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Unread 23-06-2002, 22:09
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Re: Capacitors on speed controllers and antenna attenuators

Posted by Thomas A. Frank, Engineer on team #121, The Islanders/Rhode Warrior, from Middletown (RI) High School and Naval Undersea Warfare Center.

Posted on 3/26/99 10:54 AM MST


In Reply to: Re: Capacitors on speed controllers and antenna attenuators posted by Jerry Eckert on 3/21/99 6:43 PM MST:




: Another note -- make sure you use the attenuator on the receiver antenna.

: Even with the capacitors we had severe problems with data corruption errors which left the
: robot essentially dead. These problems occurred only when operating from one position on
: one field: it worked fine in the pits and from the other three positions. FIRST claimed
: this was due to the attenuator not being used on the receiver antenna. I can't verify
: this was actually the problem as we didn't have another match in that position after
: correcting the problem, but I wanted to give everyone a heads up so they can make sure
: they don't run into the same situation.


The attenuator may not be the solution to the problem. We went through this last year...first
we used the 10 dB attenuator, no luck. Then we went to a 20 dB attenuator, still no luck. Then
I got that feeling like maybe we had done something 'weird', so I ran a signal strength meter
up and down the structure of our machine; what I found was that we had placed the antenna at an RF "null"
on the robot. Which is to say, with a transmitter operating in the RNet freq range nearby, the signal
strength on the robot at the antenna was virtually zero. I moved the antenna several inches, and the
entire problem disappeared. Several other teams were experiencing the same problem in NH, and after
they moved their antennae the problem seems to have sorted itself out.

In our case, I suspect we accidentally made a waveguide (the RNet's work at about 460 MHz, lambda = 26"
at this freq, which was very close to the dimensions of our structure); or perhaps the shape yielded
destructive interference at our antenna location.

Regardless of the cause, moving the antenna solved the problem...we did make it to the finals in FL...while
we really had problems in NH...

So I would strongly suggest that you also try moving the antenna. It's simple, free, and it might solve the
problem for you.

I'd also check the cable from the RNet to the receiver box. FIRST has experienced some QC problems in this area
over the years.


Tom Frank


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