Actually, Jim, a "dipole" antenna can be as short as about 0.1 normal
size and still be over 90% efficient in radiating the power it is fed.
(Those are very approximate values.)
The idea that an antenna must be "resonant" in order to radiate
efficiently is one of the unfortunate urban legends of amateur radio.
Broadcast stations, of course, are an excellent example of shortened
antenna structures that are efficient enough to be licensed and cover
their service areas. Aircraft HF antennas are another example of
efficient shortened designs. They also make short work of the notion
that a "ground" connection is always necessary for an antenna system to
operate efficiently.
The difficulty with any short antenna is the fact that its driving point
impedance is one of low resistance and high capacitive reactance. This
makes for a difficult matching situation, usually leading to significant
loss in the matching network, primarily in the required matching
inductance(s). But, the loss is *there* and not in the ability of the
antenna to radiate efficiently.
Any antenna will radiate all the power it receives less any that it
dissipates in its own or in nearby structures.
The notion of "resonance" being either desirable or necessary or both is
one essentially of convenience in matching: a resonant antenna has a
purely resistive driving-point impedance. Usually, this is an easier
condition to match than the more general case of a complex impedance.
But, therein can lie a fallacy, in that away from the magic resonant
frequency, the impedance presented to the transmission line by the
antenna will become reactive and cause the antenna to fail to accept
power as readily unless a change is made in the matching system.
This is most widely seen on the 160 and 80 meter bands where the
percentage bandwidth is quite large. Normally, special steps have to be
taken on those bands to obtain efficient antenna *system* operation over
the entire band. This notion is quantified in the "SWR bandwidth" spec
that we see for antenna systems, such as the 2:1 limits. A common
solution to this problem is the use of tuned feeders to obtain a
resonant antenna *system*. In this solution, matching is done in the
shack with a convenient matching network instead of at the antenna. The
tradeoff for this convenience is added loss in the feeders.
All this business also wraps up into the myth of "low SWR" on the
feedline as being necessary for an antenna to radiate. While it is true
that an SWR larger than 1:1 will always introduce additional feedline
loss, the amount of such loss is usually overstated, especially with
open-wire or ladderline types. And apart from decreasing the power
actually delivered to the antenna, an SWR larger than 1:1 has no effect
whatsoever on the ability of the actual antenna structure to radiate the
power it does receive.
So, we are not bound always to use "resonant" antennas if shorter ones
will do the job, or must do the job. The problem ultimately becomes one
of delivering power to the antenna and that is a matching task. Here is
a rich area for Smith Chart activity to both design a matching network
and to understand what it is doing and how. Lots of examples in the
books . . .
72/73, George W5YR - the Yellow Rose of Texas NETXQRP 6
Fairview, TX 30 mi NE Dallas in Collin county QRP-L 1373
Amateur Radio W5YR, in the 55th year and it just keeps getting better!
Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)
Jim Reid wrote:
>
> In H. Hertz's paper of 1889*, he did not require that
> the radiating element be "resonant". In fact, that idea
> did not then exist!
>
> *1. Hertz H., "On the relation between light and electricity"
> Gesammelte Werk, 1, 340 (1889).
>
> 2. Hertz H., "The force of electrical oscillations treated with
> the Maxwell theory"**, Ann. Phys. 36, 1 (1889).
>
> ** Maxwell, J.C., "On physical lines of force: Part 1. The
> theory of molecular vortices applied to magnetic phenomena;
> Part 11. The theory of molecular vortices applied to electric
> currents." Phil. Mag. 21, 161, 281, (1861).
>
> Hertz's work was completely depended upon the work of both
> Maxwell and Lamor. Lamor, who in 1897 was able, at last,
> to write down the expressions for the fact that radiation
> occurred only form Accelerated Point Charges, Phil. Mag. 44,
> 503 (1897).
>
> Perhaps of interest to some. But, certainly to me!
>
> 73, Jim, KH7M
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