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 _______________________________________________ IRCA mailing list [email protected] http://montreal.kotalampi.com/mailman/listinfo/irca Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: [email protected]
