my SWR reading
 

Highland Ham wrote in
:

Try transmitting up and down the band to see where your lowest swr is.
Then you can shorten or lengthen the antenna a bit to get a low swr in
the 28.4 mhz range. If you can't quite get it to 1:1, coiling the coax
at the feedpoint, 4 or 5 turns about 6 inches in diameter can get you
pretty close to 1:1.

2:1 isn't bad, but you're probably not getting full power output if
your rig is solid state.
=================================
When SWR is 2:1 the reflected power is only 11% of the tx output
power,
which will hardly be noticeable at the receiving end .
Even when SWR would be 3:1 only 25% of the transmitter power would be
reflected , still resulting in only a fraction of an S-point at the
receiving end.
But a low SWR will make the solid state PA of your radio feel happier !

Frank, your analysis ignores the fact that the PA may deliver other than
its rated power into the actual load, it could be higher or lower power,
but in radios that incorporate VSWR protection of the PA, it is most
likely to be lower, and at 3:1, substantially lower.

VSWR protection helps protect radios operated by operators with the view
that 'anything works'.

To me, 2:1 seems a bit poor for such a simple antenna and probably
readily capable of improvement to 1.5:1 or better. More importantly, the
OP might expand their knowledge in the process.

The original description was scant on information about the
configuration, and I guess that sometimes, knowing how to describe the
configuration / problem is the first step of knowing how to solve it.

Others have identified missing elements of the description, the use or
otherwise of a balun is relevant in indicating the extent to which common
mode feed line current plays a part in determining the load presented to
the feedline.

Owen

my SWR reading
 

2:1 isn't bad, but you're probably not getting full power output if
your rig is solid state.

=================================
When SWR is 2:1 the reflected power is only 11% of the tx output power,
which will hardly be noticeable at the receiving end .
But a low SWR will make the solid state PA of your radio feel happier !

Frank GM0CSZ / KN6WH

Frank, I know that you know this, but I want to comment to the
group...
Assuming the transmitter does not fold back, the 11% reflected power
will not change the signal strength... The reflected 11% will be 88%
radiated on the return trip (minus any line losses) and 88% of that on
the next round trip, etc... So, in the end the decrease in
transmitted power is only a fraction of 1% for feedlines with low
losses to start with... W2DU's very readable book REFLECTIONS, would
be a good place to start for those who are a bit hazy on transmission
lines, reflections, conjugate mirrors, etc....

Now, if we take transmitter foldback into consideration, we will need
a lot of information about the various rigs before we can even begin
to discuss how much a 2:1 SWR will change radiated power...
Personally, for a situation where there is SWR on the line, I prefer a
DX100B - it's pi-net could not care less and will still put out full
power into 2 or 3 to 1... As well as some more modern rigs such as
TS-830, etc... Tubes have a lot going for them... The Omni series of
solid state rigs from Ten Tec also do not fold back under 2:1, though
3:1 does have some effect...

cheers ... denny / k8do

my SWR reading
 

Assuming the transmitter does not fold back, the 11% reflected power
will not change the signal strength... The reflected 11% will be 88%
radiated on the return trip (minus any line losses) and 88% of that on
the next round trip, etc... ]

That will only be true IF the Tx output stage has an infinite SWR looking
back in to it, AND the reflection is in phase with the forward signal. In
practice this will not happen. In fact the re-reflected signal may be
180degrees out of phase and so diminish the signal by its total value. The
real story is that a much smaller proportion of the reflected signal will be
re-reflected and the phase will depend on the system in use.

In reality the impedance of a transistor o/p stage looking back in will not
be too far away from 50 ohms due to the broadband matching networks.Some
transmitters that I have worked on even had a spec on the impedance looking
back in, a real pain to measure under full power conditions.

73
Jeff

my SWR reading
 

"Denny" wrote:
Assuming the transmitter does not fold back, the 11% reflected power
will not change the signal strength... The reflected 11% will be 88%
radiated on the return trip (minus any line losses) and 88% of that on
the next round trip, etc... So, in the end the decrease in
transmitted power is only a fraction of 1% for feedlines with low
losses to start with ...

Now, if we take transmitter foldback into consideration, ...
____________

If, as reported, more than 99% of the output power of a transmitter
ultimately is absorbed/radiated even by a mismatched antenna system, then
why would a transmitter need power foldback for such loads?

RF

my SWR reading
 

Denny wrote:
Assuming the transmitter does not fold back, the 11% reflected power
will not change the signal strength...

True if a match has been achieved by a network.
A 100 watt source will provide 112.4 watts of forward
power with 12.4 watts of reflected power.

Power delivered to the load is 112.4w - 12.4w = 100w

Not true where a 100 watt 50 ohm source is not matched
and provides 100 watts forward with 11 watts reflected.

Power delivered to the load is 100w - 11w = 89w
--
73, Cecil http://www.w5dxp.com

my SWR reading
 

Jeff wrote:
Assuming the transmitter does not fold back, the 11% reflected power
will not change the signal strength... The reflected 11% will be 88%
radiated on the return trip (minus any line losses) and 88% of that on
the next round trip, etc... ]

That will only be true IF the Tx output stage has an infinite SWR looking
back in to it, AND the reflection is in phase with the forward signal. In
practice this will not happen.

How about this?

50 ohm
100w TX-------1/2WL Z0=100 ohm feedline--------50 ohm load

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

my SWR reading
 

On Oct 25, 11:11 am, Cecil Moore wrote:
Denny wrote:
Assuming the transmitter does not fold back, the 11% reflected power
will not change the signal strength...

True if a match has been achieved by a network.
A 100 watt source will provide 112.4 watts of forward
power with 12.4 watts of reflected power.

Power delivered to the load is 112.4w - 12.4w = 100w

Not true where a 100 watt 50 ohm source is not matched
and provides 100 watts forward with 11 watts reflected.

Power delivered to the load is 100w - 11w = 89w
--
73, Cecil http://www.w5dxp.com

Ahhh, ya namby pambys... My open wire feed line can lick your coax!
And the tuner is a conjugate mirror to boot...
nyaa, nyaa, nyaa...

denny / k8do

Oh yeah, and to add something constructive for the one questioner:
Foldback on transistor amps is to prevent RF voltages rising high
enough to puncture the transistor junction...
Tube radios with pi-net or pi-L tanks had enough matching capability
to handle significant impedence mismatch compared to broadband
transistor amps... Today, that function is served by autotuners, etc...

my SWR reading
 

My homebrew Field Day rig monitors only the "forward power" with a
single directional coupler and adjusts the output level to keep the
"forward power" constant. That rig delivers 83% of the power into a
2.38:1 SWR load as it does into a 1:1 SWR. This represents a 0.8 dB
power reduction. Big deal.

But my 25 year old Icom 730 delivers a full 100 watts to the output
regardless of the SWR up until it starts shutting down at around 3:1.
For example, with a 2.38:1 SWR, the "forward power" is 120 watts and the
"reverse power" is 20 watts. Aren't modern rigs able to do this, or have
they gone to the simpler system like I use in my homebrew rig?

Roy Lewallen, W7EL

my SWR reading
 

Roy Lewallen wrote in
:

....
But my 25 year old Icom 730 delivers a full 100 watts to the output
regardless of the SWR up until it starts shutting down at around 3:1.
For example, with a 2.38:1 SWR, the "forward power" is 120 watts and
the
"reverse power" is 20 watts. Aren't modern rigs able to do this, or
have
they gone to the simpler system like I use in my homebrew rig?

Roy,

Modern rigs often employ a range of protective devices that influence the
power delivered to the load.

Many of the modern Icom HF radios control:
- 'maximum output power' using either the forward output of a directional
coupler or a non directional sample of output;
- VSWR protection to limit maximum reflected power indicated by the
directional coupler, typically to a value that is equivalent to the
reflected power at rated output and specified maximum VSWR (eg 11W for a
transmitter rated at 100W and max VSWR=2:1);
- maximum collector current.

The actual power delivered to a severely mismatched load is affected by
all of these. It is likely for such a radio that with a 5 ohm load, the
radio will level 'reflected' power to 11W, 'forward' power would be
16.5W, output power would be 5.5W.

Depending on the way in which maximum output power is detected /
controlled, it is indeed possible to get more than rated output power, so
mismatch doesn't necessarily result in lower radiated power (despite
popular opinion).

Owen

my SWR reading
 

Richard Fry wrote:

If, as reported, more than 99% of the output power of a transmitter
ultimately is absorbed/radiated even by a mismatched antenna system,
then why would a transmitter need power foldback for such loads?

Egad, here we go again.

Foldback has nothing to do with "reflected power". It's simply that a
mismatch results in higher voltage or current at the output which could
damage the output device or circuitry. That's why foldback is used.

And, for that matter, "reflected power" isn't radiated *or* absorbed by
the transmitter. The transmitter produces power which is sent to the
antenna. All the power produced by the transmitter arrives at the
antenna less whatever is lost as heat in the transmission line. There
are no waves of average power bouncing back and forth on a transmission
line. Mathematically separating the power moving down the line into
"forward" and "reverse" components doesn't mean that waves of average
power actually exist.

Roy Lewallen, W7EL