On 05/14/2018 01:53 PM, Bill Hurne wrote:
At WGCR, we used a couple of 'banana boxes.' I wrote a short article
for Radio Magazine in October of 2000 (although I think it was actually
for Broadcast Engineering at the time.... I'll have to look in my email
archives on the subject; see
) for the new transmitter building. The online article doesn't have any
of the photos, but I think I could probably find them if you were
interested. The original email was a reply to John Battison's request
for more information on what we had built, and it had to do with our
odd, but working, negative-pressure ventilation system at the time (with
the 50kW in place it is no longer just ventilated, but
air-conditioned). I found the email I sent, but not the email from
John. Lost to bit-rot, I'm afraid.
With the tower only a couple hundred feet away it would be a very
significant source of RFI unless the shipping container if very well
shielded. I would expect more RF from the tower than from a hole in the
transmitter. At only 200 feet and 5KW the field strength off the tower is
probably several volts per meter. Learned this when I maintained a studio
in a converted metal mobile home only 200 feet from the 5KW AM tower. RFI
was very significant.
When WGCR upgrade to 25kW and then to 50kW they kept the new transmitter
in the closer of the two containers, which is within 50 feet of the
tower. Thanks to the two-layer construction of the refrigerated banana
boxes (galvanized outer shell, welded-seam high-grade stainless steel
sheet on the inside with a think layer of polyuerthane between) there is
almost no RFI inside the box with the door mostly closed. Thanks to the
waveguide-beyond-cutoff effect, even opening the end door doesn't
increase the RFI significantly inside the boxes. Both inner and outer
skins are silver-brazed to the radial system's 6-inch strap.
Now, we also ran the station for a while out of a converted double-wide
about 500 feet from the tower, and we had lots of RFI in that; the
banana box is much thinker material and a better RFI attenuator, and it
is far better grounded than the double-wide ever was.