[Antennas] VSWR on Log Periodic.

[George, W5YR]

SWR is a complex comparison of the driving-point inpedance of the antenna
to the characteristic impedance of the feedline. You are correct, Chris.
Complex not in the sense of being "hard" but in the sense that the Zo of
the line can be complex (Ro+jXo) as is usually the imput Z of the antenna.
Both impedances are vector quantities and full treatment of their
relationship requires the use of  vector math. Since Zo is usually almost
totally real at HF with common feedlines, simple numerical SWR measures
have some value, especially at resonant frequencies where impedances are
real.

Thus, knowing the feedline Zo, the SWR does "say something" about the
driving-point Z of the antenna. But, it does not *define* or *specify* it.
If the SWR at the antenna is 2:1 and��#� feedline is 50-ohm coax, the
antenna impedance could be 25 or 100 ohms at resonance.  The frequency at
which the antenna impedance is purely real is by definition the resonant
frequency of the antenna and at that frequency the SWR will be at a minimum
- not necessarily 1:1 but lower than at any other frequency.

Generally, SWR is useful primarily for pointing out resonant frequencies of
antennas and for aiding in estimating feedline losses. Given information
about the line, SWR values also can be used to calculate other parameters
of the antenna system.

For reasons such as this, SWR can be useful but becoming overly concerned
with SWR is highly unproductive. And whatever it is at t��#�ntenna can be
calculated from what it is at the feedline input, probably with greater
accuracy.

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


Chris BONDE wrote:
> 
> At 08:04 AM 2002-04-25 -0400, Dave Shrader wrote:
> >Don Havlicek wrote:
> > >
> > > In my opinion, the ONLY meaningful SWR measurements are AT THE ANTENNA..
> > > NOT at the shack!
> > > Don - N8DE
> 
��#� I need help here.  I thought that the SWR was a measurement of how well the
> feed point impedance of the antenna is matched to the characteristic
> impedance of the feed line or transmission line and does not exist as such
> of an antenna or describe any property of the antenna.
> 
> Tha is all I know of this so please help me.
> Chris opr VE7HCB
>