Jeff wrote:
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.

Subtracting any reflected power will give the power
being delivered to the load. Power reflected from
the load is power that is NOT delivered to the load.

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.

Consider the following example assuming a lossless
tuner and transmission line.

100W source+tuner--x--50 ohm coax-----291.42 ohm load

Assuming the tuner is properly tuned, what forward power
will a Bird read on the 50 ohm coax at point 'x'? What
reflected power? What is the net power being delivered
to the load?

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.

You're right. The operator must measure the forward power
and the reflected power and do the subtraction manually.
In the above example, the Bird will read 200w forward and
100w reflected. The operator must subtract those two values
to determine the net power delivered to the load.

This was my guess as to why the Bird was reading 200w in
the original posting. Note that a Bird between the source
and the tuner does NOT read the SWR on the transmission
line between the tuner and the load.
--
73, Cecil http://www.w5dxp.com

On Sun, 25 Feb 2007 07:14:54 -0800, dansawyeror
wrote:
resonates at about 7.7 MHz.
On low power the tuner creates a very low SWR.
according to the SWR Watt meter the system
appears to transmit well over 200 watts.
When it is transmitting the SWR reads about 1.1 to 1.

Hi Dan,

The Meter is either not as good as you claim it to be (it doesn't
accurately perform at low power); OR
you have common mode problems (classic).

Do you choke the dipole?

73's
Richard Clark, KB7QHC

Thank you.

I think it may be a common mode problem. There is no choke on the
dipole. I will bring it closer to resonance.

- Dan kb0qil

Richard Clark wrote:
On Sun, 25 Feb 2007 07:14:54 -0800, dansawyeror
wrote:
resonates at about 7.7 MHz.
On low power the tuner creates a very low SWR.
according to the SWR Watt meter the system
appears to transmit well over 200 watts.
When it is transmitting the SWR reads about 1.1 to 1.

Hi Dan,

The Meter is either not as good as you claim it to be (it doesn't
accurately perform at low power); OR
you have common mode problems (classic).

Do you choke the dipole?

73's
Richard Clark, KB7QHC

dansawyeror wrote:
I think it may be a common mode problem. There is no choke on the
dipole. I will bring it closer to resonance.

Dan, are you measuring that power on the input or
output of the tuner? If at the output of a transceiver
with autotuner, for instance, the forward power is
the sum of the transmitter power and reflected power.
--
73, Cecil http://www.w5dxp.com

On Mon, 26 Feb 2007 08:10:49 -0800, dansawyeror
wrote:

I think it may be a common mode problem. There is no choke on the
dipole. I will bring it closer to resonance.

Hi Dan,

One very simple test is to fire up your rig and note the SWR. Patch
in a short, loose length of transmission line to the existing run
(about an eighth wave) and fire up the rig again.

Did the SWR change?

If so, you are being SWR whipped by Common Mode currents.

You can keep introducing different lengths of line until you find 1:1
(unlikely, but perhaps close) and let it go at that. This will be a
single frequency solution, but take care that it does NOT cure Common
Modality. Instead, it adds a vertical component to your transmission
(maybe).

This is not always good as that same transmission line (plus
extension) may run past RF sensitive devices (like intercoms, VCRs,
light dimmers, etc.) that go whacko. It may also fill in the nulls of
your dipole (sometimes good, sometimes bad).

73's
Richard Clark, KB7QHC

dansawyeror wrote in
:

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

Dan,

I assume in all these scenarios, the VSWR meter is between the
transmitter and ATU, and the ATU is adjusted for low VSWR.

You seem to say raise two issues:
-you have adjusted the tuner for a "very low SWR" on "low power"
(whatever each of those means), and when you transmit at "well over 200
watts" the SWR is 1.1:1; and
- your transmitter rated at 100W indicates "well over 200 watts" into a
1.1:1 load.

Re the first issue: If this is to mean the VSWR is higher on higher
power, the most common reason (but not the only one) that VSWR meters
read better VSWR on very low power is to do with the voltage drop across
the diode(s) in the meter. The scale may be calibrated at higher power
where the diode drop is less significant, perhaps even insignificant, and
when you adjust the meter for maximum sensitivity the diode drop
introduces significant error.

Re the second issue, if the instrument is a typical directional
wattmeter, the power output is calculated by deducting the "reflected
power" from the "forward power", but at VSWR=1.1 the "reflected power is
0.2% of "forward power" and insignificant. Otherwise, it might just have
an RF voltmeter sampling the line and calibrated in watts, and which is
only valid at very low VSWR. Transmitters don't often exceed their rated
power by over 100%, so your reading casts doubt on your meter. It sounds
like you need to make another measurement with another instrument to
locate the problem.

Owen