[BC] Another new antenna for broadcast

[Mark Humphrey]

I'm working on an interesting project today.

Two 1 kW AM stations in upstate New York's dairy country want to share
the same 401 foot tower, but their frequencies are only 20 kHz apart.
 A waiver was granted by the FCC to allow this unusually close spacing
(because of the co-location) but after talking to several vendors
about building a combiner, none of them thought a conventional design
would provide enough isolation -- so they sent me to Prof. Lirpa.

I mentioned that the tower site was formerly a farm and showed him
some photos of the property.  He noticed four abandoned Harvestore
blue metal silos next to the milking shed, and suggested that we
convert them into huge pass-reject cavities, tuned for the medium wave
band.  What a brilliant idea!

So I just measured the dimensions of the cavities on a Wacom 2-meter
duplexer, and it appears that we can simply scale all of the necessary
parts by the same factor.  I'm fabricating the coupling loops from 6
inch beryllium copper strap -- and for the tuning rods, we will use 20
foot sections of steel culvert pipe, inserted in the top of each silo
and adjusted for minimum intermod with a crane.

Our only concern is that the cavities may go out of tune on cold
winter nights, so we're planning to duct the hot air exhaust from the
twin Gates Vanguard II transmitters into the base of each of the
silos.  I'll let you know how it all works out.

Mark

On Thu, Apr 1, 2010 at 10:13 AM,  <RichardBJohnson at comcast.net> wrote:
>
> Hello all,
> A new antenna has been designed which swaps dielectric and metal. It consists of a broadband impedance transformer of proprietary design that matches the free-space impedance to the impedance of the air interface.
>