All,

I am trying to install a trapped 40m dipole in the attic, the antenna is
in place however it is short and resonates at about 7.7 MHz. I decided
to try it by using a tuner close to the transmitter in the shack. The
feedline is 50 ohm coax. On low power the tuner creates a very low SWR.
The transmitter is a solid state 100 watt Heathkit.

However when I transmit according to the SWR Watt meter the system
appears to transmit well over 200 watts. It pined the meter on a 200
watt range. I repeated the test twice and then stopped. When it is
transmitting the SWR reads about 1.1 to 1. The meter works very well and
does not exhibit strange readings on other setups.

My questions a What is happening? What is causing it?

Thanks - Dan

dansawyeror wrote:
I am trying to install a trapped 40m dipole in the attic, the antenna is
in place however it is short and resonates at about 7.7 MHz. I decided
to try it by using a tuner close to the transmitter in the shack. The
feedline is 50 ohm coax. On low power the tuner creates a very low SWR.
The transmitter is a solid state 100 watt Heathkit.

However when I transmit according to the SWR Watt meter the system
appears to transmit well over 200 watts. It pined the meter on a 200
watt range. I repeated the test twice and then stopped. When it is
transmitting the SWR reads about 1.1 to 1. The meter works very well and
does not exhibit strange readings on other setups.

My questions a What is happening? What is causing it?

When the transmitter is putting out 100 watts and the
forward power reading is 200 watts, it means that the
SWR is 5.83:1, i.e. the voltage reflection coefficient
is 0.707 and the power reflection coefficient is 0.5.

To get the power delivered to the load, you must subtract
reflected power from forward power. In your case that seems
to be:
Pfwd - Pref = Pload = ~Psource
200w - 100w = 100w = ~100w
--
73, Cecil http://www.w5dxp.com

In article ,
Cecil Moore wrote:

dansawyeror wrote:
I am trying to install a trapped 40m dipole in the attic, the antenna is
in place however it is short and resonates at about 7.7 MHz. I decided
to try it by using a tuner close to the transmitter in the shack. The
feedline is 50 ohm coax. On low power the tuner creates a very low SWR.
The transmitter is a solid state 100 watt Heathkit.

However when I transmit according to the SWR Watt meter the system
appears to transmit well over 200 watts. It pined the meter on a 200
watt range. I repeated the test twice and then stopped. When it is
transmitting the SWR reads about 1.1 to 1. The meter works very well and
does not exhibit strange readings on other setups.

My questions a What is happening? What is causing it?



two things pop into mind could be your antenna is radiating towards
and into or near your radio and causing some feedback that might effect
the radio and or the meter

there is also the possibility that you have some bad (common mode? etc)
current flowing back down the coax which could also wreck havock


just my guess and double check the above and grounds

put a dummy load into your tuner see if you get proper readings

To get the power delivered to the load, you must subtract
reflected power from forward power. In your case that seems
to be:
Pfwd - Pref = Pload = ~Psource
200w - 100w = 100w = ~100w

How can that be? If the meter is basically a directional coupler then the
forward power is just that. Subtracting any reflected power will just give a
stupid answer. The only errors will be due to the directivity of the
coupler, which will give a band of uncertainty which varies with VSWR, and
the error due to the accuracy of the detectors.

My Bird does not subtract any reflected power to give a forward power
reading!! It can't I need to rotate the slug to read reverse power.

73
Jeff

"Jeff" wrote in
.com:

How can that be? If the meter is basically a directional coupler then
the forward power is just that. Subtracting any reflected power will
just give a stupid answer. The only errors will be due to the
directivity of the coupler, which will give a band of uncertainty
which varies with VSWR, and the error due to the accuracy of the
detectors.

My Bird does not subtract any reflected power to give a forward power
reading!! It can't I need to rotate the slug to read reverse power.

Jeff, without commenting on whether Cecil's assertions are right or
wrong, you seem to have some misconceptions about what is measured with
your Bird (presumably 43).

The so called "forward power" and "reflected power" are notional values,
but not actual power "components". The only power is the average rate at
which energy passes a point, and it is in one direction or the other.

In fact the power can be calculated taking "forward power" minus
"reflected power", but only in the case where the sampler is calibrated
for Zo being real (as it is in a Bird 43).

My article at http://www.vk1od.net/VSWR/VSWRMeter.htm describes the
operation of a Bruene type of VSWR meter and discusses the power
measurement issue. Though the sampler in the Bird is different to the
Bruene sampler, the Bird samples V and I in a very small region (
regarded a point ) and sums them in the same way as the Bruene circuit.

Owen

The so called "forward power" and "reflected power" are notional values,
but not actual power "components". The only power is the average rate at
which energy passes a point, and it is in one direction or the other.

I am sorry, but I disagree, forward power is real and can be measured, or if
you wish separated out with a circulator or isolator. What you are
describing could be called 'transmitted' power or power delivered into a
mismatched load, but that it different from forward power, or the power
delivered by the source.

73
Jeff

On Mon, 26 Feb 2007 10:25:10 -0000, "Jeff" wrote:

I am sorry, but I disagree

Hi Jeff,

Your appologies aside, it is the convention you are disagreeing with.
The injection of such terminology as
'transmitted' power
is not part of conventional usage in this discussion. The trap here
of inventing terms is that your term would not account for Ohmic loss
as either forward or reverse power in the balance sheet (and this loss
could well be the source of mismatch); and yet this loss would have a
definite impact on what is "transmitted."

A simple instance proves this. Add a 14 Ohm resistor in series at the
feed to a perfect quarterwave radiator. The reverse reading would be
nada, the forward reading would NOT be "transmitted" power. Further,
the conventional usage of terms seen in this thread would still be
accurate.

73's
Richard Clark, KB7QHC

"Jeff" wrote in
.com:

The so called "forward power" and "reflected power" are notional
values, but not actual power "components". The only power is the
average rate at which energy passes a point, and it is in one
direction or the other.

I am sorry, but I disagree, forward power is real and can be measured,
or if you wish separated out with a circulator or isolator. What you
are describing could be called 'transmitted' power or power delivered
into a mismatched load, but that it different from forward power, or
the power delivered by the source.

Jeff,

You dropped a number of terms he
- 'transmitted' power;
- power delivered into a mismatched load;
- forward power;
- power delivered by the source;

The power delivered to a load (of any kind) from a lossless transmission
line section, is the same as the power delivered by the source. In the
case of the lossy line, then the line characteristics and load impedance
also need to be taken into account to calculate the power lost in the
line section, and it is not as simple as using up a dB/100' rating in a
table (unless the line is matched).

You assertion that you have travelling forward and reflected power waves
on the transmission line runs into a problem when you try to analyse the
combination of both at a point (eg the input to the line) as power
doesn't combine vectorially.

When you devise configurations with circulators, isolaters, directional
couplers, hybrids etc to "trap and reroute" reflected power, you have
probably changed the nature of the load on a line section and that
accounts for why the reflected power seems to have been isolated from
forward power.

Owen

Owen Duffy wrote:
The so called "forward power" and "reflected power" are notional values,
but not actual power "components". The only power is the average rate at
which energy passes a point, and it is in one direction or the other.

That statement depends upon the definition of "power"
being used. The IEEE Dictionary has a different
definition of power than does a physics textbook.
The net power is the average rate at which net energy
passes a point. The net power is the difference between
the forward joules/sec and the reflected joules/sec.

Instead of using the word "power", let's switch over to
the dimensions of power, i.e. "joules/second". Those
joules are indeed *actual energy components*.

The so called "forward power" is a forward traveling
EM wave containing energy moving at the speed of light.
There are indeed actual forward joules/sec moving past
a point on the transmission line.

The so called "reflected power" is a rearward traveling
EM wave containing energy moving at the speed of light.
There are indeed actual reflected joules/sec moving past
a point on the transmission line.

Note that an EM wave cannot stand still. According
to the theory of relativity, EM waves always move at
the speed of light (taking VF into account).

Standing waves consist of a forward traveling wave
containing joules/second and a reflected traveling
wave containing joules/second. The joules/second in
those two waves are supplied during the transient
power-on state. During steady-state, that energy has
not yet reached the load. But the total energy contained
in the transmission line during steady-state is exactly
the amount of energy needed to support the forward
traveling wave and the reflected traveling wave. Standing
waves would not be possible without those two real EM
wave energy components traveling in opposite directions.

At power-down, assuming the source is disconnected
from the transmission line, all of the forward wave
energy and reflected wave energy stored in the lossless
transmission line is eventually dissipated in the load.
That happens during a time when the source is supplying
zero energy.
--
73, Cecil http://www.w5dxp.com

"Jeff" wrote in
.com:

To get the power delivered to the load, you must subtract
reflected power from forward power. In your case that seems
to be:
Pfwd - Pref = Pload = ~Psource
200w - 100w = 100w = ~100w

How can that be? If the meter is basically a directional coupler then
the forward power is just that. Subtracting any reflected power will
just give a stupid answer. The only errors will be due to the
directivity of the coupler, which will give a band of uncertainty
which varies with VSWR, and the error due to the accuracy of the
detectors.

Depends if the meter is before or after the tuner. If the meter is after
the tuner, then the tuner is taking reflected power and adding it to the
transmitter's contribution.

My Bird does not subtract any reflected power to give a forward power
reading!! It can't I need to rotate the slug to read reverse power.

If you put your bird AFTER a tuner on a line that's near 6-to-1, it will
also show twice as much power as the transmitter is putting into the
tuner. Or a bit less if the tuner is not efficient.


--
Dave Oldridge+
ICQ 1800667