is the truth?
It all started with discussion on the TopBand Reflector
(see the archives on eHam.net for May 2003) with thread "160
m Mobile Antenna Suggestions"
Barry, W9UCW pointed
out his findings based on real life measurements of decreasing
current across the loading coils. This agreed with my "unscientific"
experience, when I fried the loading coil with 600W into Hustler
resonator, melting heat-shrink tubing and wire at the bottom of
the coil. There was an exchange of arguments on the subject of
current in the loading coils, with W8JI insisting on constant
current across the coil. Unfortunately, the TopBand reflector
moderator terminated W9UCW responses, so W8JI had his famous "right"
Tom, W8JI on his web page http://www.w8ji.com/mobile_and_loaded_antenna.htm states:
current distribution for 1-ampere applied at the base (in 1-foot
2 ft= 1.0091
8ft = .89522
Measuring the current
into and out of the loading coil with a small thermocouple RF
meter, I detect no difference This is in close agreement with
the model. "
“Clearly there is
no basis to the claim current is high only in the first few turns
of an inductor, or that current tapers in relationship to "electrical
degrees". The most accurate way to state the effect would
be to say: "When the loading coil is short and the capacitance
of the antenna beyond the coil is reasonable (in this case 3000
ohms Xc or less), there is an immeasurable reduction in current
in the coil."
is a problem in his modeling with current increasing from the
base towards the coil. That should be the flag telling him that
2 + 2 is not sometimes 4.04. Second, EZNEC has no provision for
incorporating physical length of coil. It just considers LC parameters.
Roy Lewallen, W7EL, author of EZNEC and Richard Clark, KB7QHC
recommend workarounds to replace the coil with cylinder of similar
size or breaking the coil to number of physical segments with
appropriate inductances. W8JI "findings and measurements"
reflect the reality. As someone said,
one measurement is better than thousands of theories. The question
is, how was W8JI measuring the current, and getting more current
than it was applied at the base?
Barry, W9UCW among other arguments and explanations
measurements, we used long and short coils and the current taper
was was almost identical if the topmast capacitance was held the
same. If the
"make up" was above the coil, there was slightly less
taper down of current, due to the larger capacitance above.
It would also resonate lower in freq. It appears to us that the
current decrease in the coil has most to do with the section of
the quarterwave element that it effectively replaces. The actual
decrease in our tests was always a little more than the decrease
calculated for the "replaced" section, no matter what
coil was used. I hope that answers your questions."
your position (W8JI) is clear....that under the condx described,
current reduction in a loading coil can't, won't, never did, never
will happen. My
position is that it always does, and I've measured it. Neither
of these hypotheses will go far
to satisfy the real objectives of our study."
Barry, W9UCW was kind
to provide proof in the form of some measured data and photographs
showing the antenna setup, loading coil and RF ammeters installed
at the top and bottom of the loading coil, which could be reversed:
"Here are some actual measurements
of current below and above loading coils.
92" mast, using a HI-Q coil (openwound airdux,
2 1/2"d) with small thermocouple type meters mounted
on the insulated coil support. First for 40m, moving the coil
in the mast from base to center to top (with hat) and reresonating.
--100ma below & 66ma above
--100ma below & 45ma above
--100ma below & 37ma above
Then, same test but for 30m
--100ma below & 75ma above
--100ma below & 60ma above
--100ma below & 52ma above
On a long, skinny 160 resonator with 25pf of top hat
and whip, mounted on an 8' mast, I read 100ma below and 65ma
above the coil.
Because of the constant claim that this must be due
to the fact that the coil is so big compared to a wavelength, I
measured the in and out current on a TOROIDAL loading
coil used on a 20m mobile antenna. It was a 78" base mast
(including spring and mount) with a 38" top whip (including
12" of alum. tubing for adjustment).