Yaesu FT-847 Operating Manual

Also see for FT-847: Technical supplement Technical supplement Technical supplement Modification

Contents

36 FT-847 OPERATING MANUALRECEIVER FEATURESRECEIVER INPUT ATTENUATORFor situations where extremely high signal levels arepresent, the receiver’s input ATTENUATOR may be acti-vated, to reduce the sensitivity and total gain of thereceiver system.Å To activate the receiver ATTENUA-TOR, push the [ATT] key. The “ATT”icon will become illuminated on thedisplay. You should also hear a de-crease in the incoming signalstrength (and background noise)with the ATTENUATOR on.Ç To switch the ATTENUATOR off, press the [ATT] keyonce more; the “ATT” icon will disappear from thedisplay.AGC (AUTOMATIC GAIN CONTROL)The AGC system limits the audio output produced byincoming signals, so as to prevent damage to your earsand overload of the receiver components. The AGCsystem works instantaneously when an incoming sig-nal is detected, while the receiver recovery time maybe adjusted from the front panel.When tuning around the band (look-ing for signals), it usually is best tohave “FAST” AGC recovery time se-lected (the [AGC-F] switch should bepressed momentarily so that the “ AGC-F” icon appears at the bottom right-hand corner of the LCD’s meter area).In this mode, the receiver gain recovers quickly afteryou tune past strong signals. Once you have a signaltuned in, unless it is very weak, you may find recep-tion more comfortable with “SLOW” AGC selected(press the [AGC-F] key once more to turn off the“ AGC-F” icon).RF PREAMPLIFIERS (INTERNAL)For weak-signal work, it is essential to secure the bestsystem Noise Figure possible. The receiver’s inputpreamplifiers provide low Noise Figure for best sen-sitivity in weak-signal situations. Individually-opti-mized preamplifiers are provided for HF, 50 MHz, 144MHz, and 430 MHz. In noisy locations or when verystrong signals are present, the input preamplifiers maybe switched off, thereby reducing the receiver systemgain and minimizing the chances of degraded recep-tion due to overload or intermodulation distortion.Å To activate the receiver preampli-fiers, push the [RF AMP] key. The“ R F A M P” icon will become illu-minated on the display. You shouldalso hear an increase in the incom-ing signal strength with the pream-plifier on.Ç To switch the preamplifier off, press the [RF AMP]key once more; the “ RF AMP” icon will disappearfrom the display.RF PREAMPLIFIERS (EXTERNAL)The function of the [RF AMP] switch can be modi-fied to allow the use of external tower-mounted pream-plifiers on the 144 MHz and 430 MHz bands, by sup-plying +12V DC via the ANT jack for each respectiveband. If you own a preamplifier capable of acceptingits DC power via the coaxial line, you may wish toexercise this option so as to place the preamplifier inthe line right at the antenna, where the gain and lowNoise Figure of the preamp can be used to overcomethe insertion loss of your coaxial cable feedline.Menu #29 controls the selection for the 144 MHz band,while Menu #30 controls the selection for the 430 MHzband. The available choices are:l INT: The preamplifier internal to the FT-847 willbe selected.l EXT: The internal preamplifier will be bypassed,and +12V DC will instead be sent up the co-axial line for powering an external preamp.These selections are not provided on the HF and 50MHz antenna jacks, as there is little chance that thesystem Noise Figure can be significantly improved bymounting a preamplifier at the tower. If you wish touse an external preamp on 50 MHz, you must supplythe necessary DC voltage via a separate power cable;just remember not to activate the internal preamp, asthe system total gain will probably be much too high,leading to increased probability of receiver overloadand intermodulation.OPERATIONNOTEThe maximum allowable current drain for pow-ering external preamplifiers is 300 mA. Checkyour preamp’s specifications to ensure that thisspecification is not exceeded.