On Mon, 15 Aug 2005 16:48:54 -0400, "MD" wrote:


"chuck" wrote in message
news
Can you tell us what you want to do with it?

I have an R7 up for 40-10. Two maple trees on the property, one in the front
yard and one in the back but not far enough apart for an 80m dipole. The
trees runs east west over the house. I would prefer to run the dipole north
south, using the backyard maple for a support. The lot at this point is 66'
wide. One other thing to mention I don't think I can run ladder line or open
wire feeder into the house. I am pretty much limited to coax.

Max, is that to mean you want to feed the 66' dipole with 50 ohm coax
(all the way from the rig to the centre of the dipole)?

Assuming the feedpoint impedance that others have modelled is about
right, and that was 11-j961 at 3.8MHz... the losses in 20m (60') of
RG213 would be around 18dB. That is, less than 2% of the power into
the cable reaches the feedpoint.

That isn't the end of the story, there will be tuner losses... but the
good news is that partly as a result of the very high coax losses, the
impedance presented to the tuner will be well within the range of
loads that can be transformed at fairly low loss.

So... less than 2% of the transmitter power reaching the feedpoint...
is that acceptable?

BTW, if you considered a shorter line, say half the length at 10m (or
30'), the losses would be around 13dB (note considerably more than
half of the losses for 20m of line - the loss/unit-length is not
constant along the line).

It turns out that in practice, it is very hard to get most of the
transmitter power to the feedpoint of a dipole when the dipole length
gets much below 35% of the wavelength. Using lossy feedline (like
coax) exacerbates the problem.

Owen
--