On Wed, 30 Nov 2005 21:35:32 GMT, Cecil Moore wrote:

Owen Duffy wrote:
Now, I think you have told us over several articles that you are using
an SGC500 into a 30 ohm load on 7.15MHz.

Now please be a gentleman and please don't go putting words in my mouth.

Well, there was some uncertainty, and it is why I opened with "I
think...".

Here a quote of my exact words:

"Speaking of indirect measurements - let's say the feedline Z0 is 380
ohms with a VF of 0.9 and a length of 90 ft. The measured resistance
at the current maximum point is 30 ohms on 7.15 MHz."

Clearly, "let's say", is a hypothetical postulate. I freely admit that
I pulled those values out of thin air. Going back to my web page reveals
that the feedpoint impedance on 40m for my 130 ft. dipole was really 38
ohms. Nonetheless, I can still make my point assuming the 30 ohm value
which would have been perfectly acceptable to me.

If your transmitter was delivering 500W to the feedline, about 100W is
lost in the feeder.

With a 1.7:1 SWR???? Maybe you should reprogram your calculator to take
the square root???? The ratio of Pref/Pfor for an SWR of 1.7:1 is 0.067.
Methinks you might be using the voltage reflection coefficient?

Ok, I saw your later post where you have note that you were on the
wrong track here.


500(0.067) is 34 watts, not 100 watts. The SGC-500 laughes at 34 watts
reflected. (I swear that is true. I have heard it laughing to itself in
the wee hours during a contest.) Seriously, that amp is not known as
"The Brick" just because it looks like a brick.

Do you know how much power your amplifier delivers to the feedline? It
is likely that with a load VSWR of 1.7 it may have reduced output, it
is also possible that it is delivering even more than 500W to the low
Z load.

An SWR of 1.7:1 is nothing to worry about unless you think the percentage
power reflected is the same as the percentage voltage reflected. Don't feel
bad, many others have made that same mistake. Most people are programmed not
to think within a power/energy context and it gets them into trouble with
such concepts as "reflected power just sloshes around from side-to-side" and
"gobbledegook" applied to any attempt to track energy in a transmision line.

The SGC-500 is speced to tolerate an SWR of 6:1. That means that it can
dissipate more than half of its output power and keep on ticking. I
don't recommend allowing that to happen but that spec is why I don't worry
at all about reflected power unless the SWR is in excess of 2:1.

Lets leave that issue alone.


If we keep arguing, one of us is bound to make a mistake that the other
catches. I would guess that your above mistake bothers you a lot more
than it bothers me. :-)

Well, I think we are agreed that you made a mistake in identifying a
mistake, if I am not mistaken!

No, back on track, I thought you might have measured forward and
reflected power at the amplifier output on the 30 ohm load, to deduce
the net forward power, then by allowing for the line loss, you would
have the net power at the feedpoint (most of which will be radiated in
some direction or another).

One could then calculate the performance of the feed configuration
compared to what would be delivered to an ideal nominal load with no
feed loss.

The whole excercise goes nowhere, because it seems that the 30 ohms
scenario is "hypothetical".

Owen
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