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.
--
73, Cecil http://www.w5dxp.com

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 in
:

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?

One of the numbers that is hard to come up with is the loss through a
balun. The only number I could find for a voltage balun is a matched loss
of 0.075db, this would be times 2 in your setup. So there is loss, and it's
well known that the loss goes up with miss match. I think you are
neglecting the loss through the baluns and the miss match loss. Remember
your antenna will not be 50 ohms over a very wide frequency range.
Going open wire line all the way or changing to RG8 size coax will likely
work just as well and be simpler to do.

John W3JXP

11 - 20 watts represents only 0.5 - 1 dB, which would virtually never be
discernible by the other station. As others have noted, even this
insignificant potential gain would be reduced by balun loss. But what no
one else has mentioned is that the ladder line loss can increase
significantly, possibly even dramatically, when it gets wet. Any
improvement you'll see from the proposed change will be due entirely to
placebo effect.

Roy Lewallen, W7EL

On May 28, 9:16 am, Sonny Hood wrote:
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?

If you switch from coax to ladder line and baluns or tuners, you are
going to lose a bit of efficiency I bet.
I always have when I've compared the two on that band.
You are converted from a pure and simple system which is a 50 ohm
radio, feeding 50 ohm line, which BTW at 75m is very low loss,
into an appx 50 ohm load. This "pure" system is probably 95%+
efficient on the 75m band as long as the coax is not junk.
In my opinion there is no better method of feeding a single band
dipole. Anything else is pretty much a step down as far as I'm
concerned. The *only* reason I would ever run that way is if I
wanted multi band use using ladder line and a tuner.
But in my case, I prefer multiple dipoles fed with a single coax.
The decrease in loss switching from coax to ladder line *on 75m*
is so small as to be nothing. But then you have baluns and tuners.
This is what is going to spoil the food. You will actually end up
with
a net loss at the end of the day. Maybe be a small loss, but I
bet it will be measurable on a meter if you used an A/B switch.
It was in my cases. Then you have the moisture issues, etc..
Do what you want, I prefer coax when feeding dipoles.
I usually use 213 on HF here at the house.
But at 3 mhz, even RG-58 is fairly low loss.
The mini 8 you are using should be a bit better than rg-58.
I think it's slightly larger and using foam if I remember right.

Sonny Hood wrote in
:

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?

Sonny,

I have resisted the temptation to tell you that the question is wrong
rather than to answer the question... however...

You are trying to compare two configurations that are not optimal.

An optimal coax configuration would have a balun, usually at the antena
feed point so your coax loss budget is short changed a balun.

I assume from the fact that you have proposed coax that this is a one
band antenna. You haven't made it clear whether you can use an ATU to
transform the load to suit the tx, I will assume you can.

Why do you need two 4:1 baluns / transformers? The dipole is already an
approximately balanced load and loss on open wire line with a VSWR of 4:1
is still quite low. You could just attach the feedline directly to the
antenna, and use a 1:1 balun at the ATU. Alternatively, you could
lengthen the feedline a little so that it was an electrical half wave and
dispense with the ATU (still need the balun).

In the case of the latter (an electrical half wave of open wire line, 1:1
balun at the tx) you will achieve the lowest loss, but it is just a few
tenths of a db better than the balun/RG8X config if you work them
through.

The open wire line might not be as easy to route into and around the
shack, and depending on construction, more affected by water.

Owen

On Wed, 28 May 2008 10:16:45 -0400, Sonny Hood wrote:

Thanks Guys, I got some good replies and it cause me to
reconsider the change. I find that the best and most simple change
will be to remove the RG-8X and it's high attenuation and install all
JT-2015 (RG-8 Type) from R&L Electronics. Also to put a good 1:1
balun at the feed point.
I have a 160M windom and a G5RV and will dedicate this
inverted vee antenna to 75M. This will be not used on 40M due to the
high resistance on the second harmonic with the current balun.
Yes the added baluns for the ladder line would increase the
insertion loss and yes the wet ladder line is a poor conductor. And
the cost will be spent for a more efficiency designed system.
K4WYS

"Sonny Hood" wrote in message
...
On Wed, 28 May 2008 10:16:45 -0400, Sonny Hood wrote:

Thanks Guys, I got some good replies and it cause me to
reconsider the change. I find that the best and most simple change
will be to remove the RG-8X and it's high attenuation and install all
JT-2015 (RG-8 Type) from R&L Electronics. Also to put a good 1:1
balun at the feed point.
I have a 160M windom and a G5RV and will dedicate this
inverted vee antenna to 75M. This will be not used on 40M due to the
high resistance on the second harmonic with the current balun.
Yes the added baluns for the ladder line would increase the
insertion loss and yes the wet ladder line is a poor conductor. And
the cost will be spent for a more efficiency designed system.
K4WYS

YOu have had many answers to your question, but have drawn the wrong
conclusion. If you replace the rg8x with some of the best low loss coax
(lmr400 for my example) you go from about .7 db to .2 db of loss on 80
meters. That is .5 db less loss. You will almost need an electron
microscope to see the s-meter move . Also 80 meters has so much noise on it
most of the time, you will not be able to hear the difference. I think it
was one db that was suppose to be the minimal detected difference in hearing
when the db scale was first put into use.

It may seem that if you have a 100 watt output transmitter you will go from
about 85 to 95 watts in a matched system. This may seem to be a lot , but
on the receving end it will not be noticed.

On maybe 20 meters and higher, especially 2 meters and up it does make good
use of the money to go to a higher grade of coax for weak signals.

Spend your money on something more productive.

Sonny Hood wrote in
:

On Wed, 28 May 2008 10:16:45 -0400, Sonny Hood wrote:

Thanks Guys, I got some good replies and it cause me to
reconsider the change. I find that the best and most simple change
will be to remove the RG-8X and it's high attenuation and install all
JT-2015 (RG-8 Type) from R&L Electronics.

Sonny, run the numbers at http://www.vk1od.net/tl/tllc.php before you
spend the money.

BTW, it will make NO difference on receive since S/N ratio on 80m is
almost entirely determined by band noise and small change in TL loss will
make no significant difference. Change in TL loss will affect the
strength of your transmitted signals.

Also to put a good 1:1
balun at the feed point.

Good move.

Owen

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 -