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Author Topic: Getting Classic Intermod Interference Despite IAS Coordination, RF scan, etc.  (Read 5489 times)

Bob Charest

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Hi all,

I've sent a support request to PWS, and I'm sure they will respond pretty quickly as they have in the past, but I also thought I might get some input from those on the ProSoundWeb forum.

We use PWS IAS for our RF coordination. We play mostly in Maine where the RF environment is not overly complicated (now that we've vacated the 600Mhz range.)

We have Shure ULXD (G50 band – 470-534Mhz) and Sennheiser G2 (A band – 518-554Mhz) for our In Ear Monitors, one EW365 mic, and one SKP100.

We are using the Shure supplied ½ wave antennas with the ULXD4 receivers. We are using the Sennheiser ¼ wave antennas for the EW365 & the SKP100.

For our IEM setup, the IEM transmitters feed a Shure PA821A. The output from the PA821A feeds a PWS helical antenna.

We work on fairly small stages (24’ across, 12’ deep) usually. When my singer is center stage, he experiences classic intermod interference (whoosh with a pronounced thump at the end.) When I am at center stage I have the same thing happen to me. It is not always in the exact same place, but is quite common. I’ve used my TTi PSA1301T scanner to ensure that no frequencies not accounted for are present. When this interference occurs, there is no activity on the meters at the board, so this suggests that whatever is happening is occurring after the IEM mixes are sent to the transmitters.

In the past, I’ve used a user-defined Bank/Channel model for our EW365 mics, the Evol G2 IEM devices, and the SKP100. When we first got the ULXD equipment, I also created a user-defined Group/Channel model. I had some problems with that model (couldn’t create a simple coordination with just a small number of Shure devices) and corresponded with Jason Eskew about it. I ended up just using the IAS supplied model for a while after that.

I’ve since created another user-defined model for the ULXD gear, copying the IAS supplied model, and just inserting Group/Channel comments according to the Shure documentation.

Today, after having had interference issue on the past two gigs, I created a new coordination using only the IAS supplied models, and got quite different coordination results. I haven’t yet been able to test the results in the field.

This might be a silly question, but, has anyone here experienced that the algorithm for calculating intermod could be affected by using user-defined models?

I examined the properties (bandwidth, 3rd’s Spacing, 3 TX 3rd’s, 5th’s Spacing) for both the IAS-supplied models and the user-defined models, and they are identical.

I’ve sent PWS support the IAS coordinations using my user-defined models and the IAS-supplied models and also my user-defined models.

I’m going to be unloading the van to get at the PWS helical antenna so I can check for any possible internal damage. We take very good care of our equipment, but one never knows.

All input welcome, as things should be really squared away but appear not to be.

Best regards,
Bob Charest


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Bob Charest

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...This might be a silly question, but, has anyone here experienced that the algorithm for calculating intermod could be affected by using user-defined models?...

Forget about this question. I just tried doing an intermod calculation using the frequencies that are part of the Sennheiser Bank/Channel & Shure Group/Channel models I'd created. I got the exact results that I got when using my user-defined models.

It must be something else... :-(
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Henry Cohen

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. . . When my singer is center stage, he experiences classic intermod interference (whoosh with a pronounced thump at the end.) When I am at center stage I have the same thing happen to me. It is not always in the exact same place, but is quite common. I’ve used my TTi PSA1301T scanner to ensure that no frequencies not accounted for are present. When this interference occurs, there is no activity on the meters at the board, so this suggests that whatever is happening is occurring after the IEM mixes are sent to the transmitters.

Actually, that sounds more like weak RF and the FM receiver recapturing the carrier. Check all the coax, including interconnect jumpers and the antenna(s) for integrity and ensure your coax losses are not so great that insufficient RF energy is getting to the antenna.


Quote
This might be a silly question, but, has anyone here experienced that the algorithm for calculating intermod could be affected by using user-defined models?

The "algorithm" is the basic addition and subtraction of frequencies:
2A-B
2A-C
2B-A
2B-C
2C-A
2C-B
A+B-C
A+C-B
C+B-A
. . . ad infinitum

This math function is not affected by the frequency spacing chosen for IM to frequency, or frequency to frequency spacing. But the results are certainly affected.


Quote
I’m going to be unloading the van to get at the PWS helical antenna so I can check for any possible internal damage. We take very good care of our equipment, but one never knows.

Some folk tend to use the internal tube of the helical as storage for soft items; this should be avoided. Changing the geometry of the tuning stub or tearing/deforming the copper tape element can render the antenna virtually useless.
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Henry Cohen

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Scott Helmke

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The Shure PA821A is known to be rather un-Shurelike as far as reliability goes, and I've seen images here on PSW of much weird intermod from a bad combiner. You might try using your TTi to verify that all the inputs on the PA821A are indeed working correctly.
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Paul McDermott

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We have Shure ULXD (G50 band – 470-534Mhz) and Sennheiser G2 (A band – 518-554Mhz) for our In Ear Monitors, one EW365 mic, and one SKP100.

We are using the Shure supplied ½ wave antennas with the ULXD4 receivers. We are using the Sennheiser ¼ wave antennas for the EW365 & the SKP100.

For our IEM setup, the IEM transmitters feed a Shure PA821A. The output from the PA821A feeds a PWS helical antenna.

One suggestion on your coordination. Your IEMs are the devices most likely to create IMDs, try to pack all 8 G2s in 1 6Mhz TV channel to keep the impact of the generated IMDs to a minimum amount of spectral band. This can be done in IAS early just pick frequencies that are located in on TV channel. Then keep you microphone transmitters at least 12 Mhz away from your IEMs this will make a big difference.

Another suggestion is on stages that small you can transmit at low power mode with your IEMs, you already have extra gain from your helical antenna. Keep your Tx and Rx antennas at least 10 apart and put your helical closer to the stage to get your receive antennas behind all that forward gain on your IEM antenna.

Next time you are out with this rig take a scan from the pass through port on the ULXD receivers to check for IMDs generated by the IEMs. Use your scanner to understand when IMDs are created and adapt your setup accordingly. The more devices you try to coordinate the more important it is to not only use software to avoid IMDs but to use practices and procedures that minimize the creation of them.

Good luck.

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Bob Charest

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Actually, that sounds more like weak RF and the FM receiver recapturing the carrier. Check all the coax, including interconnect jumpers and the antenna(s) for integrity and ensure your coax losses are not so great that insufficient RF energy is getting to the antenna.

Will do... Scans done recently show good levels, but this weekend, I will have a bit more time between setup and performance.

The "algorithm" is the basic addition and subtraction of frequencies:
2A-B
2A-C
2B-A
2B-C
2C-A
2C-B
A+B-C
A+C-B
C+B-A
. . . ad infinitum

This math function is not affected by the frequency spacing chosen for IM to frequency, or frequency to frequency spacing. But the results are certainly affected.

Of course, poor wording on my part :-)

Some folk tend to use the internal tube of the helical as storage for soft items; this should be avoided. Changing the geometry of the tuning stub or tearing/deforming the copper tape element can render the antenna virtually useless.

The PWS domed helical is fortunately protected from that sort of thing. I did take it apart and inspected it to verify that nothing was amiss.

(As always) Thank you, Henry.

Best regards,
Bob
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Bob Charest

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The Shure PA821A is known to be rather un-Shurelike as far as reliability goes, and I've seen images here on PSW of much weird intermod from a bad combiner. You might try using your TTi to verify that all the inputs on the PA821A are indeed working correctly.

Yes, Scott. I followed that fairly recent thread on the forum. I will be doing exactly what you've suggested this weekend.

Thank you!

Best regards,
Bob Charest
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Bob Charest

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One suggestion on your coordination. Your IEMs are the devices most likely to create IMDs, try to pack all 8 G2s in 1 6Mhz TV channel to keep the impact of the generated IMDs to a minimum amount of spectral band. This can be done in IAS early just pick frequencies that are located in on TV channel. Then keep you microphone transmitters at least 12 Mhz away from your IEMs this will make a big difference.

I will give that a try - our highest frequency in use for the ULXD devices was 492.525, with the lowest Sennheiser frequency at 518.850, so 26.325 Nhz distance between.

Another suggestion is on stages that small you can transmit at low power mode with your IEMs, you already have extra gain from your helical antenna. Keep your Tx and Rx antennas at least 10 apart and put your helical closer to the stage to get your receive antennas behind all that forward gain on your IEM antenna.

Our Sennheiser IEM transmitters are the SR300 not SR350's, and I don't see anything on the back (like the SR350 switch) or in the menus that let me get at that. The SR300 manual specifications sections states that it transmits at 20mW internally adjustable to 10mW. In any case, they are feeding a Shure PA821A combiner before the signals get to the PWS helical antenna. I've looked at my PA821A manual and searched online but don't see transmission output power listed anywhere, so that's a bit inconvenient. The PWS helical specs state 9-11 dBi (on axis).

Next time you are out with this rig take a scan from the pass through port on the ULXD receivers to check for IMDs generated by the IEMs. Use your scanner to understand when IMDs are created and adapt your setup accordingly. The more devices you try to coordinate the more important it is to not only use software to avoid IMDs but to use practices and procedures that minimize the creation of them.

Good luck.

Thanks, Paul - I'll be using WWB to scan when we next set up. That and a thorough check of all coax (again!) should help localize the problem. I've attached a graphic showing location of our antennas, so please let me know if that seems OK.

I really appreciate your suggestions!

Best regards,
Bob Charest
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Pete Erskine

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What IM options did you use?

For IEMS in a combiner use 3rd, 3tx and 5th order with a maximum IM window at 50MHz.  Use the standard models and do not use any manufacturer table entries, use all custom frequencies.  do all the IEM freqs first.

for wireless mics just use 3rd and 5th order not 3tx.

Band plan and put your iems in one spectrum area and wireless mics in another, ideally separated by a tv channel.  do not intermingle them.

Mic antennas should be at least 15' from any TX antenna and 6' to the nearest other RX antenna.  use low power for the IEM, especially if your coverage area is less than the stage area.

after setting your frequencies, war game the entire set.  Turn on everything with mics at least 6' apart spread out on the stage.  Turn of each tx and with a spectrum analyzer look closely at the freq for any underlying carriers and check that the RX rf indicators show no rf.  If they do change that freq and start the entire wargame again.
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Paul McDermott

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I will give that a try - our highest frequency in use for the ULXD devices was 492.525, with the lowest Sennheiser frequency at 518.850, so 26.325 Nhz distance between.


Hey Bob,
Glad I was of some help.

What is the highest frequency in use on your IEM’s?  the closer you pack your IEMs together the smaller the spectral footprint of the IMD they create. There is also no reason no to move the farther away like above 540Mhz. With just a few devices you can be more conservative with your coordination.

Our Sennheiser IEM transmitters are the SR300 not SR350's, and I don't see anything on the back (like the SR350 switch) or in the menus that let me get at that. The SR300 manual specifications sections states that it transmits at 20mW internally adjustable to 10mW. In any case, they are feeding a Shure PA821A combiner before the signals get to the PWS helical antenna. I've looked at my PA821A manual and searched online but don't see transmission output power listed anywhere, so that's a bit inconvenient. The PWS helical specs state 9-11 dBi (on axis).

The Power Setting on a G3 is in the menu under Advanced/RF Power. I am not sure about the G2 but the difference between 10mW and 20 mW in only 3dB. I wouldn’t worry about it if you have to open the units up.

The PA821 does not add any gain.

To troubleshoot or if the venue is to tight to get distance between your Tx and Rx antennas you could put a 1/2 wave whip on your PA821A and see if that helps.

Thanks, Paul - I'll be using WWB to scan when we next set up. That and a thorough check of all coax (again!) should help localize the problem. I've attached a graphic showing location of our antennas, so please let me know if that seems OK.

I really appreciate your suggestions!

Best regards,
Bob Charest

If you are using WWB you can use inclusion groups to place your devices in the exact spectral band you want. There is a link in the thread “Wireless Workbench How to do everything videos”, watch the one on inclusion groups.

As a rule of thumb you should have your Tx and Tx antennas  at least10 feet apart. You can also position the Helical a little more on stage to keep your Rx antennas out of the coverage of the “hot” part of the Tx antenna.

I will be performing some testing of the impact of Tx antennas on the creation of IMD in antenna splitters later this week. It will also cover how the distance and position between antennas, Power of transmitters, and the third order intercept point affect the IMD creation.
When I publish the results on my website I will post to this thread.
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