|[AMRadio] DRM from Las Vegas this week|
garyschafer at comcast.net
Fri Apr 28 20:44:31 EDT 2006
Some of this seems a little backwards. The typical CB 1/4 wave antenna is a
rather low angle radiator not a high angle radiator. The major lobe is very
low, less than 10 degrees. Many higher angle lobes do occur however.
Antenna height as a great effect on take off angle, typically lower angle at
Ground conductivity has a great effect on sky wave signals as part of the
signal is reflected from the ground and adds to the direct part of the
signal. Even antennas mounted high off the ground. It is not only the ground
conductivity in the close in area but also in the far field that matters,
especially with a vertically polarized antenna.
The "local" coverage is line of sight so it has little influence by ground
conductivity as you say.
Ground wave propagation (more correct, surface wave) is highly attenuated
much above 3 Mhz so at 11 meters there is almost no ground wave at all.
"Suppressing the sky wave signal on 11 meters"? Sounds like a job for a
fractal antenna. :>)
The better you make the "local" wave the better you are going to make the
sky wave signal as Steve notes. I would have to agree with Steve, sounds
like a good DX antenna!
> -----Original Message-----
> From: amradio-bounces at mailman.qth.net [mailto:amradio-
> bounces at mailman.qth.net] On Behalf Of Larry Will
> Sent: Friday, April 28, 2006 6:59 PM
> To: Discussion of AM Radio
> Subject: Re: [AMRadio] DRM from Las Vegas this week
> 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).
> 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
> >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
> >the MF wave being in contact with the ground reduces that somewhat, but
> >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
> >lower as the antenna is well above the ground (space wave rather than
> >wave). In any case, if you put a lot of energy out toward the horizon
> >have great DX.
> >I could design an antenna that would put the minimize the DX by aiming
> >main lobes down from a tall tower or mountain into a valley - as is done
> >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
> >when 11m opens up.
> >Even the minor lobes on a beam-tilted antenna would propagate long
> >on 11m. For "local" broadcasting 26 MHz seems a poor choice. Remember
> >4-watt CB was meant to be local, but when ever the band opens you'd hear
> >might roar of thousands of those rigs "skipping" in.
> >Steve WD8DAS
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