[SFDXA] One Man's Vertical Experience...Ray W8LYJ

[Bill]

 From the Butternut Reflector:

After 50 years in Broadcast engineering including being chairman, 
organizer of the AM directional antenna coalition that worked with the 
FCC on changing the rules permitting computer modeling I have a little 
bit of experience with AM and end fed monopole antennas.
It’s important to determine if the “vertical” antenna being looked at is 
end fed or a dipole. Many of the Gap antennas are dipoles.

AM stations that have antennas close to a 1/4 wave have a radial system 
of 120 1/4 wave radials. AM stations with radiators close to 1/2 
wavelength have 120 0.400 wavelength radials. That’s underneath each 
tower. The tower height depends on many factors, money, FAA concerns 
required theoretical efficiency, etc. Those radial system result in a 
theoretical efficiency of 100%. Incidentally, The advantage of a longer 
radiator is lower angle of radiation, less sky wave interference. AM 
stations dohave a matching unit at the tower base.

The radial system resistance is in parallel with the ground resistance 
so two resistors in parallel results in less loss, better efficiency, 
more power radiated. There are charts in the older ARRL antenna book 
showing efficiency vs # of radials, radial length etc. it’s not linear 
so beyond about 60 radials does not result in significant increases in 
efficiency. No radials has an efficiency of about 15% so yes no radial 
systems do work.
The length of the radials is determined by the number. 0.15 wavelength 
is a good number for 60 or less, hence the 37 ft recommended length. 
It’s base on 80m.

The theoretical impedance of a 1/4 end fed is about 34 ohms plus a bit 
of capacitance so not resonant. I have never seen a resonant AM array. 
As radials are added the impedance goes down closer to 34 ohms so SWR 
tends to goes up with added radials.

Best to think of an antenna as a capacitor with a resistor in series 
representing loss. The displacement currents forming the field flow 
between the plates. With an end fed, one plate is the radiator, the 
other is ground, a poor conductor of RF. Hence the need for the radial 
system in parallel with the ground loss resistance. Straight out of the 
older ARRL antenna books.

Oh, bottom line is, do what you can do. A end fed with no radials, 15% 
is better than 0%, not enjoying the hobby.
Ray
W8LYJ
Formerly Westinghouse/Group W which bough and became CBS