.net barcode reader library Forward versus reflected power graphs in Software

Creator Data Matrix in Software Forward versus reflected power graphs

27-20 Forward versus reflected power graphs
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540 Measurements and adjustment techniques
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Dummy loads
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A dummy load is a substitute antenna for making measurements and tests In fact, British radio engineers often refer to dummy loads as artificial aerials There are several uses for these devices Radio operators should use dummy loads to tune up on crowded channels and (only then) transfer to the live antenna The other use is in troubleshooting antenna systems Suppose we have a system in which the VSWR is high enough to affect the operation of the transmitter You can disconnect each successive element and connect the dummy load to its output If the VSWR goes down to the normal range, then the difficulty is downstream (ie, toward the antenna) You will eventually find the bad element (which is usually the antenna itself) Figure 27-21 shows the most elementary form of dummy load, which consists of one or more resistors connected in parallel, series, or series-parallel so that the total resistance is equal to the desired load impedance The power dissipation is the sum of the individual power dissipations It is essential that noninductive resistors be used for this application For this reason, carbon-composition or metal-film resistors are used For very low frequency work, it is permissible to use special counter wound wire low-inductance resistors These resistors, however, cannot be used over a few hundred kilohertz Several commercial dummy loads are shown in Figs 27-22 through 27-24 The load in Fig 27-22A is a 5-W model, and is typically used in Citizens Band servicing The resistor is mounted directly on a PL-259 coaxial connector These loads typically work to about 300 MHz, although many are not really useful over about 150 MHz A higher-power version of the same type is shown in Fig 27-22B This device works to the low VHF region, and dissipates up to 50 W I have used this dummy load for servicing high-VHF land mobile rigs, VHF-FM marine rigs, and low-VHF land mobile rigs Very high power loads are shown in Fig 27-23 These devices are Bird Electronics coaxial resistors, and operate to power levels up to 10, 30, and 40 kW These high-power loads are cooled by flowing water through the body of the resistor and then exhausting the heat in an air-cooled radiator Our final dummy load resistor is shown in Fig 27-24 The actual resistor is shown in Fig 27-24A, and a schematic view is shown in Fig 27-24B The long, highpower noninductive resistor element is rated at 50 , and can dissipate 1000 W for
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27-21 A very basic dummy load
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27-22A CB-style 5-W dummy load
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27-22B 15-W dummy load
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27-23 Termaline RF wattmeter
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27-24A Drake DL-1000 dummy load
several minutes If longer times, or higher powers, are anticipated, then forced air cooling is applied by adding a blower to one end of the cage The device is modified by adding the BNC sampling jack This jack is connected internally to either a twoturn loop, made of no22 insulated hookup wire, or brass rods that are positioned alongside the resistor element It will therefore pick up a sample of the signal so that it can be viewed on an oscilloscope, or used for other instrumentation purposes
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50J1 SO-239 RF input
noninductive resistor
J2 BNC sampling 100
Brass tubing or 2 3 turn loop of no 22 wire 27-24B HF dummy load with pickup loop
CHAPTER
General antenna mechanical construction techniques
ANTENNA INSTALLATION CHORES VARY IN COMPLEXITY FROM THE SIMPLE, WHICH A teenager, with no assistance, can execute properly and safely, all the way up to largescale projects that are best left to professional antenna riggers If your only antenna installation chore is a dipole handing between the peak of your house roof and a nearby tree, then little need be said besides Be careful The purpose of this chapter is to cover both wire and intermediate installation jobs that can be done by amateurs if instructed properly Keep in mind when planning an installation, however, that the information given herein is merely informal guidance Always have your plans reviewed by a professional mechanical or structural engineer licensed to practice in your state In many locales, there are good insurance reasons for this step Also, learn and abide by local mechanical, electrical, and building codes It is simply not worth the pain, and risk, to skirt around local laws
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