Topband: Deep earth rod vs radials - FS measurement beyond the nearfield zone

[Guy Olinger, K2AV]

> To me it looks that to measure a low angle radiation (say 10 dgrs)
> field strength at 1km distance from the vertical radiator, the sense 
> antenna
> needs to be at 176m height from the ground surface (calculation based on
> using right triangle formula and assumes that a small magnetic loop sense
> antenna would be used as per N6LF field strength experiments).
>
> Any ideas as to what other sense antenna could be used for the low angle
> radiation field strength measurements beyond the near field zone, which
> would not require an impossible antenna placement height? How can we be 
> sure
> that (at least most of ) the field 'visible' to the sense antenna is 
> namely
> in the desired radiation angle range, say 8 to 12 dgrs if the sense 
> antenna
> is placed on the ground?

This is a real quandary of ham radio MF use vs AM MF broadcast use where 
broadcasters are obliged to maintain field strength and patterns as measured 
(conveniently) at the ground level. Validating patterns at all elevation 
angles creates the need to accumulate actual measurements at places and 
altitudes only practical with a helicopter.

At WCPE, 89.7 MHz, in Wake Forest, NC we were experiencing multipath in some 
areas from a side-mounted antenna (why sidemounted is a long story).  Though 
the side mount was always cast as the villain, there was no proof, hence no 
solid engineering data for use in proceedings (including board meetings to 
OK capital expenditures...).

I was deputized to ride a helicopter with a sense antenna suspended beneath, 
and GPS for altitude, position.  We rode to selected points in the 
Raleigh/Durham area (not in airport approach patterns) in line to troubled 
areas, and then held at map point while gradually sinking and making 
measurements on the way down.

Nulls were clearly imposed over the registered pattern of the antenna at 
elevations independent of ground interference proving the tower's 
involvement. Similar measurements on other local FM stations with topmounted 
antennas showed no such variations and also proved that the helicopter was 
not affecting the measurements.

Proving far field patterns at elevation is difficult and expensive. That is 
why some "unspecified validating measurements in the past" are accepted as 
gospel, we accept model extrapolation of ground measurements to unmeasurable 
sky wave, and we go on.

Anybody own their own private helicopter?  The rest of us will pay for 
gas....

73, Guy.