Gerald is correct here. I think one has to start with the feed line. IF
W7WHY wants his antenna resonant at 7.125 it might be in his best interest
to find out what the velocity factor is for his coax. Then, cut the cable to
have the swr the same at BOTH ends based on the freq and the velocity
factor. I am scratching my head but I think this works out to be even
multiples of 1/4 waves or something like that ( sorry, it's been too long
since I cared about this stuff). In that way, the swr will be the same at
the antenna end as it is at the transmitter end ( or should be), and if the
cable is cut to the correct length, it should be about flat. Trouble is
when he then changes freq this will go away as the coax is then not cut for
a resonant freq. any longer. How much difference it will make is anyone's
guess. But this thread began I THINK with him saying he had swr of 1.5 and
1.7 etc. I don't know what he is worrying about anyway. At hf freqs the
loss is pretty negligible. Merry Christmas all.
-Mike
-----Original Message-----
From: towertalk-bounces@contesting.com
[mailto:towertalk-bounces@contesting.com] On Behalf Of TexasRF@aol.com
Sent: Saturday, December 24, 2011 8:53 AM
To: wa8jxm@gmail.com
Cc: towertalk@contesting.com
Subject: Re: [TowerTalk] Elevated vertical
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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