Tony,
If it helps with your SPICEing I've attached a screenshot of the complete circuit definition that I used in my QUCS simulation. It includes modest allowance for strays and losses. My 0.5ohm for the diodes is just a guess. Note that here the .s1p blocks are de-activated and the toroids are simply specified as lumped inductors.
The four 'opt' blocks on the far right show component values results derived from some 'quick and dirty' optimisations, whereby I optimise the 14MHz rejection for a number of different passband return losses. The first number in the title of each block? is the notional target return loss (in dB) and the second is the 14MHz rejection achieved. (These results are not rigorous because the R.L. is only assessed at a small number of frequencies. But it gives you an idea of the possible trade-offs.)
The issue of acceptable LPF return loss might be important given the number of PA failures reported under poor VSWR conditions. Because depending on the antenna load phase the LPF's own return loss function (whose own spec assumes a perfect 50ohm load) may worsen the VSWR seen by the PA.
- Nick.
