[RFI] Noisy transmitters...somewhat long...

Kenneth G. Gordon kgordon2006 at frontier.com
Tue Sep 23 13:26:12 EDT 2014

Gents: I need some advice and information which will either corroborate or 
its opposite my contention that modern transmitters output broadband hash, 
and that SDR transmitters are worse in this regard.

I made the assertion on another list when the subject of SDR came up that I 
wouldn't want one in the entire neighborhood. I was immediately, and IMHO 
severely, taken to task about this, and, essentially, accused of lying about, or 
possibly misunderstanding, the details.

In the run up to the last Field Day, I was working with our local club for the 
first time in over 30 years. I was told that "recently", they had never been 
able to operate two transmitters simultaneously, and that if they had one 
transmitter operating on, for instance, 20 meters, they could not have a 
second one operating on any other band without suffering intolerable 
interference to both stations.

After questioning them about the placement and types of antennas they were 
using and finding those "acceptable" in my opinion (such as it is), I 
mentioned that I had been involved with a group in Missoula, Montana at 
least 40 years ago which had run 10 transmitters, 2 on every band, and had 
not experienced any noticable interference between them.

We were using the all tube-based Drake, Heathkit, Johnson, and Collins 
gear of the era.

I then spent some time out on the web reading up on various possible helps 
for the local's situation, and in addition to discovering that some big 
multi-multi stations were routinely buying and installed bandpass filters 
(many of those being the W3NQN types available from ArraySolutions) in the 
outputs of their transceivers and amplifiers to alleviate the same kinds of 
mutual interference our local group had experienced, I also found several 
reports that almost all modern transceivers, EXCEPT, apparently, the 
Elecraft K-3 (or possibly the KX-3: I don't remember exactly) were commonly 
outputting huge quantities of "trash" that slopped over onto adjacent bands, 
and that the SDR transmitters were far worse in this regard than even those 
"ordinary" transmitters. Also, that the manufacturers had been, at least up to 
now, ignoring this problem.

I am not talking about IMD or other commonly understood forms of output 
garbage: I am talking about broadband "hash" or generalized "noise".

This all made sense to me, since the early tube-based gear I used "back 
then" had fairly high-"Q" output tanks which had to be retuned for even short 
frequency-excursions within a band, whereas very few, if any, modern rigs 
that I know of need the output circuits tuned for even wide excursions, being 
"broadband", the antenna SWR being the limiting factor.

At our FD site this year, in addition to being very careful about antenna 
placement and polarization, I bought, for $75, a W3NQN bandpass fitler for 
40 meters for the group to use, and Ed Wetherhold W3NQN very kindly lent 
us a pair of filters designed to help prevent interference between two stations 
operating CW/Digital and SSB simultaneously in the 80 meter band. We also 
used a three-fllter triplexer for our 20, 15, 10 meter beam, and this year had 
no mutual interference problems that we could detect.

So: am I wrong about the unusually dirty output of SDR transmitters (and to 
a lesser extent, the dirty output of modern transceivers), am I mis-reading or 
misunderstanding what I have discovered, or am I correct?

Due to the horrendously bad RFI I am experiencing at my QTH (which I have 
mentioned here previously), I have, perhaps, become too sensitive to ALL 
sources of broadband noise, and this may be coloring my view of things.

Thank you, and vy 73,


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