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

Thursday, July 7, 2011

I have an HTX-202 hand-held transceiver. I would like to rebuild the battery pack ...

Q Paul, W5PDA, asks “I have an HTX-202 hand-held transceiver. I would like to rebuild the battery pack, but
I’m not sure how to go about doing it. Can you give me some
advice?”


A Speaking as one who has rebuilt several battery packs (mostly for laptops), I have to caution against attempting it unless you have no other option. (Keep in mind that replacement packs are available from several aftermarket manufacturers. See the ads in this issue of QST.) If you choose to go ahead, proceed with caution.

The pack was assembled in such a way as to prevent disassembly. In order to disassemble it, you have to break it—there’s just no way around this. However, if you are very careful, you can break it in such a way that reassembling the pack is still possible, and with a reasonable appearance to boot. Start by studying the pack carefully. Try to figure out what holds it together. Come up with an idea of how to take it apart, then try to come up with reasons why that won’t work very well. When you have an idea with the least “won’t work” reasons, that’s the one you should use. 


Once you have the case apart, the rest is fairly easy. You will find multiple NiCd cells connected together with thin metal strips, usually spot-welded to the cells. Your next task is to find cells of the same size (typically nonstandard, but usually obtainable—try Digi-Key for one source). There is a caveat here: the original cells were probably matched according to their charge and discharge characteristics. If you buy unmatched cells, you won’t get as much use from a rebuilt pack because you’ll have the “weakest link in the chain” effect. 


Once you have the replacement cells in hand, you have to connect them together in some fashion. You can do this with wire and solder, or you can use the original strips if you can yank them off the old packs without cutting yourself (been there...).


To solder the wire or strips to the new batteries is a difficult
task because solder doesn’t like to stick to stainless steel or shiny aluminum (which is what most new batteries use as contact plates). First, warm up your soldering iron to its maximum temperature (if you have a 300-W iron, use it, although 60 W will do). Sandpaper or file the contacts on the new batteries to rough them up a bit so that the solder has a place to stick. Take your iron and heat the contact plate up as quickly as possible— a 3-second or longer “dwell” time will probably damage the battery, so keep it shorter than that. Apply solder to the contact to make sure it sticks. Once you get the solder to stick to the contact, add your wire/metal strip and “reflow” the solder.



Of course, when you do this, you also have to make sure that: (1) the new cells go together with the exact alignment of the old cells and (2) that the solder you added doesn’t cause the resulting pack to be too big to fit back in the old case.


Assuming you are able to connect all of the cells in a way that allows them to fit in the battery pack case, you’ll have to find a way to get the pack back together (glue, small screws, etc).


If this sounds like a lot of work, that’s because it is, but it is the cheapest way to get close to your original pack capacity. There is a much cheaper and easier alternative, but the price is in time per charge. Buy a battery case for the rig (made for alkalines) and just put NiCds in it. The capacity (and possibly the voltage) will be less than your original pack, but it is definitely a cheap and sweat-free alternative.

From QST April 2001

I’m just getting started with the HF digital modes, but I’ve run into trouble already...

Q I’m just getting started with the HF digital modes, but I’ve run into trouble already. The “help” file in one of the software packages I recently downloaded from a source in Europe suggested an operating frequency on 160 meters between 1.838 and 1.840 MHz. I tried this and quickly discovered— the hard way—that I was in the middle of the 160-meter CW DX window. Was the software programmer mistaken in his choice of suggested frequencies?


A For the IARU Region in which he lives, I’m sure his suggestions make perfect sense. For amateurs living in Region 2 (North and South America), it will cause no end of headaches. See the ITU/IARU map in Figure 1.


Figure 1—ITU zone/IARU region map. The IARU regions are
bordered in black.


You’ll find a comprehensive chart of IARU Region
bandplans on the Web at www.iaru-r2.org/p11e.htm. The
chart is too large to publish here, but let’s take a look at just
the 160-meter portion.




As you can see, on 160 meters in Region 1 the recommended digital segment is 1838-1840, so 1838 is okay in that part of the world. For hams in Region 2, however, 1830-1840 is for intercontinental operating only. Routine domestic operation is 1800-1830, so Region 2 digital operators should be lower in the band. It pays to remember that our world is host to a diversity of bandplans; what works in one location may not be appropriate in another. When in doubt, double check.

From QST April 2001

Saturday, April 16, 2011

The V terminals are reading +5 volts. What are the S terminals for?

Q Clayton, KE4RTM, asks, “I have a simple question regarding a 5-V Lambda power supply. On the output side there are –S and –V, ground, and +S and +V terminals. The V terminals are reading +5 volts. What are the S terminals for?”

A On a power supply with +/–V and +/–S terminals, the S terminals are very likely “sense” inputs. In circumstances where the current draw might cause a significant voltage drop in the cable you are using to connect the power supply to the load, you would connect the sense terminals to the load via separate wires. The sense terminals would read the voltage at the load and relay that information back to the regulator circuit. If the voltage at the load drops, the sense circuit detects this and adjusts the output of the supply to a higher voltage so that the voltage on the load comes back up to the proper supply voltage.

Because there is very little current draw in the sense circuit itself, the wires connecting the S terminals to the load can be small gauge, even if the wires that supply power from the V terminals are quite large.


For many applications, the voltage drop is not so critical, so the manufacturer often includes shorting bars that connect the V and S terminals together at the supply.



From QST March 2001