[Lowfer] Guided Waves

[Lyle Koehler]

Bill Ashlock wrote:

>This is true, but 200 km (125mi) is really not an area that would be
>expected to be supported by skywave propagation. By 400 km (250mi) the
>situation improves by 15db.

True, but the "guided waves" article talks about very low ionospheric
absorption, and also says that low-frequency waves are returned at all
angles of incidence, including normal. If both of those things were actually
true at LowFER frequencies, we *would* expect good sky-wave propagation at
200 km. Near vertical incidence skywave (NVIS) propagation is the dominant
mode used by hams during the daytime on 75 meters. On 75 meters, where the
surface wave is good only to 25 or 30 miles over poor soil, I expect strong
daytime signals from almost anywhere within a 200 mile radius of my station
because of NVIS propagation. NVIS is somewhat less reliable on 40 meters,
but as several HiFERs have noted, sometimes it can be observed even on 20
meters.

I'm not saying that sky-wave propagation is never observed on LowFER signals
within a few hundred miles. If surface waves were the only effective mode,
we would never see the diurnal differences in signal strengths that have
been known to happen even at fairly short distances. But I don't think that
happens very often.

Regarding the radiation patterns of our antennas: As Doug points out, a
vertical antenna isn't very good for NVIS use because it has a null in the
vertical direction. On the other hand, the pattern for a small loop has its
peak straight up, which makes it a good "cloud warmer". At 45 degrees, which
would be about the right takeoff angle for skywave propagation over a
distance of 200 km or so, the patterns of both the vertical and the loop
would be down 3dB. It would indeed be interesting to compare signal strength
plots for co-located vertical and  loop antennas, to see whether there is a
difference in skywave interference effects. But I'm too lazy to provide the
signals myself!

Thanks to Lewis for bringing the article on guided waves to the attention of
 the LowFER group. It has opened up an interesting discussion and might lead
to some fun experiments.

Lyle, K0LR