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September 25th 03, 09:54 PM
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In article , Paul Keinanen
writes: On 24 Sep 2003 22:04:07 GMT, (Avery Fineman) wrote: The effect should not be there with or without diodes, with or without any resistors...unless there is some VERY big RF source out of the receiver's tuning range that is supplying energy to the diodes and thus causing the "mixer" effect. It should also be noted that when several quite strong out of band signals are present at the antenna, say ten signals, each with S9+60 dB, which is 50 mVrms (71 mVpeak) into 50 ohms and -13 dBm. On average, these signals produce a combined signal ten times as large at -3 dB and the rms voltage is about 150 mV. However, from time to time, the vectors for each individual signal add up, so you have to add the _voltages_ for that moment, so the maximum theoretical peak amplitude is 700 mV (10x71 mV), thus, a single silicon diode starts to conduct, causing all kinds of mixing products. True enough. Some friends of mine live near the transmitter site of AM broadcast transmitter of KMPC in the San Fernando Valley section of Los Angeles, CA. KMPC is the only high power station in the "Valley" at 50 KW _into_ the antenna. :-) Within a few blocks of the KMPC transmitter, ANYTHING is possible insofar as IMD products, from tube type to solid-state receivers, some telephones, a few computers, intercoms, etc. :-( Using two (or more) 1N4148 type diodes in series instead of a single diode in the each back to back pair, will prevent any diode conduction as long as the peak voltage is larger than 1,4 V in either direction. Good point. The maximum number of diodes in series is determined by the amount of voltage the following stages will tolerate without disintegrating. :-) I once had a zener that became a sort of LED on a breadboard. For about 5 seconds or so. Since most likely there will be some selectivity between this diode clipper and the first amplifier stage, the amplifier stage will never see voltages as the limiting voltages in normal operation, but the diodes will still cut out some abnormal peaks e.g. induced by lightnings. True enough, but an electrostatic pickup during a storm MAY reach as high as 200 Volts or so. That's a static charge effect during the build-up period for lightning. A friend of mine living in the mountains decided he would "scientifically" measure the electrostatic charge build-up with VTVM connected to a small strip-chart recorder on a long-wire antenna. Lived at a 4000 foot elevation. Observation resulted in the "200 V" value. Unfortunately, he had so much trouble with the cheap strip-chart recorder that storm season was over by the time he got the recorder working. :-( The diodes should not have any effect on anything but a few millivolts of any signal arriving on your antenna. A non-conducting diode simply shows a junction capacitance to the rest of the world. That's a minor reactive discontinuity to the antenna connection. Putting multiple diodes in series in the back to back combination also reduce the capacitances, since the capacitances in each string are in series. 2 pFd at 10 MHz is only an 8 KOhm reactance. Len Anderson retired (from regular hours) electronic engineer person |
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