Re: low output only on 40m

Bill Cromwell


Did you run this sweep through the filters with a spectrum analyzer or a wobbulator> Or did you just use the transmitter itself? Just because the filter labeled 40 and 30 meters works on 30 meters does NOT mean the attenuation curve is right for 40 meters. I think Allison mentions counting the turns in her latest post. She has posted about this exact thing before.


Bill KU8H

On 1/15/19 11:41 AM, Curt via Groups.Io wrote:
thanks for communication - I know slightly more about symptom now --
first, it likely can't be what I thought - LPF - as this LPF also is used on 30 meters!  my 30m power is okay.
I just ran a sweep across all the bands into a dummy load -- every band except 40m is producing a lively output (my power meter is reading 10 watts on 160m * down to 4 watts on 10m -- its got a QRP scale, but it was calibrated a couple decades ago).  So I am convinced I have a substantial dip (reading less than 2 watts now on 40m).
since I am using a dummy load - I will get some data on the width of this notch.  it is almost like a choke isn't working around 7 MHz.
brief history: my ubitx has always been weaker on 40m.  I will confess it has one modification - an extra 45 MHz xtal filter placed on the other side of its IF amplifier.  this network made the 40m outage worse.  I saw a tip from Farhan to reduce R26 from 470 to 220 ohms to compensate for this filter loss - I made this mod.  but I can't blame it directly -- I got nice output on all other bands.
we have 22 other ubitx from the same or recent lot in our club -- maybe mine is acting different that the population -- we are still gathering data from builders.
the funny thing - these kind of problems always increase education, getting 'me' to examine places I would not be compelled to examine. let's see if I can learn anything, and get this thing fixed.
* yes I realize I have no low pass filtering of the second harmonic when operating at 1.8 MHz ... but my 160m vertical presents a dreadful match at 80m.  a nearby RBN receiver conveys I got at least 40 dB rejection by its non-report on 80m!  know your antenna before doing this - but several of us in NA operating a stock ubitx on top band.
73 Curt
bark less - wag more

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