|
Tom,
Thanks very much for sending the copy of the posting. I’d appreciate it
very much if you’d post the following on the same reflector for me:
——
It was brought to my attention that the following statement was recently
posted on this reflector by Yuri:
“I will leave it here, as the rest of it. W7EL, author of EZNEC measured
toroid coil and found that it HAS different current at its ends, roughly
proportional to the part of antenna that it replaces.”
This is not true. I assume the statement was made due to careless
reading of the postings I made on rec.radio.amateur.antenna, rather than
a deliberate distortion, so I’ll very briefly describe the results again
here. I made measurements of the current into and out of two different
toroids in series with an antenna at its base. The first measurement was
made using a 33 foot vertical with seven ground radials, whose feedpoint
impedance measured 35 – j185 ohms at 3.8 MHz, and a toroidal inductor
with reactance of 193 ohms and Q a bit over 300. I found that the
difference in current between input and output of the inductor was 3.1%
in magnitude and with no measurable phase shift, despite the short
antenna. The 3.1% current difference between input and output can be
explained simply by 6.8 pF of stray capacitance between the coil output
and ground and/or current probe, or a bit greater value distributed over
the coil. I repeated the measurement on the bench, with a series
resistor and capacitor in place of the antenna, and measured 2.3%
difference between input and output current. This is within measurement
error of being the same as when connected to the antenna. (One would
expect the stray C to be a bit different, also.) So in the one case I
did the measurement with the inductor “replacing” a very significant
part of an antenna and in the other with the inductor replacing no
antenna at all (or an entire antenna, if you choose), and got
essentially the same result. How this can be interpreted as my having
drawn the conclusion stated by Yuri is beyond me.
Unfortunately, the fact that the antenna is spaced only about 1/4″ from
a 4 foot mounting pipe (which altered its input impedance), in addition
to the abbreviated ground system, left a great deal of wiggle room for
people proposing alternate theories, so I did a second test with a more
idealized antenna. For this test I constructed a vertical antenna that
was 33 feet high, made of #16 insulated wire, and strung 23 radials out
on the surface of the wet ground. The feedpoint impedance of this
antenna at 3.8 MHz was measured as 15.8 – j437 ohms. This is close to
the theoretical impedance, assuming about 8 ohms of ground loss
resistance. The inductor had a reactance of 387 ohms. As described in my
posting on rraa of November 11, the inductor “replaces” about 33
electrical degrees of the antenna.
The result from the second test was a current difference of 5.4%, again
with no measurable phase shift. And again, this small difference can be
explained by about the same amount of stray capacitance. It’s nowhere
near the 16+% that the “cosine rule” (that the output current equals
the
cosine of the “replaced” antenna) proposed by Yuri and others would
predict. In no way did my measurement validate his theory — 5.4% isn’t
even “roughly” more than 16%.
Frankly, I’m a bit embarrassed to have bothered to make these
measurements at all — it’s a lot like making careful measurements to
validate Ohm’s law in order to refute someone’s measurements that
“prove” it wrong. As it is, it’s turned out to be even worse than just
a
waste of time, since the results are now being distorted to support the
very theory they clearly refute. I hope this will clarify just what
measurements I made and what they showed.
For anyone who can stomach the waffling, backpedaling, and insults which
characterized the original discussion, look up the thread “Re: Current
in antenna loading coils controversy” and its variants in the google
archives of rec.radio.amateur.antenna. Postings describing my
measurements were made between November 8 and 11, 2003. If you read the
thread, particularly the reaction to my posting of the measurements,
you’ll see why I’ve given up on that newsgroup as a forum for rational
communication.
Many thanks to Tom, W8JI for bringing the misleading quote to my
attention and for posting this for me.
Roy Lewallen, W7EL
|