RF

[Brad Harris]

So I normally use anywhere from 4-24+ channels of wireless (mics, IEM, IFB, comm) on a regular basis.

I rarely have issues with setting it up, scanning and coordinating, but once every few dozen shows, things don't play nicely together while setting things up.



Case in point, last weekend, 7ch of wireless, 6 UHFR (with an antenna distro, 1/2 whip antenna), 1 G3IEM (separate rack with 1/4 whip antenna).

Nothing major or of concern picking up on the SA (no DTV, just some TV lower out of band and no other RF in use - in the middle of the mountains), and nothing picking up on the hardware scans from the UR.

As I start syncing and turning on Tx (using G/Ch frequencies), more and more Rx Hash starts popping up on the meters, and audiable noise on Rx that don't have a Tx from the few Tx turned on while syncing.

While testing, 2 of the Tx consistently caused more interference than others, even when changing frequencies and when looking at the SA with just those Tx on, nothing seemed to seem out of the ordinary.


This was one of the 'worst' in recent memory, but everything worked flawlessly the whole day, just when all the Tx were in close proximity to each other and the Antennas things didn't seem right. ** I find the Shure G/Ch lists are pretty rock solid unless there is outside interference **


Any thoughts/tips/tricks to isolate Tx a bit more realistically while setting up and verifying (War Gaming) everything?


BRad

 

[Steve Milner]

Re: RF

... just when all the Tx were in close proximity to each other and the Antennas things didn't seem right.

BRad

I'm sure others will chime in here with more questions about your setup and settings, what type of Intermodulation coordination you have done etc. The short answer that I will offer to the line I quoted above from your OP is.... wait for it.... you're gonna have that issue. If you pile up a bunch of transmitters together, especially in close proximity to the Antenna, things will tend to go a bit sideways.

Hopefully more folks will comment though.

 

[Ben Lawrence]

Re: RF

bump up the squelch a notch if you are in a jam to at least eliminate the audio bleed.

 

[Cameron Stuckey]

Re: RF

Any thoughts/tips/tricks to isolate Tx a bit more realistically while setting up and verifying (War Gaming) everything?

As mentioned above, multiple transmitters in close proximity to the antennas(receivers) will cause the noise floor to raise and stronger intermodulation products to occur. When setting up I've become a big fan of foil bread pans for each transmitter while waiting on my staging table. It isn't a magic bullet solution, but certainly helps attenuate some intermodulation products. I first learned about the bread tins from James Stoffo who also suggested getting an insulator of some kind to keep the transmitter from directly touching the foil. Apparently it will preserve about an hour of battery life that disappears when touching the foil. But I haven't been able to verify that myself.

 

[Jason Glass]

Re: RF

As mentioned above, multiple transmitters in close proximity to the antennas(receivers) will cause the noise floor to raise and stronger intermodulation products to occur. When setting up I've become a big fan of foil bread pans for each transmitter while waiting on my staging table. It isn't a magic bullet solution, but certainly helps attenuate some intermodulation products...

+1 on that.

I gave a seminar at Belmont University on Saturday, with an emphasis on employing SA for frequency coordination on gig day. I did a live demonstration of intermod products displayed on the SA, using a pair of UR1 TX set up to intentionally IM. Placing the TX's into tins side-by-side attenuted IM by more than 30dB, down into the noise floor. You could hear a collective "aah" from the students as they watched the change on the SA display and the concept sunk in.

I had never heard about the battery life issue with the tins before. Thanks for that!

 

[Brad Harris]

Re: RF

I'm sure others will chime in here with more questions about your setup and settings, what type of Intermodulation coordination you have done etc. The short answer that I will offer to the line I quoted above from your OP is.... wait for it.... you're gonna have that issue. If you pile up a bunch of transmitters together, especially in close proximity to the Antenna, things will tend to go a bit sideways.

Hopefully more folks will comment though.

~:-o~:lol: I know that, but why does it only happen every few weeks, and not every day then?

I can set up 12+ Channels worth of UR Tx and IEM, comm, all within inches of each other, a few feet from the recievers and have virtually no RF hash on the Rx receivers and no audio interference, but some rare days, it's nothing but dancing chistmas tree lights going on (like last weekend with only 6 channels).

My usual method of setting up Wireless is as follows:

- Do some scans in various locations on site with my SAs (multiple)
- Determine all wireless equipment on site (Comm is usually another independent supplier)
- Have wireless comm guys set up their station (99% of the time it is out of band of what I'm using, and is usually the least flexible system)
- Network multiple networkable systems
- Start doing hardware scans on my largest wireless system (some days it is IEM, others Mics)
- Program that system and check for RF Hash (War Game)
- Start on my next largest wireless system .....
- Rinse wash repeat

On days where there is over 24+ channels of wireless, I usually crack out my version of IAS, import the scans from step 1 and let it do its magic instead of having the hardware do it, but for under 24 channels, I find the hardware is faster and more reliable.



Jason and Cameron, foil pans as in the cheap, bake once and throw away tins, or the sheet metal pans?


Thanks again,

BRad

 

[Cameron Stuckey]

Re: RF

These things:
1 lb. Foil Bread Loaf Pan 500/CS

Don't expect them to be made out of steel and you can get dozens of uses out of a single tin.

 

[Jason Glass]

Re: RF

~:-o~:lol: I know that, but why does it only happen every few weeks, and not every day then?

I can set up 12+ Channels worth of UR Tx and IEM, comm, all within inches of each other, a few feet from the recievers and have virtually no RF hash on the Rx receivers and no audio interference, but some rare days, it's nothing but dancing chistmas tree lights going on (like last weekend with only 6 channels).

It might be because on those days, local TV could be strong enough to intermod with your TX and your software is only calculating intermods between your TX units.

+1 on Cameron's tins. Thin aluminum is perfect. Dollar General and other grocery stores have them in smaller quantities for dirt cheap.

 

[john sulek]

Re: RF

So I normally use anywhere from 4-24+ channels of wireless (mics, IEM, IFB, comm) on a regular basis.

I rarely have issues with setting it up, scanning and coordinating, but once every few dozen shows, things don't play nicely together while setting things up.



Case in point, last weekend, 7ch of wireless, 6 UHFR (with an antenna distro, 1/2 whip antenna), 1 G3IEM (separate rack with 1/4 whip antenna).

Nothing major or of concern picking up on the SA (no DTV, just some TV lower out of band and no other RF in use - in the middle of the mountains), and nothing picking up on the hardware scans from the UR.

As I start syncing and turning on Tx (using G/Ch frequencies), more and more Rx Hash starts popping up on the meters, and audiable noise on Rx that don't have a Tx from the few Tx turned on while syncing.

While testing, 2 of the Tx consistently caused more interference than others, even when changing frequencies and when looking at the SA with just those Tx on, nothing seemed to seem out of the ordinary.


This was one of the 'worst' in recent memory, but everything worked flawlessly the whole day, just when all the Tx were in close proximity to each other and the Antennas things didn't seem right. ** I find the Shure G/Ch lists are pretty rock solid unless there is outside interference **


Any thoughts/tips/tricks to isolate Tx a bit more realistically while setting up and verifying (War Gaming) everything?


BRad

The hardware scan on different gear is just looking for open spectrum for that unit's preprogrammed frequency list but it is not factoring in any intermod between those frequencies and other equipment. Sometimes it may work great, other times you get a combination of frequencies that doný play as well together.

You might also be overlaoding the rf input of one of the Rx units which will produce even more spurious rf. This could be the extra hash you are seeing.

 

[Henry Cohen]

Re: RF

Several folk have correctly pointed out the issue of having multiple transmitters very close to one another, while possibly too close to one or more of the RX antenna(s), will result in the anomalies you've experienced, and to use foil pans to reduce mixing in the TX power amplifier sections as well as IF and LO mixing. (Remember to make sure the pans don't touch each other or have some other physical barrier.)

The other observation I would make is what was the antenna used for your SA? Ideally, you'd like to connect the SA to an output port on the same antenna distribution feeding the wireless receivers so it sees what the receivers see. The front end of the SA will then see the IM products created in the antenna distribution as well as create the identical ones additionally created in the receiver front ends (unless you're fortunate enough to have a laboratory grade SA with IM suppression >100dB).

 

[Brad Harris]

The other observation I would make is what was the antenna used for your SA? Ideally, you'd like to connect the SA to an output port on the same antenna distribution feeding the wireless receivers so it sees what the receivers see. The front end of the SA will then see the IM products created in the antenna distribution as well as create the identical ones additionally created in the receiver front ends (unless you're fortunate enough to have a laboratory grade SA with IM suppression >100dB).

Hi Henry,

The SA, I usually grab whatever antenna is sent with the wireless package (1/2 whip, LDPA, etc). I do have a small collection of antennas at the ready if one isn't available.

I never quite put the SA into the distro practice, into practice, but that will be added into my workflow when possible.

Just briefly remembering that those days with having more IM problems setting up than not (again, setting up, Tx and Rx in too close of proximity), were days with UA845-SWB splitters, so maybe there is too much gain with that unit than without in the system.

Antennas range from 1/2 whip off the front face of the 845, 874 active LPDA, or 1/2 whip mounted on 830, using 25-50ft lengths of FLL400 cable, depending on RF world and proximity to the performance area.

 

[Brad Harris]

Well, it just so happens that 99% of the equipment on the last 2 shows that I noticed this on was in my local warehouse (even though those shows came out from 2 different locations).

The pic below says everything to me. 2 sets of Rx, the left through a UA845-SWB with 1/2 whips on the front pannel, while the rack on the right is just 1/2 whips off of thier front pannels.

Each rack is set identically.

Only 4 Tx is turned on at the moment (top 2 Rx) while the bottom Rx is showing some hash (~20dBu more) with the set of Rx fed through the UA 845.

What this means (to me), is now when I have the 845, I'm just going to have to use WWB to see the Rx meters from a different location away from the antennas when checking coordinations.

So new question, would that be a case of reflections or something similar with the Rx cables and the UA845? I remember a thread a while back talking about RF Isolators and the like, would this be an application to help minimize the effects of what is going on?

 

[Henry Cohen]

Re: RF

. . . while the bottom Rx is showing some hash (~20dBu more) with the set of Rx fed through the UA 845.

dbuV or dBuW? If the former, 20 does not look right according to the RF meters on the left side channels 5 & 6. If the latter, 20 seems a bit low, but okay.

What this means (to me), is now when I have the 845, I'm just going to have to use WWB to see the Rx meters from a different location away from the antennas when checking coordinations.

So new question, would that be a case of reflections or something similar with the Rx cables and the UA845? I remember a thread a while back talking about RF Isolators and the like, would this be an application to help minimize the effects of what is going on?

First, remember that the antenna distribution unit is a non-linear device and therefore is a[n additional] source of IM products. This does appear to be an issue of inadequate or improper frequency coordination.

Second, try remoting the RX antennas so they're in free field and not next to electronics.

How close were the transmitters to each other?

RF isolators are used only in transmission systems, not for receive.

 

[Jens Bacher]

Re: RF

Just another example that you would often get a healthier system with a pair of passive splitters like http://217.34.103.131/pdfs/ZFSC-3-1W+.pdf

If you only use 6-12 systems you could calculate you own frequencies with more roboustness/ "IM room" than the stock Shure groups, which are a compromise between roboustness and max. transmitters.

The isolators would only work if you could build them in to the transmitters. They work by acting as one way "valve" to avoid RF entering though the antenna of an transmitter, and creating IM products in the RF stages in the transmitter. In highpower systems (think FM broadcast) they also avoid the RF amps in the transmitters from destroying themself if the antenna is disconnected.

 

[john sulek]

Re: RF

What this means (to me), is now when I have the 845, I'm just going to have to use WWB to see the Rx meters from a different location away from the antennas when checking coordinations.

QUOTE]

First off, my apologies if I am misunderstanding your meaning but here goes....

If you use WWB to remotely monitor these racks you will see the same Rx meter display as on the actual receivers. WWB is getting all it's info from the receivers and redisplaying that in a different format. I use WWB mostly to do just this on larger shows where it is easier to look at one screen vs a whole rack of receivers.

What the meters on the bottom left receiver are telling you is that you have some intermod frequencies (caused by any of the reasons already stated above by others) landing very close to the frequency these recievers are tuned too or that one of your transmitters is tuned very close to the frequnecy of these receivers. As in within 150Khz if the transmitter is close to the rx.

If you "war game" this setup by turning off one transmitter at a time that should help narrow down the culprit.

 

[john sulek]

Re: RF

One other thought... When you do the initial scan with your SA, are you looking at a wideband sweep or are you stepping through the spectrum in smaller spans?
A lot of things are hard to see or pinpoint on a large span but show up nicely when looking at a smaller slice of spectrum.
There may be some local rf that is not showing up on a wideband sweep and not being accounted for which would throw your coordination results off.

 

[john sulek]

Re: RF

What this means (to me), is now when I have the 845, I'm just going to have to use WWB to see the Rx meters from a different location away from the antennas when checking coordinations.

QUOTE]

First off, my apologies if I am misunderstanding your meaning but here goes....

If you use WWB to remotely monitor these racks you will see the same Rx meter display as on the actual receivers. WWB is getting all it's info from the receivers and redisplaying that in a different format. I use WWB mostly to do just this on larger shows where it is easier to look at one screen vs a whole rack of receivers.

What the meters on the bottom right receiver are telling you is that you have some intermod frequencies (caused by any of the reasons already stated above by others) landing very close to the frequency these recievers are tuned too or that one of your transmitters is tuned very close to the frequnecy of these receivers. As in within 150Khz if the transmitter is close to the rx.

If you "war game" this setup by turning off one trasmitter at a time that should help narrow down the culprit.

 

[Brad Harris]

One other thought... When you do the initial scan with your SA, are you looking at a wideband sweep or are you stepping through the spectrum in smaller spans?
A lot of things are hard to see or pinpoint on a large span but show up nicely when looking at a smaller slice of spectrum.
There may be some local rf that is not showing up on a wideband sweep and not being accounted for which would throw your coordination results off.

John,

I generally do a wideband just to see what the overall picture is, and then look through it in 12MHz spans below, including, and above my operating area.

As for WWB, just to see the meters when I can't see the actual recievers.

The scans I import are in 25kHz steps into WWB or IAS (havent tried smaller into IAS, but WWB wont support non integer steps).

It was the 4th Tx that added in the noise (interacting mainly with the 3rd Tx) that the last 2 Rx were seeing. It was less apparent on the non distro rack than the rack with the distro, which was my original question (why some days had more rf noise than others).

Tx were approx 8 ft away from both racks, and about 5-6in from each other (no change moving the Tx closer to one rack than the other due to offsets)