Maximum NVIS signal occurs when the antenna is 1/4 wave
above ground (33 -35' on 40 meters) - not at 1/8 wave (16').
Any analysis that recommends heights at or below 1/8 wave
is flawed in that ground losses are being ignored (work
done by the DOD).
At an antenna height in the 20 to 25' range a 40 meter
dipole will have a feed impedance in the 50 Ohm rage ...
There is no justification for the very low 8-15' heights
when elevations as high as 45 feet will provide increased
high angle radiation (> 60 degree TOA), lower ground
losses, and higher feedpoint impedances.
73,
... Joe, W4TV
> -----Original Message-----
> From: towertalk-bounces@contesting.com
> [mailto:towertalk-bounces@contesting.com] On Behalf Of Steve, W3AHL
> Sent: Tuesday, August 25, 2009 2:28 PM
> To: Tom Osborne; Towertalk
> Subject: Re: [TowerTalk] Folded dipole
>
>
> One of the best technical presentations on NVIS propagation
> and antennas is
> at:
>
> http://groups.yahoo.com/group/wl2kemcomm
>
> Look in the files section for NVIS_Presentation_R1-4-8a.pdf
> by WA5BEN. You
> need to join the group to access the files section. There are older
> versions of the file at other sites, but get the latest rev.
>
> Optimum height is 1/8th wave (30' for 80M, 16' for 40M).
> Anything less just
> reduces efficiency with excessive ground losses.
>
> I use a 80/40 fan dipole at 20'. 80M needs a tuner to cover
> the band, but
> 40M covers 7.1 - 7.3 at less than 1.8:1 SWR without a tuner.
> 7.1 MHz is as
> low as I need to go for Winlink Pactor -- don't do CW. I
> have an identical
> antenna in my go kit for portable use.
>
> Steve, W3AHL
> ----- Original Message -----
> From: "Tom Osborne" <w7why@verizon.net>
> To: "Towertalk" <towertalk@contesting.com>
> Sent: Monday, August 24, 2009 10:22 PM
> Subject: [TowerTalk] Folded dipole
>
>
> > Hi All
> >
> > Looking to put up another 40 meter antenna. Need a NVIS
> antenna for
> > close-in work.
> >
> > Is there any reason to put up a folded dipole instead of a
> > tuned-feeder dipole?
> >
> > I think when this antenna was popular, it was touted to have more
> > bandwidth
> > than a dipole. But, if you have to use an antenna tuner to tune it
> > anyway,
> > there doesn't seem to be any advantage over a regular
> antenna, bandwidth
> > wise. 73
> > Tom W7WHY
> >
> > _______________________________________________
> >
> >
> >
> > _______________________________________________
> > TowerTalk mailing list
> > TowerTalk@contesting.com
> > http://lists.contesting.com/mailman/listinfo/towertalk
>
> _______________________________________________
>
>
>
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