NOISE
**June of 2009 important update**
It is never too late for learning they say... Indeed, after a few members from the
Perseus newsgroup alerted me about the strange behaviour found on my wav files (available
for download thanks to Ken Baird in Scotland), I was left intrigued. Actually, when
the preamp was engaged, it was contributing in lowering the noise floor while the
signal was enhanced by a large margin. I was really convinced that the RPA-
Then, the key message (#4966) that hit me came from Bill W4ZV in response to the following (beginning of quoted text):
>>What *is* interesting is that the noise drops while the signals increases, like
magic....A >>"normal" amplifier can not see any difference between wanted signals
and un-
>I suspect the problem here is common mode noise being fed to the Perseus but being
>rejected at the RPA-
Yes, common mode noise rejected by the RPA-
This is the choke
used for
preliminary testings. I made it according to Jürgen's design using a FT140-
While I was locating the test choke in shack hoping to find where it could make a difference, I found that inserting the choke between preamp and phaser made a huge change on the noise floor. Actually, it dropped a lot. I also found that engaging the preamp made both noise floor and signal to rise. It wasn't "recovering" the signal as it was before. Bingo I said to me. Now I'm getting a proper s/n ratio from my system. Nevertheless, the noise was still a little high. I knew that I had to pursue in that direction with further testings.
The next morning, I was horrified to find that even with the choke in place, the
bad behaviour was back. I began to move the cables around the choke and the problem
was on and off. It was definitely something around there. First, I thought of a
faulty PL259 socket on the MFJ phaser chassis but after it was replaced, the problem
was still there. Now, only a 30cm piece of cable that is used to connect the RPA-
Indeed, THAT was the problem. Replacing the cable, s/n ratio was back to normal and
I found that there was no need to have an in-
An apparently harmless cable causing all that trouble! There must have been a fault in my coax preparation or a hidden imperfection when the cable was manufactured. One thing for sure is that I couldn't find anything unusual after a visual inspection over both connector areas.
Now, with this major problem fixed, the time has come to "really" play with chokes. Noise floor was still a little high so there was room for improvements. I decided to start from feedpoint and later compare from the shack. So, I brought my laptop and Perseus outside straight at my feedpoint. I connected the receiver with a 1m cable to the matching transformer. This way, I had no long feedline involved in the measurement. I used a 470 ohm resistor to ground instead of the regular 950ft wire. The two Perseus shots bellow are explaining the effect of the choke.
The choke mounted in a waterproof plastic cabinet It literally connects through the feedline close to feedpoint. One leg of the matching transformer coax side has to be grounded for the choke to be efficient. Connectors are simple F female chassis mount. Hot glue has been used to tie up the choke to the box.
This without the choke inserted, noise floor @ 895 KHz was at -
This with the choke inserted showing how the noise peaks were cut besides bringing
the noise floor down to -
This shot has been taken with the Perseus in shack using the faulty cable. Antenna
switchers (in shack) were in the "off" position leaving a 75 ohms dummy load for
the receiver through both preamp and phaser that were in their bypass position. A
total of ±50cm cable run. The noise bellow 750 KHz was coming from the external
HDD switching power supply. Note that @ 930 KHz, you can read -
So, -
Now, what I have left in order to get the noise lower is moving the feedpoint farther
which is out of question. Don't expect me to make another trench in the forest and
get the feedline buried. However, I still have that MFJ-
Initially, this phaser was used to null out adjacent or very close MW stations helping
out revealing weaker TA . Frankly, I rarely thought in using it for cancelling noise.
My only concern is during a Perseus wideband recording. I doubt that over both LW-
I really want to thanks Jürgen Bartels for his great availability in this quest and also Leif Asbrink SM5BSZ for taking time analyzing my on/off preamp file (provided by Ken Baird) with his legendary strictness and kindness.
I've been triggered after numerous comments on the Perseus newsgroup about the "miraculous"
effect of the RPA-