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.

The magnitude of both components is small relative to the load
impedance and the loop's own reactance, and does not influence the
loop current (and therefore the load voltage) much.

What you are saying is the antenna is a voltage source, and power
efficiency can go to the devil. For receivers, they forgive such
blasphemy. Just how much appears to be unknown at this point (we will
say things are in limbo). However, if you add a capacitor, you should
be able to observe the loss in the expressed Q. This is an untuned
loop, is it not? Somewhere there's the devil to pay otherwise. }:-)

Whilst it would be interesting to know if it is a fault in modelling,
or just numerical stability in the computation engine, I suspect the
effect you observed probable does not give great concern for the
accuracy of my receive loop models.

Turning the antenna into a voltage source gives rise to many nuances
that would otherwise be smothered.

I would like to have modelled insulated wire in the loops, but don't
have NEC-4... so I will build the loops from bare wire, although I
think it would make very little difference, even more so at the lower
frequency side of the range.

This alone appears to be a safe conclusion.

73's
Richard Clark, KB7QHC