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PostPosted: Sat Jun 11, 2011 1:52 pm 
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As part of a receiver-based spurious emission measurement technique I'm considering building a simple shielded receiver (DC or maybe even regenerative) preceded by a step attenuator. My concern is to prevent stray RF from leaking into the receiver, and to ensure that the only RF entering the receiver is through the attenuator (which itself, of course, also needs good shielding between sections).

I was planning on enclosing the receiver inside a wooden box lined with copper foil tape with a single RCA jack for the RF input. I imagined that this would give a similar level of shielding to building a box using copper-clad PCB board. However, considering the rather high level of shielding I would need for low-level signal measurement, I'm guessing I should aim for 100 dB of RF shielding. But is this achievable using the techniques I mention, or does 100 dB of shielding require more drastic measures like thick metal casing? How about the tiny hole in the RCA jack - is it enough to seriously compromise the RF shielding?

NOTE: the measurement idea is to send an RF signal of known power through a known amount of attenuation into a sensitive receiver and measure the audio output power. Then an RF signal of unknown power (the spurious emission being measured) is sent through a variable amount of attenuation into the same receiver to give the same amount of audio output power as the known-power signal. The difference between attenuation of known-power signal and unknown-power signal allows calculation of the unknown power.


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 Post subject: -100dB Shielding
PostPosted: Mon Jun 13, 2011 4:18 am 
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Lining the inside of a wooden box with copper tape won't give you -100dB. You need to use something that will give you a continuous conducting surface on the *outside* of the box. Your RCA jack will work just fine when terminated. Use a single hole mount jack with the shoulder contacting the outside surface of the shield box. If power is from an external source you will need to bring it into the box using feedthrough capacitors on both the positive and negative leads. An rf choke on both positive and negative leads on the outside of the box is a good idea also. If you use a reference ground it should also be attached to the outside of the box.

The whole idea behind this is to prevent currents other than signal from getting into the box. Ideally interfering currents should only be allowed to flow on the outside of the box.

You can build a fairly decent shield box with double-sided copper clad board but you will probably find a good die cast aluminum box works better.

73,

'Bear' NH7SR


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PostPosted: Mon Jun 13, 2011 10:52 am 
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Hi qrpbear,

Welcome to theradioboard! I've enjoyed your posts in the other Yahoo group, that many of us seem to visit as well.

Thanks for your detailed reply. A couple of more questions:

When building a shield box out of double-sided copper-clad circuit board, is the outer foil surface electrically separate from the inner foil surface? Is there any reason a slab of medium-density fiberboard with copper foil on both sides can't be used exactly like a double-sided copper-clad circuit board for building a shield box?

Also, is it feasible to homebrew an openable, but still RF-tight, lid for the enclosure? If so, I could keep the power supply (battery) and tuning controls for the measurement receiver all inside the box, tune the receiver with the lid open, and close the lid only when measuring. I would prefer an openable lid for convenience; if however that's not easily possible, then I have to run all the receiver control lines and power supply lines outside the box, which seems more complex with drilling holes, using feedthrough capacitors (which I don't have), chokes, etc.

Finally, your comment about outside vs. inside shielding raises another question - I now wonder if I may be doing the front-panel shielding wrong on my current wooden-box regen serving as my station receiver. I've got the circuitry mounted on the inside side of a wooden box lid:
Image

I added shielding on the inside of the lid by taping copper foil to the inside and soldering the corners together:
Image

The shield foil is grounded to the main circuit ground at the tuning capacitor. There is no shielding foil on the outside side of the lid, where the knobs are mounted. Now, in this case, should the shielding, located on the inside of the lid, help against hand capacity effects when touching the knobs, or must the shielding be on the outside?


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 Post subject:
PostPosted: Mon Jun 13, 2011 3:34 pm 
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Joined: Thu Mar 12, 2009 4:09 pm
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Location: Sonoma County, CA
In my opinion, copper foil or sheet forms a Faraday cage. Whether the copper is inside the box or outside the box is im-material. Since the wooden box is non-conductive it does not play a role in shielding.

Getting a good continuous layer is most important. All surfaces bonded together and no gaps at seams.

Copper is a good RF shield, but not very effective for low frequencies (60Hz) where you need some thickness of iron to absorb the magnetic field.

Rich

_________________
Homebrew Radio ex-Silicon Valley


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 Post subject: Shielded Enclosure
PostPosted: Mon Jun 13, 2011 5:35 pm 
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I don't have time to write much this morning.

Here is the URL for a fairly good article describing construction of a shielded enclosure.

http://www.compliance-club.com/currenti ... ?artid=554.

I am trying to find an old article written by Byron Goodman or Don Mix that appeared in QST magazine some 40 or 50 years ago. It had pictures illustrating the flow of circulating currents inside and outside shielded enclosures, proper mounting of connectors and methods for bringing connecting leads into or out of the box. No luck so far... it may not even be on the web.

Think of your shield box as having two separate conducting circuits. One is the outside surface of the box and the other is the inside surface of the box. The aim is to minimize the potential for currents flowing on the outside to find their way into the box and vice versa.

Attenuation of -100dB is a very tall order. You should be able to achieve -30dB fairly easily but for every -10dB further attenuation expect a 10-fold increase in attention to details and materials selection.

I'll try to write a bit more when I get home from work.

73,

'Bear' NH7SR


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 Post subject: Re: Shielded Enclosure
PostPosted: Tue Jun 14, 2011 3:22 am 
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qrpbear wrote:
Attenuation of -100dB is a very tall order. You should be able to achieve -30dB fairly easily but for every -10dB further attenuation expect a 10-fold increase in attention to details and materials selection.


Thanks for putting things in perspective. So for the additional -70dB attenuation to go from -30dB to -100dB we're looking at a ten-million-fold increase in attention to details. :shock:

I did some browsing on the costs of ready-made -100dB boxes and they cost more than buying a new rig! Part of the idea of this project (shielded receiver with stepped attenuator signal measurements) was supposed to be a "poor-man's" way of measuring transmitter spurious emissions down to low levels (-60dB required where I live), without a spectrum analyzer. I still think this idea could be cheaper than buying a spectrum analyzer, but measuring low-level spurs seems hard any way you go about it.


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 Post subject: TX Spurious & Harmonics
PostPosted: Tue Jun 14, 2011 4:52 am 
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If you have a receiver with a reasonably calibrated S-meter you can still make some meaningful measurements. Feed the tx output into a shielded dummy load. Make a not of the S-meter reading on the fundamental then again on the 2nd-5th harmonics (if you can go that high). You can sweep the receiver across the bands while keying the tx on/off to see if there is anything showing up that is not harmonic related. You can use your step attenuator on the rx to keep signal strength within a reasonable range. If the tx is qrp you can also use the step attenuator between the tx and the dummy load.

It's all relative, of course, but generally considered accurate enough for most purposes... especially with qrp transmitters.

73,

'Bear' NH7SR


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PostPosted: Tue Jun 14, 2011 6:08 am 
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Probably the easiest way to achieve -60dB harmonic rejection is to use a 7th order elliptic filter. For 7MHz with the cutoff set to 10MHz the 2nd and higher harmonics are all >60dB down. Add that to whatever harmonic suppression the tx already has. I use the AADE Filter Designer for this sort of thing. Use 500WV silver mica caps and build the filter in a separate enclosure of its own for best results.

73,

'Bear' NH7SR


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 Post subject:
PostPosted: Tue Jun 14, 2011 9:00 am 
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Joined: Sat Oct 13, 2007 1:46 pm
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Quote:
and build the filter in a separate enclosure of its own for best results.


For that, return to the start post of this thread :=)


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PostPosted: Tue Jun 14, 2011 12:07 pm 
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qrpbear wrote:
If you have a receiver with a reasonably calibrated S-meter you can still make some meaningful measurements. Feed the tx output into a shielded dummy load. Make a not of the S-meter reading on the fundamental then again on the 2nd-5th harmonics (if you can go that high).


Sounds very interesting, but I'm not sure I follow the idea. Is there any physical connection between tx and rx in this scenario? And don't we still need massive shielding on the rx?


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PostPosted: Wed Jun 15, 2011 4:05 am 
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Joined: Sun Jun 12, 2011 6:42 pm
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You can do it without a direct connection to the receiver. Use a shielded dummy load on the tx located about 3 meters distant from the receiver. Use a 50 ohm resistor on the input side of a switched attenuator with the output connected to a receiver with S-meter. Key the tx and set the attenuator so that the rx S-meter reads S9+30dB on the tx fundamental freq. Tune the rx to the 2nd harmonic and make a note of the S-meter reading. Harmonic suppression of 65dB will be roughly an S1 reading.

You can also do this with a T-connector on the dummy load and coax connecting to the rx switched attenuator. In that case you should also put a 10dB attenuator between the tx and the dummy load.

So long as you maintain about 3 meters distance between the tx and rx shielding becomes less of an issue.

73,

'Bear' NH7SR


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PostPosted: Wed Jun 15, 2011 11:22 am 
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qrpbear wrote:
So long as you maintain about 3 meters distance between the tx and rx shielding becomes less of an issue.


Excellent - so increasing distance can effectively add attenuation, reducing the amount of leaked signal reaching the measurement receiver. Is there a rule of thumb for this, like e.g. 1m of distance adds 6dB of attenuation?


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 Post subject: Attenuation vs distance
PostPosted: Wed Jun 15, 2011 5:08 pm 
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It's called the Inverse Square Law. Each time you double the distance the signal level decrees by a factor of 4 times (-6dB). For instance if you measure a signal strength of 100 microvolts at 3 meters it will decrease to 25uV at 6 meters, 6.25uV at 12 meters, 1.56uV at 24 meters and so on.

Use of the 3 meter distance for free field measurements has been an accepted practice for nearly a century. It's defined as 'near field' measurement as opposed to 'far field' measurement which is performed at 30 meters distance.

73,

'Bear' NH7SR


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