Re: Plane Wave Antenna - Appology to Terry

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Gerry

At 10:48 PM 1/2/2006, you wrote:
> Hi Terry and all that have been following this thread,
>
> I made a recent post in response to your post on this 
> subject.   Upon rereading the post, I agree with Mike that the first 
> part might have sounded a little harsh.  I meant it in a joking way 
> and hope you took it that way.  I wasnt feeling well at the time and 
> maybe I should refrain from posting during these moments.

No problem.  I am sorry I have been taking so much time responding to 
this.  I have been ill too and have two other projects going, so I 
was not able to spend any time at all on this stuff...

> I never meant that the plane wave antenna had a problem.  In fact, I 
> believe this is a creative method of making a difficult 
> measurement.  By my very nature, I like to throw stones at a 
> solution in an attempt to find potential holes.  The only potential 
> hole I thought I found was the termination issue.  At the time I 
> believed this wasn't an issue since the frequency the termination 
> started dissappearing at seemed too low for the cable length.  I did 
> a spice simulation using the transmission line model that gave odd 
> results.  The only thing I could think of was some weird phenomenum 
> involving a coax with little termination at both ends (I have had 
> many real cases of these things happening at work that seemed to 
> defy explaination).  Based on my simulation, I was suspicious of the 
> transient response and mentioned this to the group hoping that 
> someone had tested the antenna for this.  Since my post, I have 
> given the termination issue more thought using both an AC and PULSE 
> methods of analysis.  I believe I have resolved this in my mind and 
> the concern I was first worrying about doesn't seem to hold 
> water.  I appologize for seeding doubt in the validity of your 
> antenna design.  Keep up the great contributions that you make to 
> the science of our hobby.

Termination is an issue, but the problems would all occur at higher 
than 100MHz frequencies and those problems are fairly well 
behaved.  You were right about the velocity factor frequency of 
109MHz for a 6 foot cable I divided when I should have 
multiplied...  But all that is right with the time delay of 9.25nS in 
the model on page one here:

http://hot-streamer.com/temp/PlaneWave-01.pdf

Of course, all those computer models can easily be tested with signal 
generator and a short wire antenna close to the plane antenna.  Here 
is the sine and squarewave input and output signals at 10kHz:

http://hot-streamer.com/temp/pw-001.gif

http://hot-streamer.com/temp/pw-002.gif

100kHz:

http://hot-streamer.com/temp/pw-003.gif

http://hot-streamer.com/temp/pw-004.gif

1MHz:

http://hot-streamer.com/temp/pw-005.gif

http://hot-streamer.com/temp/pw-006.gif

So the antenna happily has the correct amplitude, phase, group delay, 
and impulse response over the frequencies we care about.

The 50 ohm resistor and 20nF capacitor do not form a filter because 
the line is open loaded which vastly reduces the current in the 
resistor foiling the -3dB filter effects.  The transmission line 
parameters are insignificant at <10MHz.  It might "look" like a 
filter, but it is not loaded right to act as a filter.  It is just a 
capacitive divider with a high impedance voltage probe.  The 50 ohm 
resistor is "lost" when in series with the 1M scope input.  Simply 
remove the transmission line and look at it that way.  Adding the 
line does nothing until you get to frequencies close to it's physical 
wavelength where the resistor damps the standing waves.  If you think 
about it all a very long time, one can get very confused %:-))  But 
it really is simple when it is all used at frequencies at 1000th of 
the cable's Fo frequency ;-))

Transmission line stuff is a messy business and I used to be 
surrounded all day long buy true gurus in the art.  The little I know 
about it was due to osmosis from being around them.  They mostly just 
stared of blankly into space for a few moments to solve such 
transmission line problems in their head...

Cheers,

        Terry

> Gerry R