Wednesday, October 19, 2011

Over the past two years or so, I have received dozens of QSL cards from DX stations ...

Q Joe, WT7V, asks, “Over the past two years or so, I have received dozens of QSL cards from DX stations that I’ve never worked. In fact, many of these QSLs confirm contacts supposedly made when my rig was completely off the air for weeks at a time. Do you think someone could be bootlegging my call sign?”


A Bootlegging is always a possibility, but it is rare. If the
cards seem to arrive in spurts, there is a more likely explanation.


It is not at all unusual for a call to be consistently misrecorded in contests. For example, K0NS gets several cards per year intended for K0DI, a very active CW contest operator. If you sound out the suffixes of both call signs in Morse, you can understand how someone could blur the two together. Early this year, NT1A inquired about some cards that were apparently meant for our own Dave Patton, NT1N, here at Headquarters. In the heat of a contest, missing or transposing the individual letters is easy to do.


From QST June 2001

Tuesday, October 18, 2011

What does the ‘dBi’ mean when used to rate an antenna

Q Jerry, KC8OTH, asks, “What does the ‘dBi’ mean when
used to rate an antenna



A I will presume that you are somewhat familiar with the decibel. If not, at least a few numbers will put it into perspective. Decibel is a term that can compare two powers or voltage levels. The formulas are:


dB = 10log (P1/P2) where P1 and P2 are two power levels.


It can also compare voltages, if the voltages are at the same impedance. The formula is:


dB = 20log (V1/V2) where V1 and V2 are the two voltage levels.


(For “extra credit,” if the resistances are not equal, you can
use the formula dB = 10log ((V12/R1)/(V22/R2)).


Now, in antenna gain, there are two common references. The first is an imaginary antenna called an “isotropic” radiator. This is an antenna that radiates equally in all directions. An isotropic radiator placed at the center of a sphere would illuminate the sphere equally. No such antenna exists in real life. A practical example of what is nearly an isotropic radiator is a light bulb. 


When gain is expressed in dBi, it indicates how much louder a signal from that antenna will be in the main beam of the antenna than it would be if the same amount of power were applied to an isotropic radiator in free space. The thing to remember about gain is that an antenna develops gain by concentrating energy in one direction and not radiating energy in other directions. Two examples of gain are flashlights, and the technique of cupping your hands when you shout to make the sound louder in the desired direction.


A directional antenna such as a Yagi can have considerable gain. Typical HF Yagi beams can have 8 dBi gain or more; a large VHF or UHF beam can have 20 dBi gain, or even more. Some easy numbers to remember are:


1 dB = 1.25 × power
2 dB = 1.6 × power
3 dB = 2 × power
10 dB = 10 × power



A 20-dBi-gain antenna would have 10 × 10 or 100 times the power gain of an isotropic radiator. One watt fed into a 20-dBi gain antenna would be as loud as 100 W fed into an isotropic source, but only in the direction the antenna is beaming.


Decibels also work in the other direction, too. An antenna with –3 dBi “gain” actually has a loss of 3 dB—it will lose half of the power applied to it. An antenna that is –10 dBi is radiating 1/10 the signal of one with 0 dBi gain; one that is –20 dBi is radiating 1/100 the signal and so on. A –20 dBi gain antenna with 1 W fed to it would sound as loud as an isotropic antenna being fed with 10 mW


Most H-Ts have antennas that are not very efficient. A gain of –10 dBi would be about typical. This can work very well if you are near a repeater, but if you are right at the edge of a repeater’s range, or operating simplex over a few miles, this will not give a very good signal; it will sound “scratchy” on the receiving end.


Another reference point is dBd, or referring the gain to a half-wave dipole in free space. The half-wave dipole in free space has a gain of 2.15 dBi, so gain expressed in dBd is always 2.15 dB less than gain expressed in dBi. Don’t worry, the gain of the antenna is the same in both cases, only the reference has changed. If you want to compare an antenna whose gain is in dBd to one whose gain is in dBi, add 2.15 to the gain of the antenna in dBd.


I don’t want to make it too complicated, but I will add that most antenna gain figures tell you what the antenna would be if it were in free space—infinitely far away from the Earth. In the real world, the ground affects the antenna performance by reflecting signals upward. This actually adds up to about 5 dB to the gain of an antenna. So, a half-wavelength dipole over ground can actually have about 5 dBd of gain! Slick, eh? The half-wavelength dipole over ground has 5-dB gain over a halfwave dipole in free space.

From QST April 2001

My son wants to run sophisticated gaming software on the computer ...

Q My current shack PC is a 333-MHz Pentium II with 64 Mbytes of RAM and a 3-Gbyte hard drive. My son wants to run sophisticated gaming software on the computer when I’m not using it, and my wife would like to do a few things on the machine with PhotoShop. I’m considering the idea of pulling the motherboard and replacing it with a new 133-MHz bus board and 1 GHz CPU. Is this the most cost-effective approach?

A Motherboard and CPU prices have been plunging lately, but you’ll still shell out about $600 for a good-quality 1-GHz motherboard/CPU combo. You can probably save more than $100 by purchasing a 933-MHz package. Believe me, you won’t notice the performance differential between 933 MHz and 1 GHz. Clock speed isn’t everything!

motherboard chipset is “old” as far as the PC market is concerned, but I’ve measured it to be as fast, if not faster, than the newer 815E chipset. Go with a 440BX-chipset motherboard and you’ll save a little cash without sacrificing performance. 


You might consider expanding your memory to 256 Mbytes to accommodate future needs. Beware of cheap memory, however. You can find 256-Mbyte SDRAM for under $100, but there is memory and there is memory. Bargain-basement memory can drop data when you’re cycling it at 133 MHz on your new motherboard. Just one dropped bit is enough to corrupt data and possibly trigger the dreaded “blue screen of death.” This is the last thing you want to see, say, in the middle of a contest!

Definitely upgrade your hard drive. With drive prices falling through the floor these days, you have no excuse. You can find 30-Gbyte drives for less than $125. Stick with the highspeed (7200 RPM) drives and your programs will load at the speed of thought!


 From QST April 2001

I operate slow-scan TV (SSTV) using sound card software.

Q I operate slow-scan TV (SSTV) using sound card software. Last week I upgraded my PC to the Windows ME operating system and now my SSTV software is acting up. According to what I see on the display, I am grossly overdriving the sound card input. Reducing the LINE INPUT volume control on the sound card mixer helps, but I have to practically take the level to zero (and it is very touchy). Is this a problem with Windows ME?

A In an indirect way, yes. If you did a full installation of
Windows ME (not just an upgrade), chances are it loaded
a Microsoft sound card driver automatically. Depending on the type of sound card you own, the Microsoft driver can cause some strange behavior. If you still have your original sound card software, I suggest that you reinstall the original drivers. If not, go to the Web site of the company that made your sound card and download the drivers from there. 


From QST April 2001