Radio in general is a technologically advanced field, and those who have obtained their ham radio licenses can tell you that learning some of that technology can be challenging. We've collected some links that can help you understand both the basics behind radio and some of the in-depth engineering that goes into the physics of radio, along with ways to improve your setup.

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Study and Licensing education is the topic of another page (including our slide show PPTs for the courses)
Repeater education is a sub-topic of another page
DIY education is a sub-topic of another page
The ham radio glossary is on another page
Training topics for nets are on another page
Morse code / CW education is a sub-topic of another page
History is a sub-topic of another page
Outdated educational is a sub-topic of another page
Satellite, ISS, solar, and space communication is a topic of another page
Troubleshooting is a topic of another page

Quick Links

Just starting out in ham radio Antennas [general, VHF-UHF, HF] Grounding and bonding Propagation [general, solar, other] Signal transmission [tuners, transmission lines, reflection, return loss] SWR [education, effects, myths, other] Connection [coax, connectors, wires, loss] Technology Chokes, filters, baluns Digital Issues [noise, shack RF, damage, other] Good-to-know [general, tips, how-to, best practices, other] Solar [general, panels, charge controllers, batteries, other] Repeater [general, isolation, other] Specialized Electrical [electricity, batteries, circuits, electronics, advanced, other] For beginners [electronics, electromagnetics, RF, other] Tubes [theory, amplifiers, other] Presentations Sane preparedness Safety [general, medical, other] Advanced topics Antennas PhD required Physics [general, electrodynamics, other] Other Debunked myths Other [glossaries, Hallas, other]

Just starting out in ham radio

Taking your first ham radio steps (by Noji)
Beginner's Guide to Ham Radio (TechWholesale)
New ham tips (SB-ARC)
New ham podcast (ARRL)
Ham Radio for Beginners (Massachusetts Community Colleges)
A telecommunications timeline (brief, non-amateur history)
DX Code of Conduct
The full ham radio experience (by Noji)

Antennas

General

Antenna basics
Understanding antenna resonance (by Noji)
Antenna gain (by Noji)
How antennas radiate (PDF)
Antennas and radiation patterns, Part 1
Antennas and radiation patterns, Part 2
What is angle of radiation?
Vertical vs. Dipole Angle of Radiation (important to understand)
Why (some) antennas look the way they do (IW5EDI)
Antenna education (Tom Rauch W8JI)
Antenna theory (K9ERG)
More than you ever wanted to know about antennas [PDF]
Understanding Antennas for the Non-technical Ham (N4JA)
How Yagi beams work (non-technical, no-theory)
How a Yagi antenna works (by Noji)
There's no free lunch (ARRL - radial-less verticals, miniature HF antennas, wild gain claims, etc.)
Open-wire Feed Lines (QST PDF)
Ground planes (by Noji)
Non-resonant lengths (by Kurt Sterba, K5JIB - too long = inductive, too short = capacitive)

VHF / UHF

How ground plane antennas work
Why 5/8-wave is near-optimal
Why a 5/8-wave vertical must have a counterpoise [toward the end of the page](Tom Rauch W8JI)
Do tiger tails work? (yes, in most cases) (01-24-2016 by Mike Kaiser K4ISR)
Laboratory study of tiger tails (big improvement in some cases) (10-21-2018 by John Huggins KX4O)
How J-pole antennas work (Wiki)
Why J-pole antennas fed by coax must have a choke balun [feedline radiation](Wiki)
More on J-pole antennas
Even more on J-pole antennas (IW5EDI)
Technical analysis of J-pole antenna performance (Tom Rauch W8JI)
Technical argument on why a J-pole should not be grounded (John S. Huggins, 01-28-2012) and a follow-up discussion on masting J-poles (John S. Huggins, 02-27-2015)
How high to mount a ground plane antenna (higher than 1.5 wavelengths)
Thoughts on 5/8-wave antenna construction (ON6MU)
How close to mount two VHF antennas (10 dB/ft vertical proximity, 1 dB/ft horizontal proximity)
Video on 2 meter light bulb dipole [YouTube]

HF

Dipole

Your first HF dipole antenna
How high to mount a dipole antenna (around 0.4 wavelengths for local, higher than 1.0 wavelengths for DX)
Another article about dipole antenna height (between 0.15 and 0.20 wavelengths presents a 50 Ω impedance)
Dipole feed point impedance (73 Ω)

End-fed

Understanding Zepp and other end-fed antennas (Tom Rauch W8JI, excellent)
An end-fed antenna must have a counterpoise (Tom Rauch W8JI, excellent)
An end-fed halfwave antenna must have a counterpoise (by Steve Yates AA5TB)
How an end-fed antenna can work without a counterpoise (by Steve Yates AA5TB)
Discussion about end-fed antennas presentation [PDF]
Can an end-fed antenna without a ground connection (counterpoise) work? (Yes, but betwen 0.10λ and 0.40λ is best)

Radials

Effect of radials on HF vertical antennas (good article — the Hustler 5-BTV and 6-BTV in particular)
Followed these guidelines to install my Hustler 5-BTV
The purpose of radials (Part I)
The purpose of radials (Part II)
Design of radial ground systems (Rudy Severns, N6LF)
Optimal radial lengths (more radials are better than correct lengths)
The myth of 120 radials (Tom Rauch W8JI, excellent), plus
Radials should be insulated, not bare
Radials should be at least as long as their short verticals are tall
Radials should come in direct contact with Earth, while counterpoise wires should not

Random-wire / long-wire

Understanding long wire / random wire antennas (Tom Rauch W8JI)
Random wire antennas (and more)
Understanding and simplifying random wire antennas (also called long wire antennas)
Best random wire lengths

NVIS

Understanding NVIS antennas (Pat Lambert WØIPL)

Other

The isotron antenna (and an analysis [PDF])
The doublet antenna (all HF bands, requires a tuner) (another and yet another and more still)
Rhombic antennas (Tom Rauch W8JI)
The cubical quad (WA2OOO — poorly written, but contains many facts)
The inverted-L (L.B. Cebik W4RNL)

More

Amplifiers vs. antennas for better HF
If you're not being heard, chances are you need a better antenna or location, not an amplifier
How to read a cross-needle SWR meter
Traps (Tom Rauch W8JI — very good and in-depth)

Grounding and bonding

Understanding shack grounding and bonding in your house (see also station grounds and ground systems) (Tom Rauch W8JI)
Grounding myths, facts, and solutions (excellent article — must read)
ARRL's common-sense take on grounding
Articles on lightning protection (list of links on ARRL)
Complete lightning protection begins with your home (QST, June 2017)
RF Grounding [YouTube]
Station grounding — will we ever be able to dispell the myths? (excellent article by Steve Katz, WB2WIK)
Supported by Grounding and Bonding for the Radio Amateur page 5.3 and Reflection III, Figure 5-1 caption
And Desk Radio Equipment Ground (Tom Rauch W8JI)
The un-ground and resonant ground mitigation (02-05-1997)
Motorola / BLM R56 Manual [PDF]
Collection of links [GitHub]
Grounding and Bonding for the Radio Amateur (Ward Silver NØAX) [Amazon] and Errata for the 2nd Edition (ARRL)
NEC 810 (grounding and more, for the amateur antenna system)

Propagation

General Skywave propagation (by Noji) Skywave propagation nuts and bolts including ionization and recombination (Brittanica) How far can two-way radios communicate? Understanding 10-meter Sporadic E (excellent article) Solar Understanding solar indices (ARRL - excellent article) Quick Guide to HF Propagation Using Solar Indices (what it all means) How the Sunspot Cycle affects radio propagation (by Noji) Sunspot Cycle 25 forecast (possibly the best ever, 06-27-2020) Other How weather affects radio propagation (it can affect the conditions, but no set answer fits all) Essentially, rain , snow, and fog typically affect frequencies over 2 GHz very heavily, while affecting frequencies under 30 MHz very little On the other hand, lightning is the single largest contributor to atmospheric noise Furthermore, temperature inversions can result in tropospheric ducting, which typically affect frequencies greater than 90 MHz

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Signal transmission

Tuners

Antenna tuner basics (1994 ARRL PDF)
How a tuner "tunes" (excellent General-level article, including the math, by Davide Achilli IZ2UUF)
Video of the same, Part 1 (excellent for the lay person, by the same author)
Video, Part 2 (excellent for the lay person, by the same author)
Tuners (by Noji)
Tuner effect at the antenna (W5DXP)

Transmission lines

Coaxial cable dissected (Owen Duffy, 06-25-2019)
Coaxial cable and shielded wires (Tom Rauch W8JI)
Feedline discontinuity analysis (W5DXP)
Coaxial cable leakage (Tom Rauch W8JI) and another (Charles Brainard, WA1ZRS, QST April 1981)
Online coax loss calculator (VE3SQB) and a copycat calculator (CO8TW) and yet another (more comprehensive, by KV5R)
TLD (Transmission Line Details by AC6LA)
My feedline Tunes My Antenna! (SWR is constant in a feed line, 03-1956, by Byron Goodman, W1DX)

Reflection

Complete understanding of reflected power (book Part III by Walter Maxwell, W2DU) and a link on my website to his book and some reviews
Page 1-2 : All power reflected from the feed line-to-antenna mismatch is conserved, not dissipated
Page 1-5 : Reflected power is not dissipated in the transmitter
Page 1-9 : A tuner allows for greater bandwidth for a given antenna
Page 7-3 : Why match at the feed line input?
Page 7-3 : A transmitter can be damaged by overloading or underloading
Page 13-3 : The antenna tuner really does tune the antenna to resonance
Page 17-1 : A tuner changes the antenna impedance by conjugate matching
Page 17-2 : Feed line length does not matter if using a tuner
Page 17-3 : The logical place for a tuner is at the operating desk, not at the antenna
The myth of reflected power (excellent General-level article, including the math, by Davide Achilli, IZ2UUF)
Is your tuner only to "fool" the transmitter or to "make it happy"?
Copy of the permission email to use this material
The myth of high SWR causing feed line radiation (excellent General-level article, including the math, by Davide Achilli, IZ2UUF)
Conjugate match myths (Steve Stearns, K6OIK, 10-16-2011) [PDF]
Reflected power will NOT damage your radio and his original page (good reading, but sloppily written, by Steve Ward, WC7I)
SWR and reflection (Darrin Walraven K5DVW, QST 11-2006)

Return loss

Relationship of relection coefficient and VSWR (Ron Hrnac, Broadband Library)
Real-world transmission lines and return loss (Owen Duffy, 03-11-2020)
Facts about real-world transmission lines with loss (PDF by Steve Stearns, K6OIK, 10-17-2010)
Description (Wikipedia)
Answer on QRZ (Karl-Arne Markstrom, SMØAOM, 07-20-2020)

SWR

Information on SWR can be categorized under antennas, transmission lines, and reflections, among other things, but is a topic often singled out for discussion, and so deserves its own section

Education

SWR Demystified (AD#28) (30:22 YouTube™ video, by Dave Casler KEØOG) [excellent, includes the math]
SWR basics [PDF]
Understanding SWR by Example [ARRL, QST Nov 2006, PDF]
SWR article (needs revising) and SWR presentation (PPT by Noji)
SWR and reflection (Darrin Walraven K5DVW, QST 11-2006)

Effects

Myths

The myth of high SWR causing feed line radiation (excellent General-level article, including the math, by Davide Achilli, IZ2UUF)
SWR myths and mysteries [PDF] (Andrew Barron, ZL3DW, 09-2012)

Other

Connection

Coaxial cable

Characteristic impedance calculation (Huber+Suhner AG, p. 16, 07-2007)
Why differential-mode currents are on the braid interior (StackExchange, Phil Frost, W8II)

Connectors

Wires

Common coax chart
Bandwidth per wire gauge (L. B. Cebik W4RNL, QRP Quarterly 1997)

Loss

PL-259 vs. N on 430 MHz (Davide Achilli, IZ2UUF) [about the same]
Insertion loss of SMA, N, TNC, and BNC connectors to 500 MHz (John Huggins 08-06-2011) [very little loss among all of them]
The PL-259 / SO-239 connectors, on the other hand (John Huggins 08-06-2011) [inferior to N and SMA connectors]

Technology

Basics of AM and FM (by Noji)
How coax is 50 ohms (by Noji)
Why 50 Ω ? (arithmetic mean compromise between lowest loss and power handling for air dielectric)
Why 50 Ω ? (compromise between voltage tolerance and power handling for telephone cable)
Understanding ALC (Rob's web - PA3CJD)
Understanding Superheterodyne radio receivers (for the lay person)
Superheterodyne radio receivers (excellent book excerpt)

Chokes, filters, baluns

Baluns 101 (by Noji)
Using a 1:1 balun to mitigate common-mode radiation (3:01 YouTube™ video, by Davide Achilli IZ2UUF)
Understanding proper baluns (excellent article, by Charlie MØPZT)
The OCFD balun (excellent article by Rick Westerman DJØIP)
Understanding baluns (excellent article) see also The Ugly Balun (good article, but poorly written)
Baluns: What They Do and How They Do It (ARRL, Roy Lewallen W7EL) [PDF] and on EZ-NEC
All About Baluns: Ask Dave #73 (Dave Casler) [YouTube 25:47]
Fundamentals of filters (by Noji)
Choke effectiveness of different ferrites (Steve Hunt G3TXQ) [frequency graph]
Understanding common-mode and how to choke it (excellent article by Serge Stroobandt ON4AA)
Common-mode Chokes (by Chuck Counselman W1HIS, 04-06-2006) [PDF]

Digital

Getting started with digital modes [YouTube]
Digital radio and the openSPOT hotspot [Dave Casler on YouTube 15:28]
Fundamentals of direct sampling (by Panoradio)
Comparing D-STAR, DMR, and System Fusion (by Mike Myers, K3DO)
Digital radio waves (by Noji)

Issues

Noise

Understanding noise, noise sources, and how to eliminate or reduce noise (by Noji)
Phase noise (excellent article)
Common-mode noise (Tom Rauch W8JI)
Understanding noise and power line noise (technical, by Tom Rauch W8JI)
House appliance RFI (video by Josh Nass, KI6NAZ) [Youtube 13:26]
Common-mode Chokes (to decrease receive noise) (by Chuck Counselman W1HIS, 04-06-2006) [PDF]

Shack RF

Solving shack RF from a doublet (Tim Hier, G5TM, 01-05-2020) [YouTube™ 5:24]

Damage

MOSFET avalanche and parasitic BJT latching (page 7, Abdus Sattar of IXYS)
MOSFET damage due to both the open-circuit case and the low-impedance case (StackExchange, Phil Frost, W8II)
He also mentions that SWR will not change with transmission line length

Other

Understanding intermodulation distortion (IMD) (Ed Hare, KA1CE, QST, 08-1996) [PDF]
Intermodulation distortion (by Noji)
HT blocking and desensitization (YouTube 9:35 by Richard Newstead, 12-28-2019)

Good-to-know

General

How far one of these things can transmit (by Noji)
Repeater fundamentals (by Noji)
Repeaters and how to use them
Radio football (Understanding the Fresnel Zone by Noji)
The magic of 6 meters
Measuring quarter-wave and half-wave coax stubs (with velocity factor calculation)
Understanding transmission lines for amateurs (Tom Rauch W8JI, excellent)
Wind load explained (rough but good)
Thoughts from ARRL on Anderson Powerpole™ connectors [PDF]
Install Anderson™ Powerpole connectors on YouTube™ (9:40)
Using alternate phonetics (by Noji)
Line-of-sight calculator (everything RF)

Tips

How to quiet a noisy receiver by AB4OJ
Mic gain and compression tips by W6LG on YouTube™ (14:53)
RF feedback and its prevention (Elecraft, Nov 1999)
RF feedback discussion (ARRL - good Q&A article)
Selecting your feed line (by Noji)
Why get a better antenna and coax (by Noji)
Radio preparation prior to an emergency (by Noji)
What steps to take during an actual emergency (by Noji)
How to tie bare wires together by thaitrick on YouTube™ (6:08)
Mobile radio installation considerations (technical, by Jim Aspinwall NO1PC)

How-to

How to use a waterfall (Icom IC-7300) by Eric of HamRadioConcepts on YouTube™ (14:58)

Best practices

Don't repeat your entire call sign by Callum of DXCommander on YouTube™ (5:22)
EmComm best practices by Marin Amateur Radio Society on YouTube™ (6:08)

Other

FT8 on 10 meters, on a budget (Tom Costello KD9CPB)
Signal attenuation due to vegetation (graph photo from ITU)
2-meter example through a leafy, 22-foot diameter tree:
At 146 MHz, attenuation for a vertical antenna is about 6 × 10^-2 dB/m, or
−0.06 dB/m = 1 − 10^-0.006 = 0.0137 attenuation/m = 1.37 % att/m
For a 7 m diameter tree with thick leafage : 7 m × 1.37 % att/m = 9.6 % att
5-watt HT : 4.5 watts get through / 20-watt mobile : 18.1 watts get through

Solar

General

Solar power (by Noji)
Calculating your solar setup (BatteryStuff)

Panels

How Solar Panels Work (UCS, 09-11-2015)
How Solar Cells Work (HowStuffWorks)
How Does a Photovoltaic Cell Work (planetEenergies)
Monocrystalline vs. Polycrystalline which is better? (Mono is, but more expensive.)

Charge controllers

What is a Solar Charge Controller (altEstore, PWM vs. MPPT)

Batteries

How to choose the best battery for a solar energy system (EnergySage, 05-15-2020)

Other

The photovoltaic effect (PVEducation.org, 2019)

Repeater

General

Isolation

Repeater performance and receiver / transmitter isolation (Mike Morris, WA6ILQ) [must-read for serious repeater considerations]
How duplexer cavities work (by John Portune, W6NBC)

Other

Specialized

Understanding IMD (excellent article)
Understanding ERP (effective radiated power) [more] (KB6NU)
Understanding radiation resistance (Tom Rauch W8JI)
Radiation resistance vs. ohmic (loss) resistance (short but good article)
Radiation resistance (very technical article)
Understanding CW bandwidth (Tom Rauch W8JI)
Understanding image response by KA7OEI (indepth and very well-written)
Voting repeater systems (excellent article)
Legal setup of crossband repeaters (ARRL - click How can I legally make my VHF/UHF station into a crossband repeater?)
Understanding desensitization (technical article from a smartphone perspective)
Background behind CTCSS, DTMF, and PL tones (in-depth, somewhat historical)
Background behind CTCSS, DTMF, and PL tones (PDF version of the PPT version of a summary of the previous link)
ALC adjustment procedure by Alan Applegate KØBG (well-documented)
How we mis-use ALC by Leif Asbrink SM5BSZ (technical, well-documented)
Don't fuse the mobile negative battery terminal (Tom Rauch W8JI)
Image modes (analog photos and video on ARRL)
Q65 description (WSJT-X digital mode, 12-10-2020) and Quick Start Guide (data sheet)

Electrical

Electricity

Understanding voltage (excellently written lay article)
What Is Voltage in a Battery? (Sciencing)
Understanding current (excellently written lay article, including current density and velocity)
The speed of electricity (excellently written lay article, including electron velocity, drift velocity, and signal velocity) [Dr. Christopher Baird]

Batteries

How batteries work (by Noji)
How a battery stores energy (doesn't store charge) [Kenneth Buckle, 05-29-2006]
Rechargeable battery chemistry (Lead-Acid and NiCd) [06-05-2019]
Non-rechargeable battery chemistry (alkaline, zinc-carbon, mercury) [LibreTexts, 06-05-2019]
Battery chemical composition (lists half-reaction potentials - excellent lay article) [science.org, 03-06-2018]
Disposable (non-rechargeable) battery comparison video (ProjectFarm) [YouTube™ 10:17]
BMS (battery management system)
Why battery internal resistance is important (Painless360) [YouTube™ 13:52]

Circuits

Kirchoff's Laws (ScienceFacts)
Residential electrical circuits explained (HomeAdvisor)
KiCad (free online schematic drawing tool)

Electronics

How transistors work (by Noji)
Introduction to diodes and rectifiers
Filter fundamentals (by Noji)

Advanced

Download LTspice™ (circuit simulator from Analog Devices) and
Electrical length (complete transmission line math)

Other

Spark-gap transmitters (by Noji)

For beginners

Electronics

Electronics for Dummies (book - Wylie, 2005)
Grob's Basic Electronics (book - Mitchel E. Schultz, 2011)
Basic electronics (Instructables - Circuits)
Basic DC circuits (Electronics Tutorials)
Capacitance (1943 Navy training video [YouTube 25:43])

Electromagnetics

Electric potential (simple physics)

RF

Introduction to RF (Huber+Suhner AG, p. 8, 07-2007)

Other

How transistors work (fun video that explains n-type and p-type materials [Facebook 5:46])

Tubes

Theory

How vacuum tubes work (by Noji)

Amplifiers

Loading and adjustment tips (Tom Rauch W8JI)
Using a 'pecker' to tune up on high PEP (by John Tait)

Other

Presentations

The ZS6BKW antenna [PDF] and YouTube (26:12)
End-fed secrets [PDF] (Yuma 2019 Hamfest, on dipole, end-fed, Zepp antennas)
Hertz's experiment (YouTube™ video 4:09 by Ludic Science) and slide show of the same (PPT by Noji)
SWR (PPT by Noji)

Sane preparedness

Solar 1234 (Steve Harris Radio Show)
EMP (by Noji)
Home Faraday cages (by PREPSTEADERS)
Effective Faraday cages (by Dr. Arthur Bradley)
Whole-house EMP protection (by Dr. Arthur Bradley)
Emergency communication training (ARRL)
Handyman Tips (by Noji)

Safety

General

FCC statement on RF safety (no evidence of risk)

Medical

ARRL's take on pacemaker safety

Other

FCC RF Safety FAQ

Advanced topics

Antennas

Vertical antennas and real-world radial analyses (technical, in-depth)

PhD required

Ultimate antenna gain definition [PDF] (Roger F. Harrington, 1959)
RF Field Exposure (NIH)
The Construction of Electromagnetism (Lund University, 02-13-2020) [PDF]

Physics

General

Electrodynamics

Maxwell's Equations and on Wikipedia

MathJax reference (math formatting)
1. Gauss's Law and on Wikipedia
Charge creates a proportional electric field
2. Gauss's Law for Magnetism and on Wikipedia
The net outflow of a magnetic field through a closed surface is zero
3. Faraday's Law of Induction and on Wikipedia
A changing magnetic field will induce a voltage in a conductor exposed to the field (Faraday's Law)
A changing magnetic field will produce a changing electric field (Maxwell-Faraday Law)
4. Ampere's (Circuital) Law and on Wikipedia
An electric current will produce a magnetic field (Ampere's Law)
A changing electric field will produce a magnetic field (Maxwell's addition)
(A changing magnetic field will produce an electric field)
(A changing magnetic field will produce a current in an exposed conductor)
Biot-Savart Law and on Wikipedia
An electric current creates a proportional magnetic field
Lenz's Law and on Wikipedia
A current that is induced in a conductor by a changing magnetic field will oppose changes to the magnetic field that produced it
Coulomb's Law and on Wikipedia
The force of one point charge acting on another point charge is proportional to the inverse-square of the distance between them
Electromagnetic wave equation
Second-order partial differential equation that describes the propagation of electromagnetic waves through a medium or in a vacuum

Other

Other

CW bandwidth (technical, in-depth, by Mark Amox W8XR)

Debunked myths

Folded dipole myths (Tom Rauch W8JI)
Busting old myths surrounding end-fed half-wave antennas (they do work : by Peter Waters G3OJV)[YouTube 13:10]
The Myth of RF Ground (Ward Silver, NØAX, QST, 01-2015)
The myth of reflected power (excellent General-level article, including the math, by Davide Achilli, IZ2UUF)
Copy of the permission email to use this material
Coaxial cable length does not change antenna SWR (by Gordon Hudson, AD5GG)
My feedline Tunes My Antenna! (03-1956, by Byron Goodman, W1DX)
He also mentions that SWR will not change with transmission line length
Collection of busted myths (ARRL, Terry Graves, K7FE, 2009)
SWR myths and mysteries [PDF] (Andrew Barron, ZL3DW, 09-2012)
Modern batteries are safe to store on concrete

Other

Ham radio glossaries

The Ultimate Ham Radio Glossary
Amateur Radio Dictionary Wiki
ARRL glossary
AC6V glossary
QRZ glossary
Yet another glossary (radio wave and solar)

Joel Hallas articles in QST

Former ARRL technical editor (W1ZR), in his columns Getting to Know Your Radio

Receiver Gain Control (June 2006, p. 59-60) [PDF]
Fighting Interference and Noise with Filters (Oct 2006, p. 65) [PDF]

Other

OCF HF antennas The Windom antenna (Tom Rauch W8JI) The G5RV antenna (Tom Rauch W8JI) The G5RV antenna (and more details) (WØBTU) Split operations APRS SolarHam HamSCI (amateur radio for university students) RF Wireless World (good, educational material that's not strictly amateur) Do not use Belden 9913 for repeater (email from John Lloyd to Richard Bateman, 12-04-2017) Don't use Belden 9913 or LMR cable for repeater (Mike Morris WA6ILQ, 1995) [due to dissimilar metals]

General atmospheric opacity chart