> In those days (early '60s), field strength was referenced at
> a mile. km was simply a shift to the metric system.
It's been more than a few years since I read the AM Rules and,
even then, I was not involved with an AM station much less
antenna proofs <G>. However, "field strength at one mile" is
three to four wavelengths even at the bottom of the AM band and
that should be more than enough to get into the far field.
Of course, with AM broadcast the licensee only cared about the
groundwave except for the need to limit skywave in certain
directions to avoid cochannel interference (thanks to guys like
Pete and Carl who "packed the band" <G>). Even then, the skywave
pattern was entirely theoretical unless there was actual interference.
73,
... Joe, W4TV
On 2021-09-05 4:21 PM, Jim Brown wrote:
On 9/5/2021 6:31 AM, Joe Subich, W4TV wrote:
If one takes a lesson from the FCC/broadcast "proof of performance"
measurements, they measure field strength at one kilometer (one
mile?). That (1 km) works out to be 2 wavelengths at the lowest
frequency in the broadcast band and 6 wavelengths at what used to
be the top of the band.
As a young EE student, I worked in the consulting office of Pete
Johnson, who with Carl Smith (who also ran CREI), wrote the FCC's AM
technical Rules after WWII. Pete's practice was designing arrays to fit
new stations into a band that had been full for 25 years. Some weekends
were sitting with other EE students with slide rules, Bessel tables, and
25 column accounting spreadsheets computing patterns for a design, every
5 degrees of Az and 5 degrees of elevation. During the week, I would
compute and plot on aero maps the resulting ground wave contours using
the graphics and conductivity contours in the Rules, with different
graphics every 5-10 channels. (Wavelength in the AM band varies by more
than 3:1.) In those days (early '60s), field strength was referenced at
a mile. km was simply a shift to the metric system.
73, Jim K9YC
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