[Flexradio] Odd RFI Problem

[Scott Myers]

Tim,

 

You and I have it hooked up EXACTLY the same... right down to the ICE
lightning arrestor.  (Overkill on a butternut with the lightning arrestor,
but you can't be too safe.)  But you know, I should check on the condition
of ground connection of the shield of the bulkhead connectors.  I did this a
few years back and after I got done, I remember thinking that I could have
done it better.  The Hoffman box is now rusting at the hinge and I had to
repaint it to slow the rust down.  I'd like to change it out to N-type
connectors on the bulkhead and a larger PVC box with screw on cover.  The
PVC boxes are much nicer for longevity outside.

 

I pulled out the photos of the bulkhead box when I made the last changes and
can definitely use to make some improvements.  I need to replace the
bulkhead connectors with flanged N-type and get everything bonded better.

 

But that point where they are shunted to ground is within 1 foot of the
closest 8' copper ground pipe, of which there are three, all of which have
been soldered together with braided ground strap and there is about 2 feet
between pipes.  Yes, solid copper strap would have been better.

 

My equipment order is exactly; antenna, ICE lightning arrestor, bulkhead
with shield ground, balun design choke in shack, Heathkit SA-2040 tuner,
Diawa CN-801 SWR Meter, antenna switch for dummy load, AL-80B amp,
Flex-3000.

 

73,

Scott AC8DE

 

From: Tim Ellison [mailto:tellison at itsco.com] 
Sent: Saturday, December 05, 2009 2:14 PM
To: Scott Myers; FlexRadio at flex-radio.biz
Cc: 'Dudley Hurry'
Subject: RE: [Flexradio] Odd RFI Problem

 

Wow.  There is a Ph.D dissertation in there somewhere!

 

All kidding aside, this is a great post about how to methodically track down
a problem and open yourself up to new ways of thinking about a problem.  The
scientific method is alive and well.

 

Scott, I have a question.  Do you shunt the coax shield on the run into your
shack from the antenna before the choke?  I have done this with some great
success.  When you choke those common mode currents with a choke, giving
that potential a low impedance path to ground will help with the efficiency
of the choke.  On my coax egress it looks something like this

 

Antenna-àICE lightning arrestor (grounded)àcoax shield grounded at
bulkheadà|(enters house)à1:1 RF chokeàswitches & radios

 

 

-Tim

 

From: Scott Myers [mailto:ac8de at ameritech.net] 
Sent: Saturday, December 05, 2009 1:55 PM
To: FlexRadio at flex-radio.biz
Cc: 'Dudley Hurry'; Tim Ellison
Subject: RE: [Flexradio] Odd RFI Problem

 

Thanks for the responses Tim, Dudley and Brian.  I apologize as this is a
longwinded e-mail as I went through some troubleshooting.  The eureka moment
is near the end, if you would like to skip all the rambling and details.
Bottom line: RFI causes the printer driver to disconnect the printer from
the network.  When the printer software re-establishes the connection to the
printer, it causes PowerSDR to freeze.  RFI source has not been determined.
Read on (painfully) for details.

 

Well, I did some more detective work.  I moved my choke outside my shack.
It was right in the shack before the feedline went outside.  The choke I am
using as a Balun Designs 1115ud.  The details of which are at:

http://www.balundesigns.com/servlet/the-63/balun-designs-1-cln-1-Isolation/D
etail

The choke is now about 15 feet away from the shack.  Probably about 20 feet
from the amp as the coax runs.  This made no difference.  So I added a
second 1115u (not D version) choke back inside the shack.  This also had no
effect, but I am choked to the max to stop any common mode, that is certain!

 

For the record, the antenna in use is a Butternut HF9V-X that is about 100
feet from the shack/house.  The shack is in the basement below grade.  The
feedline is pulled through conduit underground below the very extensive
radial field; 64 radials 40 feet long.  I use 9913 foam (RG-8U) coax
throughout my system.

 

I do use the DPC latency checker.  My latency never gets above 100 uS under
the most busy clicking sessions.  The machine is built for quality and
speed.  Even under this dropping of communications with the Flex post TX,
the latency doesn't rise, so that has been ruled out.

 

I have added another ferrite choke to the firewire cable that was large
enough to loop the cable through twice.  This was no help.  I put this at
the computer end, as the two on the stock Flex provided firewire cable are
at the rig end currently.

 

Another observation of note; The TX must be in excess of 20-30 seconds for
the problem to occur.  On a short TX, the problem doesn't occur, even at PEP
750 watts out on RTTY.  The networked printer I mentioned doesn't lose
communications for until at least that 20 second mark is reached also.  It
is as if the RFI has to build up.  Seems odd.

 

Certainly, going to a better grade firewire cable is a good idea and I may
do this.  But I will work on getting the RF out of the shack first, as I
should.  I want to try nipping it at the source.  I need to get myself a RF
sniffer in here or build something capable of doing it.

 

I changed a 2 foot coax jumper between the transmatch and the SO-239 on the
wall to a 60 foot cable I had lying around.  No change.  So feedline length
had no effect.  I changed back to the 2 footer.

 

As far as grounding the computer to the station, I read the article and was
hesitant at doing this.  WAS is the operative word there.  I have taken some
care to insure that the house AC line grounds are not common with the RF
grounds and my metal sealtight conduit that runs up the wall and out of the
shack.  My thoughts would have been before this that by grounding the
stations RF ground to the computer, I am making the computer the common
point between the house ground and my station's RF ground.  I would have
thought it would be necessary to use a converter plug to lift the ground of
the computer from the house's AC ground circuit.  But upon further
consideration, I got out what schematics I have of equipment.  I noted right
off in the schematic for my AL-80B that the AC ground is common with the RF
ground of the SO connectors ,at least by its schematic.  Hmm.  I do not have
the schematic for the Flex-3000, as it is not in the manual.  But looking
under the hood, (and correct me if I am wrong) it would appear that the AC
ground is common with the BNC connector's ground, making RF and AC grounds
common in this too.  So while we all take great care to try to eliminate
ground loops, it would appear that the standard practice (probably due to UL
and NEC requirements) in ham gear is for the AC ground to be common with the
RF ground point on the back.  So while I have tried to isolate my house
ground system from my RF ground system, those efforts would appear to be for
naught, as it is all made common through the rig and amp anyway.  What does
it mean?  Probably nothing.  This was just a revelation that I didn't
realize and from an engineering standpoint and something I hadn't considered
before now.

 

The big weakness in my shack is the length of the my main ground strap with
runs to my RF ground system.  It too is 15 feet long and runs out that
conduit along with my feedlines.  All components attach to the strap it in a
star pattern with equal length braided 1" wide conductors.  The entire
ground strap is a 1" braided strap.  Not ideal.  One theory would be that I
have gotten to a situation that a resonance/high impedance is set up on it
and now that more power is added, it is rearing its ugly head.  Looks like I
could be drilling into the basement floor.  Still a few things to try, but
it's not looking good for the home team in avoiding that intensive bit of
labor and expense.  If I do so, 3-4 copper pipes and 3" wide copper strap
will be involved.

 

So now to drill a hole in the back of the computer and add a strap and
grounding screw.  Such fun.  But maybe I'll get lucky and this will solve
the problem and I won't have to reinvent my RF ground system.

 

I added a big snap-on MFJ ferrite to the printer's network cable at the
printer end with the cable through it 4 times.  No change on helping the
printer's connection.  At this point, I almost overlooked that fact that
when I plug the printer back into the network that the PowerSDR will freeze!
PowerSDR isn't doing anything but sitting there idling in RX mode.  There is
no rise in DPC during this.  So I watched closely after a TX and it is true
that PowerSDR only freezes when the printer re-establishing its connection
with the computer's driver.  (It's an HP Officejet 7310 running their Hp
Solution center driver software)  If I leave the computer unconnected from
the network where it doesn't lose communications because it isn't connected
in the first place, then PowerSDR never freezes!!!!  Just turning it off
works too.  Eureka!  So it seems I need to get rid of the RFI to keep some
parts of the network from having problems that then stops the printer driver
and then freezes PowerSDR upon reconnect!  But in the short term, I can at
least unplug the printer/turn off and keep using RTTY on 20M and not have
any PowerSDR freezes.

 

So I can now conclude that something about the printer software
re-establishing communications with the printer after being interrupted by
RFI causes PowerSDR to freeze, even though there is no DPC latency issue.

 

There are some of the network wires that run through the wall within 4 feet
of the coax conduit run.  I estimate that the conduit runs within 3 feet of
the data closet where the DSL modem, router and switch reside.  I have now
tried ferrite chokes on every part of the printers network cable, tried it
on different wall connections and put ferrite on the printer's power supply
cable, both on the high voltage and low voltage side.  No changes.  I must
now look for the source of the RFI.

 

Ok, I'm now officially mentally exhausted for the day with this one but I
can now operate RTTY with the amp on 20M without PowerSDR freezes.  Now to
sniff out where the RFI comes from exactly.

 

73,

Scott AC8DE