I'm not disagreeing with the fact that for a small number of wires close to or
on the ground that they will not have a resonant less than a physical 1/4
wavelength. What I am saying is that for a large number of radials, whether
they are resonant or not is irrelevant since they are now forming a ground
screen - not a counterpoise.
Quoting from ON4UN, fourth edition, page 9-14..."As soon as you use a larger
number of equally spread radials the resonance effect disappears, and the
radials form a disk, which becomes a screen with no resonance characteristics.
In this case, we no longer talk about length of radials, but about the diameter
of a disk hiding the lossy ground from the antenna...." (emphasis added)
Hence the goal in this case should be to create an efficient screen and not
resonance in the radials.
I do agree that for a few radials on the ground or elevated couterpoise
radials, achieving resonance is important in order for the 1/4 wavelength
radiator to have something to work against. However, for muliple ground
mounted radials, resonance is irrelevant.
Of course, one can discuss which works better - a ground screen or a
counterpoise system. But that's a whole new can of worms, one that has been
debated ad nauseum without consensus agreement.
73, Jeff
W3KL
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: RE: [TowerTalk] 160 radials
To: w3kl@w3kl.com
Date: Friday, December 5, 2008, 7:44 PM
Nonsense. Radials on the ground behave as lossy transmission lines and have a
natural resonant frequency. Their Vf is approx 0.6
Run the experiment. You will find a nicely defined resonant point on a 10
meter dipole laying on the ground. X (or j) will =0 at around 20 meters.
I make no other claim….all the extrapolations of what I said were not said by
me. I make no claim regarding efficiency – I just offer up the idea that if
you want an easier impedance to match you can get it with radials shorter than
135 feet.
I have supplied more intelligent errata to ON4UN and N6BV on their books than
most folks…
73
Hal
N4GG (several advanced degrees and founder of 14 electronics companies. I
consider it superfluous at best to list it all in the context of ham radio
conversations)
-----Original Message-----
From: w3kl@w3kl.com [mailto:w3kl@w3kl.com]
Sent: Friday, December 05, 2008 10:29 PM
To: Hal Kennedy; towertalk@contesting.com
Subject: Re: [TowerTalk] 160 radials
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 PMFor 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 wireon the ground is approx. 0.5. Quarter wave radials are
actually approxa half wave electrically - which is why it takes so many of
them to geta monopole down to 35 ohms - each radial presents a high impedance
ifits 135 ft long and on the ground. BC stations use quarter
wavelength(mechanical quarter wave) radials because 120 of them will provide a
lowimpedance when placed in parallel and current share well since each area
high impedance. You can easily prove this to yourself. Lay an 80 meter (or
higher inQRG) dipole on the ground and check it with an MFJ. It will be
resonantnear 160 meters. Do it quick with a 10M dipole - it will be
resonantaround 20M. You can't do that test with a 160 dipole as it will
beresonant around 900 KHz and the MFJ won't go that
low. Want that inverted L impedance down where it should be with very
fewradials? Elevate them to eliminate the above effect, or put them on
theground and make them close to an electrical quarter wave - which isaround 80
feet long - not 135 ft. Having an inverted L "too long" is just right. It
moves the maximumcurrent up off the ground and is easily tuned out with series
C. 73HalN4GG(Running just great with a low feedpoint impedance from four 80
footradials under the vertical, on the ground)
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