Plane wave antenna thoughts

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Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi,

I have been sort of quiet on this, but now I will speak :o))

The plane wave antenna was originally designed with spice.  Been 
there, done that...  The only reason the 50 ohm resistor is in there 
is to match and damp out the 50 ohm coax cable.  The scope end is 
pretty much "open load" so the resistor gives that magic .7071 "Q" 
value to prevent ringing in the cable system to the scope.  One could 
eliminate all that if the scope were right at the antenna...  Dmitry 
asked why the "Q" was not "1" but a Q of one still rings a little 
bit...  I used to design speakers too ;-))
With a six foot cable and assuming a propagation of 1 foot / nS, pure 
cable reflections start at 6nS or about 170MHz.  But the cable's 
propagation velocity is really 66% so we can really get a value 
of  1/ (6 x 0.66 / (186282.397 x 5280)) = 248MHz...  That is afar 
above what the antenna is "speced" at...  So, I would refer those 
that worry about the cable and attenuation things to page "one" here:...

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

But if the 50 ohm resistor and cable match right (not all that 
easy!!) then the cable reflections will be damped too.  If the 
resistor is say 45 ohms, then the higher frequencies start to ring up 
very dramatically.  Thus the note about trying to get the resistor 
"just right".  But all that really is at only the super high 
frequencies which really "don't matter" anyway...  I know that most 
50 ohm cable systems deliver "power" where a 50 ohm "load" is needed 
too.  But we don't care about "power" in this case...  We just care 
about delivering an accurate voltage to the scope's vertical voltage 
amplifiers...

The frequency response comes out to 7.9Hz to 245MHz...  At say 
250MHz, eddy currents and other effects can start to "matter", thus 
the funny etched pattern...  At below 1MHz, the pattern is not 
important at all...  I can and have tested the thing with a signal 
generator to 15MHz and it is "really flat" over that range.  I am 
real confident that the frequency response is flat over say the 1kHz 
to 10MHz range and am very unconvinced to the arguments 
otherwise...  My good pal Gerry talks of a problem at 80Khz...  "Show 
me"... ;-))  Perhaps eddy currents where never a big deal 
either...  I just copied known designs that did worry about such stuff...

Calibration is really best done with a signal generator at Fo.  That 
takes into account the secondary coil's voltage profile too...  But 
it takes equipment 95% of us don't have...  But the error is pretty 
small unless you are really trying to split hairs...

Space Charge - All that DC stuff ;-))  We can raise the lower 
frequency response easily enough if the space charge effects seem 
important...  But I don't think those effects have the Power or 
Energy to effect things much.  The system's frequency response (scope 
too!) tends to drastically discount such effects.  If the scope's 
input were buffered to say 1G ohm, then all that 0.1Hz stuff would be 
very apparent(!)...

Streamer effects - worst case - streamer hits the antenna 
=:O  Massive errors are noted!!!  Don't do that!! :o))  Streamers are 
dynamic and sort on unpredictable "noise" in the antenna's 
pickup.  Perhaps one could simply do digital averaging to "eliminate" 
their effects.  But "I" think if you have sort of small streamers 
directed far away from the antenna, it is not a "big deal".  I also 
thing "streamer loading (to the air) is not a "great" load on the 
coil anyway...  Or spice coil models would fail drastically if they 
were that far off...  It really depends on how accurate you are 
trying to be...  In many cases, like 50% if really good ;-))

Backplane - It is supposed to block stray signals.  In retrospect, 
there are NO stray signals within like 5 orders of magnitude :o)))  I 
stoled the idea from ones that measured 5V computer noise signals 
:o)))  It does help to "direct" the antenna's sensitivity zone.  That 
might be useful...  But you probably don't need any back plane at 
all...  The 20nF cap provides all the divider load needed...  So you 
can use just a single sided PC board or metal plate...

The plane wave antenna things was really just meant to be better than 
the "wire off a scope probe" thing...  It really is MUCH 
better!!  But it is not perfect and you can trick it if you try 
easily enough...  But really, even a wire antenna with the cable 
loading and matching "tricks" added would really do very 
well!!!!  Maybe that is the best solution in the long run...  I am a 
little intrigued by using an "array" of them to see a "bigger picture"...

All things considered, I think we can figure out top voltages darn 
well now-a-days... :D

Cheers,

	Terry