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Re: Re: AM antenna switch for 5kW ?
To: CRTech <crtech@crtech.org>
Subject: Re: Re: AM antenna switch for 5kW ?
From: Lamar Owen <lowen@pari.edu>
Date: Tue, 15 May 2018 01:21:48 -0400
Content-language: en-US
In-reply-to: <005701d3ebac$75a9a230$60fce690$@pilgrimradio.com>
References: <alpine.WNT.2.10.1805111649050.3300@Lyle-Dell-D630> <259A7239-C5DF-44A0-8432-9AA6580D3D10@bu.edu> <alpine.WNT.2.10.1805121115410.3300@Lyle-Dell-D630> <005701d3ebac$75a9a230$60fce690$@pilgrimradio.com>
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On 05/14/2018 01:53 PM, Bill Hurne wrote:
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.
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 https://www.radiomagonline.com/industry/an-alternative-to-constructing-a-transmitter-building ) 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.

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.

References: AM antenna switch for 5kW ?
(RADIO DOCTOR <lylehenry@fastmail.fm>, 12 May 2018 00:20:15 -0000)
Re: AM antenna switch for 5kW ?
(RADIO DOCTOR <lylehenry@fastmail.fm>, 13 May 2018 04:35:57 -0000)
RE: Re: AM antenna switch for 5kW ?
("Bill Hurne" <billhurne@pilgrimradio.com>, 14 May 2018 17:53:45 -0000)
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