Mounting Antennas

[wolfman005]

Hey All,
My Highschool has some older Shure handheld mics which we took out of commission last year because they were dropping out. We would like to recommision these so we can take the replacement mics for the upcoming Musical.
To do so we need to figure out why they are dropping out. I was wondering if these antennas look like they are positioned correctly. I've never dealt with antennas not permantly mounted to a receiver. I do know that there is no amp/distro. 1 pair of antennas per reciver (2 receivers) If you have any questions I will do my best to find you answers.

 

[BillESC]

Usually antenna should be in a 90 degree V formation and should also not be surrounded by metal.

Try relocating them at the proper angle and if at all possible also line-of-sight.

 

[wolfman005]

I'll have to see if I can get maintenance to do that. If that is all the length of cable we have for the antennas, could we add more without needing an signal amp? I know there is a hit or miss aspect when it comes to needing an amp/distro for this stuff. Sometimes it works without, sometimes it needs one. At least that's what I have gathered from other threads.

 

[Chris15]

Those antennas look like they are probably VHF, which may be a factor in all of this.
The two that are parallel are probably interfering with one another - there does not appear to be an adequate separation between them to not have IMD issues.

What model are these mics that we're discussing?

 

[wolfman005]

Shure L4. Manual states they run 169.445 MHz and 216.100 MHz which as far as I can tell are legal frequencies. That's based on a quick google search. All of those numbers make my head spin.

 

[Repeater]

Shure L4. Manual states they run 169.445 MHz and 216.100 MHz which as far as I can tell are legal frequencies. That's based on a quick google search. All of those numbers make my head spin.

I believe they are still legal.

The antennas in the picture are badly mounted, being parallel and extremely close to the metal supports. They need, at minimum, to be roughly 90 degrees to what they are mounted on. If possible the nearest metal, at VHF, should be several feet away. They individual antennas also need to be separated to avoid interaction between each other and provide diversity so that the receiver antenna port voting system can work properly. At VHF where you are, this would mean probably a minimum of 4-6 feet; more is better.

There should be no IMD issues in your current setup however, since these are receiving, not transmitting.

 

[MikeJ]

Line of sight will be a big help. You can use BNC cable to extent them but it should be 50ohm cable, not the more common 75ohm.

Also you should scan the mics for open frequencies often, if the mics suddenly started having problems dropping out, you are probably getting stomped on by other equipment in the same range.

 

[FMEng]

Those mics use the high VHF TV channels. In St Louis, there are TV stations on channels 8 and 12, so avoid tuning the mics to 179-187, and 203-211.

 

[FMEng]

Line of sight will be a big help. You can use BNC cable to extent them but it should be 50ohm cable, not the more common 75ohm.

Cable, connector, and antenna impedance is irrelevant on receivers. If anything, 75 ohm cable always has slightly less loss compared to 50 ohm cable of a similar diameter.

 

[MikeJ]

Cable, connector, and antenna impedance is irrelevant on receivers. If anything, 75 ohm cable always has slightly less loss compared to 50 ohm cable of a similar diameter.

interesting, Can you provide some references for me, I would like to look into this. In training, I'm pretty sure, Shure recommends 50ohm cable, though I don't know the exact science behind it.

 

[MikeJ]

Also I was thinking UHF, it may be different at lower frequencies, like the VHF the OP has.

 

[FMEng]

interesting, Can you provide some references for me, I would like to look into this. In training, I'm pretty sure, Shure recommends 50ohm cable, though I don't know the exact science behind it.

No references because this is common knowledge among RF engineers. The antennas are not tuned and matched to 50 ohms, and neither are the receiver inputs, therefore the cable doesn't matter. Impedance matching is critical for transmitters because it affects power transfer and heat dissipation. Receivers only care about signal voltage, not power.

 

[wolfman005]

MikeJ- I'd scan for channels if they were able to do that but the cover only has two knobs.

Thanks to all who replied but the powers decided to just bite the bullet now and buy 2 new wireless mics. I'm guessing we will stick with the Telex FMR-500 series as that's what we have in our "performance space". Over the summer or sometime next year they will get a whole new system but I will have graduated then.

Thanks again!

Also just a random though, does anyone know of people who rebuild mics? The choir director, who acts as a tech director thinks that we should have the old L4s rebuilt.

 

[Floobydust]

MikeJ- I'd scan for channels if they were able to do that but the cover only has two knobs.

. . .
Also just a random though, does anyone know of people who rebuild mics? The choir director, who acts as a tech director thinks that we should have the old L4s rebuilt.

See the folks at Microphone Madness: http://microphonemadness.com/

 

[BillESC]

MikeJ
Thanks to all who replied but the powers decided to just bite the bullet now and buy 2 new wireless mics. I'm guessing we will stick with the Telex FMR-500 series as that's what we have in our "performance space". Over the summer or sometime next year they will get a whole new system but I will have graduated then.

That makes almost no sense at all. Both bands of the FMR-500 systems are in the pending FCC auction and could become obsolete shortly. On top of that, the FMR-500H hand held systems MAP at $ 735. Good mics to be sure but better are available for less IMHO.

 

[wolfman005]

Bill, thanks for the catch. Any alternatives you'd suggest?

 

[MikeJ]

No references because this is common knowledge among RF engineers. The antennas are not tuned and matched to 50 ohms, and neither are the receiver inputs, therefore the cable doesn't matter. Impedance matching is critical for transmitters because it affects power transfer and heat dissipation. Receivers only care about signal voltage, not power.

Thanks, But I don't think most of the members here are RF engineers though, so it may not be common knowledge on this forum. I'll look around, I'm sure I can find some information that may be helpful to myself and others.

 

[Les]

The antennas in the picture are badly mounted, being parallel and extremely close to the metal supports.

...and should also not be surrounded by metal.

Just thought I'd point out that this appears to be stained 2x4 bracing they are mounted to.

 

[BillESC]

Wolfman,

I'm fond of Audio Technica's ATW-3000 series. I've installed hundreds of them in schools, theaters and churches and service issues are next to none.

 

[wolfman005]

I'll run that by the powers.
Les, you are right.
Today I also remembered there is a Wifi router right next to the rack.