[TenTec] Re: Rudimentary SWR question...

Mon Oct 20 13:48:18 EDT 2003

Interesting . . .

I use my 88 ft 20-meter EDZ on 75 SSB and most stations say it is equal to
or exceeds the signal from my resonant 80-meter full-wave horizontal loop.
The first time I used it on 75 there was total disbelief among the group
that an antenna that short and non-resonant at that could even work, much
less put out such as strong signal.

It turns out that any wire antenna that is at least 0.3 wavelengths long on
a given band will radiate a signal whose strength is almost immeasurably
different from that of a half-wave dipole.

My experiences with the EDZ on 75 support and confirm that.

Resonance of the antenna is a convenience for line matching. It has little
to do with radiation efficiency. This is not an opinion but a well
established fact in the professional antenna field. Consider also that in
the Yagi beam, only the driven element is "resonant" while the other
elements are intentionally made either too long or too short to be resonant.
And in some designs, none of the elements are resonant and the driving point
impedance is complex requiring such approaches as T, gamma and beta matches.

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"Starting the 58th year and it just keeps getting better!"
w5yr at att.net

----- Original Message ----- 
From: "Bill Fuqua" <wlfuqu00 at uky.edu>
To: <tentec at contesting.com>
Sent: Monday, October 20, 2003 11:08 AM
Subject: Re: [TenTec] Re: Rudimentary SWR question...

>       The reason why half wave dipoles are half wave length is not for a
> specific impedance but that they are resonant.
> Small antennas like the half wave dipole are not effective radiators
unless
> they are resonant. Resonance can be provided by a combination of the
dipole
> antenna and other elements such as inductances, capacitances or
> combinations ( antenna tuner and transmission line).
>      Resonance is a means of storing oscillating energy.  That is what Q
is
> all about. Q =  (total stored energy)/ (energy entering and leaving the
> system each cycle).  At resonance the energy stored in the half wave
dipole
> manifest itself as larger currents and voltages than those feeding it. The
> alternating current and voltage build up until the amount of radiated
> energy equals the energy that  is being applied. This is called
equilibrium
> ( power in = power out).
>      This is also why effective short antenna systems have higher Q than
> half wave dipoles. Magnetic loops, short antennas (whips etc) with antenna
> tuners or what have you all will have higher Q than half wave dipole
> antennas due to the fact that higher currents and/or voltages must exist
in
> the antenna for the same radiated power.
>       There are effective non resonant antennas but they are larger than a
> half wave dipole. Horn antennas are an example. Helical antennas or
Discone
> ( sort of a circular horn) antenna.  And antennas that are a collection of
> resonant antennas that have overlapping resonances such as a Log Periodic
> Dipole array.
>
> 73
> Bill wa4lav
>
>
> At 08:49 AM 10/20/2003 -0400, you wrote:
> > > Watts Reflected at the Mismatch.................At VSWR indicated of:
> > >
> > > ..........1 watt..................................................
> > > ......... 30 watts.................................................
> > > 3.4:1
> >
> > > If you use an antenna tuner,  most of the above indicated
> > > reflected power will be re-reflected back up to the antenna,
> > > and most of that will then be radiated!  Losing only the
> > > indicated reflected percentages once again and a bit lost
> > > in I^2R loss in the transmission line (a real tiny amount).
> > > Antenna tuners also have some loss in the coil,  again little.
> > > This is absolutely true even though the vswr  between
> > > the tuner and the transmission line/antenna system will remain
> > > at the indicated numbers.  The tuner "isolates" the rig from
> > > the returning reflected voltage,  so the rig will continue
> > > to put out all the power it can at a set of  control/drive/
> > > tune up settings.
> >
> >My understanding (from reading Reflections II) is that this power is
> >re-reflected regardless of whether a tuner is present.  Assuming that
your
> >transmitter can handle the mismatch w/o folding back output power, there
> >should be no difference in performance SWR wise.
> >
> > > And this explanation is the ENTIRE story/difference between
> > > an antenna which is resonant at a particular frequency,  and
> > > another antenna which is not resonant and introduces the
> > > above examples of mismatch and vswr as a result.  That is
> > > the ONLY difference between resonant and non-resonant
> > > antennas!
> >
> >However, one thing I have noticed (empirical data only) is that the
> >radiation pattern of an antenna is affected when not operating at the
> >designed frequency. For example, I found that my 2 meter beam (vertically
> >polarized) did not have the same directivity when using it to receive FM
> >broadcast. Similarly, if you use an antenna tuner to "force" an antenna
to
> >work on a band for which it is not designed, don't expect the radiation
> >pattern to be the same. Thus, while the antenna may be radiating almost
all
> >the energy, the communication may not be as effective as some energy will
be
> >radiated in other directions.
> >
> >Mark
> >
> >
> >
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