The abbreviation “IF.” Can you help?

Q I’m studying to upgrade my license and I am having difficulty with some of the terminology that keeps popping up. In particular, the abbreviation “IF.” Can you help?

A In a superheterodyne receiver, the radio frequency (RF) signal picked up at the antenna must be converted to a lower frequency prior to demodulation. This conversion takes place in the mixer stage of the receiver when the RF signal is mixed with another signal generated by the local oscillator (LO). This mixing process produces sum and difference signal frequencies. The difference frequency is amplified and becomes the Intermediate Frequency, or IF (see Figure 2). The IF is usually high enough to still be considered RF, but it may be substantially lower than the signal at the antenna. For example, FM receivers commonly convert to an IF of 10.7 MHz. AM broadcast receivers often use an IF of 455 kHz. The exceptions are so-called “up conversion” receivers that use IFs that are higher than the highest received signal frequency. To complicate matters further, superhet designs may also include more than one mixer/IF section (Figure 3).

It’s interesting to note that in a direct conversion receiver the RF conversion takes place in one huge step—mixing the signal from the antenna with a local oscillator signal at nearly the same frequency. This puts the difference frequency in the audio range for immediate demodulation.

Figure 2—A basic block diagram of a superheterodyne receiver.

Figure 3—Block diagram of a double-conversion superhet with two IF sections. 

From QST December 2000 
 

When I was working as a broadcast engineer I once had the misfortune to touch a high voltage terminal in a transmitter power supply...

Q When I was working as a broadcast engineer I once had the misfortune to touch a high voltage terminal in a transmitter power supply. Thank goodness I wasn’t killed outright, but the shock threw me across the room, seriously injuring my back when I slammed against the wall. To this day, I’ve wondered how the electricity was able to propel me through the air in such a fashion. Do you have the answer?

A Prepare yourself for another shock: the electricity didn’t propel you anywhere—your own muscles did! When a large electrical current runs through your body, your muscles are stimulated to contract powerfully—often much harder than they can be made to contract voluntarily. 

Normally the body sets limits on the proportion of muscle fibers that can voluntarily contract at once. Extreme stress can cause the body to raise these limits, allowing greater exertion at the cost of possible injury. This is the basis of the “hysterical strength” effect that allows mothers to lift cars if their child is trapped underneath, or allows psychotics the strength to overcome several nursing attendants.

When an electric current stimulates muscles, these built-in limits don’t apply, so the contractions can be violent. The electric current typically flows into one arm, through the abdomen, and out of one or both legs, which can cause most of the muscles in the body to contract at once. The results are unpredictable, but given the strength of the leg and back muscles can often send the victims flying across the room with no voluntary action on their part. Combined with the unexpected shock of an electrocution this feels as if you are flung, rather than flinging yourself. 

A common side effect of being thrown across the room by an electric shock, apart from bruising and other injuries, is muscle sprain caused by the extreme muscle contractions. This can also damage joint and connective tissue. 

From QST December 2000 
 

My Kenwood TS-570D transceiver has a 13-pin receptacle for accessories. They provide a 13- pin DIN plug with the radio ...

Q Carl, W3MAO, asks, “My Kenwood TS-570D transceiver has a 13-pin receptacle for accessories. They provide a 13- pin DIN plug with the radio. This plug has 13 stubs on the wiring side that do not appear to be meant for soldering. Is there some kind of adapter that I can use with this plug to make an otherwise difficult solder job easier?”

A Through the years, I have encountered DIN plugs like the ones you describe. MFJ sells their 5213 open-end-adapter for $9.95, but if you opt to make your own, careful soldering is required.
 
If the wire is thin enough, and you are not connecting to adjacent pins, you can form a small loop at the tip of the wire, bend it 90° and slide it onto the pin for soldering. More often than notyou are forced to “tack” solder the wire on, and when you are done, “pot” the connector with silicon rubber or another sealing compound to insulate the wires and pins from each other.

From QST December 2000 
 

I left my H-T exposed to sunlight for several hours. When I finally retrieved it, I was horrified to see that the LCD was completely black...

Q I left my H-T exposed to sunlight for several hours. When I finally retrieved it, I was horrified to see that the LCD (liquid-crystal display) was completely black. However, after it cooled for a few minutes, the display returned to normal. What happened to the display, and how did it recover?

A Liquid crystals used in most LCDs are long, straight mol ecules that tend to line up with each other, and anything else that’s nearby. If you sandwich a film of liquid crystals between glass plates that are ridged like a miniature corrugated roof, the molecules will line up with the ridges. If you rotate one plate by 90° the molecules near that plate will orient themselves at right angles to the molecules near the other plate. Between these plates, the rest of the crystal lattice forms a smooth one-quarter twist. This twist rotates the polarization of light by 90° as it passes through the liquid crystal. 

The liquid crystals used in displays are electrically unbalanced: one end of the molecule is slightly negatively charged, the other end slightly positively charged. So applying a small voltage across the glass plates causes all the molecules to “stand on end,” and the liquid crystal loses its ability to twist the polarization of light. Switch off the voltage, and the lattice returns to its previous state.

To create a display, the glass plates are replaced with polarizing filters, also out of alignment by 90°, and a reflecting surface is put behind them. Incoming light is polarized by the first filter, twisted 90° by the liquid crystals, passes through the second filter, is reflected and reverses its journey.


Apply a voltage, however, and the incoming light passes unchanged through the liquid crystal and so can’t pass through the second polarizing filter. Consequently, the display goes black. By using segmented electrodes, letters, numerals and other shapes can be displayed.


The liquid crystal state is a phase between solid and liquid cool it and it solidifies; heat it and it melts. Melted liquid crystals lose their ability to change the polarization of light, becoming ordinary liquids. That’s when you see the display going completely black or blue. When cooled, it returns to the liquid crystal phase and reflects light again.


Don’t make it a habit to leave your rig in the sunlight for prolonged periods of time. Repeated abuse can, over time, permanently alter the chemical properties of the display and render it inoperable.

From QST December 2000 

Ground plane circuit construction. What does this mean?

Q In QST I often see references to “ground plane” circuit construction. What does this mean?

A Ground-plane construction is a point-to-point technique that uses the leads of the components as tie points for electrical connections. You may also see it referred to as “dead bug” or “ugly” construction. (The term “ugly construction” was coined by Wes Hayward, W7ZOI.) “Dead-bug construction” gets its name from the appearance of an IC with its leads sticking up in the air. In most cases, this technique uses copper-clad circuit-board material as a foundation and ground plane on which to build a circuit using point-to-point wiring, hence “ground-plane construction.”

Ground-plane construction is quick and simple: You build
the circuit on an unetched piece of copper-clad circuit board.
Wherever a component connects to ground, you solder it to the copper board (see Figure 1). Ungrounded connections between components are made point-to-point. Once you learn how to build with a ground-plane board, you can grab a piece of circuit board and start building any time you see an interesting circuit.

A PC board has strict size limits; the components must fit in the space allotted. Ground-plane construction is more flexible; it allows you to use the parts on hand. The circuit can be changed easily—a big help when you are experimenting. The greatest virtue of ground-plane construction is that it is fast.

Circuit connections are made directly, minimizing component lead length. Short lead lengths and a low-impedance ground conductor help prevent circuit instability. There is usually less intercomponent capacitive coupling than would be found between PC-board traces, so it is often better than PCboard construction for RF, high-gain or sensitive circuits.

Figure 1—Typical ground plane construction. It may look ugly, but it is quick and easy.

From QST December 2000

Coping with Cabrillo By Dave Pruett, K8CC

Coping with Cabrillo

Few developments have affected Amateur Radio contest operating as much as the development of logging software for personal computers. Such programs quickly replaced paper logs, dupe and multiplier sheets on the operating desk. These same programs make it a simple task to submit your log electronically to the contest sponsor.

Standardization Needed
 
In early 1999, computer professional Trey Garlough, N5KO, worked with many of the major developers of Amateur Radio logging software to develop a standardized electronic contest entry specification for the ARRL. The result was the Cabrillo File Format Specification, which in late 1999 was adopted by the ARRL as its standard format for electronic contest entries. Beginning with contests in November 2000, all logs for ARRL contests that are electronically generated must be in the Cabrillo file format. The ARRL will continue to accept paper logs written by hand. However, contest entries generated using a computer must submit the electronic Cabrillo file. 

A Look at Cabrillo

Information about the Cabrillo File Format Specification is available on-line at: http://www.kkn.net/~trey/cabrillo/. An example of a Cabrillo file is shown below. Each line in a Cabrillo file begins with a keyword ending with a colon. This keyword identifies the data contained in that line. The file begins with the “START-OF-LOG:” keyword. Other keywords identify summaryinformation defining the contest entry. Non- QSO data lines can appear anywhere in the file; however, QSO data lines must appear in chronological order.

The format of each QSO data field is defined in the Cabrillo specification, and there is at least one blank space between adjacent data fields. These fields must be positioned in a specific order. The line starts with the “QSO:” keyword, followed by the frequency (in whole kilohertz for HF contests, or a letter designating the band for VHF/UHF) and mode of the contact. Next is the date (in YYYY-DD-MM format) and four-digit UTC time. The entrant’s call sign and sent information comes next, followed by the call sign of the station worked and the received information.

The log file ends with the “END-OFLOG:” keyword, which is very important. On occasion, the ARRL has received electronic log files that have been cut off or truncated. This sometimes happens during the e-mail process, usually beyond the control of either the entrant or the ARRL. With a Cabrillo log file, if the “END-OF-LOG:” keyword is missing, it is obvious that the file has been truncated and the entrant can be contacted to send another copy. With the non-Cabrillo ASCII files generated by the popular logging programs today, this truncation can be difficult, if not impossible to detect.

Generating Cabrillo Files

Recent versions of most popular contest logging programs can generate Cabrillo files. Specific instructions for these programs and the Cabrillo-compatible version number follows. If you have a pre-Cabrillo version, you should contact your software provider about obtaining a current version.

• CT by K1EA—As of version 9.49, CT supports Cabrillo files for the CQWW, ARRL DX (either domestic or DX), Sweepstakes and ARRL 10-Meter contests. A Cabrillo file can be created from within the program by typing the command WRITELOG in the call sign field of the logging screen. The Cabrillo file will be created along with the other log output files. It will be named yourcall.TXT, where yourcall is the call sign used during the contest.
• NA by K8CC—NA has supported Cabrillo since version 10.43. A Cabrillo file can be created when exiting the program. On exit, a screen prompt appears saying “END PROGRAM: rite Log to Disk, rint, xit”. Press “W” to write the log to disk. The Cabrillo file will be created along with the other log output files in the NA output directory. It will be named yourlog.LOG, where yourlog is the base filename of the NA log being processed.
• TRLog by N6TR—The first Cabrillo compliant version of TR is 6.50. A Cabrillofile is created using POST, the separate post-contest program provided with TR to generate entry files. Run the POST program, select “C” from the menu of commands and follow the prompts on the screen. The Cabrillo file will be created in the same directory as the log file being processed. It will be named yourlog.CBR, where yourlog is the base filename of the TR log being processed.
• SD by EI5DI—A Cabrillo file can be created using SDCHECK, the separate post contest program provided with SD to generate entry files. The first version of SDCHECK supporting Cabrillo is 9.68. Start up SDCHECK then select Option 4 - Entry File. The Cabrillo file will be created in the same directory as SDCHECK. It will be named yourlog.LOG, where yourlog is the base filename of the SD log being processed. 
• WriteLog by W5XD—To create a Cabrillo file with WriteLog, pull down the Contest menu and click on Cabrillo File.
• In the screen that appears, make sure your sent information (ARRL section, category, power, etc.) is all entered correctly, then click OK. The Save As window appears showing the directory where the Cabrillo file (named yourcall.LOG) will be created.Change the destination directory if desired, and then click OK.

Preparing the Entry
 
Cabrillo files can be easily viewed or edited using the DOS Editor program (EDIT.EXE) or Windows NotePad. A word processor program is not recommended since such programs often insert hidden formatting characters into the file without the user’s knowledge.

Opening the Cabrillo file allows the entry information to be quickly reviewed. One very important item for the ARRL Contest Department is the “ARRL-SECTION:” field, which is used to compile the score listings in QST, which are by ARRL section. Be sure to check this field to ensure that your score will appear under the correct section in the QST listings. Remember that some states have multiple sections, so include the correct section if you live in one of those areas.

A common problem with electronic logs is incorrect information. For example, many popular logging programs allow a default location (such as your state) to be set. However, many contests use different entities for the geographic locator, in which case the default may be incorrect. Program bugs can also cause the QSO information to be incorrect. A few minutes reviewing the QSO information in your entry file can catch these types of errors quickly.


Prior to submitting your electronic entry, it may be necessary to rename the Cabrillo file. The ARRL requires your entry file to be named yourcall.LOG, where yourcall is the call sign used by the entry during the contest. Some programs name the Cabrillo file in this way, while some do not (to avoid the possibility of inadvertently overwriting a prior copy of yourcall.LOG from another contest.) If necessary, rename the Cabrillo file to yourcall.LOG using either the RENAME command in DOS, or using Windows Explorer.


One issue with using the call sign as a file name is that the forward slash character (/) used in portable call signs is not a valid file name character. Use the underscore (_) character as a substitute, or omit the character entirely.


Submitting the Entry
 
Your electronic entry may be submitted one of two ways. One method is to copy the Cabrillo file to a floppy and send it to the ARRL via regular mail. However, most entries are sent as e-mail attachments. E-mail programs typically support attachments as a way to send an electronic file as a separate, detachable part of the e-mail To submit your entry, prepare an e-mail addressed to the ARRL for the specific contest to be entered. The address is always found in the rules for each contest or online at http://www.arrl.org.contests. The subject line should contain your call sign, the name of the contest and your entry class. Nothing needs to be included in the body of the e-mail because the Cabrillo file is a complete entry in itself.


Attach the Cabrillo log file, send the e-mail and you’re done! Don’t send the files as the text of the e-mail, as this causes problems in detaching and saving the file information.


Submitting an electronic log is easy once you’ve done it a few times. Electronic logs allow the ARRL logcheckers to do their job more quickly and accurately, and Cabrillo allows them to spend less time doing data translation and more time checking. The entrant also benefits from Cabrillo through improved integrity of their entry file. Electronic log submittal is here to stay, and it sure beats killing a tree to print your entry!

An example of the Cabrillo file format.

From QST November 2000

I use a station clock that has large, red LEDs. I’ve noticed that if I am chewing on something (a mid-contest snack!) and happen to glance at the clock ...

Q I use a station clock that has large, red LEDs. I’ve noticed that if I am chewing on something (a mid-contest snack!) and happen to glance at the clock, the numbers seem to be jumping or flickering. Assuming that this isn’t the symptom of some dreaded disease, what really causes the flickering?

A If you’re chewing on something hard (crunchy potato chips, candy, etc) you set up vibrations in your jaw that propagate to your eyes, shifting their positions ever so slightly. The LED segments are “refreshing” themselves at a high rate of speed and, because of the movement of your eyes, the bright “moving” segments are in different places from where the visual centers of your brain expect them to be. You may see the same effect while watching your computer monitor.

This phenomenon involves something called the critical fusion frequency, which is the point where we begin to perceive things that are flickering as if they are solid. Different factors influence that frequency, including the size of the object, its brightness, and which part of the retina it is seen by. The brighter the background, for example, the greater the flicker. The action of chewing jars the visual axis and changes your line of sight relative to the particular point you are focused on, moving it far enough off the central retina to change your ability to perceive a flickering image as a stable one. 

From QST November 2000 
 

I know that VOX is voice-operated switching, but what is “MOX”? ...

Q I know that VOX is voice-operated switching, but what is “MOX”? I see this popping up in transceiver feature lists from time to time

A MOX is manually operated switching. It is a front panel button that places the rig in the transmit mode. MOX is handy when you need to transmit, for antenna tuning purposes, for example, but don’t have a mic or key connected to the transceiver.

From QST November 2000 
 

Can I still find RTTY on the HF bands? What about VHF? What do I need to get started with this mode?” ...

Q Don, WB5UIA, asks, “Can I still find RTTY on the HF bands? What about VHF? What do I need to get started with this mode?”


A RTTY as a digital mode is still very much alive, although it is primarily used for DXing and contesting these days (PSK31 has taken over the lion’s share of the “conversational”
HF digital activity). You’ll find RTTY on just about every HF band, but it is mostly heard on 20 meters between approximately 14.080 and 14.095 MHz. As far as VHF is concerned, RTTY was once heard on 2 meters—there were even “RTTY repeaters” but VHF RTTY activity today has all but disappeared in the US.

To operate RTTY you have two options: purchase an external multimode interface for your computer, or purchase software that will send and receive RTTY signals using your computer’s sound card. The external interfaces are still popular, but the software approach is gaining ground. (See our review of RITTY 4.10 by Brian Beezley, K6STI, elsewhere in this issue.) Beyond that, all you need is an SSB transceiver and you’re good to go.

To learn more I’d strongly recommend that you pick up a copy of the ARRL HF Digital Handbook. You can purchase this book at your favorite dealer, or order directly from the ARRL. See the ARRL Publications page in this issue.

From QST November 2000 
 

I’m confused about the concept of “SWR bandwidth.” Can you explain?

Q I’m confused about the concept of “SWR bandwidth.” Can you explain?


A “SWR bandwidth” is a term you’ll often encounter when you’re reading about antenna designs, or checking the specifications of commercial antennas. Basically, the SWR bandwidth is the frequency range after the antenna has been tuned at one frequency, over which the SWR is 2:1 or less. This is easier to explain visually, so take a glance at Figure 3. Let’s say that we have a 40-meter dipole antenna that is tuned to resonance at 7100 kHz. If our dipole has an SWR bandwidth of 200 kHz, we’d expect the SWR to rise to 2:1 at 7000 kHz and 7200 kHz.

Some types of antennas such as compact tuned loops have extremely narrow SWR bandwidths when tuned to resonance. Trap dipole and vertical antennas will have varying SWR bandwidths for each band, usually becoming narrower on the lower bands. Be wary of an antenna that claims a 2:1 SWR bandwidth covering all of a wide band, such as 80 meters. This band covers 3.5 to 4.0 MHz, a percentage bandwidth of more than 13%. While a wide SWR bandwidth may seem ideal, it’s often the hallmark of an inefficient design with high losses. After all, dummy loads have the “best” SWR bandwidths of all! Read all about broadband antennas in Chapter 9 of The ARRL Antenna Book.

From QST November 2000 

I live on the top floor of an apartment building. We have a small balcony, but I can’t hang wire antennas ...

Q I live on the top floor of an apartment building. We have a small balcony, but I can’t hang wire antennas for HF because they’ll droop onto the balconies below. I also need an antenna that I can remove quickly. Can you help?

A You actually have more options available than you think. You could try a compact tuned loop antenna such as those sold by MFJ. Other extremely compact antennas such as the Bilal Isotrons (http://www.rayfield.net/isotron) may help. You might also try using a lightweight mobile antenna such as a Hamstick. You could mount the Hamstick on the balcony railing, for example, and attach a counterpoise wire to the ground side of the antenna mount. (The counterpoise wire should be 1/4 wavelength for the desired band.) Just route the counterpoise wire along the floor of the balcony. Be sure to stay away from the ends of these counterpoise radials, where high RF voltages can exist even at modest transmitter power levels. 

All of these antenna options are, of course, compromises. They sacrifice efficiency to save space. Don’t expect any of them to outperform even a full-sized dipole mounted high in the clear, but they will get you on the air and provide many enjoyable contacts. 

From QST November 2000 
 

I learned that the station was 500 miles away from me. Was this sporadic E propagation? ...

Q Last night I heard a strange CW signal on 6 meters. It was hissing and buzzing, but I was still able to copy. To my astonishment, I learned that the station was 500 miles away from me. Was this sporadic E propagation?

A My guess is that you heard auroral propagation. The clue is your description of the signal as having a hissing or buzzing characteristic.

Those of us who reside at the higher latitudes are occasionally treated to the visual spectacle of the aurora borealis, better known as the “northern lights.” (Yes, there are “southern lights” as well, visible occasionally in South America and Africa.) The aurora is caused when the Earth intercepts a stream of charged particles ejected from the Sun, resulting in a “geomagnetic storm.” These fast-moving particles funnel into the polar regions of the Earth thanks to our magnetic field. As the particles interact with the upper atmosphere, the air glows, which we see as an aurora. The shimmering, ghostly curtain of light is not only a treat for the eyes, it can reflect radio signals like a giant mirror (see Figure 2). 

Like sporadic E, you’ll encounter auroral propagation more
often on 6 meters than on 2 meters. Nevertheless, 2-meter aurora is far more common than 2-meter sporadic E. You can also work distant stations using auroral propagation on 222 and 432 MHz. 

As you’ve discovered, auroral DX signals are very distorted.
That’s why CW is the most commonly used mode, although you’ll hear SSB from time to time. Auroral CW signals have the raspy, buzzing quality you heard. (It sounds like the other guy is operating an ancient spark-gap transmitter!) Just listen carefully and you’ll be able to decode the signals. 

You do not need directional antennas and high power to work
aurora on 6 meters. The Doctor has done it with dipoles and 100 W. Many hams have even enjoyed success with 6-meter aurora from mobile stations! 

From QST November 2000 
 

I have a 10-year-old Realistic 13-inch color TV that I use with my ATV station. Recently the TV went completely dead...

Q I have a 10-year-old Realistic 13-inch color TV that I use with my ATV station. Recently the TV went completely dead. It won’t turn on when I press the ON button on the front panel, or when I try to turn it on from the remote. I checked the power supply fuse and it is okay. The power supply appears to be working as well. Any ideas?

A Many TVs operate in what you might call a “sleep” mode. That is, there are circuits within the TV that are active continuously—even when the rest of the TV is off. Usually the primary microprocessor is always active, waiting for the command to switch on the rest of the set. If the microprocessor isn’t responding to manual or remote “on” commands, the microprocessor could be defective. If you have a volt-ohm meter and a schematic diagram, measure the voltage at the Vcc pin of the microprocessor. Is it receiving power from the power supply (probably 5 V)? If so, find the microprocessor pin that produces the output signal to turn on the rest of the TV. Do you get a reading at this pin when you press the TV’s “ON” button? If not (and I suspect you won’t), the microprocessor is probably dead. On the other hand, if you do get a reading, it’s time to troubleshoot the rest of the circuit that is responsible for switching on the set. This is likely to include a couple of switching transistors and possibly an optoisolator. 

From QST November 2000 

I am using a G5RV on 80-6 meters. How efficient is this antenna on 6 meters?

Q Dave, WD8DK, asks, “I am using a G5RV on 80-6 meters. How efficient is this antenna on 6 meters? I have been told that it is very inefficient on this band. In fact, I have been told that a 1/2 wavelength dipole is more efficient than the G5RV on 6 meters. Any comments?”

A On 20 meters, where the G5RV was designed to operate, it boasts a little gain over a conventional half-wave dipole. Given a reasonably efficient feed line (450-Ω line) and a good antenna tuner, there’s no reason why the G5RV can’t be at least as “efficient” as, say, a coax-fed dipole in the HF bands. 

However, on 6 meters the G5RV acts as a long-wire antenna,
with an azimuthal pattern with multiple, very narrow lobes. The
narrow lobes are what give it gain, but also what make its performance compared with a regular garden-variety dipole inferior in direc-tions other than the ones it favors. The EZNEC plot shown in Figure 1 assumes that the antenna is mounted as a flat top at 50 feet above average ground. The G5RV has significantly more gain than the simple dipole, but it achieves this mainly in four, narrow-beamwidth directions. For the rest of the azimuths, its pattern has nulls that the dipole covers well. 

Any multiband antenna is a compromise, but most of us can’t have five or more dipoles hanging in our backyards. On 6 meters I would recommend a separate antenna designed for that band. There are a couple of inexpensive 6-meter wire antenna designs on the ARRL TIS Web site at http://www.arrl.org/tis/. Go there and click on “Antenna Projects,” and then “Other VHF Antennas.” 

Figure 1—This is an EZNEC plot of a G5RV antenna on 6 meters compared to a dipole cut for 6 meters. The solid line represents the G5RV pattern while the dashed line represents the dipole. Notice that the G5RV is creating numerous narrow lobes of radiation.

From QST November 2000

Many solid-state timers function by producing a logic “high” at the output within a specified time after the timer is triggered ...

Q Many solid-state timers function by producing a logic “high” at the output within a specified time after the timer is triggered. For my application, however, I need a timer that “goes high” as soon as it is triggered and remains high for about 60 seconds before dropping back to zero. Can you steer me in the right direction?

A How about trying the one-shot multivibrator shown in Figure 1? This one uses a garden-variety 555 timer chip and a couple of components. The trigger pulse causes the output (Q) to go positive and capacitor C to charge through resistor R. When the voltage across capacitor C reaches 2/3 of Vcc, the capacitor discharges to ground and the output returns to zero. You can calculate the values of R and C with the equation T = 1.1(RC) where T is the duration of the output pulse in seconds, R is resistance in ohms and C is capacitance in farads. For a 60-second pulse, you’ll need a 56-kΩ resistor and a 1000 μF capacitor. This works out to be about 61.6 seconds. Of course, you could use a potentiometer (a 100-kΩ pot, for instance) in place of R to tweak the pulse length and compensate for the tolerance range of the capacitor.

Figure 1—A simple one-shot multivibrator built around a common 555 timer IC.

From QST October 2000 
 

I am using a Hustler 4-band trap antenna fed with coax. At the antenna the coax is attached not by a connector ...

Q KT4SP asks, “I am using a Hustler 4-band trap antenna fed with coax. At the antenna the coax is attached not by a connector, but by the shield and center conductor to screws. I assume that this is proper, since the antenna does not have an SO-239 connector. The antenna is mounted about three inches above ground with no radials. I have been using this setup for a number of years, but my power output on phone is only about 45 W from a 100-W transceiver. Would the addition of a 1:1 balun to the system improve or degrade the output? Is this type of vertical considered a balanced or an unbalanced antenna?”


A While a vertically oriented half-wave dipole would be a balanced antenna, a quarter-wavelength vertical—with its “missing bottom half” made up using an image antenna reflected in the ground plane—is indeed an “unbalanced antenna.”

However, just because this is an unbalanced antenna fed with an unbalanced (coax) feed line doesn’t mean that everything is hunky-dory in the installation. The real clue to the nature of this problem is that your system doesn’t have ground radials. Two concerns immediately arise: (1) power reduction in the radio due to common-mode currents and (2) poor radiation efficiency.


If the ground plane or radial system is inadequate under the vertical, then there’s a really good chance that common-mode currents are being radiated onto the shield of the coax cable running on the ground under the vertical. Such common-mode currents can fool a transceiver’s SWR sensor and cause it to reduce the output power. Putting a balun at the feed point may, or may not, reduce the level of common-mode currents, but this isn’t really the proper approach—putting down radials is what is really required here!


Consider this: a quarter-wave vertical with a perfect ground system should show a feed-point impedance of about 36 Ω, and this is only a 50/36 = 1.388:1 SWR—not enough to cause an SWR shutdown unless common-mode currents are involved. Indeed, because of the lack of radials, the feed-point impedance due to losses might be even closer to 50 Ω, even if the radiation efficiency is poor. So, unless common-mode currents are involved, the SWR alone wouldn’t cause a power reduction to protect the radio.


Adding ground radials would improve the radiation efficiency and increase your output power (by eliminating common-mode currents that are falsely activating your radio’s SWR sensor).

From QST October 2000 
 

When talking about computers, what does SCSI mean?

Q When talking about computers, what does SCSI mean?


A SCSI is an abbreviation for Small Computer System Interface. Pronounced “scuzzy,” it is a parallel interface standard used by Macs, PCs and many Unix systems for attaching peripheral devices to computers.

SCSI interfaces provide faster data transmission rates (up to 40 Mbytes per second) than standard serial or parallel ports. In addition, you can attach many devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface. 

Although SCSI is an ANSI standard, there are many variations of it, so two SCSI interfaces may be incompatible. For example, SCSI supports several types of connectors.

While SCSI is the only standard interface for Macintoshes, PCs support a variety of interfaces in addition to SCSI. These include IDE, Enhanced IDE and ESDI for mass storage devices, and Centronics for printers. You can, however, attach SCSI devices to a PC by inserting a SCSI board in one of the expansion slots. Many high-end PCs come with SCSI built in. Note, however, that the lack of a single SCSI standard means that some devices may not work with some SCSI boards.

The following varieties of SCSI are currently implemented:

SCSI-1: Uses an 8-bit bus, and supports data rates of 4 MBps.
SCSI-2: Same as SCSI-1, but uses a 50-pin connector instead of a 25-pin connector for 16-bit transfers, and supports multiple devices. This is what most people mean when they refer to plain SCSI.
Wide SCSI: Uses a second cable (called a B-cable) to support 32-bit transfers.
Fast SCSI: Uses a 16-bit bus, but doubles the clock rate to support data rates of 10 MBps.
Fast Wide SCSI: Uses a 16-bit bus and supports data rates of 20 MBps.
Ultra SCSI: Uses an 8-bit bus, and supports data rates of 20 MBps.
SCSI-3: Uses a 16-bit bus and supports data rates of 40 MBps. Also called Ultra Wide SCSI. Ultra2 SCSI: Uses an 8-bit bus and supports data rates of 40 MBps.
Wide Ultra2 SCSI: Uses a 16-bit bus and supports data rates of 80 MBps. 

From QST October 2000