Re: Are these output watt rating crazy?

Jerry Gaffke


I assume you posted to this old thread because
you wanted to tell us about your simulation results.

Yes, the uBitx should not be used on 160m without a suitable LPF
inline with the antenna.  Without that LPF the third harmonic will be
nearly as strong as the fundamental.

Not sure exactly how you went about simulating the filter
or exactly what corner frequency you saw.
Regardless of what your filter program thinks, the 80m LPF
is being used in a more or less 50 ohm environment.
Changing the impedances will change the frequency response.

Attenuating some harmonic by 13dB may or may not be sufficient.
If that particular harmonic coming out of the final amp is already 30dB down
from the fundamental, then the extra 13dB of attenuation from the filter
will be sufficient to meet FCC regs.  Due to the push-pull configuration
of the IRF510 final, it is mostly the 3rd harmonic that gives us grief.

Would be interesting to measure (not simulate) the response of the 80m LPF
using something like the Antuino, driving the filter from a 50 ohm source
and terminating it into 50 ohms.

I believe the 3dB corner frequency of the 80m LPF is around 5mhz,
so if you tried to use it on 40m you would not be putting out much of a signal.
Here's the post you quoted
It happens to be in the same thread as our current conversation,
though this thread hasn't been active for over 6 months.,

Jerry, KE7ER

On Sun, Nov 10, 2019 at 05:30 PM, Mark - N7EKU wrote:
Jerry Gaffke
Apr 25  
The 80m LPF is also used for 60m, so the cut-toff freq is high enough to pass the third harmonic of 160m.This is not a sine wave:

I just entered the lowest ubitx LFP into a filter program and see that it is actually a 40m LPF.  It has a pretty good cutoff, so it cuts off what it need to for 60m and 80m, but would do almost nothing for 160m as it won't cutoff anything until the 4th harmonic and will only do about a 13dB cut there so not really effective until the 5th harmonic.  So basically useless on 160m.

Besides that, the input impedance to the filter on 160m comes out to about 10 ohms so not what the PA is designed to see.  On 60m it shows as 20 ohms a little better, but not great.



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