Re: [IRCA] Metal matching transformer box

[Rick Kunath <k9ao@xxxxxxxxxxx>]

Patrick Martin wrote:

> OK, 1/10th of a wavelength.

I wouldn't want to see anything larger than 1/50 wavelength hole in the 
box. Still bigger than the box at mediumwave, even at the high end.

Thickness of the box also makes a difference when you are drilling holes 
in it and affects the attenuation of any hole. Seams are bad, holes are 
good, that's why I wanted to make sure you had metal to metal contact 
with the cover and the box.

Since there is no way this is going to look like a waveguide at 
mediumwave, we are safe in assuming you are well below any Fco/3 
frequency, so the hole won't be acting like a waveguide.

So for a box 1/4-inch thick and a hole 1/16-inch diameter (and no 
conductor going through the hole), you'd have something like 120 dB of 
attenuation with the hole in the box. If you decreased the box thickness 
to 1/8-inch with that same 1/16-inch hole (and no conductor going 
through the hole), you are down to only 60 dB attenuation, not enough. 
Go to a 1/32-inch hole with the 1/8-inch box thickness (and no conductor 
going through the hole) and you are back in the area of 120 dB again.

As a general guide for holes with no conductor and at a frequency of at 
least Fco/3 (of the hole or slot size) the formula is:

dB attenuation approximates 30 * (t/L)

Where:

t = Thickness
L = Length or diameter of the opening

Since L = the larger dimension of the two of either "diameter of the 
opening" or the "length of the seam", you can see why open seams are 
really bad. They look like holes with a diameter of the seam length. A 
cover without metal-to-metal contact to the rest of the box is next to 
useless for shielding.

Since your quad-shield coax will probably have shielding of somewhere in 
the neighborhood of 100 dB (hardline is even better shielded), this is 
OK. Though you can likely increase the shielding effectiveness of the 
coax with the use of a sleeve-type choke on each end of the run at the 
connectors. Still you in good shape with a small hole in a fairly thick box.

Keep in mind that you are looking for some extreme attenuation in your 
shielding, lots more than the average enclosure designer. Thick boxes 
are good. Small holes are good.

Again, I'd cement some aluminum screening over any hole on the inside of 
the box to keep the crawlies out of the box.

Hopefully some of this makes sense.

Rick Kunath

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