Fun with VSWR

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Bill_G
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Fun with VSWR

Post by Bill_G »

I need one of those ice cube NM-NF matching networks with trim cap. I think Celwave made them. Maybe DB. I just dont recall the specific name for the little passive device. I used to see them between mobiles (used as bases) and their antenna to match the impedance better. Unfortunately, this will be for the 700M band. So, no one makes one. But, I may be able to adapt a UHF or 800M.

Here's the conundrum - brand new system, top tier 700M P25 base stations. Five to twelve stations per site into Celwave combiners using half inch superflex. Every station reports some amount of reflected power in the xmit diagnostics. It varies. One station will report 98/6, and another will report 108/3. When we terminate the stations into a service monitor for cal, different test cables gives us different forward and reflected power readings. We tried every coaxial cable type and length imaginable, and each gave us a different, but reproducible, reading. We never got full power into the service monitor. And we could never get zero reflected power even into several different dummy loads.

When we put a wattmeter in line, none of the loads, cables, or combiners showed reflected power, but we noted the stations did not vary as much when we changed cables to loads. When we connected the wattmeter directly to the back of the station through an NM-NM adapter, suddenly we had zero station reported reflected power. We also had greater reported forward power, and we had more power into the service monitor. We could actually get 100w on the General Dynamics and Aeroflex displays instead of 78 to 80 something. Woohoo.

Through some more experimentation, we were able to identify three adapters coupled together (one NM-NM to a NF-NF to a NM-NF right angle) that killed the reported VSWR and coupled the most forward power into any cable, any combiner, and resulted in greater forward power to the antenna.

The OEM is reluctant to accept our findings, and states three to five watts reported at the station into a combiner is normal.

cough

So, rather than cobble a bunch of adapters together, or experiment with tuned short lengths of coax, I want to find those ice cubes I've seen in the past to see if they can help.
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xmo
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Re: Fun with VSWR

Post by xmo »

DB used to make them - they called it the "Z-Matcher"
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

Thanks!
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kcbooboo
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Re: Fun with VSWR

Post by kcbooboo »

You'll find that adapters of any kind plus coax (type and length) make a BIG difference on anything above 500 MHz. Make purpose-built cables with the correct connectors at each end. Make them as short as possible. Do NOT use any adapters. Even inserting a thru-line wattmeter will present a significant portion of a wavelength to screw things up. I found that on 900 MHz, where a wavelength is about a foot, that the 4 inches of the wattmeter made the transmitter very upset.

Bob M.
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

I tried making critical length cables out of foil shield RG58, but could never get them perfect across the band (769 to 775). It's somewhere between a quarter wave and half wave. The several three quarter wave coils of RG58 trick did not work either. However, this one cluster of adapters did get us zero reflected in the station with pretty good overshoot on the power (110/0). Those adapters gave us 100W into the service monitor with almost any test cable including crummy ones. So, there is a clear impedance mismatch in the pa section.

I played with it some more today. Went out with lots of adapters. Some of them have been rolling around in toolboxes for a while. None of them were as perfect as our magic stack, but darn close. They all got the reported reflected down to a watt. We could get reproducible measurements with different test cables. And we got at least 10% increase in forward power which brings the stations into acceptance. Sometimes we got almost 20% more power. Measured power to the antenna through the hybrid combiners also increased by the same amount proving the adapter stacks were doing something.

We'll see what happens.
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Wowbagger
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Re: Fun with VSWR

Post by Wowbagger »

I agree with the analysis that the PA isn't 50+0j ohms - I've had luck tuning the PA's output to get a better match, can you try that?
This is my opinion, not Aeroflex's.

I WILL NOT give you proprietary information. I make too much money to jeopardize my job.

I AM NOT the Service department: You want official info, manuals, service info, parts, calibration, etc., contact Aeroflex directly, please.
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

Well, there's nothing to tune. There's a LLPA that drives dual parallel 60W boards that are combined. Those jumper to a LPF / directional coupler board with the NF connector soldered directly to the board. The alignment procedure lets you adjust the drive to the finals, but nothing else. There are no trim caps or snubbing resistors. There is no matching network stripline after the directional coupler etched on the board. It goes straight out the door.
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FMROB
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Re: Fun with VSWR

Post by FMROB »

My short easter response (Happy Easter Everyone) is that I have experienced the same issue in the UHF band on the TB9100 stations, Quantars, and the MTR. Quantars and MTR units causing the trouble beeps. When read with meter, acceptable and quite lower VSWR than what was reported.
More recently on the TB9100 stations that actually provide a reading, I had a reported a 1:7:1 vs. an acutal reading of almost no detectable meter movement.

I tried changing the duplexer to lightning arrestor cable lengths, connectors, jumpers and just plain gave up. With no way to actually calibrate the meter within the unit (not that I found in the manual quickly) I simply set the alarm reporting thresholds for the known constant. I figure if it reads over 1:8:1 there is a problem and it will alert us via emai. As a note I almost always use RG 142 for rack to lightning arrestor jumper cables of less than 5'.
mike m
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Re: Fun with VSWR

Post by mike m »

This sounds like a design problem in the base station itself, high harmonics in particular and the lack of a proper diplexed low pass filter.

I had a similar problem with a 118 to 137 MHz PA that I was designing for an avionics company down here in Salem.

This is a 80 watt PEP 8.33 KHz AM comm radio, optimizing the pa output match for close in spectral mask to meet 8.33KHz requirements along with lowering the far out broadband noise that falls in the 108 to 118 NAV band resulted in a Zout for my low pass filters input side that was not 50 ohms, as a result I had to change the low pass filter from a simple 7th order elliptic to a diplexed low pass design which fixed the issue.

The diplexed filter just dumps harmonic energy into a small 50 ohm load resistor and lets the fundamental go to the antenna as it should.

The diplexed filter adds 3 or 4 inductors and a similar amount of capacitors to the filter but you can then let the PA see a different Z than the 50 ohms that a typical LPF wants to see.

You could add a diplexed LPF externally to each station but the mfg really should address this.

Is this a Harris base/repeater by the way ?

The advantage of a designed in diplexed LPF as opposed to an externally added one is that it also helps with directional couplers in the PA output path that have poor directivity and this could be part if not most of your problem with a PA output that isn't seeing 50 ohms and a power control loop responding to some harmonic energy.

This is like peeling back the layers of an onion trying to fix a problem like this, you may find a Low Pass filter design issue and a power control/directional coupler design issue in the Power Amp's themselves.


Mike
down Salem way
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wavetar
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Re: Fun with VSWR

Post by wavetar »

Bill_G wrote: Here's the conundrum - brand new system, top tier 700M P25 base stations. Five to twelve stations per site into Celwave combiners using half inch superflex. Every station reports some amount of reflected power in the xmit diagnostics.
The OEM is reluctant to accept our findings, and states three to five watts reported at the station into a combiner is normal.

cough

So, rather than cobble a bunch of adapters together, or experiment with tuned short lengths of coax, I want to find those ice cubes I've seen in the past to see if they can help.
Ummm, although this has been an informative thread, the bottom line in my opinion is the OEM needs to come to the plate & realize it shouldn't take a bunch of adaptors or specific lengths of coax to stop their base stations from failing it's own internal diagnostics, and not producing rated output power when terminated into 50-ohm loads. It's a fairly obvious mis-match or harmonic filtering issue as Mike pointed out.
No trees were harmed in the posting of this message...however an extraordinarily large number of electrons were horribly inconvenienced.

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SlimBob
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Re: Fun with VSWR

Post by SlimBob »

I assume that the combiner and antenna have been tuned to 50-ohms using terminators on all ports?

Interesting, very interesting... is the antenna port tuned or the PA output?, indeed. Thanks for sharing guys.
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

FMROB wrote:My short easter response (Happy Easter Everyone) is that I have experienced the same issue in the UHF band on the TB9100 stations, Quantars, and the MTR. Quantars and MTR units causing the trouble beeps. When read with meter, acceptable and quite lower VSWR than what was reported.
More recently on the TB9100 stations that actually provide a reading, I had a reported a 1:7:1 vs. an acutal reading of almost no detectable meter movement.

I tried changing the duplexer to lightning arrestor cable lengths, connectors, jumpers and just plain gave up. With no way to actually calibrate the meter within the unit (not that I found in the manual quickly) I simply set the alarm reporting thresholds for the known constant. I figure if it reads over 1:8:1 there is a problem and it will alert us via emai. As a note I almost always use RG 142 for rack to lightning arrestor jumper cables of less than 5'.
I've never had this problem with Quantars and MTRs, and I have zero experience with TB9100s. Quantars will report real time fwd and ref power, but I don't recall if MTRs will except as an alarm. If the VSWR is wrong, the MTRs shutdown, beep, and turn on a red light. I get to work the problem. However, I did have a problem with a VHF MTR recently that I am sure was caused by site RF weirdness. Wireline base station with ant relay. Dispatch keys, and total garbage comes out. Phone lines in sound perfect. Swapping out first the wireline card, and then the whole station, did not fix the problem. It took a ferrite bead on the rf jumper between the relay and the rcvr input to fix it. Very strange. Came up out of the clear blue, and as far as I'm concerned, it's not fully resolved - just masked by a ferrite bead. It's a busy site, and they don't want to spend the money to find it. OK.

Anyway, I've never had an MTR or Quantar report reflected power into a combiner that I couldn't confirm with a wattmeter. I've had site jumpers and combiner ports go bad. I've also had mobiles used as base stations burn through RFPAs because they weren't designed to look into hundreds of feet of feed line squirreliness. An isolator and a matching network usually fixed that.
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

mike m wrote:This sounds like a design problem in the base station itself, high harmonics in particular and the lack of a proper diplexed low pass filter.

I had a similar problem with a 118 to 137 MHz PA that I was designing for an avionics company down here in Salem.

This is a 80 watt PEP 8.33 KHz AM comm radio, optimizing the pa output match for close in spectral mask to meet 8.33KHz requirements along with lowering the far out broadband noise that falls in the 108 to 118 NAV band resulted in a Zout for my low pass filters input side that was not 50 ohms, as a result I had to change the low pass filter from a simple 7th order elliptic to a diplexed low pass design which fixed the issue.

The diplexed filter just dumps harmonic energy into a small 50 ohm load resistor and lets the fundamental go to the antenna as it should.

The diplexed filter adds 3 or 4 inductors and a similar amount of capacitors to the filter but you can then let the PA see a different Z than the 50 ohms that a typical LPF wants to see.

You could add a diplexed LPF externally to each station but the mfg really should address this.

Is this a Harris base/repeater by the way ?

The advantage of a designed in diplexed LPF as opposed to an externally added one is that it also helps with directional couplers in the PA output path that have poor directivity and this could be part if not most of your problem with a PA output that isn't seeing 50 ohms and a power control loop responding to some harmonic energy.

This is like peeling back the layers of an onion trying to fix a problem like this, you may find a Low Pass filter design issue and a power control/directional coupler design issue in the Power Amp's themselves.


Mike
down Salem way
I'm tempted to buy some single section isolators, put them directly to the rfpa connector, and see what happens.
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

wavetar wrote: Ummm, although this has been an informative thread, the bottom line in my opinion is the OEM needs to come to the plate & realize it shouldn't take a bunch of adaptors or specific lengths of coax to stop their base stations from failing it's own internal diagnostics, and not producing rated output power when terminated into 50-ohm loads. It's a fairly obvious mis-match or harmonic filtering issue as Mike pointed out.
Agreed.
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

SlimBob wrote:I assume that the combiner and antenna have been tuned to 50-ohms using terminators on all ports?

Interesting, very interesting... is the antenna port tuned or the PA output?, indeed. Thanks for sharing guys.
If by tuned to 50 ohms you mean - are the combiners tuned for center - yes. They are purpose built and stacked by Celwave. They arrived racked and ready. They made it as difficult as possible to get at the threaded slug, and they put locktite on the nut. The project mgr shipped a whole rack back for retest and they gave it a blessing.
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FMROB
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Re: Fun with VSWR

Post by FMROB »

I do recall that the Quantar will report actual figures. I do agree, pulling out the bird or the telewave was the best way to see what was going on. Even a sweep of the line with the anritsu or antenna analyzer through the soft jumper, duplexer, lightning arrestor, and hardline puts all pretty much to bed. Just RF wierdness at the site and within the base stations internals.
SlimBob
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Re: Fun with VSWR

Post by SlimBob »

Bill_G wrote: I'm tempted to buy some single section isolators, put them directly to the rfpa connector, and see what happens.
That sounds best. The isolators should provide excellent isolation from the PA to the antenna and vice-versa.

The above comments about using the Z-matcher really make me wonder about using single and dual circulators, for the above reasons. If I have tuned the isolator with a 50-ohm load on all ports but the transmitter, and I tune that port for a 50-ohm load using the other ports, then any mismatch should be dealt with between the circulator and the transmitter with a Z-match.

That's what I'm thinking, anyway. In a perfect world...
Last edited by SlimBob on Thu Apr 12, 2012 11:38 pm, edited 1 time in total.
Will
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Re: Fun with VSWR

Post by Will »

Note:

Some coax cable(s) are better at 700 800 than others.

Foil shield cables should not be used.
AEC
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Re: Fun with VSWR

Post by AEC »

This problem may have an external solution....

When testing, are any other transmitters up and running at the same time?
What's closest to your system, channel/s, power/frequencies?
You might have coupling issues on top of PA troubles.
Even order, third order products....Adjacent channel IMD/mixing.
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

This problem occurred during individual station testing as part of the ATP. There was only one station keying at a time, and the problem occurred even into a dummy load.

Not to worry - my observations and concerns were poo-poo'd. I was assured numerous times this was normal behavior. Absolutely nothing to worry about. Move along. There's nothing to see here. Pay no attention to those erroneous readings.

This is me not worried.
jhooten
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Re: Fun with VSWR

Post by jhooten »

How much loss are you getting in a few feet of 1/2" superflex with a 1.66 swr?
What is the insertion loss of the Z-Matcher?
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Bill_G
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Re: Fun with VSWR

Post by Bill_G »

I'll have to get back to you. My notes are in the project file.
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