K3 Shields - Theory of Operation?

[Dan Levin]

Naïve question...

I'm looking at the shields on the inside of my K3 (the one that separates
the RF compartment from the front panel, and the one that segregates the
KPA3 from the rest of the system), and I'm wondering why they work :-)

My questions fall into three areas:

1) These shields are full of holes and slots.  For example, the KPA shield
has big rectangular cutouts where material was folded down to make the tabs
that the board mounts onto.  It also has a 1-2mm slot along the bottom left
edge (close to the key and paddle connectors).  The front panel shield has
holes where the tabs were formed in the upper corners to mount to the 2D
connectors, and slots for clearance around the connectors that pass through
to the front panel board from the RF board in the lower corners.  Why don't
these slots allow unacceptable RF leakage?  Are they just too small at these
frequencies?

2) The PC board forms the "bottom" of the shield box in both cases.  Yet the
shields aren't connected electrically to the boards.  Doesn't this make the
interior of the PCB itself a big slot antenna where the bottom of the shield
runs along the board?  Does the board have a solid enough ground plane that
it provides a good bottom for the shield?

3) The connectors that pass through the shields, especially the ones that
pass through the front panel shield, don't appear to be RF proofed in any
way.  They don't use pass-through caps or anything - there is just a big
hold in the shield with the connector sticking through it.  Why does this
work?

I'm asking because I'm considering homebrewing some stuff that would require
separation between the RF area and the PIC microcontroller area, and I'm
trying to learn from the K3's obviously excellent design.

Thanks much!

                   ***dan, K6IF

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[Alan Bloom]

On Fri, 2008-05-16 at 12:50, Dan Levin wrote:
> Naïve question...
>
> I'm looking at the shields on the inside of my K3 (the one that separates
> the RF compartment from the front panel, and the one that segregates the
> KPA3 from the rest of the system), and I'm wondering why they work :-)
>
> My questions fall into three areas:
>
> 1) These shields are full of holes and slots.  

As long as the longest dimension of the hole is much smaller than a
wavelength (less than 1/10 wavelength is the rule of thumb) it doesn't
degrade the shielding very much.

> 2) The PC board forms the "bottom" of the shield box in both cases.  Yet the
> shields aren't connected electrically to the boards.

It looks to me like the PA shield is indeed connected to the RF board
(motherboard) and also to the top cover.  (Notice how the paint has been
removed on the top cover to make a good connection.)  It looks like the
front-panel shield is connected to the front-panel board through the
chassis.

> 3) The connectors that pass through the shields, especially the ones that
> pass through the front panel shield, don't appear to be RF proofed in any
> way.  

That can indeed be a path for RFI to couple through the shield.

I think the general answer to your question is that the best shielding
is a solid aluminum box with all power and signal lines going through
coaxial feedthrough capacitors and all coax cables using connectors
mounted to the box.  Sometimes you have to do that kind of thing if you
are designing something like a microwave spectrum analyzer (I used to
work for HP/Agilent) where (1) the wavelengths are very short, and (2)
all detectable spurs must be surpressed over a very wide bandwidth.  But
for an amateur transceiver operating in the HF range, the requirements
are far less stringent.

Al N1AL


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