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

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 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

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

"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

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