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
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