[Amps] Input Impedance of RF MOS amp

[Stefan]

Dear collegues

Recently I was lucky enough to buy two KL300 
(http://www.rmitaly.com/scheda.asp?IDGr=1&cat=0&tipo=94) amplifier at a flea 
market for a reasonable prize (2USD each) and I got them to work again. I 
have checked the devices with my FT817 and they work rather well (I know, it 
does not have any output filters but I only wanted it for my FT817 to give 
it a punch, out there in the "wilderness" when carrying the appropiate 
battery).

These devices are claimed to be useful for the whole HF band but I have 
noticed a quite non uniform output power over the HF bands. I think the 
reason is because my rig folds back its ouput power by noticing an unmatched 
input impedance (the antenna is tuned or even a dummy load). Therefore I 
have the feeling that this amplifier does not have a flat input impedance 
(50 ohm). How can I flatten this impedance or what is done ususally to solve 
this problems. I suppose that the input impedance is the input impedance of 
the transistors (class B push-pull) plus the resistance put between the 
transistor's base and emitter (10ohm here).

I would appreciate your suggestions and solutions. On the above web is not 
only the amp itself but also its very simple diagram.

Thanks for your help.

Kind regards,

Stefan
EA5FY