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Re: [TowerTalk] Elevated vertical

Subject: Re: [TowerTalk] Elevated vertical
From: Jim Lux <>
Date: Sat, 24 Dec 2011 08:09:31 -0800
List-post: <">>
On 12/23/11 7:36 PM, Paul Christensen wrote:
>> "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."
> How are you defining the terms "tuned," and "matched?"  What is tuned and
> what is matched?  How is your use of "tuned" different than "resonant?"

> then your 130 ft dipole and transmission line are resonated, where the
> system reactance is zero. That's a resonant antenna system - where circuit
> reactance is zero.  In some circles, it's also called "tuned."  See below.
> Tuned = Resonant = 0 reactance = X(0).

This is the classical resonance condition for a resonant system (e.g. 
mass on a spring, single RLC).  Where we get wrapped around the axle is 
when we try and use a RLC conceptual model for an antenna.  Antennas 
look like a RLC only over a narrow range, and it's not a particularly 
good model either (a classic resonant circuit will be symmetric in 
squared frequency, antennas, in general, are not)

> That's the X value but R may be a value well above or well below 50 ohms,
> but your antenna is still resonant.  However, is it also matched at the line
> input (50+j0)?

"match" is even more ambiguous, especially when talking about a source 
which may not have a "resistive" or "constant" output impedance.

Does it mean that the impedance happens to be equal to that of the 
transmission line? Or that maximum power is transferred to the load from 
the source?  Or that we are making most efficient use of the power 
supplied to the system (so transmitter efficiency gets in the game)

We like to think in terms of all devices having 50 ohm resistive Z, so 
hooking up 50 ohm line to a 50 ohm source and a 50 ohm load makes things 
"matched" in both senses.

However, consider a source that has zero output impedance.  Maximum 
power is probably transferred at a load impedance other than 50 ohms, 
even if there is 50 ohm line.

> I just assisted in the design and installation of a 160m "T" radiator
> consisting of 85' ft of vertical section and a 130 ft. flat top section.
> Using 4Nec2 software, the feed-point Z was close to 50+j200.  This value was
> confirmed with a Vector Network Analyzer.  Under this condition, tuning the
> antenna to resonance and matching the antenna to 50 ohms resistive requires
> a low-pass L network, or simply a series capacitor to cancel inductive
> reactance by an equal but opposite amount of capacitive reactance.  In this
> case, one series capacitor will tune the system to resonance, while ensuring
> that the input remains matched at 50 ohms resistive (50+j0) for a 1:1 VSWR.
> It may have also been possible to achieve the same result by changing the
> feedline length to also tune the system into resonance.  But tuning to
> resonance may not have yielded an R=50 value.

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