On Dec 24, 2010, at 3:37 PM, Nick Smith wrote: > I've looked through the mailing list archive and found a few messages related > to this, but they referred to signal generators and power in dBm. > In my case, I plan to use it with an antenna and eventually a pre-amp. > However, I've found that by measuring the voltage from even a 3ft (90cm) > antenna using a multimeter with a probe on the antenna and a probe to ground > gives a reading of four volts RMS. With a 25ft (7.6m) antenna, I can slightly > light up an LED (I don't remember the voltage), and with a 100ft (30.5m) > antenna, I got about 140V.
A multimeter has very high input impedance. If you attach a 50 ohm resistor(*) between the antenna and ground to simulate the input impedance of a radio receiver, then you should no longer see a measurable voltage with a multimeter. Also, a multimeter won't properly measure RF voltage. You're probably just seeing 60 Hz power line noise, unless you're very close to a high-powered transmitter. If there's a 100kW broadcast transmitter next door, then all bets are off. :) (*) or any other value you can get your hands on between around 25 and 500 ohms -- Mark J. Blair, NF6X <n...@nf6x.net> Web page: http://www.nf6x.net/ GnuPG public key available from my web page. _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
