[BC] Directional Antenna Proofs - Ground systems.

[Tim Sawyer]

I have over the years seen very few AM stations that hold up (pattern rms or
service) over say 25-30 years using the original ground system - seriously
how long do you really expect it to remain intact? Shifting soils, tractors
cutting the field, trees, bushes, and other surface disruptions cause
serious damage to soft-drawn #10 wire. System losses about the base of the
tower are critical too - ground screens get ripped, trampled upon and not
repaired, etc. 

Play with the on-line FCC figure 8 calculator -
http://transition.fcc.gov/mb/audio/bickel/figure8.html and you can see what
the FCC thinks happens when you remove part of the ground system.  

90-degree tower, 120 90-degree radials 1 kW = 305.8 mV/m/kW 
90-degree tower,   90 90-degree radials 1 kW = 296.1 mV/m/kW
Effective radiated power is about 937 watts, not a big deal

Now let's look at what happens if you shorten the radials (broken radial
wires)
90-degree tower, 120 90-degree radials 1 kW = 305.8 mV/m/kW 
90-degree tower, 120 60-degree radials 1 kW = 280.0 mV/m/kW
The effective radiated power has dropped to about 838 watts <now we got some
system losses>

Broken radials are far more serious than missing radials wires because the
surface area of the ground system has been reduced (assuming somewhat
equally spaced radials).  A large number of missing radials on one side of
the ground system can of course cause some pattern distortion
(non-directional).  

This is not necessarily true for directional antennas because during the
adjustment of the array we compensate for the ground system (or lack of)
based on measured field values - i.e., we adjust the radiated pattern to fit
the pattern envelope, if the ground system is really bad - then we run into
meeting minimum pattern rms because of the high system losses. If you're
looking at a M-O-M proof, well, we just "assume" that the ground system is
all there and functioning correctly - thank you very much, and off you go.

So if you crank up the power (illegal) to recover from ground system
deterioration will the coverage area be the same as 25 years ago? Nope -
we've also got to consider the changes that have occurred in the soil
conductivity over the years - as we pave over the planet, adding concrete,
asphalt and draining swamps, the surface (soil) conductivity changes
(drops). 

Or maybe it's also the lack of chemicals/pesticides we use to use in our
soils? 

What to do? Now that we have dial-a-power (my phrase) instead of the
discreet power levels of days of old - perhaps measurement data can prove
that you can increase the station power to recover service area (daytime
only, as nighttime power is based on skywave interference not groundwave).
Keep in mind that the FCC requires a minimum power increase of 20% for AM
stations unless you are changing the site  - for the application to change
power to be acceptable. See 47 CFR §73.3571 (e)(1). 

I've taken many of 1-kilowatt stations to 5-kW and 5 kW to 10 kW  (or
somewhere in-between) over the years due to changes in soil conductivity.
For a number of stations (daytime service) was established using just the
FCC M-3 map of soil conductivity, and we all know how wrong that assumption
has become - once upon a time it was not that far off - but as I said as we
change the planet's surface the M-3 map is not correct in most areas of the
country, the conductivity is about half what the M3 map shows.

I apologize if I have been too verbose or otherwise long winded - I'm old
and grumpy and possibly senile -- and new to the list.

---------------------------------------
T Z Sawyer
Mullaney Engineering, Inc.
202-642-2130 (direct line) or 301-921-0115 (General office).