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Author Topic: RF Overload Situation  (Read 3403 times)

Curt Sorensen

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RF Overload Situation
« on: March 22, 2018, 06:20:20 pm »

Hi,
We recently did a production at a venue using four of their Shure handheld transmitters with ULXD4Q receivers and two of our Lectrosonics IFBT4 systems using SNA600 Antennas. The mics were in the 400s, the IFBs in the 600s, coordinated with a scan into WWB. They had a new Shure Antenna DA, probably a UA844+SWB getting signal from conventional Shure paddles, reportedly with no gain added. Iíd estimate 10 feet between the antenna systems. When our IFBs were transmitting, the DA indicated RF overloads. I canít find any info on the level that would overload those DAs. To complicate matters, the house tech had connected the DA outputs to the receiversí Cascade outs. Fortunately we got through the event, as that mistake was noticed afterwards. A recommendation was made that we attenuate the IFBT4s by 3dB and maintain 15 feet of separation between the systemsí antennas, but that person was unaware of the Cascade connections. This is an annual event, and it is planned that we will re-block our IFB systems to vacate the 600MHz-and-up band. Due to the re-pack, we are considering staying below 500MHz as well, however that moves us into the band where the venueís wireless mics live.

So my questions are:

How likely is it that the the mis-connection of the DAs to the Cascade outputs would contribute to the overloads?

Should we consider attenuating the IFBT4s anyway? They are rated at 250mW output. It seems unlikely a Lectrosonics device would be too powerful, but I donít know.

Any advice welcome, and thanks, as always.
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Curt Sorensen
Madison, Wisconsin

Ike Zimbel

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Re: RF Overload Situation
« Reply #1 on: March 22, 2018, 08:08:19 pm »

Hi,
We recently did a production at a venue using four of their Shure handheld transmitters with ULXD4Q receivers and two of our Lectrosonics IFBT4 systems using SNA600 Antennas. The mics were in the 400s, the IFBs in the 600s, coordinated with a scan into WWB. They had a new Shure Antenna DA, probably a UA844+SWB getting signal from conventional Shure paddles, reportedly with no gain added. Iíd estimate 10 feet between the antenna systems. When our IFBs were transmitting, the DA indicated RF overloads. I canít find any info on the level that would overload those DAs. To complicate matters, the house tech had connected the DA outputs to the receiversí Cascade outs. Fortunately we got through the event, as that mistake was noticed afterwards. A recommendation was made that we attenuate the IFBT4s by 3dB and maintain 15 feet of separation between the systemsí antennas, but that person was unaware of the Cascade connections. This is an annual event, and it is planned that we will re-block our IFB systems to vacate the 600MHz-and-up band. Due to the re-pack, we are considering staying below 500MHz as well, however that moves us into the band where the venueís wireless mics live.

So my questions are:

How likely is it that the the mis-connection of the DAs to the Cascade outputs would contribute to the overloads?

Should we consider attenuating the IFBT4s anyway? They are rated at 250mW output. It seems unlikely a Lectrosonics device would be too powerful, but I donít know.

Any advice welcome, and thanks, as always.
Hi Curt,
It is unlikely that the cascade connection mix up had anything to do with it, as those are on the output side of the DA.
The IFBT4 is just about the most RF power (short of walkie talkies) that you will encounter on any gig. Not only should they be separated from other RF gear by at least 15 feet, they should also be separated from each other by several feet. If their antennas are in close proximity, they become great intermod generators.
Going forward, you might want to look at the VHF version of the IFBT4 as they are; a) in the VHF band, and b) limited to 50mw output (by FCC regs) so should play better with the other kids. If you stay in the UHF band, there is also an internal jumper setting (actually a solder bridge on some very closely spaced pads, a bit tricky to do) that can lower the output to 100 or 50mw. If you are using them indoors, and your talent is not wandering all over the building, the lower settings should work just fine. And finally, I believe you actually are supposed to have a Part 74 license to operate at 250mw anyway.
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~Ike Zimbel~
Wireless frequency coordination specialist.
Manufacturer's Representative (Canada)
Alteros Inc
Radio Active Designs
~416-720-0887~
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Henry Cohen

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Re: RF Overload Situation
« Reply #2 on: March 23, 2018, 11:07:21 am »

How likely is it that the the mis-connection of the DAs to the Cascade outputs would contribute to the overloads?

Possible damage: The cascade out is fed by an LNA and reverse power of sufficient level could damage it. Easy enough to test. Surprised it actually functioned at all.


Quote
Should we consider attenuating the IFBT4s anyway? They are rated at 250mW output. It seems unlikely a Lectrosonics device would be too powerful, but I donít know.

Absolutely. 250mW (+24dBm) is far too much power for a confined area. These should be attenuated by at least 6dB, resulting in about 62mW (+18dBm) RF power. Better yet, if the performance area will be a normal stage size, take two 3dB attenuators (one on each IFBT4 output) and connect to a 2-way splitter/combiner as a combiner rated for at least 500mW dissipation (not power handling); the result will be a very reasonbale and effective 55-60mW per carrier and one antenna. Still keep the TX and RX antennas spacially separated by at least 8'-10'.
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Henry Cohen

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Henry Cohen

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Re: RF Overload Situation
« Reply #3 on: March 23, 2018, 11:11:41 am »

If their antennas are in close proximity, they become great intermod generators.

Actually, the T1 and T4 models have isolators built in, so IM production is reduced significantly.


Quote
Going forward, you might want to look at the VHF version of the IFBT4 as they are; a) in the VHF band, and b) limited to 50mw output (by FCC regs) so should play better with the other kids. If you stay in the UHF band, there is also an internal jumper setting (actually a solder bridge on some very closely spaced pads, a bit tricky to do) that can lower the output to 100 or 50mw. If you are using them indoors, and your talent is not wandering all over the building, the lower settings should work just fine.

Much easier to add inline attenuators on the BNC connector.


Quote
And finally, I believe you actually are supposed to have a Part 74 license to operate at 250mw anyway.

Correct.
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Henry Cohen

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Radio Active Designs   www.radioactiverf.com

Jerome Malsack

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Re: RF Overload Situation
« Reply #4 on: March 23, 2018, 11:13:06 am »

is that 8 to 10 feet or 8 to 10 inch.  my understanding 8' to 10' is feet and 8" to 10" is inch.
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Henry Cohen

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Re: RF Overload Situation
« Reply #5 on: March 23, 2018, 11:38:25 am »

is that 8 to 10 feet or 8 to 10 inch.  my understanding 8' to 10' is feet and 8" to 10" is inch.

8 to 10 feet, as written.
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Henry Cohen

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Curt Sorensen

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Re: RF Overload Situation
« Reply #6 on: January 28, 2019, 05:41:41 pm »

Henry Cohen wrote:
Better yet, if the performance area will be a normal stage size, take two 3dB attenuators (one on each IFBT4 output) and connect to a 2-way splitter/combiner as a combiner rated for at least 500mW dissipation (not power handling); the result will be a very reasonable and effective 55-60mW per carrier and one antenna. Still keep the TX and RX antennas spatially separated by at least 8'-10'.

I'm revisiting this thread as I'm back soon to an annual event that began my original post.

First, I'm having a hard time finding the 500mW dissipation spec while shopping for a proper combiner, anyone able to help with this?

Second, as a broadcaster I believe we're legal to use higher power transmitters under a part 74 license, but I haven't found whether we need to do some additional paperwork or if we're good to go under our existing license. That said I'm not planning to use higher power unless it's appropriate.

Thanks to any and all,
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Curt Sorensen
Madison, Wisconsin

Ike Zimbel

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Re: RF Overload Situation
« Reply #7 on: January 28, 2019, 08:26:27 pm »

Henry Cohen wrote:
Better yet, if the performance area will be a normal stage size, take two 3dB attenuators (one on each IFBT4 output) and connect to a 2-way splitter/combiner as a combiner rated for at least 500mW dissipation (not power handling); the result will be a very reasonable and effective 55-60mW per carrier and one antenna. Still keep the TX and RX antennas spatially separated by at least 8'-10'.

I'm revisiting this thread as I'm back soon to an annual event that began my original post.

First, I'm having a hard time finding the 500mW dissipation spec while shopping for a proper combiner, anyone able to help with this?

Second, as a broadcaster I believe we're legal to use higher power transmitters under a part 74 license, but I haven't found whether we need to do some additional paperwork or if we're good to go under our existing license. That said I'm not planning to use higher power unless it's appropriate.

Thanks to any and all,
I think it's looking for the minimum spec that's a problem. This one, from PWShttp://www.professionalwireless.com/product/2-way-high-power-splittercombiner/ or this one https://www.minicircuits.com/WebStore/modelSearch.html?model=ZSC-2-4%2B from Mini-Circuits will work.
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~Ike Zimbel~
Wireless frequency coordination specialist.
Manufacturer's Representative (Canada)
Alteros Inc
Radio Active Designs
~416-720-0887~
ca.linkedin.com/pub/ike-zimbel/48/aa1/266

Henry Cohen

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Re: RF Overload Situation
« Reply #8 on: January 28, 2019, 08:27:42 pm »

First, I'm having a hard time finding the 500mW dissipation spec while shopping for a proper combiner, anyone able to help with this?

https://www.minicircuits.com/WebStore/dashboard.html?model=ZAPD-900-5W-N%2B


Quote
Second, as a broadcaster I believe we're legal to use higher power transmitters under a part 74 license, but I haven't found whether we need to do some additional paperwork or if we're good to go under our existing license. That said I'm not planning to use higher power unless it's appropriate.

If you're a licensed Part 73 broadcaster, you're permitted to operate up to 720 hours annually without actually having a Part 74 license. Certain restrictions apply: ß74.24 - Short-term operation
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Henry Cohen

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Curt Sorensen

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Re: RF Overload Situation
« Reply #9 on: January 29, 2019, 04:19:34 pm »

Hi,

Sincere thanks for the assistance.
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Curt Sorensen
Madison, Wisconsin
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