)
• Ideally, all the forward power from the transmitter should be emitted by the antenna, if the impedances are matched. • We want all the forward power to radiate from the antenna and none of it to be reflected. • SWR is the ratio of the maximum voltage on the line to the minimum, ideally, 1:1. • SWR of less than 2 is acceptable. Higher than 4 indicates a problem. • Modern transmitters are designed to match a 52 Ω load. Will shut down power if SWR > 2 • Good matching improves performance!
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SWR Meters • Used to measure impedance matching of transmitter and feed line and the resonant frequency of an antenna. • Need to determine frequencies the meter was designed for. Outside that range they will not be accurate. • Problems with antennas can be found with the SWR meter: – erratic measurements could indicate loose connections. – extremely high could indicate shorts or gross dimension problems – change with time (months) could indicate corrosion.
• Tuning an antenna is probably the most common use.
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Half-Wave Dipole Antenna • The length of the halfwave dipole is calculated by: 1/2λ(ft)=468/f(MHz) 1/4λ(ft)=234/f(MHz)
• This is only accurate for frequencies up to 30 MHz (10 meters)
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Dipole Stuff • Insulators are needed for the center and the ends. Can be bought (cheap) or can be home-brewed from plastic. • Wire choice is important. Best is copper clad steel. 12-14 gauge is suitable. Small gauge wire will will stretch. • Cut wires a little longer than calculated to allow for connections and to allow fine tuning. • Use good coax with >95% shielding. RG-58 works just fine for runs up to about 100 feet. • Dipoles radiate most perpendicular to the wire. Alignment may be important. • Get it as high as you can. Preferable 1/2λ above ground. • Inverted V and slopers work just fine.
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More Dipole Stuff • Use your imagination to get lines into trees. Bow and arrow, rocks and slingshots work well. Send up a light line, then pull through the support ropes. • Don’t use polypropylene line; it deteriorates. Nylon is better, and dacron is best. • If you can, support the middle as well as the ends, it makes for a sturdier installation. • Make sure your antenna is a long way from metal things, flag poles, gutters, etc. • NEVER NEVER put your antenna near power lines.
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Tuning with an SWR Meter • Install SWR meter at antenna feed point. • Set transmitter to low power. • Adjust meter and take series of measurements across frequency band. • A “dip” indicates the resonant frequency (lowest is best!). PHYS 401 Physics of Ham Radio
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More Tuning With a SWR • If there is no dip, you must look at the slope of the SWR curve. It slopes down toward the resonant frequency. • If the minimum is at the low frequency end, the antenna is too long. • If the minimum is at the high frequency end, the antenna is too short (so make them a little long to start!!)
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Multi-Band Dipoles • A simple three band dipole can be built from ladder line in a manner similar the the simple dipole. • Advantage is that a single antenna can be used on several bands. • You will need a transmatch. • Be careful, this kind of antenna can radiate on several wavelengths. Be sure your transmitter is properly adjusted.
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Quarter-Wave Vertical • This is an unbalanced antenna, one side is grounded. • Omni-directional that tends to shoot signals toward horizon. • Radiator is 1/4λ = 234/f(MHz). • This is accurate for < 30MHz, end effects and radiator diameter to frequency ratio make it overestimate for higher frequencies • Connect center conductor to radiator and shield to ground.
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Ground Plane Antenna
Easy to build, good outdoors (and indoors) Gives an “effective” ground not at true ground
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Beam Antennas
• Directional antennas which provide a lot of gain in the direction pointed. Beam in direction of shortest piece! • The more elements, the bigger front to back power ratio • Driven element is 1/2 λ, making it impractical for 80 and 40 meters.
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Antenna Polarization • Polarization: Direction of the electric force lines in a radio wave • Vertical antennas are vertically polarized • Dipoles are horizontally polarized. • Best results are obtained with transmitting and receiving antennas having same polarization. PHYS 401 Physics of Ham Radio
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Cubical Quad and Delta Loop
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Other Antennas • Handy Talkies often use a “rubber duck.” This design is compact, but a compromise design. Lower performance (lots shorter than 1/4 λ) • Better performance can be had with 1/4 and 5/8 λ telescoping antennas. • Roof mounted 5/8 λ antenna has better gain than the others. Car roof is a great ground plane!
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Repeater Overview • Simplex operation- two stations are talking directly on the same frequency. (standards are 146.52 (2m) and 446.000 (70cm) • Duplex operation - two stations communicating transmitting and receiving on different frequencies. • Repeater operations - use standard frequency offsets from the receiving mode. This is automated in most VHF and UHF equipment. • There is a listing of all amateur repeaters which can be used to find useful frequencies. Many repeaters have special features. • Repeater frequencies are mostly coordinated to minimize overlap and possible interference. • Most repeaters are “open”, anyone can use them. • Often incorporate CTCSS or PL tones to avoid interference. If you don’t have the right tone set, you can hear them but they can’t hear you. Check a repeater directory. • Setting “memories” on your rig has it remember the proper tones! PHYS 401 Physics of Ham Radio
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Repeater Operations
• Don’t call CQ on a repeater, simply say: “AB5OX listening” • Turn down your squelch then back up to just cover the noise level. (be sure noone else is talking!) • To join a conversation simply say your call sign during a break and wait to be acknowledged. • “Break” means that you have emergency traffic, don’t use it unless you need it. • Most repeaters have a courtesy beep which indicates that the transmitting station has released the PTT. • Most repeaters also have a time out feature to protect the transmitter. • Be Courteous, it’s more fun that way. 127 PHYS 401 Physics of Ham Radio
CW (Morse Code) Operations • Listen before transmitting “QRL?” • Send at a speed that you can easily read. • Calling CQ, use the “3 X 3” call: – CQ CQ CQ DE AB5OX AB5OX AB5OX K • To answer, use “2 X 2” format: – AB5OX AB5OX DE K5CXH K5CXH AR • Use appropriate prosigns, Q signals and abbreviations: – K5CXH DE AB5OX BT TNX FOR CALL BT UR RST 559 BT NAME JOHN QTH CAMP STRAKE NR HOUSTON BK • Close the conversation as follows: – TNX QSO ES 73 BT CUAGN K5CXH DE AB5OX SK (Use similar prosigns for PSK31) PHYS 401 Physics of Ham Radio
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Common Prosigns
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Single Sideband (SSB) Operations • Voice communications are known as “phone”: SSB, AM, FM. • SSB is the most common phone mode on HF. • Initiate a contact with “3 X 3” call as with CW, but use phonetics for your call sign. • Reply with the calling station’s call sign, this is , then your call phonetically. • Keep your conversation plain and simple. Be courteous. • Don’t use prosigns or Q signals and don’t use CB jargon. • Signal reports are only “RS”. • Listen before you talk, use VOX or PTT and listen to make sure you are not doubling. PHYS 401 Physics of Ham Radio
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Data Emissions - RTTY • RTTY - Radio Teletype , narrow band direct printing telegraphy - continuous signal modulated between two frequencies. – FSK - frequency shift keying - CW carrier shifted 170Hz (HF) – ASFK - audio-frequency shift keying - shifting audio tone transmitted by FM (VHF). MCW (modified CW).
• Only 1 QSO can be maintained on a given frequency and it requires operator control of transmissions. • Modern systems use computers and modems. • Baud rates are typically 300 (HF) and up to 1200 (VHF). • Call CQ with the “3-6 X 3” method. • Use prosigns and Q signals. PHYS 401 Physics of Ham Radio
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Data Emissions – PSK-31 • Allows conversations with more background noise than a voice conversation (will get through when Phone won’t). Always uses UPPER sideband (USB) • Several conversations on one frequency, just using different frequency tones. See where everyone is using the “waterfall”. • System is controlled with a computer using its sound card to interpret and send. Special software, generally free or cheap. • PSK means “phase shift keying” to key the 0’s and 1’s. “31” means 31 baud (slow but efficient for HF) • Standard frequencies: – 7.017.15 40m – 14.070 (14.035 at night) 20m – 28.120 10m
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Sending photos and video • Video (Slow Scan TV) takes a higher frequency so can send data at a faster rate. • Scans the picture across, one horizontal line at a time. First was only 128 lines but now also you can get 256. • System is controlled with a computer and special software. • Tone coding. “tee-del-ee” • Standard frequencies: – – – –
3.845 7.170 14.230 28.680
80m 40m 20m 10m
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