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Author Topic: Antenna Tuner's SWR meter vs HF Radio's SWR meter  (Read 2172 times)
W1SAF
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Posts: 62




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« on: December 16, 2003, 12:06:11 PM »

How many out here tune their antenna tuner and just watch the SWR meter on their tuner and not the rig's? Did anybody ever notice when you tune the antenna tuner to the lowest SRW watching that meter, that your rig's SWR meter is not as low as it should or could be? In other words, I notice a bit of de-tuning the antenna tuner can lower the rig's SWR meter reading. A call to MFJ said that the rig's meter reading is "relative" to the antenna tuner's meter reading. HMMMM???I always thought that an antenna tuner fools the rig to thinking it has the best antenna/rig match possible. Thanks in advance for any opinions and anwers.
Stu W1SAF
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K5DVW
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« Reply #1 on: December 16, 2003, 12:21:32 PM »

I'd watch the rigs meter for the simple fact that the rig looks at VSWR to determine when to fold back power.
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WB2WIK
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« Reply #2 on: December 16, 2003, 12:31:54 PM »

A tuner doesn't fool the rig.  The tuner matches the antenna *system* (including antenna and its transmission line) to the reference impedance of the bridge used to make the SWR measurement, which is normally 50 Ohms.

There are many reasons that the meter in the tuner and the meter in the transmitter may not agree; one obvious one is that all SWR bridges are nulled or balanced when the load equals the reference resistance of the bridge circuit.  If 5% tolerance resistors are used, and one bridge measures 52.5 Ohms while the other one measures 47.5 Ohms, there will be a 10% discrepancy built into the system -- not much.

Also, although the SWR on any given transmission line is a constant* the R and X values are not.  If SWR = 2.0, the load can be anything from 25 to 100 Ohms.  It's always 2.0, but the variables within that 2.0 can cause measurement conflict, as can common mode currents on the transmission line itself.

(*Since a tuner "in the shack" does nothing to match the actual load (antenna) to the line, the actual SWR as measured at the generator (rig) will always be lower than the true SWR at the antenna because line loss will reduce the reflected power.  If your line has 6dB loss, your antenna could be a dead short and your SWR will still read pretty good at the rig end of the line.  With 10dB line loss, your SWR will never measure higher than 2:1, even if your antenna was an open or short circuit.  That "correction" in SWR causes measurement confusion, especially under high mismatch conditions.)

If you're trying to tune a system to match your rig, only the measurement made at the rig itself counts.  Then, tuning for maximum possible forward power should yield the same results, since when using a tuner, the actual SWR isn't so important: We are trying to achieve maximum power transfer from source to load, and that's all we're trying to accomplish.

For several decades of amateur radio, hams didn't own SWR bridges and still worked DXCC...

WB2WIK/6
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KT8K
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« Reply #3 on: December 16, 2003, 12:33:23 PM »

I would expect a bit more accuracy from the tuner's meter than the rig, but only a little.  The rig may be reading a slightly different SWR because it is a little farther from the antenna (jumper length can make a difference, and affects each band differently).  I agree that the rig's meter may indicate the SWR that will result in fold-back of output power, but I'd want to test it or see something proving the accuracy of the meter in the rig before I would put too much trust in it.
73 de kt8k - Tim
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N8FVJ
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« Reply #4 on: December 16, 2003, 02:08:08 PM »

I do not know how much variation the two meters read, however I would connect a 50 ohm dummy load to your radio without a tuner in-line. If your solid-state HF radio SWR meter reads very close to a perfect SWR, I would trust the HF radio SWR meter for tuning the antenna for a low SWR.

I believe the low quality antenna switches in the antenna tuners causes a difference in SWR readings. I can not use 6 meters through the MFJ Diff 'T' tuner. The tuner has a 'pass thru only' antenna select switch function (tuner out of circuit). The SWR is so high, the radio shuts down.

It was such a poor system, I actually bought a seperate radio for HF only, put up resonate HF antennas & sold the tuner.
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W8JI
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« Reply #5 on: December 16, 2003, 02:38:52 PM »

You can be almost certain the problem is a difference in the meter calibration. The SWR, unless you have a bad jumper cable or something defective between the tuner and radio, is identical at the tuner and at the radio.

If you have ANYTHING between the tuner and the radio, like a low pass  filter or amplifier, take it totally OUT of the system and look again at SWR.

If the meters still disagree and knowing you have a MFJ tuner, I'd trust the meter in the radio first (even though radios are not that accurate). I base that on the fact MFJ has NO quality control process, so it is more likely if there is a problem with calibration it would be in the MFJ meter.
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W8JI
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« Reply #6 on: December 16, 2003, 02:38:53 PM »

You can be almost certain the problem is a difference in the meter calibration. The SWR, unless you have a bad jumper cable or something defective between the tuner and radio, is identical at the tuner and at the radio.

If you have ANYTHING between the tuner and the radio, like a low pass  filter or amplifier, take it totally OUT of the system and look again at SWR.

If the meters still disagree and knowing you have a MFJ tuner, I'd trust the meter in the radio first (even though radios are not that accurate). I base that on the fact MFJ has NO quality control process, so it is more likely if there is a problem with calibration it would be in the MFJ meter.
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K5LXP
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« Reply #7 on: December 16, 2003, 02:56:46 PM »

More responses for this same question:

http://www.eham.net/forums/Elmers/52677
http://www.eham.net/forums/Elmers/51165

Mark K5LXP
Albuquerque, NM
k5lxp@arrl.net
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W9PMZ
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« Reply #8 on: December 17, 2003, 12:22:44 AM »

WB2WIK says,

"Also, although the SWR on any given transmission line is a constant* the R and X values are not. If SWR = 2.0, the load can be anything from 25 to 100 Ohms. It's always 2.0, but the variables within that 2.0 can cause measurement conflict, as can common mode currents on the transmission line itself."

This is the measurement uncertainity.  Think of the total power as a vector with the SWR effect as a rotating vector at the end of the power vector.  In order to know the true power reading you will need to know the phase angle to resolve the measurement.

Since you are matching the transmitter to the input of a device that presents a good match I would expect a more accurate reading than when you measure the power in a matching device where the the return losses are potentially very different.


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N7DM
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Posts: 671




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« Reply #9 on: December 20, 2003, 07:08:50 PM »

My "antenna tuner" has no SWR meter.. it is just a simple L/C circuit, link coupled to the rig, and having the feedline tap onto the coil..for  "1:1"...

In the coax between the rig and the two turn link on the input to the matching netork [antenna tuner], I have a Bird  Wattmeter.  I keep that set to read Reflected Power..from the network back to the rig. A 'Bird' reads in actual watts (+/- 4%)..  

So I use the Bird reading to set my matching. I tune for ZERO  power going back to the rig from the matching network. That is 100 %  TO the network, NONE back...call that  1 to 1  VSWR/ISWR.

If I look at the rig  "SWR" meter, it will read from 1 to 1 on one band up to 1.4 to 1 on the worst band.

These devices are ALL  diode driven. It is like trying to calibrate the calibrator !  What is ""RIGHT"" ?   You got me..I go with the Bird.  As long as ONE of 'em is derned close, the station works fine...

73
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