It sounds like we could debate the definition of resonance for days without
reaching a consensus. So let's get back to the original topic: is resonance
an important attribute of a good antenna? For example, consider a 40M
double-extended zepp antenna. At the feed point the impedance varies from
243-j1150 at 7.0 MHz to 165-j870 at 7.3 MHz. The reactance is not zero
anywhere in the 40M band. Yet this antenna exhibits approximately 3 dB gain
over a "resonant" dipole, and as one would expect a narrower 3dB beamwidth.
One could argue that a double-extended zepp is actually two end-fed
collinear resonant dipoles whose ends are separated by 1/4 wavelengh. But
at the feed-point, which is the only point of visibilty that we have, there
is reactance and the circuit does not appear resonant and requires some form
of tuning network to make the transmitter happy. It does have zero
reactance and is resonant around 8.64 MHz, but the pattern no longer has two
broadside lobes, but rather 4 lobes that are approximately 45 degrees from
broadside and the maximum gain is now only 1.2 dB better than the dipole.
In this case resonance may actually be hurting us.
The bottom line is that you have to look at a much bigger picture than just
the feedpoint to understand whether an antenna is effective or not.
Measuring feedpoint impedance and declaring victory when the reactance is
zero or when the SWR is below a certain point is just plain ignorant, unless
your only goal is good SWR and antenna performance is not important. If the
antenna is not resonant, you can always add a matching network to make it
appear electrically resonant to the transmission line and/or transmitter.
73,
Clay W7CE
----- Original Message -----
From: "Jeff Carter" <towertalk@hidden-valley.com>
To: <towertalk@contesting.com>
Sent: Friday, August 07, 2009 9:11 PM
Subject: Re: [TowerTalk] Resonance is over rated
> Of course. I tried to gently suggest this earlier.
>
> I'll go a step further: It's stupid to talk about resonance in a
> circuit that doesn't contain reactive components. j0 = j0 (or i0 for
> the math and physics folks). The vector diagram, or Smith Chart, or
> network analyzer is going to show that we're living on the X axis,
> firmly in Resistiveland when j (or i) is equal to 0.
>
> We can muddy the water by talking about what broadcasters do and what
> hams can get away with, but when we return to the theory and the books
> (including anything by ARRL) the math is the same, and has been
> accepted science for a hundred years or more.
>
> I will now wait for somebody to start talking about how lead length
> and "real world" factors make the math pointless. I will then answer
> that the man's meter said "j0", which means for practical purposes the
> leads can be discarded. His dummy load is a big resistor, which by
> definition are not reactive.
>
> Jeff/KD4RBG
>
> ---- Original message ----
>>Date: Fri, 07 Aug 2009 20:10:26 -0400
>>From: Blair S Balden <blair.balden@wmich.edu>
>>Subject: Re: [TowerTalk] Resonance is over rated
>>To: Clay W7CE <w7ce@curtiss.net>
>>Cc: towertalk@contesting.com
>>
>>Hi everyone,
>>Wouldn't a better definition of a resonant circuit be a circuit that has
>>reactance, but where the capacitive reactance is equal to the inductive
>>reactance? That would exclude a purely resistive circuit. The concept of
>>resonance does not seem appropriate for purely resistive circuits, where
>>the response is not frequency-dependent.
>>Blair
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