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Feeder SWR & power loss
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la7no
Antenna Theory Regular


Joined: 05 Jul 2014
Posts: 22
Location: Bergen, Norway

PostPosted: Mon Jul 14, 2014 7:17 am    Post subject: Reply with quote

E Kafeman wrote:
Have no knowledge about your hardware but typical measurement of system VSWR is done by measuring source impedance R+jX and then measuring load impedance. For 1:1 VSWR should it then be a conjugate match, RS=RL and XL=-XS.
Guess your LP-100A, if not calibrated for actual radio impedance, assumes an reference impedance 50+j0 Ohm and due to that not shows correct VSWR as there not is 50 Ohm neither as load or source impedance.
Shift input/output of the LP-100A and it do probably indicate that radio+tuner is mistuned with a VSWR of 1:3.5 but if you can read complex impedance values do it shows a not to bad conjugate match compared to measured antenna impedance values.


My point is that the LP-100A shows the same situations independent of the setting of the K3 tuner. I can even switch the K3 tuner completely off, and the LP-100A indicates exactly the same.
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R. Fry
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Joined: 06 Jun 2011
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Location: Illinois USA

PostPosted: Mon Jul 14, 2014 11:15 am    Post subject: Reply with quote

bigSteve wrote:
...For the radio to deliver power to a 50 Ohm system, it has an intrinsic 50 Ohm impedance. As a result, reflected power is absorbed within the transmitter.

Just to note that if the source impedance of a transmitter exactly matched the Zo of its load impedance, then 1/2 of the available r-f power produced by the transmitter would be dissipated within that transmitter.

Yet many transmitters using Class C/D/E r-f output stages have measured d-c input to r-f output efficiencies of 75% or more. The reason for this is that the transmitter source impedance typically is much lower than the load impedance.

As for re-reflection of energy produced by an antenna system mismatch...

Below is a test report of a UHF TV transmit antenna, using a short r-f pulse. The reflection from the antenna is clearly seen in the bottom photo, returning to the transmitter about 3 Ás after it left the transmitter. The v.p. of the transmission line in this system was about 0.996 c, meaning that length of the transmission line was about 1,470 feet (which was the physical reality).

The transmitter re-reflection of that reflection in this analog TV transmit system is radiated later than the original signal, and produces a "ghost" image displaced to the right of the main image -- present on all TV receivers tuned to that station.

That re-reflection from the antenna appears even in the display of the incident signal, as a small blip 3 Ás after the initial pulse waveform.

Transmit system reflections of 3% and less are difficult to see in normal program video, though.

R. Fry
RCA Broadcast Field Engineer (long ago)



Last edited by R. Fry on Wed Jul 16, 2014 10:16 am; edited 1 time in total
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E Kafeman
Antenna Theory Regular


Joined: 04 Feb 2013
Posts: 25
Location: Sweden

PostPosted: Mon Jul 14, 2014 11:44 am    Post subject: Reply with quote

la7no wrote:
I can even switch the K3 tuner completely off, and the LP-100A indicates exactly the same.

Yes that is what I guessed. Your LP-100A shows VSWR relative 50 Ohm, if it is that impedance it is calibrated for, not relative your K3 tuner settings.

VSWR can be measured relative a reference impedance, often is measurement instrument reference 50+j0 Ohm.

To find actual system VSWR in a non 50 Ohm system must impedance for both source and load be measured at same point somewhere along coaxial cable to be able to find out actual VSWR.

Example:
If your antenna have a perfectly impedance of 200+j0 impedance at measurement point and You have a radio with same impedance, also measured at same point as for the antenna, then will it be a perfect match, 1:1 VSWR.
In same system measuring VSWR relative a instrument calibrated for 50 Ohm, will show a VSWR of 1:4 in both directions, just as I wrote before.

In this case, is your LP100-A measuring VSWR relative 50 Ohm? As it not react if radio is on or off, do it not measure VSWR relative radio impedance. Correct?
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la7no
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Joined: 05 Jul 2014
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Location: Bergen, Norway

PostPosted: Wed Jul 23, 2014 5:19 pm    Post subject: Reply with quote

Some conclusions from my side:

(1) An RF transmitter is actually a current generator. Its internal resistance is kept low in order to reduce internally dissipated power, with consequent reduction in efficiency. It is usually designed to work efficiently into 50 ohms.

(2) It is important to know the difference between these two statements:
(A) The transmitter has a 50 ohm output impedance.
(B) The transmitter is designed to work into a 50 ohm load

(3) A transmitter will be able to put its rated power into a 50 ohm load, but this says nothing about its output impedance. Many people are confused regarding this.

(4) Any RF power being reflected back from the load toward the transmitter will mostly be re-reflected back toward the load. This is due to the mismatch between the transmitter and the load, and the phase difference between outgoing and incoming waves.

(5) The transmitter is not getting hot or destroyed by the reflected power itself. Temperature or damage is due to the load being outside the transmitters design point.

(6) The tuner placed between transmitter and feeder is not tuning the antenna. It does not influence antenna matching or VSWR on the feeder. The directional coupler used to control the tuner, is found between tuner and transmitter.It changes the load into something that is acceptable for the transmitter, so it will be able to deliver its rated power to the load.

73es,

Per-Tore
LA7NO
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Schubert
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Joined: 08 Apr 2009
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PostPosted: Fri Jul 25, 2014 11:15 pm    Post subject: Reply with quote

Your conclusions aren't valid.

You can't forget all of transmission line theory to reach those conclusions. 50 ohm receiver powers a 50 ohm load, it doesn't matter how you measure it, reflected power is absorbed within the transmitter. It doesn't matter if you accept it or not, but this analysis is pretty poor.
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la7no
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PostPosted: Sat Jul 26, 2014 7:17 am    Post subject: Reply with quote

Schubert wrote:
Your conclusions aren't valid.

You can't forget all of transmission line theory to reach those conclusions. 50 ohm receiver powers a 50 ohm load, it doesn't matter how you measure it, reflected power is absorbed within the transmitter. It doesn't matter if you accept it or not, but this analysis is pretty poor.


Could you please elaborate a bit on this?
Were did I go wrong?

P-T
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R. Fry
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Joined: 06 Jun 2011
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Location: Illinois USA

PostPosted: Sat Jul 26, 2014 11:22 am    Post subject: Reply with quote

Schubert wrote:
... 50 ohm receiver powers a 50 ohm load...

For some additional input -- I have taken part in factory tests of
high-power Class C single-tube/tuned cavity FM broadcast transmitters
driving 50-ohm test loads measured to have 1.03VSWR, showing a DC
input to r-f output efficiency of the PA to be in excess of 80% --
including the loss in the harmonic filter.

Load power was measured using calorimetric methods. In fact, 80%
PA efficiency is the published spec for this transmitter as long as
load VSWR relative to 50 ohms is 1.7:1 or less (any phase angle).

Those results don't fit well with the idea that the tx source impedance
must be 50 ohms in order to drive a 50 ohm load most efficiently, as then
the output power could never exceed 50% of the r-f power produced
by the final amplifier in the transmitter.

Also... the r-f pulse test report I posted earlier in this thread shows that
load reflections DO reflect from the output stage of the transmitter back to
to the load -- which would not be true if the transmitter presented a
50-ohm load to the 50-ohm transmission line it was driving.
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E Kafeman
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Joined: 04 Feb 2013
Posts: 25
Location: Sweden

PostPosted: Sat Jul 26, 2014 1:35 pm    Post subject: Reply with quote

Transistor-based class C stages, most simple schematics do usually include a trim-able PI network for most possible power delivered to antenna. Did build a few such in my youth. Small stages, maybe as most a few Watt. Main heat-loss was easy to find as it was the transistor that did become warm. Maybe less then 20% efficiency due to a bit too slow transistors, resulting in low gain.

A more efficient modern variant, class E, is rater interesting.
Have recently done some impedance measurements on Texas Instrument CC2520. 2.4 GHz. Max power 10 dBm. Can not really compete with big tubes but efficiency is very important as this type of chips often is used in handheld equipment. Its transistors are very fast and acts more or less as pure on/off switches with Rdson less then 1 Ohm at 2.4 GHz. Insane fast switching. It have dual stages, 180 degree phase-shifted with one external charge-coil each. A bit like class C. As transistors are either full on/off are internal losses low. Measured impedance for each transistor-stage with recommended coils is about 15+j0 Ohm and 30 Ohm between these ports, combined by a Lattice LC balun and an impedance network for 50 Ohm output impedance.
These class E stages have often problem with harmonics so a SAW-filter is rather common as next step. These filters are designed typical for an assumed impedance of 50 Ohm, or else would filter effect be less.
If measuring inward, against power-stage from SAW-filter do we have 50 Ohm load if impedance-matching is done correct, still efficiency in power-transistors are much higher then 80%.
Measured PA impedance depends mainly on what size of charging coil that is selected. Most losses are due to that DC feeding and ground/decoupling not is stable enough. Actually is at PCB printed coil recommended as first components at chip DC-pin, before ant capacitor decoupling as it reduces losses in internal DC circuit.

A similar type of circuit are the type of switched DC/DC-converters that exist in every cellphone. Basically is it a FET-transistor and a coil driven by a square-wave of 100kHz or more. They can deliver several Ampere using a minimal coil and a transistor with a size of 2-3 mm. A few percent losses and it will burn. To achieve this efficiency must transistor internal resistance be very low when it is "on". Less then 0.01 Ohm is not uncommon. Measured output impedance can be 100 times higher as most of the resistive losses are due to resistive wire-loses in the coil.

My point is that an PA-stage impedance is not necessarily same as the transistor or tube impedance and good impedance matching is not in contradiction to high efficiency.

Regarding CC2520, it is not always recommended to do conjugate impedance matching as the way it will be driven is not max output power of interest, instead is it impedance matched for the impedance that result in highest efficiency, but slightly less max emitted power. For cellphones do we have same situation, it is not always conjugate matching that is wanted, neither for TX or RX.
For the receiver is it of more interest to match for the impedance resulting in best S/N (TIS) which not always is the same as the conjugate of its input measured impedance.

RX with AGC direct at input, before any LNA stage, is not too uncommon among these small radio chips. They can show different impedance depending on what level VNA is set at. Same for TX, some of these small TX-circuits have internal DC-boosters so that actual RF impedance is depending on actual power level.
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