[Elecraft] antenna question

Sat May 25 05:24:01 2002

Good one George,

After another recent burst of alleged K2 knob deficiencies I was
almost on the point of 'unsubscribing' from this reflector, but your
very erudite delivery on non-resonant dipoles was sufficiently
refreshing to provide some balance to the trivial and so I have
decided to stay.
Keep it up

Bill Scovell / VK2SOE
----- Original Message -----
From: "George, W5YR" <[email protected]>
To: "Stuart Rohre" <[email protected]>
Cc: "David A. Belsley" <[email protected]>; <[email protected]>
Sent: Saturday, May 25, 2002 12:42 PM
Subject: Re: [Elecraft] antenna question

> 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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