1N191 compared to 1N34A in SB-401 for REL PWR measurement

[Robert Myers]

Hi,

 

On my SB-401 I replaced a blown 1N191 diode (used in measuring relative
power) with a 1N34A.  Suddenly I am reading a lot less on 15m, and
especially 10m, than on 80m and 40m.  Someone told me that since the voltage
divider used in the power detector circuitry was designed with the 1N191 in
mind, the 1N34A in its place would not necessarily give you the identical
level as before.

 

I can see how this may cause a flat shift across the band in the simple
front panel power 'measurement,' but this person added that the frequency
response of the 1N34A could cause a slant in measuring across the frequency
domain --- thus showing apparently less power at higher frequencies.

 

Over 80 to 10 this is noticeable??  I didn't think 10m was all the high in
frequency, but perhaps it depends of the capacitance of the 1N34A compared
to the 1N191 at any particular frequency combined with the resistances of
the voltage divider --- effectively forming an RC that could cause this (?).
Does this make sense to anyone?  Has anyone experienced this with the SB
series transmitters?

 

Thanks,

--- Rob

VE3JQL

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