Introduction to Digital Mobile (DMR)

This professional digital voice technology is now gaining fans in the world.

John S. Burningham, W2XAB their protocol. DMR utilizes a ment manufacturers will ever standar·dize Digital voice involves the conversion of Radio ID number that amateurs for business reasons. However, convers~ on analog voice into a digital data stream for must obtain from DMR-MARC, between the two vendor protocols is pos­ transmission over the RF or wired media. which serves as a central registry sible. Any brand DMR (Tier II) user radio This conversion process utilizes a vocoder. for worldwide amateur users. will work on any Tier II system, although The transmitted digital voice data stream some manufacturers offer additional pro­ DMR Standard is then encapsulated inside a protocol that prietary features for their infrastructure. DMR is divided into three tiers. handles the addressing and communication Tier I is a single channel specifi­ Th e Vocoder management. Both sender and receiver cation designed f<;>r the European Although it is not specified must use the same vocoder and protocol unlicensed dPMR446 service. It in the ESTI standard, by to communicate. Among the digital voice is a single channel Frequency agreement of the manu­ technologies used in the amateur bands, Division Multiple Access factures, the current im­ the most popular are D-STAR (Digital (FDMA) signal in a 6.25 kHz plementation of DMR Smrut Technologies for Amateur Radio), bandwidth. The standar·d sup­ utilizes the Digital Voice DMR (Digital ), and Sys­ ports peer-to-peer (mode I), Systems, Inc . (DVSI) tem Fusion. This article focuses on DMR repeater (mode 2), and linked AMBE+2TM vocoder. technology. repeater (mode 3) configura­ Many of the radio man­ Digital Voice on VHF/ UHF tions. The use of the Tier I ufacturers have imple­ Both D-STAR and System Fusion were standard has been expanded mented the vocoder in designed specifically for the Amateur into for uses outside the licensed software, others Radio market. DMR, developed by the unlicensed dPMR446 service. use a DVSI chip. The for­ European Telecommunications Standards ward error correction in the Tie r II DMR occupies a Institute (ETSI), is used worldwide by AMBE+2 is an improve­ 12.5 kHz bandwidth shar·ed by professional and Amateur Radio users. ment of the voice quality of two channels using Time-Divi­ Currently, only lcom manufactures hand­ older technology vocoders. sion Multiple Access (TDMA). heids, mobiles, and repeaters for D-STAR. IP Site Connect (IPSC) for in­ Yaesu is the only manufacturer for System Networks terconnecting repeaters over Fusion handhelds, mobiles, and repeaters. Amateur MOTOTRBO the Internet is vendor specific There are over a dozen manufacturers and Hytera DMR net- and is not par·t of the ETSI Figure 1 - A DMR handheld producing DMR compatible equipment, transceiver can have no display, works operate the same standar·ds at this time. Most including Kydera (Figure l), Motorola So­ a monochrome display, or a color way from the end user Amateur Radio implementa­ display like this Kydera model. standpoint. A mateur lutions, Hytera, Vertex Standard, Connect [Image courtesy of Kydera] Systems, Baofeng Telecom Technologies tions of DMR use voice on MOTOTRBO networks (BFDX), Kirisun, and Ke9wood (although both time slots. are much larger, cover many more ar·eas, and most are intercon­ not distributed in the US). Tier ill builds upon Ti.er IT, adding trunking nected. Not all of the amateur DMR repeat­ operation involving multiple repeaters at a Amateurs have implemented DMR with ers ar·e connected to the wide area networks. single site. Not all manufacturers' trunking over 1400 repeaters and 16,600 user radios Some ar·e standalone, either because they implementations are Tier ill compatible. registered worldwide. A majority of the have yet to obtain ar1 ISP connection at their Vender specific protocols have expanded repeaters are interconnected over the In­ repeater site or because they just prefer to the trunking to multiple site operations. ternet. There are a number of US amateur use the repeater for local communications. regional and state networks, and most are Amateur Radio Implementations Some standalones are operating in dual­ interconnected with the DMR-MARC Amateurs ar·e implementing DMR Tier II mode· (analog/digital), which allows the (Motorola Amateur R adio Club) and in their MOTOTRBO™ and Hytera in­ repeater to support both digital and legacy DMRX core servers. frastmcture networks. The IPSC protocols analog users. MOTOTRBO repeaters oper­ D-STAR and System Fusion both in­ used by those different brand repeaters are ating in dual-mode do not support intercon­ clude the radio amateur's call sign in not compatible, and it is doubtful the equip- nection via the Internet using IPSC.

30 October 2015 RRL. the natio1 al association for Amateur Radioia www.arrl.org Playing in the Sandbox I Some hams have installed DMR repeat- ers in vehicles, relying on 3G/4G cellular iW Signal rl12 wireless services for Internet access. Oth­ Amplitude TS2 ers have implemented remote bases to TS1 TS2 interconnect to other networks or radios. TS1 Wide area networks typically have policies TS2 prohibiting interconnected traffic, however TS1 local implementations that stay local are acceptable. Policies prohibiting interconnection to dif­ ferent types of networks help keep large Frequency networks functioning. Network operators have expended much time and great efforts 12.5 kHz Occupied to keep a linked system running smoothly. Bandwidth QS 151O-Burn ingham02 DMR-MARC maintains a "sandbox" that is separate from the main DMR-MARC Figure 2 - The DMR signal occupies 12.5 kHz, which supports two time slot channels. network, which is available for persons in- terested in developing and experimenting.

DMR Equipment and Programming by lost packets. The Internet can also drop over two standalone repeaters to obtain two Early era amateur analog repeaters origi­ UDP packets used for moving u·affic be­ separate communication channels, since nated with surplus commercial radios. Over tween repeaters and bridges, causing the only one repeater and one antenna system time, equipment designed and targeted for same broken audio effect. Analog static is is required. The two-slot TDMA DMR up­ the radio amateur reached the marketplace. a thing of the past using any of the digital link uses 30 ms for each time slot (27.5 ms Today you can find used commercial DMR voice technologies. DMR implements For­ frame and a 2.5 ms gap). Your transmitter gear, but new DMR radios in the typical ward E1Tor Correction (FEC), which can is on for just 27 .5 ms of every 60 ms, result­ ham budget range ($165-$200) are readily slightly extend the usable communication ing in extended battery life for portables. available. range. The DMR repeater transmits (downlink) Most of the DMR radios require a pro­ Two-Slot TDMA a continuous data stream even if only one gramming cable to program the radio DMR Tier 11 and Tier Ill occtipies a 12.5 time slot is being used. The 2.5 ms uplink using the manufacturer's software, while kHz bandwidth shared between two time gap is replaced with a CACH burst (Com­ other radios support programming using slots (TS I and TS2) using Time-Division mon Announcement Channel) that is used Bluetooth and even over-the-air program­ Multiple Access (TDMA), resulti ng in a for channel management and low speed ming. FPP (Front Panel Programming) is spectrum efficiency of 6.25 kHz per chan­ signaling. The 27.5 ms frame comprises available on some models. nel (Figure 2). Each time slot can cany ei­ 264 bits (108 bit payload, 48 bit SYNC, ther voice and/or data. You can think of the and a second 108 bit payload). The vocoder Any Tier Ill capable radio will also work two time slots as separate channels. This compresses 60 ms of aud io including FEC on Tier lI systems but neither will work (forward e1Tor correction) into 216 bits of on Tier L means one repeater allows two separate channels at the same time. Currently most data for transmission. The 2.5 ms gap in­ Digital vs Analog amateur DMR repeater system implemen­ cludes guard time to allow power amplifier If you operate on analog FM repeaters, you tations utiliz.e both channels for voice and ramp up and for propagation delay. will have noticed that the audio quality de­ some limited text messaging. Typically Talk Groups grades as a signal into the repeater (uplink) one channel (time slot) is used for wide­ Talk Groups (TG) allow groups of users gets weaker. As distance to the repeater area and the second is local and regional. to share a time slot without distracting and increases, the signal from the repeater For repeater operators, a single two-slot disrupting other users of the time slot. Only (downlink) also gets weaker until you can TDMA repeater offers a significant savings one TG at a time can use a time slot on a no longer hear the repeater. A combination repeater. Unless your radio is programmed of the weak signal into a repeater and a to listen to a TG, you will not hear that TG weak signal from the repeater can rapidly traffic. The DMR-MARC MOTOTRBO degrade the audio quality and usability. Table 1 Some DMR-MARC MOTOTRBO network suppo1ts a number of Talk Groups A basic difference with digital repeaters Network Supported Talk Groups (Table l). Check with your local repeater is that the audio quality remains the same Slot Talk Group Description operator to find out what Talk Groups/ on the uplink and downlink until the very Time Slots are available on a repeater. TS1 TG1 World Wide edge of the coverage range. Then the audio TS1 TG3 North America Although the DMR standard supports on DMR systems starts sounding broken TS1 TG13 World Wide English · TS2 Local, State, private calls (one-to-one), encryption, and (missing portions of the speech) caµsed and Regional data, private calls are not allowed by most

QST"') - Devoted entirely to Amateur Radio www.ant.org October 2015 31 Table 2 Table 3 DMR Information Sources Recommended DMR simplex frequencies and radio settings: OMA-MARC (Motorola Amateur Radio Club) www.dmr-marc.net (Use TG99 / CC1 /TS1 /Admit Criteria: OMA-MARC Canada www.va3xpr.net/dmr-marc-canada/ Always I In Call Criteria: TX or Always) OMA-UK www.dmruk.net Digital Communications Interconnect Group (DCI) www.trbo.org Channel Fre~ , MHz DMRX www.dmrx.net Digital Mobile Radio Association VHF 1 145.790 (Professional DMRA) www.dmrassociation.org VHF 2 145.510 Amateur Radio Guide to Digital Mobile Radio (OMA) www.k4usd.org/guide.pdf UHF1 441 .000 UHF 2 446.500 Regional DMR Grou_,,.JJ_s _____ UHF 3 446.075 UHF 4 433.450 Central Michigan Emergency Network (CMEN MIS) w8cmn.net K0USY Group (Kansas) kOusy.strikingly.com K4USD Network www.k4usd.org Massachusetts Interconnect Team (MIT) www.mitcom.com can also contain a contact List of Radio IDs, NC-PAN (VA/NC/SCfTN) www.ncprn.net New England Digital Emergency Communication call signs, and names to be displayed. You Network (NEDECN) nedecn.org can find copies of con.figured code plugs on NJ-TRBO Network (NJ/NY/PA) www.n2jti.net NorCal OMA www.-norcaldmr.org the web for different radio models. [Search NOCO OMA Group (Northern Colorado) www.wOdmr.org "DMR code plugs" in your browser. - Rocky Mountain Ham Radio www.rmham.org Ed.]. Check out the different Yahoo! DMR groups. All DMR radios with an alphanumeric display support a limited number of en­ of the amateur networks. Many consider violation of FCC rules and regulations, and tries in the Contact List. Radios without private calls not amateur friendly, and enforcement action could be taken against an alphanumeric display do not support they can tie up a large number of repeat­ you. If you have a VHF model, you could Contact Lists. ers across the network. Encryption is not program a Zone for the National Weather Talk-Around and Simplex permitted in .Amateur Radio in most parts Service broadcast channels (again, in re­ DMR Talk-Around refers to simplex op­ of the world. Data and text messaging are ceive only). Zones are just a way to manage eration on a repeater output channel. This suppo1ted on some networks. large numbers of channels. allows direct communication while still TGs are implemented for individual states Color Codes being able to hear the repeater. Amateurs, and regions on many networks. Some DMR repeaters use Color Codes (CC) however, typically use dedicated simplex are avail able all the time, while others much like analog repeaters use cress or cbanne~s (Table 3) so as not to interfere are avail able at preprogrammed times DCS. To access a repeater, you must pro­ with repeaters. Do not use 146.520 or or require a local user to activate it for a gram your radio to use the repeater's CC. 446.000 MHz - they are the national period of time. Only one TG can be active There are 16 different CCs (CCO - CC 15). analog simplex channels. Operating DMR at a time on a time slot, so many systems The use of Color Codes is not optional on on these commonly used analog frequen­ will disable other TGs when a local user DMR systems. If your Color Code is not cies will just cause dishannony within the is active on a TG in the time slot. Be ham set correctly, you will not be able to access amateur community. Also, avoid repeater friendly and tty to use TGs that tie up the the repeater. inputs and outputs, locally used non-DMR fewes t number of repeaters ifyo u are going simplex channels, satellite subbands, and to be chatty. Further information about DIVIR Channels any other frequencies that could disrupt specific Talk Groups can be found on the A DMR radio channel is a combination other amateur communications. DMR-MARC, DMRX, and regional group of frequency, CC, TS, and TG. A single Admit Criteria websites (Table 2). repeater may occupy six or more pro­ grammed channels depending on the num­ The Admit Criteria determines when your Zones ber of TGs available. radio is allowed to transmit. The recom­ DMR radios support Zones. A Zone is a mended setting for repeater channels is gro.uping of individual channels. Some Code Plugs Color Code Free; this confi gures your radio models limit the number of channels A code plug is simply the radio con.figura­ radio to be polite to your own digital sys­ per Zone and the number of Zones allowed. tion file. You configure the channels and tem. You should configure your Jn Call You can program Zones for local chan­ operating parameters of a radio using a Criteria to Follow Admit Criteria. Simplex nels (DMR or analog), another Zone for a manufacturer's programming software. channels should be configured as Always neighboring state, and a Zone for business Upload this file to the radio and save a copy for both Admit Criteria and Always or Fol­ and government channels. If you program on your computer as a backup. You can low TX for In Call Criteria. non-amateur channels in your radio, make also download the code plug from a radio Accessing a DMR Repeater sure they are receive-only unless you are to modify it. Building a code plug can take licensed and authorized to use them as per many hours, especially if you want to pro­ You must have the frequency, CC, TS, FCC 90.427(b), otherwise you can be in gram hundreds of channels. The code plug and TG set correctl y to access .a DMR

32 October 2015 ARRL, the national association for Amateur Radio® www.arrl.org repeater. Keying your transceiver sends ing on one of the interconnected repeaters Repeater operators should also be support­ a signal to the repeater, and the repeater is relayed over the IP network to each of ing their bridge operators. Besides the cost responds to you with an acknowledgment the other repeaters. The Peers will first of the infrastructure equipment, there are that you can transmit your message. If you establish a connection with the Master also recurring monthly expenses for rent, do not receive an acknowledgment from and obtain the database of the other Peers utilities (power and Internet), insurance, the repeater, your radio will stop trans­ along with their IPv4 and port addresses. and maintenance. mitting and you will hear a negative The more repeaters in this fully meshed User Radios confirmation tone. This is an advantage IPSC network, the more IP network band­ of TOMA- allowing bidirectional com­ There are many sources for new and used width required for each repeater. To expand munications between user radio and the DMR radios, but as of this date, you can't beyond the limits of basic IPSC network repeater when transmitting. The DMR walk into an Amateur Radio store and buy requires the utilization of a b1idge to in­ repeater can signal your radio to stop a DMR radio. AU current DMR radios are terconnect the different IPSC networks. transmitting if there is contention on the professional (commercial) radios marketed Rayfield Communications (c-Bridge™) network because more than one station is primarily to commercial radio users. You and BridgeCom Systems (TL-Net) are transmitting at a time. can easily find a dealer if you want to the current preference in North America. purchase a new DMR radio for ham use. Not all DMR repeaters are interconnected SmartPTT is common in Europe. These Some dealers are "ham friendly" and will on the Internet. Some repeater operators bridges require static IPv4 addresses and offer reasonable discounts to hams. Check may just prefer to keep their repeater for larger network bandwidths than individual with other DMR users or on-DMR related local use, without connecting to the larger repeaters. websites for further information. regional and worldwide networks. The bridges suppo1t individual managers Search eBay, online flea markets, and ham­ IPSC and Bridges for each repeater (micro-segmentation), fests for both new and used radios. Here IP Site Connect (IPSC) is a vendor­ which is an improvement over having the are a few things to know before buying a specific repeater feature offered by some bridge connected to a network of IPSC DMRradio. man,ufacturers. MOTOTRBO repeaters repeaters. This reduces bandwidth require­ (see Figure 3) will interconnect over the ments and allows TG 'customization for New or Used Internet only with other MOTOTRBO re­ individual repeaters. The bridge manager Buyer beware of used DMR radios! Re­ peaters. Motorola Solutions MOTOTRBO can serve as either a Master or Peer on an member that you will not be able to repair IPSC implementation (not part of the IPSC network. a non-working DMR radio unless you ETSI Specification) allows up to 15 have the technical skills and necessary test Remember, someone is paying for all of MOTOTRBO repeaters operating in DMR equjpment. That test equipment can cost the infrastructure and monthly operating hundreds of times the cost of the radio. The mode to be connected on a fully meshed costs. Ifa club is operating your local DMR IPv4 network, with one of the repeaters street price for new DMR radios ranges repeater, join and support the operation. If from $165 - $200. Name-brand used or a bridge serving as a Master and all of an individual is operating the local repeater, the others are Peers. Any traffic originat- radios, such as those from Motorola or donate to support their ongoing expenses. Hytera, typjcally sell for more than brand­ new radios cost from newer entrants into the DMR market. You typically get what you pay for. Higher-priced radios usually have more features, are better constructed, and can handle more abuse than less expen­ sive radios. For the average amateur, one of the new lower-cost models makes a good initial radio. VHF, UHF, or, 900MHz. UHF is the most commonly used band in the US and worldwide for DMR, but because of military radar in some US areas VHF repeaters may be the local choice. There are ·few 902 - 928 MHz amateur DMR repeaters in the US. If you are purchasing UHF equipment, make sure it covers the amateur band (420 - 450 MHz) from the factory. No one currently sells a multi-band DMR radio in the US. Programming Software Some manufacturers supply programming Figure 3 - A pair of Motorola DMR repeaters. [Image courtesy of F1 ZOI repeater] software at no cost. Motorola Solutions

QST® - Devoted entirely to Amateur Radio www.arrl.org October 2015 33 charges around $260 for a 3-year subscrip­ keypad. MOTOTRBO repeaters support External Antenna on Portable tion (which covers all their models within an optional proprietary autopatch feature Not all portable radios support connection a region) to their software and updates. (Digital Telephone Interconnect) that to an external antenna, except for testing DMR radios, because they are professional works only with MOTOTRBO radios. and alignment. Using an adapter to connect radios, typically do not allow keyboard an external antenna can place undue stress GPS programming. If a vendor charges for the on the portable antenna connector, which programming software, do .not ask another OPS is available on some models, but may result in premature equipment failure ham to bootleg a copy for you. If you have DMR does not support APRS (Amatem and expensive repair. If you intend to use an an authorized copy, you may program Packet Reporting System). On professional external antenna adapter, I recommend an radi0s for others, but you cannot legally networks, one of the time slots is typi­ adapter cable that uses R0-17 4 size coax distribute the software. Software piracy is cally allocated for location reporting and cable to reduce stress on the radio connec­ against the law. is interconnected to server-based dispatch tor. Some MOTOTRBO models, such as applications. If enabled, GPS will shorten the XPR6000 series, support an external Some radios use standard USB cables for battery life. microphone with an antenna mounted on programming, others use cables that can the top. cost upward of $80. Bluetooth Higher-end radios may have Bluetooth Portables, Mobiles, Amplifiers Number of Channels built in for wireless headsets. Some radios Portables are available in the 2 to 5 W Radios can have from two channels to l 000 with Bluetooth support data and program­ range, and mobiles are available with a or more channels. You will need a channel ming via Blnetooth. Some models have maximum output of 10 to 45 W. I recom­ for each frequency, CC, TS, and TO com­ Bluetooth adapters optiqnally available. mend a handheld transceiver for your first bination. You can easily use six or more Bluetooth will shorten battery life if en­ DMR radio unless you live beyond hand­ memory channels for each DMR repeater abled. held coverage of your local DMR repeater. you program into yom radio. Analog If you spend significant time in your ve­ Displays and Keypads Most radio models support analog FM. hicle commuting, you will find the mobile Some radios have just a channel selector Current FCC rules require narrowband a good investment (Figure 4). You can also knob, while others include monochrome FM for most commercial/government use a mobile transceiver as a base station (Figure 4) or color displays (Figure 1) services. For DMR radios from some with the addition of an external power sup­ to show TO and ID information. Some manufacturers, this requires a · program­ ply. Many external amplifiers will not work displays show only channel numbers. For ming entitlement key or a different version with DMR radios unless they are specifi­ visually impaired operators, consideration of the programming software if you require cally designed to meet the fast switching must be given to the channel selection wider-band FM that is still used on many requirements ofTDMA on DMR. knob on the radios. Most of the non-display amateur analog repeaters. Batteries and Chargers models have channel selection knobs that Most radios include a wall-type charger have fixed stops rather than 360 degree Some manufacturers are di.scontinuing and offer a desk charger for optional pur­ continuous rotation to allow the operator their analog' non-DMR radios, while offer­ chase. You may want to consider a mobile to frnd channel one. Some LCD display ing the DMR radios at a reduced price if chai'ger or 12 V battery adapter. Some models also have fixed stops on the channel the digital mode is not enabled. On1the se models can be charged using a USB cable, selector knob, including some Hytera and radios, the customer can later upgrade if just-like cell phones. You should rotate CSJ radios. Many models offer program­ the radio to operate DMR their needs through at least one spare battery to be pre­ mable voice announcements. change, for an additional fee. pared for extended operation in emergen­ Some radios include a 12-button DTMF cies. Follow manufacturer instructions for initial charging for maximum battery life.

Warranty and Service Contracts If you buy an expensive new radio, you may want to consider the warranty and possibly purchase an extended service con­ tract. Few amateurs have the technology or skills to repair these radios. Programming Your Radio Your new radio must be programmed before first use. Check the DMR-MARC website for basic parameters that need to be configured to get your radio working correctly on the amateur network. Figure 4 - This Vertex Standard mobile DMR transceiver has a monochrome display. First, you need a subscriber ID. The [Image courtesy of Vertex Standard] DMR-MARC website issues all subscriber

34 October 2015 ARRL, the national association for Amateur Radio® www.arrl.or°g ( IDs for amateur users and repeaters. Click New Products - ---- on CONTACT US in the top right corner The CW Geek's Guide to Having of their page (www.dmr-marc.net) and Fun with Morse Code then follow the instructions. IDs are as­ The CW Geek's Guide to Having Fun signed based on your geographic location with Morse Code, by Dan Romanchik, (country/state). If you use an unauthorized KB6NU, is intended to help amateurs subscriber ID, you may find that you can't learn Morse code, choose the right key, access repeaters or the wide area network. get on the air and make CW contacts, and Your radio may stop transmitting while use abbreviations, Q signals, and prosigns you are talking because of contention on properly. The book is available in several the network or because you have travelled different fo1mats - PDF, Nook, or Kindle beyond the range of the repeater. Your e-book - from www.kb6nu.com. It's radio is receiving control information from also available as a paperback or Kindle the repeater while you are talking. Network e-book from www.amazon.com. Price: contention occurs when more than one sta­ e-book, $2.99 or paperback, $9.99. tion is transmitting at the same time on the same TS. Operating on DMR New Books Announce what Talk Group you are on when you make an initial transmission, Electronics from the Ground Up - or when you place a call to another sta­ Learn by Hacking, Designing & Inventing tion, or make a general call, because Ronald Quan, K/6AZB for a vacuum-tube Colpitts oscillator. The some users may be scanning or may have Reviewed by Rick Lindquist, WWJ Colpitts lives on in Quan's book, but in radios without a display. Please avoid call­ ME solid-statefo1m. By the way, Quan does not ing CQ. DMR is not like HF operation, Electronics from the Ground Up is just altogether ignore vacuum tube technology. and operating DMR over the network is what the title reflects. Th.is is a terrific self­ not DXing. training/teaching manual, or perhaps even A chapter you might not expect to find, a textbook for a high school or early col­ "Video Basics, Including Vi deo Signals," Talking on one of the wide area TGs ties lege electronics course. It's also a fun read discusses both standard-defi nition (480 up hundreds of repeaters. If you are unable for those times when you find yourself at lines) and high-definition (1080 lines) to move to a more localized TG, be con­ loose ends on a contest- television signals and siderate of the other users on the network. free weekend or feel the what the "i" as in 480i, While one TG is active, other TGs on the urge to tinker. and the "p" as in 720p same time slot will be blocked. Leave mean (i = interlaced; space between transmissions so others can The reader can jump in at ELECTRONICS p = progressive scan). break in. Remember that emergency traffic FROM THE any point. Quan starts out The following chapter . always has priority over all other traffic. at the component level and GROUND covers "Video Circuits See Table 2 for sources of more DMR takes the reader through and Systems." information. a j ourney that includes UP l<•m by Hacking. Dc51gnlng /5< Inventing some rather simple labo­ While Electronics from ratory-style experiments, the Ground Up may not --- -- __,___ the use of vaiious pieces stress RF circuits suffi­ First licensed in 1970, John, W2XAB, has been of test gear, half- and ciently to suit some radio involved with amateur repeaters since the early 1970s and currently teaches computer network­ full-wave power supplies amateurs, the p~incip les ing technology after having worked in the aero­ and progressing through · are the same. TI1is book space and telecommunications industry. You can reach John at 3110 Mount Zion Rd #1302 amplifiers and oscilla- offers a practical, hands- Stockbridge, GA 30281 and [email protected]. ' tors, AM and FM detectors, the principles on approach to learning of negative feedback, the application of or .reviewing electronics basics and some . filters, circuit analysis, and some of the advanced aspects, with a little bit of the For updates to this article, math thrown in along the way. A handy see the OST Feedback page at mathematics that tie all of these things www.arrl.org/feedback. together. reference appendix tells where to obtain electronic components. · One experiment I found interesting was one that lets you "hear" the signals from a McGraw-Hill Education (TAB), New York/ remote control unit - the kind that sends Chicago/San Francisco, 2014. ISBN: a series of light pulses with frequencies in 978-0-07-183728-6, softcover, 7.2 x 9.2 the audio range. Older radio aniateurs may in, 544 pp. Available from Amazon.com, well recall having to learn the schematic softcove1; $22.68, Kindle edition, $16.50.

QST® - Devoted entirely to Amateur Radio www.arrt.org October 2015 35