[AMRadio] DRM from Las Vegas this week


Larry Will lhwill at verizon.net
Fri Apr 28 18:58:31 EDT 2006


Steve,

Your points are correct and well taken but not the whole story.  No 
doubt that F2 skip can be very low angle and rhombics used at HF are 
designed for that. (Takeoff angles of say 2 to 10 degrees with good 
suppression above that)  And local bcsting on 26 MHz would have 
trouble with F2 skip.  But E skip (the usual CB kind) and nighttime 
AM are another matter.  My station had the tall towers to protect 
co-channel stations at 350 to 600 miles AT NIGHT not to cancel 
skywave close in (35-60 miles) at critical hours.  This is not unusual.

   The typical 1/4 wave CB antenna has a very high angle of radiation 
and thus dumps most of the power at 25 to about 70 degrees thus fully 
illuminating the E layers day and night.  It doesn't mater whether 
the antenna is on the ground or on a tower, the skywave radiation has 
nothing to do with ground conductivity (nor does the local coverage 
from a 26 MHz transmitter.  The "direct" wave is what you pick up 
locally on 26 MHz, the poor GC causes the "ground" wave to not travel 
very far even with very high power.

TCI has a very solid reputation on designing HF antennas and I am 
sure that they can propose or have even installed anti-skywave 
antennas for the 12 meter shortwave band (where the DRM tests were done).


Larry


At 02:15 PM 4/28/2006, you wrote:

>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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