Ian White GM3SEK wrote:

Owen Duffy wrote: (Dieter Kiel) wrote in
news:1ihnz3n.wxm0z7nvxaivN% :
http://www.w8ji.com/vswr_reactive_power.htm I assume Dieter that
this is your recommendation of the article. That article uses the term
'reactive power' in a non-conventional way, though the term is a well
known one (ie has a conventional meaning). Conventional use is that the
term 'apparent power' is applied to the product of RMS voltage and
current flowing into a two terminal load, and the units are VoltAmps
(VA), not Watts as used in the article. Reactive power is the reactive
component of apparent power, and expressed in units of 'VoltAmpsReactive'
(VAR). 'Real power' is the real component of apparent power and
expressed in units of Watts (W). The relationship is that
apparentpower = (realpower^2 + reactive power^2)^0.5 . This is all
basic lumped component AC circuit theory, and holds at RF. True, but we
still aren't there.

It's very misleading to quote "VAR powers" in the kilowatt range, because
the only power available to melt the feedline is 100W from the
transmitter. There is no magnification of real power.

The high value of "VAR power" is a theoretical result of the large RF
currents in the system. These result from an antenna feedpoint impedance
that has a very low resistive part and is almost entirely reactive. The
large RF currents are a genuine physical phenomenon, as also are the high
voltages a quarter-wavelength back along the feedline (if the feedline is
long enough, of course)... but if there were no losses in the feedline,
these would have no further effect. In spite of the wild values of
impedance, current, voltage, VSWR etc, if there were no losses in the
feedline then all of the RF power would still reach the antenna.

In a real feedline, the effect of the high currents is to divert almost
all of the available RF power away from the antenna and into the
feedline's own resistive losses - skin-effect losses in the copper
conductor, and dielectric losses in the plastic. Both of these result in
heating and softening of the plastic, which makes the dielectric loss even
higher. This tends to divert even more of the available power into the
weak spots, where the plastic finally melts.

But there is still only 100W available to do the damage.

No argument about the final conclusion - it ain't gonna work - but I don't
care for the explanation. There's no problem with "VAR power" for anyone
who already has a firm grip on the concepts, but it is not a good way to
explain those concepts to a newcomer.

I think you are right, the referenced link of W8JI mixed up the
definitions. He used "kilowatts" instead of "1000 VAs". But for me the
text was very helpful. It also will give newcomers some information
about problems with short antennas. If you ask: What is responsible for
the losses of an antenna construction ? The answer would be: The losses
are proportional to the " Apparent Power VA" It`s "Scheinleistung" in
German language. I didn`t even know the term before it was mentioned
here.
I found this schematic in the wikipedia with the different terms
Real Power, Reactive Power, and Apparent Power:

http://en.wikipedia.org/wiki/Image:P...riangle_01.png

So if we use a short antenna, e.g. 20m for the 80m band or even the G5RV
we should use parts that can handle the "Apparent Power". I`ve seen
Sonny has already made up his mind using koax for the 80m Band. This
probably is a good idea, and I`m planning to build a monoband for my
favorite band and multiband antenna fed with a low loss ladder line to
cover all the bands. I`m not quite sure if a folded dipol as a monoband
antenna fed with ladder line would give better results as a coax fed
dipol. I have the equipment to match both kinds of feeding lines.

Sonny Hood wrote:

I have a 160M windom and a G5RV and will dedicate this
inverted vee antenna to 75M.

There's your answer. And here's the readers digest version of it.

As a rule of thumb, if you are going to use an antenna cut for a
specific band, cut it for that, and use coax, and a balun.

If you want to have multiple bands from one antenna without traps or
tricks, use ladder line and a tuner.

The ladder line can handle the wildly varying VSWR, and the tuner will,
well, allow the Xciever to see an impedence it likes.


- 73 de Mike N3LI -

Cecil Moore wrote:
Ian White GM3SEK wrote:

... a very good posting with the exception (IMO)
of the choice of the word "magnification".

There is no magnification of real power.

There is indeed no net increase in real average power.
However, "magnification" seems to be a poor choice of
words since its definition contains such words as
"apparent", "seems", "exaggerate", "overstate", ...

"magnify - to cause to *seem* greater" certainly describes
a feedline with 100W source power and 200W forward power.

But there is still only 100W available to do the damage.

True, but a 100W laser can do a lot of damage. :-)
And a *magnifying* glass can start a fire.

I have no intention of allowing *that* particular fire to start again!


--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Sonny Hood" wrote in message
...
I have an efficiency question concerning feed lines. My present
system is RG-8X to my 75 meter inverted vee which is about 85 feet
away from the shack. I propose to replace some 88 feet of coax with
300 ohm window ladder line that is inserted into the coax run with 4:1
baluns to match the coax on each end. Also at the feed point of the
antenna switch from a voltage balun to a current balun (ferrite chock
type). By my calculations with a 98 watt generator I will increase
the power to the load by about 11-20 watts and with a 985 watt
generator, 117-210 more watts will reach the load. Figuring
theoretical total system (A) against total System (B) or by just the
difference in the 88 feet of ladder line versus coax. What do you
think the increase will be?

----------

In all likelihood, no one on the other end will ever be able to tell the
difference after you make the changes. Neither will you.

Ed, NM2K

On May 30, 6:46 pm, "Ed Cregger" wrote:


----------

In all likelihood, no one on the other end will ever be able to tell the
difference after you make the changes. Neither will you.

Ed, NM2K

Maybe not from any practical standpoint, but I could see the
difference on a meter when I fed both a coax dipole, and a
ladder line dipole using the same tuner, which has antenna
switch, and meter built in.
So I could A/B real fast. I could see a slight difference on
receive switching between the two. And if it's enough to see
on a meter, I consider it a noticeable amount.
Most of the signals were in the 20-40 over 9 range being 75m,
and I'd see about 5 db worth of difference between the two
antenna/feed lines.
Course, I make no claim as to the exact DB amount of difference,
but I could see it on a meter.
After many years of 75 and 40 m operating using mostly NVIS
paths, I've come to the conclusion nothing will beat a dipole/loop/
turnstile fed with good coax for sheer system efficiency unless
say you had a run of several hundred feet or something...
Which of those three doesn't really matter much, although I
prefer the turnstile over the other two if I had a choice.
Nothing wrong with a loop, but in my cases, some sections
end up sagging closer to the ground, and you are often a
bit more prone to ground losses, and also overall performance
if sections of heavy current are low to the ground.
With a dipole or turnstile, max current is at the feed point, so
the ends of the wires height above the ground is often not quite
as critical.
As far as I can think of, only Cecil's no tuner/no choke system
would let a ladder line fed dipole equal or slightly beat a coax
fed. And in that case, I'm not sure how noticeable it would be,
being as the 213 I use is quite low loss at 3-4 mhz.
But like you say, for on the air, many would probably not
notice much. #1, you would have to do quick A/B transmit
tests for them to notice any difference, and also make sure
fading doesn't skew the results. It would be best to test with a
stable ground or space wave signal.
But... I still prefer coax.. I want every drop out I can get, and
I also much prefer the convenience of coax. IE: with most
antennas, no tuner required. And really not even a "balun"
required if you roll a choke out of the feed line itself and tie
wrap it. Weather is no issue with coax. I can have it sitting
in standing water with no problems.
The only time I use ladder line is mostly experimental
antennas where the feed point impedance is not the usual
50 ohms. If the mismatch to coax is high, ladder and the
tuner makes sense.

wrote in message
...
On May 30, 6:46 pm, "Ed Cregger" wrote:


----------

In all likelihood, no one on the other end will ever be able to tell the
difference after you make the changes. Neither will you.

Ed, NM2K

Maybe not from any practical standpoint, but I could see the
difference on a meter when I fed both a coax dipole, and a
ladder line dipole using the same tuner, which has antenna
switch, and meter built in.
So I could A/B real fast. I could see a slight difference on
receive switching between the two. And if it's enough to see
on a meter, I consider it a noticeable amount.
Most of the signals were in the 20-40 over 9 range being 75m,
and I'd see about 5 db worth of difference between the two
antenna/feed lines.
Course, I make no claim as to the exact DB amount of difference,
but I could see it on a meter.

Most of it had to be the antennas. Going from almost no loss open wire to
much higher loss rg58 would be only 1 db at the most on 75 meters.
You still need a magnifing glass to see the smeter move this much on a ham
receiver.

On May 31, 8:17 pm, "Ralph Mowery" wrote:


Most of it had to be the antennas. Going from almost no loss open wire to
much higher loss rg58 would be only 1 db at the most on 75 meters.
You still need a magnifing glass to see the smeter move this much on a ham
receiver.

What about the tuner loss? And.. I was using the least amount
of inductance needed to tune.. The difference was noticeable on a
meter.
So that would tend to indicate the difference was more than 1 db.
The 213 I use at the house would be even lower loss than the 58,
but I do use rg-58 sometimes when portable, etc..
Doesn't do any good to use a slightly lower loss feed line, if the
tuner
needed to match causes more loss than the feed line saves vs coax.
And tuners will vary also. Also setting the tuner itself can lose even
more power if the minimum inductance is not use. It's not too hard
to lose up to 20% of your power if the inductance settings are wrong.
I'll still stick with coax and no tuner.