On Wed, 10 May 2006 09:05:54 GMT, Dave Oldridge
wrote:

what the resistance per foot is of the shield on LMR400 at 3.75mhz?

Hi Dave,

With the aid of an Ohmmeter, you should be able to determine that
without need for theory or books. Unless, that is, you simply try to
determine it for one foot's length where the resistance is bound to be
so small as to be buried in the resolution of the meter's display.

If we were to simply assign a total resistance of 0.1 Ohm, then from
what follows you would find yourself in serious trouble. You should
be shooting for at least one tenth of that value IN TOTAL. The
practical reality of this is that your connections may have more
resistance than the bulk cable. Further, at this level of resistance,
you need to make four lead (Kelvin) measurements.

I made a 55-inch diameter loop out of the stuff,

More to the matter would be the radiation resistance of this in the
80M band where you use it. This sets up the necessity of knowing that
LMR's resistance (much less the TOTAL). The radiation resistance of
such a small loop runs 0.002 Ohms.

If you used the same size loop at 160M, that figures out to 112 µOhms.
However, you use something smaller, and thus that figure is following
down further, faster.

I may try something more ambitious in 3/4 inch soft-drawn copper pipe,
about 6 feet in diameter. That should get me another 3 or 4 db, I think.

Then you would find that radiation resistance now turns out to roughly
double the former values.

Still, these coax loops seem to be working amazingly well for something so
simple and relatively inexpensive.

This observation (not really explained by you as to what constitutes
"amazingly well") can be a product of two realities:
1. You don't actually need much power to communicate;
2. Tuning up has been leveraged by a lot of loss.

73's
Richard Clark, KB7QHC