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Re: [Discuss-gnuradio] Maximum antenna input voltage on LFRX

From: Mark J. Blair
Subject: Re: [Discuss-gnuradio] Maximum antenna input voltage on LFRX
Date: Fri, 24 Dec 2010 16:20:42 -0800

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 <address@hidden>
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.

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