Bird Wattmeters (several models)
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The use of a taut-band meter movement allows operation in cold temperatures. The Model 44A utilizes a set of precision directional detectors which sample forward and reverse CW power flow in a specially engineered section of transmission line. VSWR Voltage Standing Wave Ratio is easily determined by comparing these measurements and using the convenient chart on the back of the instrument. Five power scales are provided. The watt scale will test most high powered transmitters, while the 5 watt scale makes it simple to tune low powered portables.
These connectors can be interchanged in the field without affecting instrument calibration. The instrument's indicating meter is a shock-mounted microampere meter with scales of 25, 50, and ; permitting full-scale direct-power reading from milliwatts to 10, watts. Plug-in elements determine the power rating and the frequency range, and these values are marked on each element. The elements rotate to read both forward and reflected power.
There is no need for calibration charts or instrument adjustments. Elements can be purchased at any time and replaced in the field. For added convenience, Model 43 connectors and elements are interchangeable with many other Bird wattmeters. Please note that the picture may not show UHF type connectors but that is what will be supplied with this purchase.
Like similar diode devices, the Bird 43 indicates the carrier component of amplitude modulation, with very little response to side band compo- nents added by modulation. Standing Wave vs. The standing wave viewpoint, also widely used, is highly developed both in theory and in practice. This viewpoint can be traced to the early use of slotted transmission lines. The slotted line measures the standing wave ratio by mechanically positioning a voltage detector at peaks and nulls along a length of line section.
Its drawbacks are that it is usually too long, too expensive for good accuracy, not portable, and too slow. These problems grow rapidly as the measurement frequency drops below MHz. The Thruline Wattmeter by comparison is fast, convenient, and accurate. It provides the same information as a slotted line with the exception of the phase angle of the reflection coefficient distance, load to minimum. VSWR scales and their attendant controls for setting the reference point have been intentionally omitted from the Bird Experience using the Thruline Wattmeter for transmitter tune-up, antenna matching, etc.
The two meter readings, Wr and Wf, give a useful, approximate picture of the results without bothering to cal- culate the power ratio exactly. Consider that, for an antenna match- ing problem, the main objective usually is to minimize Wr. Anything done experimentally to this end will be seen when the element is turned to the reflected power position.
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Further effort is frequently not worthwhile because below this level reflected power is hard to measure, and Wl can not be significantly increased. TV and VHF transmitters are examples of systems requiring lower reflected power but for reasons other than maximizing power transmission. For accurate measurement of very low levels of reflected power, i. This method should not be used with element ranges differing by more than Example - Consider an 80 watt transmitter and a Bird 43 with and 10 watt elements. Measure Wf with the W ele- ment.
Measure Wr using the 10 W element with the arrow pointing towards the transmitter. Wr can be measured down to at least 0. Transmitter Monitoring The Thruline Wattmeter can be used for the continuous monitoring of transmitter output or reflected power, for instance in checking inter- mittent antenna or line faults. To correct for this without any calculations, simply connect the wattmeters directly, with no line between them, and adjust their zero settings until they are both zeroed. Open circuit testing is preferred to short circuit, because a high quality open circuit is easier to create than a high quality short.
Measuring Insertion Loss 1. Check forward and reverse power equality with a high quality open circuit,. Measure the line twice once down and once back.
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Compare the attenuation with published data for line type and length remember to halve Ndb or double the line length to account for the measurement technique. Make sure to note exact readings, or their dif- ference, on the initial equality check, and correct for this. Frequency Response Bird Plug-In Elements have a flat frequency response over their speci- fied operating range.
A sample set of curves is shown in Figure 6.
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Notice that for the low power element, the rolloff outside its frequency band is more pronounced than for the high power elements. Example - For a B element range 50 - MHz simply divide all frequencies by two. For an E element range - MHz multiply all frequencies by four. Harmonics or subharmonics that lie outside of the frequency range of the element may exist in the circuit under test.
Using an element for measurements outside of its frequency range is not recommended. The response curves presented are only typical, and not guaranteed. Always divide the larger impedance by the smaller, since VSWR must always be greater than 1. Example - Consider using a Bird 43 to tune a 70 ohm line. However, if the wattmeter is removed, the VSWR will actually be 1. Similarly, if the load impedance is Caution must therefore be used, since both good and bad matches can have the same measured VSWR. In this case, the correct impedance can be determined by slightly changing the load impedance.
When the load impedance is near 70 ohms, the Bird 43 will read increasing VSWR as the load impedance is increased. When transporting the Bird 43, insert the original dust plug, or an ele- ment with the arrow pointing upward, in the element socket and secure with the catch. This will shunt the meter circuit and protect the meter by dampening needle action during handling and shipping.
Also, secure spare elements in their sockets with the pivoting knob; just insert the element and twist the knob one-quarter turn.
Insert the Bird 43 in coaxial transmission lines of 50 ohms nominal impedance. The RF source can be connected to either side of the watt- meter without affecting readings. If non ohm cables are used, a mismatch will occur causing inaccu- racies in readings. However, if a mismatch cannot be avoided, refer to "Impedance Mismatch" on page 15 for instructions. It is strongly advised that this condition be avoided. Replacing Quick-Change Connectors 1.
Remove the screw at each corner of the connector. Unscrew the four flat head screws securing the back cover. Grasp the back cover by the side tabs behind the line connectors and pull straight back to remove it. Remove the two oval screws on the front of the housing. Slide the line section out of the housing. These hold the shock ring in place. Replace the cable attaching the line section to the meter with one of sufficient length to complete the remote installation. In some systems it may be desirable to have two or more line sections permanently installed. In this case, one set of elements and one meter can be used to measure several separate RF transmission lines.
Addi- tional RF line sections are available. A layout for the panel cut is given in Figure 7.
On thinner panels, build up the thickness with pads or washers to achieve a flush-face mounting. Attach the line section so that the finger catch is in the most accessi- ble position. The Bird 43 uses plug-in elements to make measurements. The transmitter test frequency should be within the band of the ele- ment used. See "Frequency Response" on page 13 for information on using the elements outside of their specified frequency range. The arrow on the element indicates directional sensitivity; i. Rotate the element to select forward or reverse power measurement.
43 Series, RF Wattmeters
With this combi- nation, readings only need to be taken in the forward direction because the reflected power will be negligible. Normal Operation 1. Insert the appropriate element in the line section socket.