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Author Topic: Time for a Faraday cage for my rig?  (Read 496 times)

WB6BYU

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Re: Time for a Faraday cage for my rig?
« Reply #15 on: April 18, 2021, 12:28:55 PM »

So, as I understand the situation...

Your radio picks up noise all the time when at the operating position,
even:

1) with the rig powered by a battery,
2) with the circuit breakers to the house shut off, and
3) with no antenna connected.

Boy, that is puzzling...


I'd suggest at a next step making sure there are no other wires
of any sort connected to the radio, then shorting out the antenna
connector under those conditions, and see if you still have the
problem.

I've run into two cases with somewhat similar symptoms.  The
first I noticed on 15m, covering 21.075 to 21.175 MHz with a
squirrely noise.  Then I found it in two different segments of
10m, and started working it backwards.  Turned out to be a
fundamental on something like 449.5 +/- 1 kHz, and I was
hearing the harmonics throughout the spectrum.  I tried sniffing
it out using an AM radio tuned to the third harmonic, and it
appeared to be radiating from an agricultural box about 70m
away, but as I got close it swamped the IF stage of the
receiver.  (There was a ham who lived even closer to it, who
didn't use the affected parts of the bands, but couldn't get
good AM radio reception for some reason...)

So if your two radios use the same IF frequency, it could be
some sort of radiation that gets picked up there, so that the
actual frequency that the radio is tuned to doesn't make any
difference.

The next case was in this house when we moved here, and
I've encountered it on other cases.  I had horrible interference
that seemed to come from everywhere, but nowhere in
particular.  A year or so ago, the power company made some
major upgrades, and the noise disappeared.  (Well, at least
the worst of it, I still have other local sources.)  Apparently
the noise was coming in on the AC power ground wire,
so shutting off the circuit breakers didn't disconnect it, but
it was still radiating throughout the house.  I've seen a couple
other cases, including one where there was a loose tie wire
on the power cable about 500m away from our farm, that
apparently was arcing.  It wasn't a problem when I used
a balanced antenna, but it was when I used a wire fed
against ground.  And it caused a loud roar when testing
an 80m DF receiver inside the house - made it easy to
trace where the wires were running through the walls.  That
one apparently got resolved when we upgraded the electric
service and got better ground rods installed.


So those are two possible mechanisms that could cause
strong radiation in the house, even with the power shut
off.

If shorting out the receiver input doesn't reduce the signal,
then it might be getting into to the IF stages directly.  (The
best check would be turning down the RF gain, but only if
it affects just the RF stage and not the IF stages.)

The quick and simple Faraday cage would be to wrap the
rig in kitchen foil, but that makes it difficult to see the S-meter.
Just leaving a hole for that may still work.

But it also points out the importance of paying attention to
every wire that connects to the radio:  if your battery is
connected to a charger during the test, noise could get in that
way even when the power is shut off to the house.


That suggests another test, however.  What if you set up the
rig on a battery down by the antenna, connect the coax to it,
then put a small antenna on the shack end and sniff around
to see how strong the noise is?  Not that it will be particularly
convenient, unless you can turn up the volume on the radio
so you can hear it at the other end of the coax, but at least
it would reduce the noise in the radio so you can tell where
it is stronger or weaker around the house.  That might, at
least, resolve whether it is radiating from the power lines
themselves, or from the AC power ground wire running in
your house.

XW0LP

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Re: Time for a Faraday cage for my rig?
« Reply #16 on: April 18, 2021, 10:54:14 PM »

@WB6BYU, thanks very much for taking the time to suggest possible sources of RFI and solutions.

I never have both rigs switched on at the same time!  The other rig is not the noise source.  The buzzing noise is wideband, from MF to 30 MHz, present on both rigs when each is separately tested using an isolated battery, no antenna connected and the mains power switched off!!!

However, your power line suggestion may be more on the ball.  I'm pretty sure that it's the nearby power lines that are the source of the problem.  It actually makes no difference if I complain to the electricity company here in north Laos.  If there's 220 volts coming into my house, then '"it is all working OK", and they will go back to sleep in their hammocks :)

So... to the Faraday cage idea.  Today, I bought some soft (bendable) fine mesh aluminium netting, and constructed a frame around my rig from metal coathangers.  Then I wrapped the mesh around the rig, all sides, the rear of the rig, and the front with a section of the mesh that can be folded up to adjust the controls (which I don't have to do much when running digital modes).  The power connector and antenna cable have extra mesh wrapped around where they pass through the mesh cage.

It's the middle of the day now, so I really have to wait until dark to do meaningful RFI tests against received stations.

Without the faraday cage, my noise level on all bands was S7-S9, with and without the antenna connected.

With the rig inside the faraday cage, the noise level is ........  S1.5 !!!!!

It ain't pretty, but so far, it's looking good.  I'll do some more tests after dark :)

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K0UA

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Re: Time for a Faraday cage for my rig?
« Reply #17 on: April 19, 2021, 06:24:01 AM »

That is the most puzzling thing I have ever heard of. The fact that the makeshift Faraday cage makes any difference at all just floors me.
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73  James K0UA

XW0LP

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Re: Time for a Faraday cage for my rig?
« Reply #18 on: April 19, 2021, 06:59:21 AM »

That is the most puzzling thing I have ever heard of. The fact that the makeshift Faraday cage makes any difference at all just floors me.

LoL!  I'm listening/calling on 80m right now (9pm local time).  The normal noise level is about S7, but it's now down at about S2....  I'll have to see if that's "opened up" the band to receive stations that were buried in the QRN before. 

Even if not, I'm happy to have a much-reduced QRN level :)

I will do more tests.  (This afternoon, I worked on 10m with this lower noise level and worked Portugal from Laos, ATNO for that band).

Update:  So what could be the source of the interference?  Well, I previously had a lot of wideband QRN from the PSU of the LED street light outside my house, no more than 15m from my rig.  Even in the daytime, that PSU has 220v supplied to it and then it powers the LED lamp automatically after dark.  I previously cut the 220v supply to the lamp (oops!), but the electricity company spotted the fault a few months later and repaired the wire!  I can't really cut it again.  So perhaps this street light PSU is the root cause of the QRN?
« Last Edit: April 19, 2021, 07:07:34 AM by XW0LP »
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WB6BYU

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Re: Time for a Faraday cage for my rig?
« Reply #19 on: April 19, 2021, 07:37:20 AM »

I’m glad it worked!  I hope it solves the problem
for the long term.

My comment about IF interference was not about having
both rigs on at once.  Rather, if the noise source happens
to be at the IF frequency and is picked up in that part
of the circuit, then it will be there regardless of the
frequency that the radio is tuned to
.  That’s what
happened with the noise source that was close to the
455 kHz IF of a standard broadcast receiver, and could
also happen if both of your rigs use a 9 MHz IF, for
example, and there was a large signal on that frequency
coming in through the ground conductor of the house
wiring (which isn’t disconnected by the circuit breaker).

XW0LP

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Re: Time for a Faraday cage for my rig?
« Reply #20 on: May 01, 2021, 03:48:01 AM »

As an interesting update, there was a 5-minute power cut today in my village.  During that outage, the background noise level on my rig (powered by battery and without the faraday cage), dropped from S7 to S1.5.....
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