Re: Resonant Antenna

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From: Bill Coleman (aa4lr@arrl.net)
Date: Wed Aug 22 2001 - 16:13:04 CDT


On 8/12/01 2:35 PM, Mike Branca at w3irz@att.net wrote:

>
>Also unless we are putting power into a resistor any antenna must be
>resonant.

A resistor is perfectly resonant at a large range of freuencies.

But, no, the ANTENNA need not be resonant. The Antenna SYSTEM often is,
because we typically use a tuner.

> The above described 100 foot antenna is resonant on 80 thru 10
>meters.

The antenna isn't resonant, the antenna SYSTEM defined by the antenna,
feedline and the tuner IS. (Or is when the tuner is properly adjusted.

> One must understand that we are talking about an antenna system
>composed of the radiating element (100 foot of wire), the open wire feed
>line and the antenna tuner. The tuner simply supplies the missing L and C
>values that the feed line and antenna need to achieve resonance.

Specifically, to acheive a conjugate match -- which results in a resonant
condition (defined as an impedance Z with no imaginary component).

>When
>compared to a dipole this shortened antenna is only slightly less efficient
>on 80 meters and comparable, in energy radiated, on the higher bands.
>
>The danger in such resonant Vs non resonant antenna discussion is for the
>new hams who are presented with the idea that somehow there is radiation
>magic in a specific length of antenna. Filling the discussion with a lot of
>math further confuses the subject. Its like asking which is bigger 12 or a
>dozen.

The key to maximizing antenna radiation is minimizing loss. Energy that
is not lost to heat MUST be radiated as a signal.

Some people confuse resonance with a low-loss condition. This is partly
true, particularly with coax-fed antennas, since complex impedances on
the coax lead to a lot more loss in the system.

But resonance is not a guarantee of low-loss. A dummy load is perfectly
resonant, but also perfectly lossy.

Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901


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