[Elecraft] Home made Sigma-GT5 & KRC2 or SGC?

[Don Wilhelm]

Let me inject yet another bit of 'wisdom' into this thread -

In ON4UN's Low band DXing book (Chapter 8) he examines the half wave
vertical dipole.  The conclusion section reports: "The 6.1 dBi gain can be
obtained with a half-wave vertical only over nearly perfect ground (sea).
Even over very good ground, the half-wave vertical will not be any better
than a quarter wave vertical (3-dBi gain).  This means that unless you are
near the sea, you may as well stick with a quarter-wave vertical  The
sloping vertical (make the sloping wire as vertical as possible) with 2
radials (5-m high for 3.6 MHz) will produce as good a signal as a half wave
vertical or sloping half-wave vertical over very good ground.  It will,
however, only require a 25 m support instead of a 35 or 40 m support for the
half-wave vertical."

Elsewhere John states that the required length for radials with a half wave
vertical is much longer than for a quarter wave vertical.  His
recommendation is for one to use radials at least 2 wavelengths long with a
half-wave vertical in order to achieve  better performance than a quarter
wave vertical.

I have a multiband half-wave vertical (Cushcraft R3) on 20 thru 10, and as a
result of John's information, I will be comparing it with quarter wave
verticals with 2 or more raised radials tuned for length.  The results may
be interesting - a full size quarter wave vertical for 20 meters is less
than 17 feet (my R3 is 23 ft long).  I will be using elevated radials rather
than trying to install a good in-earth grounding system of 120 radials. Full
quarter wave 20 meter radials will be quite practical for me, but I may be
able to see what difference shortened (and tuned) radials (and perhaps even
3/4 wave radials) produce (if any) for my particular soil characteristics
here in Wake Forest, NC.

The bottom line as I see it is that verticals are VERY sensitive to soil
conditions out to include the Fresnel zone (many wavelengths from the
antenna base), so they may work well for some installations and poorly for
others.

73,
Don W3FPR


----- Original Message ----- 

> Vertical dipoles can easily have more than 90% radiation efficiency.
> Locating them above ground reduces their ground losses somewhat as
compared
> to ground mounted verticals with a few conventional buried radials. They
> have a smaller footprint and are suitable for use for many who do not have
> enough room for conventional radial systems. Even systems with fairly
large
> numbers of radials do not exhibit significant gain over the vertical
> dipole, eg less than 1 db in modelling studies by W4RNL.
>
> Mounting vertical dipoles too high above ground causes loss of power into
> (generally) undesirable high angle lobes. So there is a tradeoff between
> ground loss and lobe loss.
>
> Vertical dipoles at the edge of seawater produce tremendous low angle
> signals - exceeding all but the most extreme beam/tower combinations. This
> can probably be replicated by a few miles of copper groundplane around the
> vertical. Reports of comprehensive tests of this kind have not been
> substantiated.
>