Original poster: "Gerry Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
Hi Terry,
I hoped you would respond.
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 ;-))
Certainly a matching resistor will kill the reflections. I'm not
sure that LCR analogies are appropriate here but they may be, given
Zo = sqrt(L/C) where L and C are the distributed parameters of the
coax (sounds like a good homework assignment). If R = Zo (matched
case), then Q = 1.0 using sqrt[L/C] / R.
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
I think with a velocity factor of 66%, the prop time is
~1.5ns/ft or 9ns not 6 x 0.66, otherwise it would exceed the speed
of light and you would get the nobel prize and then we could all
share in the profits :o)) Given 9ns to propagate one length of the
cable, the corresponding frequency would be 109 MHz. Of course the
length of the coax would load up as a 1/4 wave antenna at a lower
frequency and only if the currents in it were unbalanced. 1/4 wave
would be 27 MHz. Ah, the length of a CB (27 MHz) 1/4 wave antenna
is about 108 inches or 9 ft where the velocity factor about 0.95.
If the currents in the coax are balanced, there are no fields
outside the coax and if properly terminated the length would not be
important (other that for losses). Otherwise, how could I use 75
feet of coax for a 400+ MHz transmitter (the operative word here is
properly terminated).
The physical dimensions of the copper plate could also determine the
upper frequency limit since currents in the plate are unbalanced and
the plate will will probably load up as a 1/4 wave antenna at around
490 MHz (assumptions: length from the center tap to the edge of 5.5
inches and a velocity factor in copper of about 0.9). For the
frequencies of interest, the antenna is not resonating and acts only
as part of a capacitor divider. The self resonance of the 20nf
capacitor could also limit the upper frequency (due to what this may
do the the termination).
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...
YES, but even with 45 ohm termination, there will only be several
down and back reflections before the energy gets absorbed and
antenuated, probably dies out by the "next transition" (pulse
analysis). You could use a smith chart to figure the impedance at
the antenna end of the coax that the open circuit scope end presents
to the capacitor divider and series resister for a given
frequency. This might determine the limiting BW as well. Just thoughts
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...
I question the 245 MHz, but I also dont think it matters. Also, to
have eddy currents, the H field must have a component normal to the
surface. I believe most H field contributors (currents) will result
in tangental H field components (assuming no streamers that will
result in H field components dependent on its path). I'm having a
hard time envisioning a source of H field that would be normal to
the surface if the antenna was pointed directly at the coil.
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"... ;-))
I never said that there is a problem at 80Kz (at least I dont think
I did). 80Kz is where the 20nf cap has an impedance of 100 ohms
(matches the 50 ohms coax plus 50 ohms resister). Maybe 160 KHz
should be the frequency (where the capacitor impedance becomes 50
ohms). Anyway, the termination starts to become reactive and
imperfect at these frequencies and more so at lower
frequencies. Reflections start to have a longer settling time as
the frequency gets lower. Neglecting the 1 Mohm input impedance of
the scope, the settling time becomes infinite at DC for a lossless
cable (no energy absorption). Also, as the frequency gets lower
than 80Kz, the signal that is forward propagating toward the scope
becomes smaller.
Thevenize the capacitor divider into a source and series
capacitance. Then do a voltage division from this thevenin source
using the series coax inpedance (50 ohms) as one side of the divider
and the 50 ohm resister in series with the thevenin capacitor as the
other side of the divider. As the forward propagating voltage gets
smaller, it will take more trips down and back for the voltage at
the scope end to reach the value of the thevenin source. Also, the
lower the frequency, the more time there is for this to settle so it
may not be a problem. I just worry about things like this until it
can be proven that it is not a problem. The spice simulation I did
showed a steady state AC voltage that was accurate but had a low
frequency drift to it. This is what made me think about transient
response. The artifact could also be a problem in the transmission
line modeling of the simulator and would therefore have no real significance.
The one way I know of the find out for sure is to put a unity gain
high BW amplifier at the antenna end to buffer the coax from the
capacitor divider. This amp would have a 50 ohm source impedance so
source termination would be constant over the frequencies of
interest and down to DC. The amp would be battery powered. Both
the amp and battery would be totally shielded with an RF feed thru
capacitor to get the signal into the box and a BNC connector to get
the buffered signal out of the box.
Streamer effects - worst case - streamer hits the antenna
=:O Massive errors are noted!!! Don't do that!! :o))
Yeh, especially if you care for your scope :-))
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 ;-))
I think the effect that a streamer has on the coil is small (maybe
5%) compared to the top load capacitance. However, when you have a
capacitive divider that has only 0.047 pf between the toroid and
antenna, It seems like it wont take much streamer toward the
antenna to significantly change this. Digital averaging may very
well be the answer.
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 backplane may also cause unbalanced currents in the coax causing
it to "load up". I just dont know about this one.
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
And I too believe it is a great, low cost, and easy to make addition
to our tool box. Cant really talk "bad" about it when there is
nothing within our reach that is better. I do think the discussion
has been positive in that it suggests certain areas for exploration
and verification, and perhaps improves the awarness of what can go
wrong :o)) Thankyou Terry for this invention.
Gerry R