Hi Ken, the statement in the Antenna Book about the waves on the radiator
bouncing back and forth and being reinforced by the next wave to achieve
resonance is exactly right.
But, the point where one wave ends and the next one begins does not HAVE to
be located on the radiator proper. It can be somewhere down the feed line
as well.
And that is exactly the situation when a tuner is used to minimize vswr
(same as resonating the system). The tuner is altering the phase of the
incident wave to make it and the reflected wave in phase and hence resonate.
Of course phase is not the only component in achieving a matched condition.
Amplitude has to be correct as well and that is done by adjusting the
tuner L/C to force the R to the needed value to have the correct amplitude for
a match. We don't even think about R and X when twisting the tuner knobs.
Just watching the vswr or watt meter for lowest reflected power (resonance)
makes the magic happen.
Loss in the transmission line will alter the phase and amplitude values.
The tuner then has to be adjusted further to compensate. Again, that is
transparent when watching the reflected power/voltage and adjusting for minimum
(resonance). It all happens without thought.
We have all skipped right over the original question from W7WHY on his 40m
vertical vswr vs frequency curve having double dip. I have some insight on
that but will need some time to come up with a credible explanation.
73,
Gerald K5GW
In a message dated 12/23/2011 8:39:05 P.M. Central Standard Time,
wa8jxm@gmail.com writes:
Okay, Gerald, I will agree that you and I are talking about different
things. But most people talk about adjusting their antenna and saying it is
resonant (or not) when they get a 1:1 SWR.
As you have clarified, you are thinking of "system resonance". I'm
talking "antenna resonance".
Would you consider my 130' center fed dipole "resonant" on 60 or 30 m
because the antenna tuner inside the rig can tune the
antenna/twin-lead/balun/coax to a 1:1 SWR inside the rig? Sorry to say but
that one hurts my head
to think of as "resonant". Tuned? yes. Matched? Yes. Resonant? No.
In the chapter on Antenna Fundamentals, my 10th edition Antenna Book
describes "resonance in linear circuits" in regard to a wave traveling along
the
antenna to the end, bouncing back, and being reinforced by the next wave
coming from the feedline. It needs a specific length for that to happen:
the standard length=492/frequency (or a multiple.)
73, Ken WA8JXM
On Dec 23, 2011, at 8:18 PM, _TexasRF@aol.com_ (mailto:TexasRF@aol.com)
wrote:
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
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