[Elecraft] antenna question

[George, W5YR]

Thanks, Stuart, for the mention and agreement.

If anyone still has any thought that "resonance" is a requirement for
efficient radiation, consider that an *infintesimal* dipole - practically
speaking that means as small as you want to make it! - will transmit a
signal that theoretically is down only 0.45 db from a full-size "resonant"
dipole.

Of course, the problem is getting the power into that tiny antenna since
its driving-point impedance will have an infinitesimal real part
(resistance) and a near infinite capacitive reactance.

Back in the real world, the point simply is that the property of
"resonance" conveys no special performance capability on a dipole except
for one thing: its driving-point impedance is purely real at the "resonant"
frequency. But, move away from that frequency and the resistive component
starts changing slowly and the reactive component becomes either capacitive
or inductive fairly rapidly depending upon whether the antenna is shortened
or lengthened.

The resistive input impedance simplifies the line matching situation and
that in turn can result in reduced loss elsewhere in the system. However,
the dipole will radiate no more power than is fed into it, regardless of
its size, just like any other antenna. Everything that gets delivered to
the antenna must be radiated, turned into heat or returned to the source as
reflected power to rejoin the forward power at the tuner input and go back
to the antenna.

If the antenna is "non-resonant* then unless we happen to come up with a
magic feedline Z that both matches the antenna to the line and also
presents the proper load resistance to the transmitter, then we have to
"tune" the system to accomplish the impedance transformations required with
the usual ladderline or whatever we use.

Is this bad? Hardly ever, if done properly. A properly designed and
operated tuner will have very little loss - ever see any tuners with fans
in them to get rid of the heat? Open-wire feedline will have negligible
loss at HF; conventional ladderline about the same as long as it is dry.

So, in the real world, there is little need to worry about getting an
antenna precisely the right length unless you are shooting for the
combination of 50-ohm coax feeding a 50-ohm antenna to present a 50-ohm
resistive load to the transmitter. Nothing wrong with that setup if you
want to operate essentially a small range of frequencies within one band.
But move frequency away from that sweet spot and the SWR goes up, the
transmitter no longer sees its required load, and your resonant antenna is
no longer on your side.

Remember that we are talking about wire dipoles and verticals here. Yagi
arrays and the like depend upon critical resonances among the various
elements to obtain the pattern and directive gain for which they are used.

73/72/oo, George W5YR - the Yellow Rose of Texas         
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe   
Amateur Radio W5YR, in the 56th year and it just keeps getting better!
QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735
Icom IC-756PRO #02121  Kachina 505 DSP  #91900556  Icom IC-765 #02437



Stuart Rohre wrote:
> 
> Folks,
> George hit it on the head.  There are more reasons to use non resonant
> antennas fed with parallel line than to worry about measuring and cutting
> them to resonant length.  You are using the transmatch to make that
> unneeded.
> 
> Many handbooks and introductory texts start out talking about resonant
> dipoles just as a way to teach the basic theory.
> But that does not mean they are more efficient than a transmatch and
> parallel line fed doublet of some length.  That early intro to resonance
> gets misinterpreted by new hams as giving resonant dipoles some magical
> property over non resonant lengths.  The resonant dipole is an advantage
> only on a single frequency, and you need a transmatch everywhere else, so
> why bother with measuring to resonance? Who works only a single spot in the
> band?  Or a single band?
> 
> A resonant antenna is higher Q, and might spike the impedance such that the
> transmatch has to be adjusted if you try to cover another frequency in the
> same band that is nearby.  With a broadly tuned doublet that is not resonant
> on any band you are using, except perhaps the lowest, you can move around
> not only on frequencies in a band but from band to band with some easily
> determined settings of the transmatch, that can be returned to when you QSY.
> A simple chart sets you up to return to any spot on any band.  A few
> settings work over any one band for our Field Day big loops and Double
> Zepps.
> 
> The losses of parallel line are much lower feeding broad coverage doublets
> than if you tried to use coax for multibands.
> The measurement of doublets for many bands is simply as George says, have
> wire to stretch from one support to the other.  Fold it in half to find the
> middle, and cut it there and insert feedpoint insulator, and your feeder.
> That way you do not need to carry a long tape measure to field days, or
> emergency set up locations.
> 
> If your doublet is long enough at a given band, you can realize gain over a
> dipole. It is the truly multipurpose antenna.
> 72,
> Stuart K5KVH

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