"K7ITM" wrote in news:1147282453.462947.145900
@i39g2000cwa.googlegroups.com:

LMR400 outer conductor should be a bit less than 10 milliohms per foot
at 4MHz.

Rule of thumb: for copper, assuming thickness at least a couple of
skin depths, RF resistance in ohms per thousand feet is about
sqrt(freq(MHz))/(diameter(inches)). Aluminum will run 25% more or so.
I've assumed the aluminum foil shield of the LMR400 is at least 3 mils
thick, and I've neglected the overbraid.

OK, thanks. That pretty much agrees with my experience.

I'm not trying to compare apples and fir trees here, just apples and
oranges. The antenna I'm trying to better is a hamstick whip and this
coax loop seems to beat it by about 1 or 2 db (I have them on an AB
switch that gives me the tuned antennas at the touch of a button). With
the whip horizontal and the loop vertical, both have NVIS performance at
high angles that is quite similar, but the loop is less noisy on receive
and has a tiny bit more signal strength on actual NVIS signals.

As I said, I'm encouraged enough to go ahead now and try something in
copper that should have a lot less losses. If I can get efficiency up
into the 10% range, then it's a real winner over a mobile whip.

The coax antenna is certainly a good little field antenna for quick
deployment. Can be carried in the trunk of a car along with a radio and
its feedline. And can be hung out with only string.

Sure, if I had 40 acres and lots of cash, then a four-square and a
rhombic would be the way to go (Canadian hams are clumped along the
border, making a rhombic practical when you're at either end).


--
Dave Oldridge+
ICQ 1800667

On 10 May 2006 10:34:13 -0700, "K7ITM" wrote:

LMR400 outer conductor should be a bit less than 10 milliohms per foot
at 4MHz.

So... close to 0.140 ohms loss restance for the loop conductor, and if
the loop is resonated by the o/c stub which presents an impedance of
around 0.14+j116, and taking Richard's estimate of Rr at 0.002, the
the efficiency of the main loop (ignoring coupling loop / circuits) is
about 0.002/(0.14+0.14+0.002), less than 1%, or about 21dB of loss.

It seems that the convenience of the TL loop / capacitor comes at a
significance expense.

Owen


Rule of thumb: for copper, assuming thickness at least a couple of
skin depths, RF resistance in ohms per thousand feet is about
sqrt(freq(MHz))/(diameter(inches)). Aluminum will run 25% more or so.
I've assumed the aluminum foil shield of the LMR400 is at least 3 mils
thick, and I've neglected the overbraid.

Cheers,
Tom
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