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.
On http://gnuradio.org/redmine/wiki/gnuradio/UsrpFAQDBoards , under the
question “If I want to apply the output (50 ohm) from a function
generator directly to the BasicRX inputs, what is the allowed voltage
range that can be applied?”, it states “3V p-p should be safe, since it
won’t exceed the 3.3V supply voltage.”
While that is for the BasicRX, I assume another limit exists with the
LFRX.
I just want to confirm that I won’t break anything if I attach an
antenna.
Thanks for your time!
— Nick
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 [email protected]
Web page: http://www.nf6x.net/
GnuPG public key available from my web page.
On 12/24/2010 08:02 PM, Nick Smith wrote:
multimeter.
around 25 and 500 ohms
So, you measured above 1V with a 33ohm resistor across the multimeter?
I’m glad I don’t live in that house! Yikes!
–
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
I tried with a 330 ohm and 33 ohm resistor, and the voltage stayed the
same as without. I have no doubt that it is mostly power line and the
1kW AM station that is within walking distance. 
Also, the ground here isn’t a very good ground, so that probably throws
measurements off.
As for where it’s situated, it’s an indoor antenna. In a very noisy
house.
I long for a better location.
— Nick