[CQ-Contest] Band reject filter power limitations

00tlzivney at bsuvc.bsu.edu 00tlzivney at bsuvc.bsu.edu
Thu Mar 23 12:57:53 EST 2000

The ICE filters use a pair of parallel LC circuits coupled with a capacitor.
This topology tends to be foregiving of transmitting on the wrong band.

sume that the filter is tuned for 20m and you transmit on 40; then the   
voltage across the capacitors is "merely" a function of the mismatch your
antenna shows at the wrong frequency.

However, the Dunestar filters use a series-tuned LC circuit to couple the
pair of parallel LC circuits.  This, while giving much better filtering
capabilities (which has often been measured and reported) also is less
forgiving.  The biggest problem occurs when your rig is tuned to the same
frequency as the filter but the antenna is tuned to another band.  In this
case, the voltage across the series capacitor in the series LC section
can rise toward Q times the voltage across the load!  Since the Q will
be high (for low transmission losses), this can be catastrophic.  The
series LC circuit was used as a source of high voltage in early television

Terry Zivney, N4TZ/9
n4tz at arrl.net

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