[USRP-users] Safety Calculation

Kevin McGuire kmcg3413 at gmail.com
Thu Oct 12 20:34:03 EDT 2017


I got to doing some calculations and I realized that exceeding -15dBm on
the receiver circuitry could be a lot easy than it seems to amateurs like
me. So, I wanted to run this by some of you because I know that you guys
have far more radio engineering knowledge than me.

So, lets take a situation for me. Lets say there is a 100-watt transmitter
within proximity of me. And, I never thought much about it but obviously
that could be a problem. When I use the link budget calculation I enter in
50dBm for the transmitter power, set zero for transmitter loses, set zero
for receiver loses, set 5db for both TX and RX antennas. Then for the
distance I have to put in about 3200 feet in order to only see a link
budget of -15dBm.

I know that is only for perfect world conditions and then its even more
complicated in reality but I figure that should give me a fairly good idea.
It is actually scary because I used to tinker with my B200 less than 500
feet from an antenna that was 50-foot up around 154mhz.

Now, I am not sure what part of the RX chain is sensitive. It could be
after filtering which means that -15dBm is only for a signal that gets
through but I suppose also to be on the safe side I might need to consider
anything. Anyway, it just took me by surprise when I realized how much and
how easily I could exceed -15dBm. I know too it depends on time so that 1ns
of -5dBm might not burn something up but you know in my case this 100-watt
FM transmitter would transmit for a few seconds at a time.

What are your thoughts, suggestions, or ideas?
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