[TenTec] Re: Rudimentary SWR question...

[Bill Fuqua]

      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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