On 3/15/2021 8:26 AM, Dave Sublette wrote:
Good morning,
I recently changed my elevated 160 meter quarter wave vertical with 8 full
sized radials to having only 1/8th wave length radials and only four of
When talking about radial length in wave lengths, you need to use the
effect wavelength based on the reduced velocity factor, not the free
space wavelength. You didn't say which flavor of 1/8 wavelength you
used for length.
Did you put loading coils on your radials and tune them to resonance?
That would be a lot more effective.
It also matters a lot how high the radials are. Knowledgeable
vertical builders tend to say you need 20 feet for best performance
of a few elevated radials.
them. It is working great. So I thought adding four more radials might
"Working great" meaning what? Compared to what? A full size 1/4 wl
vertical *should* work great. How well a vertical actually works
is highly dependent on the ground it is over, no matter what the
builder does. Sand vs clay vs salt water makes a huge difference.
I use a modelling program called Antenna Model. The result of the
comparison is this:
Never heard of that program. Does it use NEC2 or NEC4 or what for the
engine? Hard to comment about it.
The 4 radial system showed a gain of 0.92 dBi with the main lobe at an
elevation of 20 degrees.
The 8 radial system showed a gain of 0.93 dBi and an identical elevation
pattern.
My question is: Why is the gain figure so low? A dipole exhibits 2.14 dBi
gain. Why doesn't the vertical show gain?
A little theory here about maximum theoretical (MT) gains:
A half wave dipole in free space has 2.14 dBi MT gain as you say.
A "short" dipole has 10 LOG 1.5 = 1.76 dBi MT gain.
A quarter wave vertical working over ground has 3 + 2.14 = 5.14 dBi
MT gain.
A "short" vertical working over ground has 3 + 1.76 = 4.76 dBi MT gain
For receive antennas, the theoretical RDF of a short vertical
is always 4.76 dB regardless of radials, etc. The actual RDF
will also usually be 4.76 dB in practice; hard to goof that up in
a RX vertical.
And lastly, I think these results tell me it isn't worth the effort to add
four more radials.
Dave, K4TO
Noted expert on radials, N6LF, says that in a perfect world you only
4 elevated radials, but experimental measurements show that in the
real world you need 8 or maybe 12 or 16 radials, no matter what modeling
shows.
73
Rick N6RK
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