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AMATEUR RADIO BY GARRY CRATT, VK2YBX How to make transmitter measurements Amateur radio operators have a responsibility to ensure that the power, frequency & harmonics of their transinitters are maintained within specified limits. This requires the use of some specialised test equipment to make accurate measurements~ gains to be had in the surplus test equipment market. For instance, a Hewlett Packard HP5245 frequency counter, which has an accuracy of three parts in 109 (ie, 3Hz per 1GHz!), can readily be obtained for a few hundred dollars. These counters offer far Watts To dBm Conversion Chart Perhaps the most obvious transmitter parameters that must be monitored are power output and frequency. Fortunately,transmitter frequency is relatively easy to check, if not with an inexpensive frequency counter, then with a receiver covering the particular band of operation. Some VHF and UHF repeaters even have a system whereby an audio tone is transmitted by the repeater, dependent upon the frequency error of the incoming signal. If we assume that a frequency error of 0.005% is accept- able on the VHF amateur bands, this equates at worst case, to an actual frequency error ofless than lkHz. Most kit frequency counters, have similar accuracy. However, professional test laboratories use frequency counters having an accuracy of five parts in SILICON CHIP +3OdBm +36.98dBm 10W +4OdBm 15W +41.76dBm 2OW +43.O1dBm 25W +43.979dBm 1010_ 3OW +44.77dNm Look for bargains 35W +45.44dBm Although a frequency counter kit having reasonable accuracy can be obtained for several hundred dollars, there are also some exceptional bar- 4OW +46.O2dBm 45W +46.53dBm Surplus test equipment can often be picked up on the secondhand market at bargain prices. This HP5245 frequency counter only costs a few hundred dollars & has an accuracy of three parts in 109 , making it ideal for frequency checks. 78 1W 5W SOW +46.98dBm 55W +47.4OdBm 6OW +47.78dBm 65W +48.13dBm ?OW +48.45dBm 75W +48.75dBm BOW +49.O3dBm 85W +49.3.dBm 9OW +49.54dBm 95W +49.77dBm 1OOW +5OdBm 1O5W +5O.21dBm 11OW +5O.41dBm 115W +5O.6OdBm 12OW +5O.8OdBm 12sw +5O.97dBm 13OW +51.14dBm 135W +51.3OdBm 14OW +51.46dBm 145W +51.61dBm 15OW +51.76dBm L2 OUTPUT ~ L1, L2 AND L3 : 2T, 1mm ENCW WOUND ON 8mm DIA FOAMER Fig.I: this simple notch filter circuit provides 30-40dB of attenuation in the 144-148MHz amateur band. The notch filter can be built on a small piece of blank PCB material & housed in a metal diecast case. superior value for money. A range of plug-in modules for these counters can extend the range of the instrument up to 12GHz. (HP-432A, etc) offer an accuracy of 0.2%, at a cost of $500-$700. Output power Amateur operators must also be able to measure transmitter output power. This is perhaps the easiest measurement to make, considering the range of low-cost HF/VHF/UHF power meters that are now -available. However, care should be taken to ensure that the rating of the power meter is not exceeded. When measuring the output power of an SSB transceiver, a "PEP" (peak envelope power) meter should be used. The "industry standard" power meter is the Bird model 43. This is an insertion type power meter, requiring plug-ins for different frequency bands. This can add significantly to the cost. Also it can only claim an accuracy of 5 % . More accurate measurements can be achieved using a thermistor mount and power meter. These instruments must be used in conjunction with an in-line attenuator. By taking the displayed power level (usually in dBm) and adding the value of the series attenuator, the transmitter's power output can be calculated. These instruments are broadband, usually offering a frequency coverage from 10MHz to 40GHz or so, and covering power levels from as low as l0µW to 10mW, when used without external attenuators. The accompanying table shows the relationship between watts and dBm. Hewlett Packard produced a series of these power meters during the late 1970s and early 80s. Models HP-43 lA/ B/C can be found at surplus test equipment stores for as low as $300 or so. Such an instrument offers a measurement accuracy of 3%. Later models Transmitter harmonics The amateur bands are harmonically related to ensure that interference from maladjusted transmitters falls within other amateur bands. Even so, good operating practice dictates that amateurs should be capable of measuring harmonic radiation levels at the transmitter output. Transmitter harmonics are most easily measured using a spectrum analyser. However, some precautions are necessary before connecting a transmitter to a spectrum analyser. A typical spectrum analyser such as the Hewlett Packard model 141, available to amateurs at a reasonable price, has a dynamic range of 50dB or so, and clearly, the transmitter cannot be con- nected directly to the analyser. So an attenuator or "pad" must be inserted between the two pieces of equipment. The value of this attenuator must be such that the spectrum analyser is not driven into overload (compression). A typical 25W transmitter (+44dBm) must be connected via 70dB of attenuation to reduce the signal level at the analyser input to -25dBm (around 220mV into a 50Q. load). Such attenuation can be achieved by connecting several discrete attenuators in series. The first must be capable of handling the transmitter output power, while the remaining attenuators can have a lower rating (dependent on the value of the first pad). In most cases, a fixed attenuator is directly connected to the transmitter and variable attenuators are used to optimise the input to the spectrum analyser. However, this approach has one serious drawback. By attenuating all signals by 70dB or so, the spectrum analyser would need a display range of over 80dB to readily observe a transmitter harmonic having a power level of -30dBm (ie, -74dB with respect to the fundamental). What is really needed is a selective attenuator, or filter. By notching the fundamental frequency by 25-40dB, without affecting the level of the harmonics, a reduced amount of additional attenuation is necessary, enabling those harmonics to be more easily observed on the spectrum analyser. Fig. l shows the circuit diagram and layout of a suitable filter. This has a notch attenuation of 30-40dB in the 144-148MHz band and only 0.ZdB attenuation at the second and third harmonics. The unit is easily constructed in a diecast aluminium box and the filter components can be mounted on a piece of blank PCB material. By drilling holes in the lid of the box, access can be obtained to the trimmer capacitors, thus making the filter adjustable. SC Hamfest 92 The "industry standard" Bird model 43 is an insertion-type power meter with plug-in modules for different frequency bands. The Northern Corridor Radio Group (VK6ANC) has advised us that their annual event "Hamfest 92" will be held on Sunday, 1st November at the usual venue of Carine College of Tafe, in Perth, WA. All are welcome. OCT0BEH 1992 79