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Author Topic: Amplifier output and antenna tuner issues  (Read 12066 times)
WA9UAA
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Posts: 314




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« Reply #15 on: September 14, 2012, 04:27:34 AM »

My balun connection, "I am running a THP 1.2 Kfx The amplifier is fine into a dummy load. Here is the issue, I run tuned feeders, 18 ga ladder line ~~ 137 ft., via a Palstar AT2K and a DX Engineering 1:1 balun on the output. The balun is connected to the tuner by about 6" of RG-8 size cable."
73,
Rob WA9UAA


HA! page 2 Grin
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W8JI
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« Reply #16 on: September 14, 2012, 08:29:49 AM »

My balun connection, "I am running a THP 1.2 Kfx The amplifier is fine into a dummy load. Here is the issue, I run tuned feeders, 18 ga ladder line ~~ 137 ft., via a Palstar AT2K and a DX Engineering 1:1 balun on the output. The balun is connected to the tuner by about 6" of RG-8 size cable."
73,
Rob WA9UAA


HA! page 2 Grin

It is a strong indication of amplifier or system instability, or abnormal sensitivity to common mode currents, when any amplifier has a problem delivering power into a 1:1 SWR as feeder length or the particular tuner settings are changed. If tuner settings or feeder length make a significant difference in amplifier output while SWR remains at or near 1:1, something very abnormal is going on in the system between the radio and tuner.

I think if it were my installation, I would place a dummy load after the tuner and tune through the tuner and see how the amplifier acts, or perhaps borrow another amplifier to try.

There is absolutely no reason to fuss with exact resonance of a ladder line fed doublet, as long as the tuner is able to match the antenna as indicated by a low SWR at tuner input.

73 Tom

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N6GND
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Posts: 369




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« Reply #17 on: September 14, 2012, 09:53:48 AM »

It is a strong indication of amplifier or system instability, or abnormal sensitivity to common mode currents, when any amplifier has a problem delivering power into a 1:1 SWR as feeder length or the particular tuner settings are changed. If tuner settings or feeder length make a significant difference in amplifier output while SWR remains at or near 1:1, something very abnormal is going on in the system between the radio and tuner.

73 Tom

Absolutely right Tom. First, the THP amps evidently have very-sensitive-to-common-mode-current fault-detection systems.

Second: In my original setup, the dipole I was using was installed as an inverted V with one end coming down to the edge of a (flat) roof to within a couple of feet of a long (80 ft.) aluminum gutter which runs along the roof on three sides. The balun-to-tuner coax ran parallel to and within a couple of feet of about 15 ft. of this gutter. No question why there was common mode RF on the outside of the coax.  Shortening the coax and installing a common-mode choke made an appropriate remedy for this admittedly compromised and unstable system. The ends of the dipole have since been raised by 15 ft. so that it is no longer an inverted V and the one end is no longer close by the gutter.

Those of us who live in urban areas on small lots have lots of such compromises to make in our antennas (as well as with other things). Metal gutters cause other problems too, like picking up and re-radiating urban noise. Had I built this house with ham radio in mind, I would have made adjustments.

I have also had common mode problems with the coax feeding a vertical antenna on the same roof. The coax runs about 15 ft. underneath the dipole at less than a right angle to it. The common mode RF affected the SSB circuits on my K2 (but did not affect CW operation). A common-mode choke at the vertical coax to the house ameliorated that problem.

My guess is that Rob's problem has to do with common mode current. It's not possible to analyze such a problem knowing only the length of the ladder line and the length of the balun-to-tuner coax. We probably need to know more about the orientation of the antenna with regard to the shack and the house wiring, the station ground(s), any antenna system switching, the integrity of any coax jumpers between equipment, the power supply setup and so forth.

A last suggestion: Occam's Razor. The simplest explanation is likely to be the one that works. Trimming the antenna and the feedline seem to me to be far too complex an approach. And I've been there too.

Best,

Mike
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WX7G
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« Reply #18 on: September 14, 2012, 10:55:34 AM »

Additional info.
All readings through the tuner matched to 1:1

Rig at 3550 Khz................................................................3950 Khz
Output to DL 83W maximum.............................................82W to Ant.
Antenna  82W to DL..........................................................80W to Ant.
Amplifier 600W to DL.......................................................480W to Ant.

Amplifier
Meter      580W to DL........................................................590W to Ant.

Metering issues ? I have seen this on another tuner but with a different antenna.

Perhaps the issue is nothing more than power measurement error. This is apparent in the last data point where the Amplifier Meter reads essentially the same power into the dummy load and the antenna.
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WA9UAA
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« Reply #19 on: September 14, 2012, 01:07:02 PM »

Hi All, have been back and forth with the second rig and the output is still low on the Palstar meter at 3950 Khz. Taking Dave's copy of what I had done before to heart, with the rig at 3943 Khz I took a quick look at the THP meter reading, 600 watts on the money at 3943 Khz, at the same time the Palstar meter said 450 watts. The THP meter at 3547Khz shows with in a needle's width of 600 watts key down, if I were to guess it would be 590 watts. The Palstar meter seems to be low at the higher frequency. The rig itself tops out at between 80-85 watts so I can't drive the amp any harder which is probably a good thing.

I took the risk of getting in with group on 75M this morning, all thought the audio was fine and offered suggestions to tweak it. One fellow looked at the signal on his scope and saw a good trapezoid pattern.  My fault for trusting one meter over another though skeptically how do I know it's "right" when the other isn't?  Thanks again to all, while I don't have the money for a spectrum analyzer Roll Eyes Does anybody know of a more linear reading output meter and more to the point is there another way of monitoring my signal? (Good scope recommendation?) They say when a THP amp fails somebody has to go out and commit Seppuku. Shocked  I hope I just saved a worker at THP. Again thanks to everybody the weighed in on this.
73,
Rob WA9UAA
« Last Edit: September 14, 2012, 01:08:57 PM by WA9UAA » Logged
N6GND
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Posts: 369




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« Reply #20 on: September 14, 2012, 01:07:28 PM »


Perhaps the issue is nothing more than power measurement error. This is apparent in the last data point where the Amplifier Meter reads essentially the same power into the dummy load and the antenna.

Occam's Razor indeed.
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WA9UAA
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« Reply #21 on: September 14, 2012, 01:10:12 PM »

I certainly prefer this compared to getting the amp fixed! Grin
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W8JI
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« Reply #22 on: September 14, 2012, 05:30:21 PM »

Is the THP a true directional coupler metering system?

How good is the meter in the PalStar and the THP?

Sounds like a meter is not within reasonable tolerances, or the amp or something before the tuner is RF sensitive.

There isn't any reason to build a common mode detector **intentionally** inside any sealed metal box with connectors bolted to the walls. If the THP is sensitive to CM currents, it must be accidental.
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W5WSS
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« Reply #23 on: September 14, 2012, 06:21:17 PM »

The THP I have works nicely. I must ask are you referencing output power from a before and after  state of tune with the tuner?I observed without foldback and such In older systems when there is an increase in swr a monimatch type meter in line can indicate an incorrect increase in power consistent with the swr increase but in reality is a false reading.
When the equipment is all located in the tuned zone by the tuner which should be the last part in the chain everything should be looking into a 50 ohm non inductive load but common mode can travel reversely along the interior of the shield and cause erroneous trip behavior to the THP prot system. But should not be responsible for less output of the amplifier. Go to a 50ohm non inductive dummy load and verify what the THP is actually outputting and I believe from that reference you can find common displacement currents a culprit.perhaps in the transmitter check both.
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WA9UAA
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« Reply #24 on: September 14, 2012, 06:52:42 PM »

No actual schematic of the Palstar coupler in the manual. The THP unit uses transformer coupling, toroidal transformers mounted around the trace on the pcb, diodes to generate DC voltages and a RFC and frequency compensating parts (I think). This appears to be true directional coupling.

The metering in the Palstar on the Low range seems linear from one end of the band to the other as does the THP metering where the full scale 1 kW. Full scale on the Palstar high range is 3KW. Hmmm A man with one watch knows the time, a man with two watches is never sure unless they are linked to the NIST. Embarrassed

I just put the Palstar mobile watt meter in line between the amp and tuner, it reads 600 watts out at the high end of the band where the circuit in the tuner still shows 450 watts. This meter is 2KW full scale.
73,
Rob WA9UAA
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WA9UAA
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Posts: 314




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« Reply #25 on: September 14, 2012, 07:09:05 PM »

W5WSS,
Thanks for your reply, the THP protection is not tripping in this case. The output now appears to be consistent with the mobile Palstar meter. BTW, the tuner is a recent purchase, I have had all the other gear for several years. The initial issue was what appeared to be a 150 watt difference in output of the amp at either end of the 80M band when the amp was presented with a 50 ohm resistive match in both cases. I was initially relying on the Palstar tuner meter as everything else I have had from them has been first rate at least to my level of expertise. The THP meter now appears to be consistent as does the older Palstar meter.

The CM current on the interior of the coax shield is a new one on me, I have always seen it illustrated on the outside of the shield . W8JI and W5DXP had an intense discussion on this current flow in an unbalanced antenna system a few years ago.
73,
Rob WA9UAA
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W5WSS
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« Reply #26 on: September 14, 2012, 10:25:13 PM »

Right on, usually it is on the outside of the sheild but depends CM "can" be simultaneously traveling on the inside of the sheild too because of skin effect whilst the differential mode is behaving properly anyway glad  your thp relative power meter is  accurate mine seems accurate enough.
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KC4MOP
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« Reply #27 on: September 15, 2012, 03:59:59 AM »

No actual schematic of the Palstar coupler in the manual. The THP unit uses transformer coupling, toroidal transformers mounted around the trace on the pcb, diodes to generate DC voltages and a RFC and frequency compensating parts (I think). This appears to be true directional coupling.

The metering in the Palstar on the Low range seems linear from one end of the band to the other as does the THP metering where the full scale 1 kW. Full scale on the Palstar high range is 3KW. Hmmm A man with one watch knows the time, a man with two watches is never sure unless they are linked to the NIST. Embarrassed

I just put the Palstar mobile watt meter in line between the amp and tuner, it reads 600 watts out at the high end of the band where the circuit in the tuner still shows 450 watts. This meter is 2KW full scale.
73,
Rob WA9UAA

Sounds like the metering circuit in the Palstar is way out in left field. Don't tell Paul that!!! He'll bite your head off!!! (personal experience!)
The last meter I would trust, would be the one in most tuners. Palstar doesn't mean 5% accuracy.
Bird Watt meters do. All of my station monitoring goes through my Bird Watt meter. The watt meter on my Kenwood, or linear amp, or 6M rigs are just indicators.
Accuracy of any wattmeter is going to depend on the circuit being 50 ohms.
It seems you are discovering some interesting facts about the amplifier. 150W difference in one band is a little too much.

nice thread BTW
Fred
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W8JI
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« Reply #28 on: September 15, 2012, 04:41:26 AM »

Right on, usually it is on the outside of the sheild but depends CM "can" be simultaneously traveling on the inside of the sheild too because of skin effect whilst the differential mode is behaving properly anyway glad  your thp relative power meter is  accurate mine seems accurate enough.

If anything in a metal enclosure with connectors mounted to the enclosure is sensitive to common mode, that device has a design problem. There is no reason at all to sense common mode as a shutdown function, and a device cannot even have common mode sensitivity without an external sensor or a third path external to the cabinet.

For example, a sloppy design might not properly bypass a lead from a keying line. If the keying line has common mode, the RF might be detected and unkey the amplifier.

For common mode currents on the cable, there is no possible way to detect them and have a shutdown (without an external detector over the cable), nor is there any logical reason to detect them in an amplifier or radio.

A good design does everything possible, like powerline bypass capacitors and bypassing on control and ALC jacks, to prevent common mode sensitivity.


Quote
Sounds like the metering circuit in the Palstar is way out in left field. Don't tell Paul that!!! He'll bite your head off!!! (personal experience!)

Nice.

Based on what he says happens, it looks either like wiring, metering, or an amplifier design/operation flaw.
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W5WSS
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« Reply #29 on: September 15, 2012, 12:46:02 PM »

THP warns against erroneous operation and PROT trips caused by a sensitivity to Common mode in their manual perhaps they should consult with you as to why or what the technical issues are before they write warnings against Common mode being a concerned. reason for their warning.
The amplifier is constructed following good electric/mechanical techniques respective to metal enclosure and as an RF Faraday shield with good electrical contact to the walls of the enclosure and the coaxial connectors etc. Common mode can find a way into the device via the shield of the coaxial cable.
So perhaps there is some design issue with the prot feature and CM.In which case we are in agreement.
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