On Feb 22, 1:48 pm, "MRW" wrote:
On Feb 22, 3:52 pm, "K7ITM" wrote:

With regard just to your last question, atmospheric (mainly from
lightening around the world), galactic, and typical man-made noise are
all much higher--40dB or more, usually--at 1MHz than at 100MHz. You
need signals above the noise for practical communications.
(Bandwidths also enter into the picture, but I believe the noise is
the key issue.)

Cheers,
Tom

Thanks, Tom! I did not know about this noise factor in this lower
frequency range. In a way, if I were operating at 100MHz using AM,
then I should be able to work with lower field strength values?

Yes, to be sure. It's interesting to note that communications in the
aviation band above 108MHz is, AFAIK, still all AM. The nav signals
are different but voice communications are AM there.

The "noise" chapter of Sams' "Reference Data for Engineers" has some
info on noise versus frequency, from various sources; included are a
map of usual atmospheric noise levels around the world, and I believe
sky-maps of galactic noise. You can also find some of that on the
web.

Cheers,
Tom

"K7ITM" wrote in news:1172204498.747073.7270
@k78g2000cwa.googlegroups.com:

The "noise" chapter of Sams' "Reference Data for Engineers" has some
info on noise versus frequency, from various sources; included are a
map of usual atmospheric noise levels around the world, and I believe
sky-maps of galactic noise. You can also find some of that on the
web.

A more up to date source of expected ambient noise levels is ITU-R
Recommendation P.372-8. I have summarised some of the info from that
document for HF frequencies at http://www.vk1od.net/fsm/FSAmbientNoise.htm
..

Owen