On Thu, 12 Oct 2006 17:17:27 -0700, Richard Clark
wrote:

Hi All,

Well, it seems enough problems have accumulated to warrant further
discussion.

On Thu, 12 Oct 2006 22:08:49 GMT, Owen Duffy wrote:

On Wed, 11 Oct 2006 16:32:50 GMT, "Frank's"
wrote:

...

One thing that has bothered me about small loop modeling. When
driven with a voltage source; loops of length 0.1 wavelengths
exhibit a radiated power ,and efficiency, of 0. In other words
the structure loss is than the input power.

To say the least. For a loop with sides of 1 meter, the radiation
resistance at the low end of the chart verges on 150 nanoOhms. If
this were copper wire, it would exhibit about 2.5 Ohms per 1000 feet.
Instead it is steel wire (about 5 to 10 times the resistance) over a
length of 13 feet or so. I will be generous and call it half an ohm
total.

Removing
the wire conductivity loading corrects the problem.

I hope you don't pack your own parachutes!

In your
particular case, at 5 MHz, the real input impedance is
more than three orders of magnitude greater than the radiation resistance.

Examining the radiation resistance again 0.0015 Ohm; copper loss
remains the same for intents and purposes.

I guess it could just be numerical rounding errors, but have gotten
identical results when using double precision.

Hi Frank,

That is interesting.

I haven't pursued modelling the loop being excited directly,

I had presumed you performed that exercise.

but I
accept your observations.

I have no idea what causes the problem. Perhaps a NEC guru may have an
explanation?

Hmmmm.

I thinking about the impact of error in the estimate of either
radiation resistance or loss resistance of the conductor in the
scenario that I did model, it should be low, probably insignificant.

You might want to revisit those thoughts.

I have also created a (non NEC) model that depends on the assumption
that current is uniform. Frank has commented on a NEC model at 5MHz,
where the loop side is 0.016 wavelengths, so current is close to
uniform. Using that frequency in my non NEC model suggests that the
radiation resistance is 0.0026, conductor loss resistance is 3.07,
loop inductive reactance is 163, and the load is 50. The circuit
impedance is 53.07+j163, and load voltage is relatively insensitive to
small changes in radiation resistance or conductor loss resistance.

The model is at http://www.vk1od.net/SmallUntunedSquareLoop/temp/1.htm
..

Changing the NEC model to be driven by a voltage source, I get loop Z
of 3.63+j173, so NEC predicts a higher loop Z. The efficiency
calculated by NEC is 1.54%, much lower than the 0.085% implied in the
simple model. The NEC efficiency implies that radiation resistance is
0.055. Doubling the number of segments in the NEC model does not
markedly change loop Z, but increases efficiency to 2.3%, so there is
something strange happening in the way it determines radiation
resistance. With a single segment per side, NEC suggests efficiency of
0.8% which is close to my simple model.

One of the data points from my NEC2 run is at 5.091MHz, and Antenna
Factor for the loop and 10m of RG58CU is 30.624dB/m. That includes an
amount of the coax loss, and that amount should be 0.309dB, leaving
the loop itself at 30.316dB/m.



Owen
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