Chapter 18
No matter what interests you in ham radio, from ragchewing to equipment design, you’ll get more out of the hobby if you have a basic understanding of a few technology details. If you want to dive in a little deeper, your license study guides will get you started. The ARRL Handbook and The ARRL Antenna Book have been reliable references for many years. Online you can use the ARRL Technical Information Service (www.arrl.org/technology
) that is available to all hams. Appendix B of this book is a radio math supplement that provides some common math formulas you’ll encounter in ham radio.
You should know each of the basic electrical units and what they represent:
The most basic relationship in electronics and radio is Ohm’s Law, which states that the voltage (V, in volts) across a resistance (R, in ohms or Ω) is proportional to the current flowing through it (I, in amperes). Mathematically it looks like this, shown three different ways:
V = I × R |
I = V / R |
and R = V / I |
The first version is why the voltage across a resistor is sometimes referred to as the IR drop. Voltage can also be represented by E.
Another fundamental equation is the Power Equation that shows power (P, in watts) being used or dissipated by a device or component is equal to the voltage across it (V, in volts) multiplied by the current through it (I, in amperes): P = V × I. (Voltage can also be represented by E.)
By substituting the Ohm’s Law relationships for V and I, we also get:
P = V2 / R |
P = I2 × R |
When calculating power in an AC circuit, use the RMS (root-mean-square) voltage measured by a voltmeter.
Transmitter peak envelope power (PEP) is the power measured at the very highest peak of the RF waveform. If you use a wattmeter, be sure it is calibrated to show PEP and not average power.
The decibel (or dB) was introduced in Chapter 12. The decibel is used to measure or specify the changes in signal level corresponding to:
Bandwidth is a range of frequencies over which a circuit or signal behaves in some specified way. Figure 18-1A shows a typical signal occupying a channel. The level of the signal is shown by the heavy black line. Two vertical lines show where the signal level is one-half that of the signal’s peak level. The frequency range between the vertical lines is the signal’s bandwidth. Similarly, Figure 18-1B illustrates filter bandwidth. The heavy black line shows the filter response, meaning how much signal is passed or removed by the filter. This notch filter removes signals over a range of frequencies. The range over which the signal coming out of the filter is one-half or less of the signal going in is the filter’s bandwidth. The bandwidths of other filters and signals are measured similarly.
A circuit that intentionally increases or decreases a signal’s strength based on its frequency is a filter. (There are other types of filters but this is the most common type.) Ham radio uses a lot of filters! Most are of four common types:
To describe how an antenna focuses a signal from desired directions or rejects signals from unwanted directions, an antenna pattern diagram is used. Figure 18-3 shows the two basic types of antenna patterns. Some common terms related to antenna patterns include the following:
You don’t have to be a radio engineer to use SWR as an indicator for tuning and troubleshooting antenna systems. It basically represents power bouncing around in your feed line until it is radiated by the antenna or dissipated as heat. Lower SWR is good (1:1 is the best), but you don’t have to get too worked up over it. Most transmitters are perfectly happy with SWR values of up to 2:1. For moderate SWR, you can use an antenna tuner. Big changes in or very high values of SWR can indicate a bad connection or other problem.
As SWR increases, so does the amount of signal lost in the feed line since some is lost on each trip back and forth. The additional feed line loss caused by SWR can become significant on the upper HF bands and at VHF/UHF and microwave frequencies.
You’ll encounter the following terms when discussing SWR and how to measure it:
SWR = (Pf + Pr) / (Pf – Pr)
There sure are a lot of batteries! You need to know which ones your radio needs as well as how (and whether) to charge them. Start by learning a few terms for when you start shopping:
For complete information on batteries, battery chargers, and other related topics, check out Battery University (batteryuniversity.com
).
You don’t have to be a rocket scientist to know when a satellite is visible to your radio signals and where it will be in the sky. There are terrific tracking programs available from AMSAT (www.amsat.org
) and other sources. (Websites like Heavens Above [www.heavens-above.com
] show you where the satellites are at any given time, as well.)
To use tracking software, you need a set of data for each satellite called its Keplerian elements. These values describe the satellite’s orbit with enough precision for you to known when and where to aim an antenna at the satellite. Here is a set of “Keps” for recently launched amateur satellite AO-91:
AO-91
1 43016U 17073D 17324.81992359 .00000855 00000-0 73563-4 0 9991
2 43016 97.6901 259.8749 0259544 229.6727 128.1552 14.77757019 356
This is the two-line NASA format. Each piece of data is defined as:
KEY: A-CATALOGNUM B-EPOCHTIME C-DECAY D-ELSETNUM E-INCLINATION F-RAAN
G-ECCENTRICITY H-ARGPERIGEE I-MNANOM J-MNMOTION K-ORBITNUM Z-CHECKSUM
Many tracking software packages can go online and acquire this information automatically without you having to type it in.
You’ll need to be familiar with the following terms to know when to begin listening for the satellite and how to adjust your receiving frequency as the satellite moves past your location: