RE: Night Time BCB Skywave/Ground Wave Cancellation/Interaction

[dan.strassberg@att.net]

I have experienced phasing in WBZ's signal while driving 
at night in Sudbury MA. Sudbury can't be much more than 
30 miles from Hull.

A hotly debated subject among more technically inclined 
AM geeks (such as me) is the effect of transmitting-
antenna height. It's well known that the maximum 
efficiency for standard (non-sectionalized) vertical 
radiators is achieved with a radiator of 225 degrees 
(5/8 wavelength). A 5/8-wave radiator produces an 
inverse-distance field at 1 km of about 440 mV/m/kW.

However, radiators taller than 180 degrees exhibit a 
high-angle lobe in their vertical radiation pattern that 
becomes increasingly pronounced as the radiator height 
is increased from 180 degrees to 225. As the strength of 
this lobe increases, phasing between the groundwave and 
the reflected skywave becomes more pronounced, limiting 
the station's groundwave service area--especially during 
the period now known as critical hours (local sunrise to 
two hours after and two hours before local sunset until 
sunset).

For this reason, it was once fashionable NOT to build AM 
antennas much taller than 180 degrees, even though the 
groundwave efficiency of a 180-degree tower is only 
about 380 mV/m/kW @ 1km. In other words, station owners 
were willing to accept the equivalent of a roughly 25% 
reduction in power (that is, (380/440)^2) to reduce 
phasing at the fringes of the coverage area during 
critical hours. This was in fact the case in 1939 when 
the WBZ towers in Hull were constructed. WBZ was then on 
990 and the towers are I believe, just 180 degrees at 
990 (or 187.3 degrees at 1030).

More recently, though, consulting engineers have become 
more venturesome. Not many tall AM towers are being 
built these days, but of the few that have been built 
within the past 20 years or so, the most popular heights 
seem to be between 200 and 210 degrees. At such heights, 
the theoretical inverse-distance field is just shy of 
400 mV/m/kW @1 km.

I've been assured by several consultants that the effect 
of the ground system on all this is rather minimal. For 
example, a ground system consisting of 240 0.4-
wavelength radials increases the inverse-distance field 
very little compared with a more standard ground system, 
which consists of 120 0.25-wavelength radials. 
Nevertheless, in the very limited area in which such 
things are possible, salt-water grounds are still much 
sought after. WBZ must have a salt water gound. I think 
WJDA does. (And what a signal the station has!) And 
according to the FCC AM database, WFAN/WCBS do.

I've always maintained that a salt water ground improves 
skywave as well as groundwave propagation. It's hard to 
imagine what vertical radiation patterns would look like 
if this were not so. At some low angle, there would be a 
discontinity in the vertical pattern of a station with a 
salt water ground. Very little in nature behaves that 
way.

I've used the supposed salutary effect of salt water on 
skywave propagation to explain the fact that, before the 
breakdown of the clear channels, WBZ was heard regularly 
at night on Florida's east coast. Others have offered a 
different explanation, however--that salt water is such 
a good reflector of medium-wave signals that impinge at 
less than the critical angle that WBZ's signal was 
reaching Florida on the second hop. That is, the signal 
touched down on the water half way between Hull and, 
say, Jacksonville.

I have the feeling that these issues will never be 
resolved but will just be fodder for threads like this 
one until all of us geeks die off.

> Have you experienced a difference with loop antennas vs. whips.  Seems to me
> I experience the fading more on loop antennas than I have in cars.