Extra Class question of the day: the RC time constant

When you put a voltage across a capacitor, current will flow into the capacitor and the voltage across the capacitor will increase until the voltage across it reaches the value of the supply voltage. This is not a linear function. By that I mean that the voltage will increase quite rapidly at first, but the rate of increase will slow as time goes on.

To see how this works, let’s consider the RC time constant. The time constant of an RC circuit is equal to the resistance in the circuit times the capacitance, or simply R x C. For example, the time constant of a circuit having two 220-microfarad capacitors and two 1-megohm resistors, all in parallel is 220 seconds. (E5B04)

The equivalent resistance of two 1 MΩ resistors in parallel is 500 kΩ. The equivalent capacitance of two 220 μF capacitors in parallel is 440 μF. The time constant is RxC = 440 x 10-6 x 500 x 105 = 220 s.

One time constant is the term for the time required for the capacitor in an RC circuit to be charged to 63.2% of the applied voltage. (E5B01) Similarly, one time constant is the term for the time it takes for a charged capacitor in an RC circuit to discharge to 36.8% of its initial voltage. (E5B02)

The capacitor in an RC circuit is discharged to 13.5% of the starting voltage after two time constants. (E5B03) Similarly, a capacitor charges to 86.5% of the applied voltage after two time constants.  After three time constants, a capacitor is charged up to 95% of the applied voltage or discharged to 5% of the starting voltage.

You can use these percentages to answer the questions about how much time it takes for a capacitor to discharge. The key is to figure out what percentage the voltage given is of the starting voltage. In one case, the starting voltage is 20 V and you must figure out how much time it will take for the capacitor to discharge to 7.36 V.

Well, 7.36 V just happens to be 36.8% of 20 V, so the time required will be one time constant. One time constant is R x C, or in this case 0.01 x 10-6 x 2 x 106, or .02 s. So, it takes 0.02 seconds for an initial charge of 20 V DC to decrease to 7.36 V DC in a 0.01-microfarad capacitor when a 2-megohm resistor is connected across it. (E5B05)

In the second case, the starting voltage is 800 V and you must calculate the time required for the voltage across the capacitor to drop to 294 V. Well, fortunately, 294 V / 800 V is again 36.8%, so the time required will be one time constant.

In this circuit, R = 1 MΩ and the capacitance 450 μF. R x C = 106 x 450 x 10-6 = 450 s. So, it takes 450 seconds for an initial charge of 800 V DC to decrease to 294 V DC in a 450-microfarad capacitor when a 1-megohm resistor is connected across it. (E5B06)


Extra Class question of the day: filters and matching networks


A high-pass filter can be made using two capacitors connected between the input and output and a shunt inductor. The cutoff frequency depends on the values of the components.

This particular filter has the characteristic of being a high-pass filter. That is to say it will pass frequencies above a certain frequency, called the cutoff frequency, and block frequencies below that frequency. A T-network with series capacitors and a parallel shunt inductor has the property of it being a high-pass filter. (E7C02) The reason the circuit acts this way is that as the frequency of a signal increases, capacitive reactance decreases and inductive reactance increases, meaning that lower-frequency signals are more likely to be shunted to ground.

A circuit containing capacitors and inductors can also form a low-pass filter. A low-pass filter is a circuit that passes frequencies below the cutoff frequency and blocks frequencies above it.

The circuit shown in figure E7C-2 is called a pi filter because it looks like the Greek letter pi. The capacitors and inductors of a low-pass filter Pi-network are arranged such that a capacitor is connected between the input and ground, another capacitor is connected between the output and ground, and an inductor is connected between input and output. (E7C01) The reason the circuit acts this way is that as the frequency of a signal increases, capacitive reactance decreases and inductive reactance increases, meaning that higher-frequency signals are more likely to be shunted to ground.

Low-pass pi filter

A low-pass filter is made from two shunt capacitors and a series inductance.

Pi networks can also be used to match the output impedance of one circuit to the input impedance of another or the output impedance of a transmitter to the input impedance of an antenna. An impedance-matching circuit transforms a complex impedance to a resistive impedance because it cancels the reactive part of the impedance and changes the resistive part to a desired value. (E7C04) One advantage of a Pi matching network over an L matching network consisting of a single inductor and a single capacitor is that the Q of Pi networks can be varied depending on the component values chosen. (E7C13)

A Pi network with an additional series inductor on the output describes a Pi-L network used for matching a vacuum-tube final amplifier to a 50-ohm unbalanced output. (E7C12) One advantage a Pi-L-network has over a Pi-network for impedance matching between the final amplifier of a vacuum-tube transmitter and an antenna is that it has greater harmonic suppression. (E7C03)

Brochure tells story of Marconi’s Cape Code station

Marconi BrochureLast spring, I reported on my trip to Cape Cod. Well, about a week ago, while going through some things that had been donated to WA2HOM, I found a brochure produced by the National Park Service that described the South Wellfleet station.

I had scanned the whole brochure and made a PDF file from the scans, but then I found a color version of the brochure on the National Park Service website. It looks much nicer than the scan I made.


WA2HOM to operate in special event mode on Saturday, February 25, 2012

Rotary InternationalI am a member of the Ann Arbor Rotary Club. The Rotary Club was founded in Chicago on February 23, 1905, and in commemoration of that event–and to promote awareness of the Rotary Club’s End Polio Now campaign–I’ll be calling CQ Polio on Saturday, 2/15/12 at WA2HOM.

I plan to operate primarily on 20m phone on or about 14.280, plus or minus QRM. If you’re in the Ann Arbor, MI area, I’d like to invite you to come down and help me. I plan to be there from 10am until at least 4pm EST (1500 – 2100Z).

If you’re not in the are, I’d like to invite you to work us. We’ll send out a commemorative certificate to anyone who’d like one.

If you’d like to know more about Rotary, you can go to www.rotary.org.  If you’d like to know more about the End Polio Now campaign in particular, go to  End Polio Now section of the Rotary website.

Ham Radio in the News – 2/23/12

Deltona youth loves to ham it up on amateur radio. Nine-year-old Mikaila, KI4DS, is perhaps the  youngest Extra Class licensee in the U.S.

Clear Lake Amateur Radio Operators Spot For National Weather Service. Houston-area amateurs particpate in SkyWarn.

Mont. ham radio operator helps with emergency communications, talks with Space Station. Doug, K7YD, is an all-around ham, not only working DX, but also providing local emergency-communications services.

Ham Radio on YouTube- 2/22/12


A modified brad point bit makes a pad in some PCB material.

Building Ham Radio Subway Style. This video shows how one ham (I couldn’t find his callsign anywhere in the video or his YouTube profile) makes prototype boards from bare PCB material and a brad-point bit. As one commenter noted, this looks a lot easier that by cutting islands out of PCB material and gluing them to another board.

Amateur Radio Newsline Report 1801, Feb. 17, 2012. This video includes reports on:

  • WRC-12: New 600 Meter Allocation
  • 2 Ghz band comes under attack in Sweden
  • Ham radio gets an exclusion from a proposed Illinois distracted driving law
  • Ham radio balloon heading to China disappears
  • Spray on antennas become a reality

Extra Class question of the day: semiconductor materials

[[I'm not entirely happy with this section, so please feel free to suggest improvements....Dan]]

While transistor theory is outside the scope of this study guide, I will attempt to at least give you a basic understanding of how transistors are put together and how they work.  Most transistors we use in amateur radio are made of silicon. Silicon is a semiconductor. That is to say, it’s neither a conductor with a very low resistance, like copper, or an insulator with a very high resistance, like plastic or glass.

You can manipulate the electrical characteristics of silicon by adding slight amounts of impurities to a pure silicon crystal. When transistor manufacturers add an impurity that adds free electrons to the silicon crystal, it creates a crystal with a negative charge. We call that type of silicon N-type silicon. N-typeis a semiconductor material that contains excess free electrons. (E6A02) Free electrons are the majority charge carriers in N-type semi-conductor material.

When you add other types of impurities to a pure silicon crystal,  you can create a crystal with a positive charge. We call this type of material P-type semiconductor material. In N-type semiconductor material, the majority charge carriers are the free electrons. (E6A16) The majority charge carriers in P-type semiconductor material are called holes. (E6A03) P-type is the type of semiconductor material that contains an excess of holes in the outer shell of electrons. (E6A15)

You can think of holes as spots in the crystal that accepts free electrons. Because of that, the name given to an impurity atom that adds holes to a semiconductor crystal structure is call an acceptor impurity. (E6A04)

Silicon isn’t the only semiconductor material used to make transistors. At microwave frequencies, gallium arsenide used as a semiconductor material in preference to germanium or silicon. (E6A01)


Extra Class question of the day: meteor scatter propagation

Perseid meteor

Amateur radio operators use many different ways to get signals from one spot to another. Perhaps one of the most interesting is meteor scatter propagation.

Meteor scatter propagation is possible because when a meteor strikes the Earth’s atmosphere, a cylindrical region of free electrons is formed at the E layer of the ionosphere. (E3A08) 28 – 148 MHz is the frequency range that is well suited for meteor-scatter communications. (E3A09)

Unfortunately, these ionization trails are relatively short-lived, so to communicate via meteor scatter, you need to either be able to detect when these paths are available or be transmitting when the paths are available. All of these choices are correct when talking about  good techniques for making meteor-scatter contacts (E3A10):

  • 15 second timed transmission sequences with stations alternating based on location
  • Use of high speed CW or digital modes
  • Short transmission with rapidly repeated call signs and signal reports

For more information on meteor scatter, go to:

Are contests good or bad for CW?

This afternoon, I got to make a few contacts in the ARRL DX CW contest. I was on 10m, using my new loop antenna, and propagation was pretty good to Central and South America. I worked a bunch of countries including Argentina, Barbados, Costa Rica, Colombia, Brazil, Virgin Islands, Aruba, Belize, and Surinam.

After about an hour, I got bored with that, and decided to QSY to 30m, where I heard a guy I’d worked many times calling CQ. I told that I’d been playing in the DX contest on 10m, and had gotten bored with it, so I was down here looking for a ragchew. He told me that he never works contests, to which I replied that I thought that contests might actually be good for CW in that it might get more hams to work CW on a regular basis.

That comment got him going. He noted that he’d seen an increase in operating practices that we use in contests in normal operation, and he didn’t think that was a good thing. The two examples he gave were responding to CQs only with one’s callsign and not using the K prosign to signal the other operator that it’s his turn to start sending.

To be honest, I have also noted an increase in these behaviors, especially the first. I’d never thought about contests as encouraging these poor operating practices, but I think he has a point.

I don’t know how we encourage operators to not use contest procedures during normal operation, but I think we should talk about how to do so. One idea that he had was to send QRZ? whenever an operator responds to a CQ with only his callsign. I’ve done this in the past, and think this is a good idea, but I’m not sure that it gets the point across as well as we think it does.

What do you think? Do you think these practices are bad for CW? If so, what can we do about it?

Australia on 5 watts and a wire

by Alfred Gruenke PE, KB3JPP

VK2GWKIt was a cold and rainy February evening in suburban Wescosville, Pennsylvania. It was “tween” time, the time between the Super Bowl and the opening of baseball Spring Training.  This is when physical and mental activity in general slow down. February is, in general, a pretty useless month. Other than a few exceptions, it’s usually cold, wet, and miserable. The month just doesn’t have much going for it. For years I’ve been advocating going from January directly to March, skipping February entirely. However, my voice of reason has been a mere cry in the wilderness, drowned out by the forces of darkness and disparagement.

It was early Friday evening and I was home recovering from surgery.  My recovery left me with a lot of time to spend on my favorite leisure activity, globe trotting with my Elecraft K1. I fired up my ICOM IC-746PRO and the K1, Ham Radio Deluxe and QRZ.com on the computer. Then I went hunting for DX on 20 meters. Utilizing two receivers really makes a difference when Dxing; monitor one QSO while searching with the other. My antenna is a mere 52 ft. G5RV Junior 25 feet above the ground.

Up and down the band, I would listen to a station, check if I had worked that country or state, and then skip to another. This went on for some time. Then I heard it! A VK prefix! Any prefix that starts with the letter V (other than VE or VA) will get my attention since it’s sure to be a pretty exotic DX. I listened for a while to verify the call, “VK2GWK”. I checked the call on QRZ.com. Yup, that’s Australia! I’d already worked VK land with 100 Watts, so I thought I’d give it a go with the K1, QRP.

After finishing his QSO he called, “CQ, CQ de VK2GWK”, and I responded. He came back, “KB3???”. I did a fist pump and repeated my call. Again he sent “KB3???”.  I repeated my call about three or four times, after which he had my call correct. My RST was a mere 339, but, as they say, a slight ripple in the cosmic ether is better than no ripple at all. His RST was a very respectable 579

VK2GWK is Henk Tobbe, New South Wales, Australia.  It’s a few miles up the coast from Sidney and 9.776 miles from Wescosville, or 1,995 miles per watt. Not bad. A few watts go a long way! I intend to apply for another 1,000 miles per watt award. Henk has a rather sophisticated website which allowed me to download an electronic QSL card. It’s nice, but I’d rather have a real card in my hand. Call me old fashioned. Besides, I don’t know if an electronic card is valid for any awards.

I don’t know if I was just lucky or whether the long-promised sun spots are coming back, but my QRP QSOs have been increasing lately. So far, I have confirmed QRP QSOs with 38 states and 62 countries. I’ve had QSOs with Oman, Nigeria, and Kiritimati, but this one is the most memorable. Just think of it, Australia, with only five watts and a wire!