[Elecraft] Re: [Icom] Opinions re: HF Verticals?

[George, W5YR]

John, this is largely off-topic for this list so this will be my last
posting on this  . . .

John Fraizer - KC4KGU wrote:
> 
> George, W5YR said:
> 
> > The absolute magnitude of VSWR is not an indicator of resonance.
> 
> Granted.  When you're operating resonant antennas and the antenna is
> supposed to presenting a 50Ohm load to your rig (via whatever _static_
> means, no adjustable transmatch here) VSWR will indicate that you're
> operating outside of resonance.  Since I operate all resonant antennas, I
> think in terms of ANY VSWR when operating at the design frequenct == BAD

Again, the basic question arises as to the supposed vs the actual merits of
operating "resonant" antennas. But, that appears to be your preference . .
.

That assumes that you have taken care of the non-Zo-match condition by some
form of impedance matching at the antenna. Almost invariably, an antenna
driving-point impedance will NOT exactly match the line Zo, certainly to
the 1.01:1 level, so something has to be adjusted somewhere to reduce the
line SWR to whatever you or the transmitter considers acceptable. Some
folks like to do it at the antenna, some folks fiddle with line lengths,
others like myself prefer the convenience of doing it on the operating desk
with a box with knobs called a tuner. Either way, the same thing gets done
with the same result: the transmitter sees a 50-ohm resistive load.

So, you have a certain VSWR - as low as possible for your taste - at some
selected frequency (not necessarily a resonant frequency, by the way) - and
any departure from that reference value you regard as a change in the
antenna system and you equate that change to BAD. It clearly indicates a
change of some sort. You are certainly within your rights to do that, but I
and several others are merely telling you that there is no engineering
basis for doing so. The change may produce miniscule effects on your
radiated power but these are readily compensated.
> 
> > With all respect, John, a review of the Antenna Book on the topics of
> > antenna resonance might be worth your time. Walt Maxwell's "Reflections
> > II" will give you an entirely new outlook on this subject. Resonance is
> > convenient for matching purposes, but that is about all it buys for you.
> 
> I beg to differ.  It will also buy you different radiation patterns.

Perhaps in some extreme cases, such as changing from one band to another,
but consider this: you have your perfect antenna set up for 14.227 MHz; SWR
is 1.007:1 and you are very happy. Your dipole antenna has the usual dipole
pattern in three dimensions. Now you QSY to 14.007 MHz to work some DX.
Your antenna is no longer resonant - assuming it was at 14.227 which is
possible but not necessarily the case (the antenna system was resonant, not
necessarily the actuall radiation "antenna") -  so the line SWR changes. 

Now tell me what change there is in the pattern, assuming that you provide
the same amount of power to the antenna as you did at 14.227. You can do
this by merely changing the impedance matching of the line input to the
transmitter and if necessary increasing the transmitter power output by an
amount equal to the (usually) negligible added line loss. Why did changing
frequency and having the radiating antenna non-resonant change the antenna
pattern: how and by how much and how does one measure this change?
> 
> > BTW, how are you measuring those 1.01:1 and 1.03:1 SWR values? And what
> > do you figure that they provide for you that 1.5:1 would not? Just
> > curious . . .   <:}
> 
> RF Applications VFD  http://www.rfapps.com/vfd.htm

I note that no accuracy specification is given at the URL for your unit.
Rather they ask you to "compare with the Bird 43 . . ." The Bird is rated
at +/- 5% of full scale. If you will work with that accuracy figure and go
through the equations for computing VSWR from forward and reflected power
measurements - which is what your instrument does wioth its microcomputer -
you will find that there is no way that it could actually compute the
difference between a 1.01 and a 1.03 SWR. The instrument may well display
such readings, but they are an artificial artifact of the processing, not
reflective of the accuracy of the raw power data. 
> 
> They provide me the piece of mind that I'm seeing 1.03:1 and not 1.5:1 or
> 2:1 or whatever else.

John, I think your statement there pretty well sums it up. You enjoy
operating your station with instrumentation and antenna systems that show
you what you regard as near perfection in transmission line operation - and
recall that VSWR concerns itself with one thing only: how well the line
matches the load - even though the facts of the matter may be significantly
different.

From an engineering viewpoint, the additional loss, change in line input
impedance, stress on the feedline, or any other attribute of "high SWR"
occasioned by changing SWR from 1.01:1 to 1.5:1 can hardly be measured with
any realistic level of accurary. Clearly the practical effects on overall
performance are negligible.

But, what makes you happy and lets you enjoy your station operation is what
counts . . .

Enjoy in good health!

73/72, George    
Amateur Radio W5YR -  the Yellow Rose of Texas
In the 57th year and it just keeps getting better!
Fairview, TX 30 mi NE of Dallas in Collin county EM13qe
K2 #489      Icom IC-765 #2349     Icom IC-756 PRO  #2121