We complete the electronics requirements for our loop antenna design with a wideband loop base amplifier to match the transmission line to a receiver.
So far in our implementation, we have a one meter loop connected to a transimpedance amplifier, with the balanced output feeding a length of CAT 7 cable. While it was possible to just use another balun to connect the CAT cable to the receiver, I wanted to keep the head end gain reasonably low and add another gain block back at shack.
To accomplish this, I designed a wideband loop base amplifier using an AD8067 op amp. The final circuit board would contain two of these amplifiers, one for each loop in a diversity system. Along the way, the wideband loop base amplifier needed to accomplish some important tasks.
First, it needed to match a 100Ω transmission line to a 50Ω receiver input. I should be able to achieve this using a differential summing circuit.
Second, and quite important, was a circuit that would reject common mode interference that found its way into the system. Because of the way op amps are made, a good differential summing amp should pass a differential mode signal (from the balanced line) while rejecting common mode signals (where the voltage is equal on both lines.)
Finally, the amplifier should add a bit of gain to overcome transmission line loss.
As you can see in the schematic above, the op amp is biased for a single 5 volt power supply. When completed, the wideband loop base amplifier board should also be able to feed this power supply to the head end board over the CAT 7 cable.
Wideband Loop Base Amplifier Parts Selection
I selected the AD8067 for the design. This op amp has a 54 MHz bandwidth, low noise and a high common mode rejection ratio, more than 80 dB. As laid out, the overall system should provide another 12 dB of gain after the head end amplifier.
My LTSpice simulation showed that signals of S9+50 should not overload this amp. If I need more or less gain, changing bias resistors would make this possible. Check out the datasheet for more information. To ensure flat response to 30 MHz, though, gain should not be too high.