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ANTENTOP- 03- 2003, # 004

Current Distribution in the A.L.C.

 

What 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"

http://lists.contesting.com/_topband/2003-05/msg00044.html

 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" last word.

Tom, W8JI on his web page http://www.w8ji.com/mobile_and_loaded_antenna.htm states:

"The modeled current distribution for 1-ampere applied at the base (in 1-foot intervals) is:

1ft= 1.0031 
2 ft= 1.0091
3ft= 1.0178
4ft= 1.0318
<Coil>
5ft= 1.0175
6ft= .97512
7ft= .92984
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. " 

and "conclusion"...

“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."

First, there 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" hardly

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 wrote:

"In our 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."

"I think 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.

Base    --100ma below & 66ma above

Center --100ma below & 45ma above

Top     --100ma below & 37ma above

 Then, same test but for 30m

 

Base     --100ma below & 75ma above

Center --100ma below & 60ma above

Top     --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).

 

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