[TenTec] Re: Rudimentary SWR question...

Mon Oct 20 15:15:02 EDT 2003

Oh yes if the directors or reflectors are too far off resonance they don't 
accomplish anything.  Just like the elements on a multiband yagi.  A 15 
meter director can't act as a 10 meter reflector because it is so far off 
resonance that very little current is induced in it.

73
Bill wa4lav

At 12:48 PM 10/20/2003 -0500, you wrote:
>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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