Hi Ken, we have a differing definition of what constitutes resonance: You
are talking about the radiating element only and I am talking about the
radiating element plus everything else that constitutes an antenna system;
matching devices, coax etc.
Your example of .28 wavelengths does indeed move the R component to 50 ohms
but the +X that happens makes the vswr about 4 to 1 until the series
capacitor is added. The series capacitor cancels the +x leaving only 50R. In
my
view and definition, that is a resonant system.
I "think" we may be saying the same thing but from differing perspectives.
73,
Gerald K5GW
In a message dated 12/23/2011 4:47:16 P.M. Central Standard Time,
wa8jxm@gmail.com writes:
Gerald,
I disagree. If a quarter wave vertical or ground plane is lengthened to
about .28 wavelength, the radiation resistance raises to about 50 ohms and
a 1:1 SWR. It is not resonant, but it has a lower SWR than a "resonant"
antenna. The reactance can be tuned out with a series capacitor. It is not
at resonance but it does have a low SWR. Moving down in frequency, to where
the antenna is resonant, the SWR will go up. (Reference ARRL Antenna
Book, 10th edition, page 125).
This is, of course, the same technique used with 160m inverted L antennas
to improve the feedpoint match: lengthen the antenna beyond the quarter
wave resonant point.
IMO, people confuse resonance and low SWR. An antenna does not have to
be resonant (or anywhere near it) to work effectively. A prime example is
the "Double Extended Zepp" which is no where near resonant.
When people say they tuned their antenna until it was "resonant with a 1:1
SWR" I believe they are mistaking matching the feedline impedance (low
SWR) with being resonant, which it is probably not.
I am confused by your suggested wording "does not UNnecessarily indicate
resonance". Perhaps you meant "does not necessarily...." but even then, if
we don't know whether or not it does, then it is a meaningless statement.
While resonance and low SWR MIGHT happen, there is no defined
relationship. I'll stand by my statements that "low SWR does not indicate
resonance"
and "resonance does not ensure a low SWR".
A 1:1 SWR indicates that the feedline impedance matches the antenna
impedance, nothing more. It does not indicate resonance. Does a dipole
change
it's resonance when you change the feedline from 50 ohm to 75 ohm to 300 ohm
or 600 ohm? Of course not. But the SWR sure changes!
73,
Ken WA8JXM
On Dec 23, 2011, at 8:54 AM, _TexasRF@aol.com_ (mailto:TexasRF@aol.com)
wrote:
Hi Ken, the statement "low swr does not indicate resonance" is a bit
narrow. Perhaps "does not unnecessarily indicate resonance" would be more
accurate.
If the feedline has a high loss, the swr would be low even with a highly
non-resonant load connected
If the feedline loss is low, then minimum swr would coincide with
resonance. Using your example of 30 ohms, at resonance the resulting 50 ohm
vswr
would be 1.67. Moving off frequency until there is 10 ohms +x results in a
vswr of 1.77 (ignoring any change in the 30 ohm value). Moving off frequency
until there is 10 ohms -x also results in vswr= 1.77 (also ignoring any
change to the 30 ohm value).
Minimum vswr then indicates resonance in that system. As you say,
resonance does not necessarily provide a low vswr. Resonance does provide
minimum
vswr at the frequency in use.
=
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