Q Ray, WA3CLD, asks, “I have an old Swan TB-3HA
tribander beam antenna. How can I check and recondition
the traps? It has 4 driven-element traps, 2 director traps
and no reflector traps. Can I adapt one of the driven element
pairs of traps for the reflector?”
A The biggest problem with old traps is corrosion, both ex ternal and internal. To “recondition” them, you’ll have to take them apart and thoroughly clean the inside. You can check a trap for internal corrosion without taking it apart by putting an ohmmeter across it—the resistance should be a relatively low value. If it is over 100 Ω, you probably have a corrosion problem.
Traps are designed to present a high impedance at the
“trapped” frequency and to act as a loading coil at lower frequencies, so you should indeed be able to use driven element traps for the reflector by adjusting the tubing lengths slightly.
If you have an antenna analyzer or a dip meter, you can check the resonance of just the reflector (it should be about 5% lower than the driven) by assembling it as a unit. You may need to get it at least 10 feet up in the air, however, because the effect of the nearby ground detunes a low antenna, changing the resonant point from where it will be when the antenna is installed later.
From QST February 2001
Monday, November 1, 2010
I’d like to try microwave operating from home, but I can’t put up antennas outdoors...
Q I’d like to try microwave operating from home, but I can’t put up antennas outdoors. Is it possible to at least receive microwave signals with an attic antenna? Will the signals make it through a standard shingled roof? I’m thinking specifically of receiving satellite microwave downlinks. Bob Bruninga, WB4APR, answered this question with an interesting experiment:
A “Since I have a Direct-TV 1-meter dish on a tripod that I
use for demonstrations, I decided to check its performance through various materials. The unit has a bargraph signalstrength meter for use during alignment. The meter scale goes from 0 to 100. Here are the results:
Outdoors in the clear: 92
1/4-inch plywood covering: 80
7/16-inch plywood covering: 77
3/4-inch plywood: 60
3/4-inch Masonite: 70
3/4-inch stack of paper: 60
1.5 inches of plywood: 43
(Note that the signal drops out completely at 35.)
“I have no idea if the scale is at all linear or logarithmic, and my arm was too short to both hold the wood and see the monitor well. So, your mileage may vary.
“With digital the picture is always perfect. You don’t lose any quality until it drops out completely. Of course, most of this margin is needed in case of rain. But it looks to me like it should be possible to receive microwave downlinks through a simple 3/4-inch roof and shingles, as long as rain, ice or snow are not involved.”
From QST February 2001
A “Since I have a Direct-TV 1-meter dish on a tripod that I
use for demonstrations, I decided to check its performance through various materials. The unit has a bargraph signalstrength meter for use during alignment. The meter scale goes from 0 to 100. Here are the results:
Outdoors in the clear: 92
1/4-inch plywood covering: 80
7/16-inch plywood covering: 77
3/4-inch plywood: 60
3/4-inch Masonite: 70
3/4-inch stack of paper: 60
1.5 inches of plywood: 43
(Note that the signal drops out completely at 35.)
“I have no idea if the scale is at all linear or logarithmic, and my arm was too short to both hold the wood and see the monitor well. So, your mileage may vary.
“With digital the picture is always perfect. You don’t lose any quality until it drops out completely. Of course, most of this margin is needed in case of rain. But it looks to me like it should be possible to receive microwave downlinks through a simple 3/4-inch roof and shingles, as long as rain, ice or snow are not involved.”
From QST February 2001
What is a switching type power supply ...
Q Keith, KF4BXT, asks, “What is a switching type power supply and what type is typically used and/or recommended for running a station? I think most of us, by now, understand that our power supplies should be regulated and filtered, but are there reasonable ways to add regulation and filtering to those that don’t already have it?”
A In a linear power supply, the line voltage goes directly into a low-frequency transformer where it is stepped down to the appropriate low voltage before rectification, filtering and regulation. In a switching power supply, the line voltage is directly rectified and filtered to produce a high dc voltage. This voltage is then “switched” at a high frequency rate (not RF, but certainly higher than audio—perhaps 50 kHz, for example) by switching transistors. It is then fed into a high-frequency transformer and the output is rectified and filtered. Regulation can be done in the output stage, but more typically, the regulation is done at the switching transistor to allow the amount of energy fed to the transformer to be adjusted as needed.
One advantage of the switching technique is that higher frequency components are much smaller and lighter weight for the same power capability than their low frequency counterparts. Another advantage is that, since the transformer is the least efficient part of the supply, controlling its input power (as is done in a switching power supply) can provide much better efficiency. The power lost as heat in a linear supply is typically 40-60% of the output power. In a switching supply, that typically drops to 10-20%.
The disadvantages of a switching supply are the increased complexity (more likelihood of a component failure), increased cost (many more parts) and tendency to create radiated RF (the switching waveform is usually pretty close to a square wave, so it contains a lot of harmonics). This last item has been the main one that has kept switching supplies out of the ham market until recently. Current designs use an extensive amount of filtering and radiation suppression techniques to greatly reduce unwanted RF.
Adding filtering and regulation to a linear supply is a simple
matter. Information on calculating filter component values for a particular desired ripple can be found in The ARRL Handbook chapter on power supplies. However, regulation will come at a cost in reduced output capacity—if you have an unregulated supply that puts out 15 V, you probably won’t be able to get more than 13 V from a regulator system attached to it.
All switching supplies have some kind of regulation, although some designs are quite crude and could use improvement. I don’t suggest trying to modify a switching supply unless you have studied switching power supply design extensively.
From February 2001
A In a linear power supply, the line voltage goes directly into a low-frequency transformer where it is stepped down to the appropriate low voltage before rectification, filtering and regulation. In a switching power supply, the line voltage is directly rectified and filtered to produce a high dc voltage. This voltage is then “switched” at a high frequency rate (not RF, but certainly higher than audio—perhaps 50 kHz, for example) by switching transistors. It is then fed into a high-frequency transformer and the output is rectified and filtered. Regulation can be done in the output stage, but more typically, the regulation is done at the switching transistor to allow the amount of energy fed to the transformer to be adjusted as needed.
One advantage of the switching technique is that higher frequency components are much smaller and lighter weight for the same power capability than their low frequency counterparts. Another advantage is that, since the transformer is the least efficient part of the supply, controlling its input power (as is done in a switching power supply) can provide much better efficiency. The power lost as heat in a linear supply is typically 40-60% of the output power. In a switching supply, that typically drops to 10-20%.
The disadvantages of a switching supply are the increased complexity (more likelihood of a component failure), increased cost (many more parts) and tendency to create radiated RF (the switching waveform is usually pretty close to a square wave, so it contains a lot of harmonics). This last item has been the main one that has kept switching supplies out of the ham market until recently. Current designs use an extensive amount of filtering and radiation suppression techniques to greatly reduce unwanted RF.
Adding filtering and regulation to a linear supply is a simple
matter. Information on calculating filter component values for a particular desired ripple can be found in The ARRL Handbook chapter on power supplies. However, regulation will come at a cost in reduced output capacity—if you have an unregulated supply that puts out 15 V, you probably won’t be able to get more than 13 V from a regulator system attached to it.
All switching supplies have some kind of regulation, although some designs are quite crude and could use improvement. I don’t suggest trying to modify a switching supply unless you have studied switching power supply design extensively.
From February 2001
Friday, October 29, 2010
What is wrong with operating FM simplex between 145.8 and 146 MHz? ...
Q What is wrong with operating FM simplex between 145.8 and 146 MHz? I hardly ever hear signals in that part of the band.
A This touches on the issue of voluntary band planning, which was addressed in “Washington Mailbox” in the December 2000 QST. You might want to take a look at that column.
The problem with operating in the 145.8 to 146 MHz segment is that the amateur satellite community uses this portion of the spectrum. There are several FM repeater satellites that operate with uplinks in this segment. If you’re chatting with a local friend on 145.85 MHz, for example, there is a chance that OSCAR 27 will hear you and relay your conversation over thousands of miles without you even realizing it! The FM satellites are crowded already, so they don’t need “unintentional signals.” Other SSB and digital satellites have uplinks here as well, and your signals could make it impossible for these birds to hear the signals intended for them.
SO-35 and AO-10 have downlinks in this portion of the band. These signals are often weak and your terrestrial FM QSOs will obliterate them.
Just because you can’t hear the satellite signals, doesn’t mean that they are not there. With the typical FM setup, you’re not likely to hear satellites in this portion of 2 meters at all.
Voluntary band planning allows everyone to enjoy Amateur Radio in all of its various forms, but it only works if we all respect the plans. It isn’t so much a legal issue as it is one of common courtesy.
From QST February 2001
A This touches on the issue of voluntary band planning, which was addressed in “Washington Mailbox” in the December 2000 QST. You might want to take a look at that column.
The problem with operating in the 145.8 to 146 MHz segment is that the amateur satellite community uses this portion of the spectrum. There are several FM repeater satellites that operate with uplinks in this segment. If you’re chatting with a local friend on 145.85 MHz, for example, there is a chance that OSCAR 27 will hear you and relay your conversation over thousands of miles without you even realizing it! The FM satellites are crowded already, so they don’t need “unintentional signals.” Other SSB and digital satellites have uplinks here as well, and your signals could make it impossible for these birds to hear the signals intended for them.
SO-35 and AO-10 have downlinks in this portion of the band. These signals are often weak and your terrestrial FM QSOs will obliterate them.
Just because you can’t hear the satellite signals, doesn’t mean that they are not there. With the typical FM setup, you’re not likely to hear satellites in this portion of 2 meters at all.
Voluntary band planning allows everyone to enjoy Amateur Radio in all of its various forms, but it only works if we all respect the plans. It isn’t so much a legal issue as it is one of common courtesy.
From QST February 2001
Every time you double your antennas, say, going from a single VHF/UHF Yagi to a stacked pair of Yagis ...
Q Every time you double your antennas, say, going from a single VHF/UHF Yagi to a stacked pair of Yagis, you realize an increase in gain, but you also lose a certain amount of power through the power divider or phasing harness. How do you manage to come out ahead?
A It is true that stacking antennas produces additional directivity and gain. You can stack antennas vertically (Figure 1) or horizontally, although vertical stacking is most common.
The “secret” is in the fact that gain can only come from taking power that would otherwise be radiated in other direction(s) and concentrating that power into the main, desired lobe(s).
The most easily understood physical demonstration is one that I’ve used for years at radio club meetings (see Figure 2). I take a balloon, blow it up so that it is roughly circular in shape and then declare that this is a radiation pattern from an isotropic radiator. Next, I blow up another balloon to the same size and shape and tell the audience that this will be my “reference” antenna.
Then I squeeze the first balloon in the middle to form a sort of figure-8 shape and declare that I’ve now created a dipole and compare the maximum size to that of my reference “antenna.” The dipole can be seen to have some “gain” over the reference isotropic. Next, I squeeze the end of the first balloon to come up with a sausage-like shape to demonstrate the sort of pattern a beam antenna would have, again comparing the gain to the reference isotropic antenna, er, balloon.
By combining antennas in a stack, you can accentuate this gain and directivity even further. In the end, you have created much more total gain in the antenna system than would be lost in the power dividers or phasing harnesses. Stacking isn’t easy or inexpensive, but the performance gain can be substantial.
A It is true that stacking antennas produces additional directivity and gain. You can stack antennas vertically (Figure 1) or horizontally, although vertical stacking is most common.
The “secret” is in the fact that gain can only come from taking power that would otherwise be radiated in other direction(s) and concentrating that power into the main, desired lobe(s).
The most easily understood physical demonstration is one that I’ve used for years at radio club meetings (see Figure 2). I take a balloon, blow it up so that it is roughly circular in shape and then declare that this is a radiation pattern from an isotropic radiator. Next, I blow up another balloon to the same size and shape and tell the audience that this will be my “reference” antenna.
Then I squeeze the first balloon in the middle to form a sort of figure-8 shape and declare that I’ve now created a dipole and compare the maximum size to that of my reference “antenna.” The dipole can be seen to have some “gain” over the reference isotropic. Next, I squeeze the end of the first balloon to come up with a sausage-like shape to demonstrate the sort of pattern a beam antenna would have, again comparing the gain to the reference isotropic antenna, er, balloon.
By combining antennas in a stack, you can accentuate this gain and directivity even further. In the end, you have created much more total gain in the antenna system than would be lost in the power dividers or phasing harnesses. Stacking isn’t easy or inexpensive, but the performance gain can be substantial.
Figure 1—An example of vertical antenna stacking.
Figure 2—Demonstrating antenna pattern gain with balloons. Take a balloon, blow it up so that it is roughly circular in shape and then declare that this is a radiation pattern from an isotropic radiator. Next, blow up another balloon to the same size and shape and tell the audience that this will be the “reference” antenna (A). Then, squeeze the first balloon in the middle to form a sort of figure-8 shape and declare that this is a dipole and compare the maximum size to that of the reference “antenna” (B). The dipole can be seen to have some “gain” over the reference isotropic. Next, squeeze the end of the first balloon to come up with a sausage-like shape to demonstrate the sort of pattern a beam antenna creates (C).
From QST February 2001
Thursday, October 28, 2010
When it comes to HF antennas, how important is the elevation angle?...
Q When it comes to HF antennas, how important is the
elevation angle?
A Presuming that you are interested in working worldwide DX on the HF bands, the vertical (elevation) angle of maximum radiation is of considerable importance. An elevation angle of 5° is very shallow, while 90° is straight up (not a good angle for long-distance communication!). You want your radiation pattern to be at a low elevation angle so that the signal energy will be refracted by the ionosphere in such a way that it propagates as far as possible (see Figure 1).
Tables 1, 2 and 3 from The ARRL Handbook show optimum elevation angles from locations in the continental US. These figures are based on statistical averages over all portions of the solar sunspot cycle.
Since low angles usually are most effective, this generally means that horizontal antennas should be high—higher is usually better.
Figure 1—The elevation angle advantage. If your signal takes off at a high elevation angle (A), it won’t propagate very far. Lower the angle (B), and the increase in distance can be considerable. A wavelength of height at a particular frequency results in a peak elevation angle of about 15°.
From QST January 2001
elevation angle?
A Presuming that you are interested in working worldwide DX on the HF bands, the vertical (elevation) angle of maximum radiation is of considerable importance. An elevation angle of 5° is very shallow, while 90° is straight up (not a good angle for long-distance communication!). You want your radiation pattern to be at a low elevation angle so that the signal energy will be refracted by the ionosphere in such a way that it propagates as far as possible (see Figure 1).
Tables 1, 2 and 3 from The ARRL Handbook show optimum elevation angles from locations in the continental US. These figures are based on statistical averages over all portions of the solar sunspot cycle.
Since low angles usually are most effective, this generally means that horizontal antennas should be high—higher is usually better.
From QST January 2001
I recently bought some radio crystals. Most are removed from 1940s Navy radios ...
Q Jon, W4BCT, asks, “I recently bought some radio crystals.
Most are removed from 1940s Navy radios. When I
was young my father had some of these, and I wanted to take them apart to see what was inside. Of course, he wouldn’t allow this. Now I have some to play with, but I was wondering if you could explain how crystals work?”
A A number of crystalline substances found in nature have the ability to transform mechanical strain (movement) into an electrical charge, and vice versa (think of a tuning fork or a church bell which can transform mechanical strain into sound). This property is known as the piezoelectric effect. A small plateor bar cut in the proper way from a quartz crystal and placed between two conducting electrodes will be mechanically strained when the electrodes are connected to a source of voltage. Conversely, if the crystal is squeezed between two electrodes a voltage will be developed between the electrodes.
Crystalline plates also are mechanical resonators that have natural frequencies of vibration ranging from a few thousand hertz to tens of megahertz. The vibration frequency depends on the kind of crystal, the way the plate is cut from the natural crystal, and on the dimensions of the plate (like the tuning fork and the bell). The thing that makes the crystal resonator valuable is that it has extremely high Q, ranging from 5 to 10 times the Qs obtainable with good LC resonant circuits.
Since the crystal has a definite resonant frequency controlled by the crystal lattice, it can be used to “regulate” an oscillator to a high degree of accuracy.
The crystals we use most often resonate in the 1- to 30-MHz region and are of the AT cut, thickness shear type, although these last two characteristics are rarely mentioned. A 15-MHz-fundamental crystal of this type is about 0.15 mm thick. Because of the widespread use of reprocessed warsurplus, pressure-mounted FT-243 crystals, you may think of crystals as small rectangles on the order of a half-inch in size. The crystals we commonly use today are discs, etched and/or doped to their final dimensions, with metal electrodes deposited directly on the quartz. A crystal’s diameter does not directly affect its frequency; diameters of 8 to 15 mm are typical.
AT cut is one of a number of possible standard designations
for the orientation at which a crystal disc is sawn from the original quartz crystal. The crystal lattice atomic structure is asymmetric, and the orientation of this with respect to the faces of the disc influences the crystal’s performance. Thickness shear is one of a number of possible orientations of the crystal’s mechanical vibration with respect to the disc. In this case, the crystal vibrates perpendicularly to its thickness. Place a moist bathroom sponge between the palms of your hands, move one hand up and down, and you’ll see thickness shear in action.
From QST January 2001
Most are removed from 1940s Navy radios. When I
was young my father had some of these, and I wanted to take them apart to see what was inside. Of course, he wouldn’t allow this. Now I have some to play with, but I was wondering if you could explain how crystals work?”
A A number of crystalline substances found in nature have the ability to transform mechanical strain (movement) into an electrical charge, and vice versa (think of a tuning fork or a church bell which can transform mechanical strain into sound). This property is known as the piezoelectric effect. A small plateor bar cut in the proper way from a quartz crystal and placed between two conducting electrodes will be mechanically strained when the electrodes are connected to a source of voltage. Conversely, if the crystal is squeezed between two electrodes a voltage will be developed between the electrodes.
Crystalline plates also are mechanical resonators that have natural frequencies of vibration ranging from a few thousand hertz to tens of megahertz. The vibration frequency depends on the kind of crystal, the way the plate is cut from the natural crystal, and on the dimensions of the plate (like the tuning fork and the bell). The thing that makes the crystal resonator valuable is that it has extremely high Q, ranging from 5 to 10 times the Qs obtainable with good LC resonant circuits.
Since the crystal has a definite resonant frequency controlled by the crystal lattice, it can be used to “regulate” an oscillator to a high degree of accuracy.
The crystals we use most often resonate in the 1- to 30-MHz region and are of the AT cut, thickness shear type, although these last two characteristics are rarely mentioned. A 15-MHz-fundamental crystal of this type is about 0.15 mm thick. Because of the widespread use of reprocessed warsurplus, pressure-mounted FT-243 crystals, you may think of crystals as small rectangles on the order of a half-inch in size. The crystals we commonly use today are discs, etched and/or doped to their final dimensions, with metal electrodes deposited directly on the quartz. A crystal’s diameter does not directly affect its frequency; diameters of 8 to 15 mm are typical.
AT cut is one of a number of possible standard designations
for the orientation at which a crystal disc is sawn from the original quartz crystal. The crystal lattice atomic structure is asymmetric, and the orientation of this with respect to the faces of the disc influences the crystal’s performance. Thickness shear is one of a number of possible orientations of the crystal’s mechanical vibration with respect to the disc. In this case, the crystal vibrates perpendicularly to its thickness. Place a moist bathroom sponge between the palms of your hands, move one hand up and down, and you’ll see thickness shear in action.
From QST January 2001
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