Hi Allison,

On 13 Lug, 04:13, wrote:


Generally if it's two cascaded 1/4wave sections as a half wave filter
they are identical. However the designer may also have the filter
going from 50ohms to some other non 50 ohm impedence.
It's not uncommon to see that. Also the middle cap for the two
sections usually is the some of the two end caps if the sections
were viewed in isolation.

In most cases when I encounter that I scale the values by 50/70 (.714)
so that the 47 pf becomes around 33 and the 100pf around 68 and same
for the coils. That maintains the same relationships and impedences
overall.

Actually I found my mistake, the formula I was using for inductance
required the external coil diameter and I was using the internal
diameter, so everything is clear now.
For instance the values you suggest are just right even calculating
with the formulas for a scaled cutoff frequency of about 90 MHz.
By the way, this filter is clearly for 50 ohms both input and output,
one side is the antenna and the other side is the output impedance
adapter filter of the final transistor.

Now the question is if I should try to use the same final (and driver)
transistor of the original 50 MHz transverter (2sc2166 driver and
2sc1969 final) which are 27 MHz transistor (working fine at 50 MHz).
On the datasheets I have been able to find there's no indication of
their ft.
I have a better final transistor but it would require me to move the
amplifier part outside of the original box (different case).

Thanks and 73

Francesco IZ8DWF