Agreeing Measurements

Independent Measurements

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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, 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

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

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 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


Many thanks to Tom, W8JI for bringing the misleading quote to my

attention and for posting this for me.

Roy Lewallen, W7EL