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Thanks adrian
There are three things that have to met to successfully use a linear amplifier with the Baby loop:
1. Power
The maximum power that can be handled by the loop is 400W up to 21 MHz, and 800W from 22MHz upwards - it is most important to ensure that these limits are not exceeded. Excessive power will lead to an RF arc on the capacitor plates, and this will result in the probable destruction of the output transistors of the PA, or, if you are using a vacuum tube amplifiers could lead to a plasma arc in the tube.
2. Controller Power Limit
The controller is only rated to handle a maximum of 200W, so the amplifier must be inserted AFTER the controller. Connect the antenna output of the controller to the RF I/P of the amplifier, and the RF O/P of the amplifier to the loop.
3. Autotune Limitation
In order for the loop controller to be able to tune the antenna, the amplifier MUST have a direct RF through path when in the receive state. In addition, any ATU in the amplifier must be bypassed in the receive state otherwise there will be confusing SWR indications to the controller and tuning will fail. In order for the controller to be able to successfully tune the loop the SWR when far from resonance has to be high and constant, if this condition is not met, for example because of an ATU in the signal path, then false nulls will be detected and you will be unable to tune correctly. A good test is to use the Antenna Auto-Detect feature after you have inserted the linear. If the controller can auto-detect the antenna type and motor sense connection, then it will work correctly under normal operation. Under normal circumstances the SWR of the loop when correctly tuned is sufficiently low that no ATU is needed.
Regards,
Adrian, 5B4AIY