On Sun, 01 Oct 2006 20:25:08 -0400, Mike Coslo
wrote:

Owen Duffy wrote:
Some months ago I put some thoughts together to assist our newly
minted 6-hour hams who seem attracted to either short end-fed wires
(although they refer to them as long-wires) or G5RVs.

Everyone must start somewhere, Owen. 8^)


Yes of course... but it is demoralising to recommend a procedure to
people when it is not within the capability. They won't be happy
sitting amidst a pile of chopped up ladder line and no result!

....
Figure 2. (system losses) was a real eye opener! I enjoyed the article
very much.

As a cross check, the G5RV's performance is easiest understood on
14MHz (where it is 3 half waves centre fed with a near 1:1 transformer
to the coax) and 28MHz (where it is 6 half waves centre fed with a
near 1:1 transformer to the coax).

Doing a back of the envelope calc for 14MHz where it is fed at a
current maximum indicates ~90 ohm load on the coax, and VSWR ~2:1, so
on the face of it, losses should be relatively low.

Doing a back of the envelope calc for 28MHz where it is fed at a
voltage maximum indicates ~2100 ohm load on the coax, and load end
VSWR ~42:1, so on the face of it, coax losses become much higher. Loss
in 15m of RG58C/U under those conditions is ~8dB, or about 15%
efficiency (coax alone). That reconciles with the graphs... the data
is believable! (You could raise the coax efficiency with RG8X (as
often recommended / supplied) to ~20% , but it still isn't pretty.)

Thanks Mike.

Owen
--