Re: NEC-LIST: Can Is as Can Do

George Hagn wrote:
>
> I am afraid that a typo (cap vs can) has led to looking under the
> wrong cap (or can).

Well, I always refer to it as a Wheeler sphere ;-) However, a quick
search shows that others have referred to Wheeler caps in recent years:

W.E. McKINZIE, III, "A Modified Wheeler Cap Method for Measuring Antenna
Efficiency," 1997 IEEE AP-S International Symposium.

R.-Y. Qiao, K. Hirasawa and K. Fujimoto,"On the Wheeler cap method,"
Transactions of National Conference of IEICE (Institute of Electronics,
Information and Communication Engineering), Japan, No.B-109, p.1-109,
March 1988. (In Japanese)

R.-Y. Qiao, K. Hirasawa and K. Fujimoto,"On the Wheeler cap method,"
IEICE Technical Report, Japan, No.AP88-47, pp.1-5, August 1988. (In
Japanese)

R.-Y. Qiao, K. Hirasawa and K. Fujimoto, "Wire Antenna Current
Distribution within a Wheeler Cap," Transactions of IEICE, Japan,
Vol.J71-B, No.11, pp1370-1372, November 1988 (In Japanese)

> We can now, with a NEC model that is based on
> a good representation of the antenna and its surroundings compute Rr
> for the small antenna structure.

I suppose it depends on the antenna. While many small antennas are
easily modeled using NEC, those which make extensive use of plates or
dielectrics are not!

> With a measured Ra, we can subtract
> and get Rl with some accuracy. The bridge accuracy may be of the same
> order as the NEC model. Harold would not have needed the "can method"
> had he had available our modern computational tools.

I'm probably reading way too much into your last remark, but few CAD
tools are good enough that I would feel comfortable skipping the
experimental validation step! As you suggest, measuring Ra = Rr + Rl
isn't too difficult. The trick is to measure or predict either Rr or
Rl. I always thought of Wheeler's method, an elegant experimental
method for measuring Rl, as replacing pattern integration, a brute force
experimental method for measuring Rr, at least for small antennas.
Especially since pattern integration is expensive and awkward when
applied to small antennas with very broad patterns.

--
Dave Michelson
dmichelson_at_home.com