A common problem with qrp transceivers is a weak frontend . Most end fed antennas use low pass tuners causing high stress due to strong signals coming from BC stations .
The High
pas tuner used here helps to reduce these problems. Put 19 turns 1mm
transformer wire on a T130-6 torroid to form a 4.8 uH coil. The trimmer used is
a 60pF type set to about 25pF . Put a
3.3K resistor between the antenna connector and the ground of the coax
connector and tune the trimmer for resonance at 14.180 MHz. Keep in mind that a thin antenna wire has a higher Z.
After tuning the unit connect the half Lambda wire ( 10.15m PVC covered stranded copper wire) and cut it for lowest swr. A 1:1.2 swr is easily obtained with this
unit. The cabinet is a PVC type G302 made by Velleman. Every end fed / vertical needs some kind of counterpoise to push
against and in this case that's the coax , so don't forget a line choke
near to your transceiver.