[AMRadio] DRM from Las Vegas this week


SBJohnston at aol.com SBJohnston at aol.com
Fri Apr 28 14:15:27 EDT 2006


Larry wrote:

>Think AM.  At WCZN we used a .42 wavelength vertical which has great 
>null suppression at medium elevation angles. (20-40 degrees as I 
>remember without looking it up.)  See any text on vertical radiators.

I understand that well, but it isn't the same situation.  One medium wave you 
want to eliminate the higher angles of radiation to avoid creating 
interference to your own ground wave coverage out toward the fringe. But your low angle 
of radiation is putting a lot of energy straight out toward the horizon, which 
would be good for DX (the skip distance is much farther out).  Losses from 
the MF wave being in contact with the ground reduces that somewhat, but not 
much.  On the Virginia coast I had a 5/8-wave tower on 1310 kHz that got reception 
reports from Africa.

In the situation on 26 MHz the same rules apply, only the ground losses are 
lower as the antenna is well above the ground (space wave rather than ground 
wave).  In any case, if you put a lot of energy out toward the horizon you'll 
have great DX.    

I could design an antenna that would put the minimize the DX by aiming the 
main lobes down from a tall tower or mountain into a valley - as is done with 
beam-tilt on FM andf TV antennas.  But the DRM rep indicated they were using a 
simple dipole on the tower - I'm pretty sure that will talk around the world 
when 11m opens up.

Even the minor lobes on a beam-tilted antenna would propagate long distances 
on 11m.  For "local" broadcasting 26 MHz seems a poor choice.  Remember that 
4-watt CB was meant to be local, but when ever the band opens you'd hear a 
might roar of thousands of those rigs "skipping" in.

Steve  WD8DAS



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