For multiple radials laid on the ground or buried just below the ground, there
is no "resonance" effect - that is, there is no such thing as a tuned radial
for wires laid on the ground or buried just below the ground.
What you are referring to IS true for a counterpoise system - aka "elevated
radials".
When you have multiple (greater than 10 say) radials on the ground they simply
form a ground screen. The "need" to extend them 1/4 wavelength out is simply
to improve the current collection efficiency of the ground screen - it isn't
because you want them to be resonant.
There are volumes written on this. I recommend ON4UN's book on low band
DXing. W8JI's website may also have something on this as well.
Finally, in order for your assertion that a length of wire in close proximity
to the ground has an electrical length greater than it's physical length would
be true only if the wire and ground formed a transmission line. This is how a
Beverage antenna works.
However, at HF frequencies (even on 160 m), I think that a wire laid on the
ground will not form a transmission line since the current and it's image
current are too close in proximity. However, again what's important is that
the radials are forming a ground screen and not the fact that they are or are
not resonant.
Said differently....a quarter wavelengtth radiator has the same radiation
resistance regardless of the efficiency of the ground. However, the radiation
efficiency of the antenna depends on minimizing ground losses. Installing an
efficient ground screen (radials) helps reduce the ground loss, thereby
increasing the radiation efficiency. This does NOT require the radials to be
resonant.
73, Jeff
Jeffrey K. Okamitsu, PhD, MBA
+1-609-638-5402
--- On Fri, 12/5/08, Hal Kennedy <halken@comcast.net> wrote:
From: Hal Kennedy <halken@comcast.net>
Subject: [TowerTalk] 160 radials
To: towertalk@contesting.com
Date: Friday, December 5, 2008, 5:13 PM
For those who believe they are putting 1/4 wave radials on the ground -
it might be important to know/remember that the velocity factor of wire
on the ground is approx. 0.5. Quarter wave radials are actually approx
a half wave electrically - which is why it takes so many of them to get
a monopole down to 35 ohms - each radial presents a high impedance if
its 135 ft long and on the ground. BC stations use quarter wavelength
(mechanical quarter wave) radials because 120 of them will provide a low
impedance when placed in parallel and current share well since each are
a high impedance.
You can easily prove this to yourself. Lay an 80 meter (or higher in
QRG) dipole on the ground and check it with an MFJ. It will be resonant
near 160 meters. Do it quick with a 10M dipole - it will be resonant
around 20M. You can't do that test with a 160 dipole as it will be
resonant around 900 KHz and the MFJ won't go that low.
Want that inverted L impedance down where it should be with very few
radials? Elevate them to eliminate the above effect, or put them on the
ground and make them close to an electrical quarter wave - which is
around 80 feet long - not 135 ft.
Having an inverted L "too long" is just right. It moves the maximum
current up off the ground and is easily tuned out with series C.
73
Hal
N4GG
(Running just great with a low feedpoint impedance from four 80 foot
radials under the vertical, on the ground)
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