Radio Mic receiver antenna

[sleah]

The topic that mentions placement of receivers reminded me of something I've been meaning to ask the collective....

 

I have a 12way L6 2.4ghz digital system and an 8way Trantec UHF 4.16 ch68/69/70 system that get used for school productions.

Last year, after having reception issues with the racks and antenna at the back of the hall (behind the audience) I though I'd try getting the antenna closer to the stage.

 

I bought some 30m bnc cables and ran them from rack position up in to the lighting grid so the antenna were above the stage.

With the L6 system it seemed to make no difference. As always they were perfect during rehearsals and started getting a bit twichy once an audience who refuse to switch phones off are in.

With the UHF system it was worse! The signal strength flickered between 3 and the full 4 leds. If I unplugged the cables from the rack it made no difference at all!

 

I was expecting full signal strength with the antenna connected and them to drop right down to one or two leds if I unplugged.

The UHF antenna have the little inline amps which form part of the mount for the antenna so they are at the antenna end, they were lit up as expected. The L6 paddles were also lit up.

 

Any thoughts on why they didn't work? Keeping the antenna at the back on shorter cables is more reliable, even with the occasional drop-out.

The cables look fine, they are undamaged. They were relatively cheap but the cable looks pretty much the same as the antenna cables supplied with the racks.

 

Confused!

[mar]

I can't comment on the UHF systems but I have a rack of Line 6 mics.

 

What sort of antenna's are you using? The little sticks that come with the mics?

I have a pair of P180 antenna (link) which have an adjustable gain to allow for longer cable lengths and are more directional. I have them mounted above the front of the audience and never seem to have any signal issues, even reaching backstage normally.

 

If you are only using the ones provided with the receivers you will find they don't have the gain to cope with long cables. Either move the whole rack nearer stage or invest in some better aerials.

 

The only other comment I would have is that I remember reading that 2" metal tube (i.e. scaffold and lighting rigs) are a bit of a problem for interference for 2.4GHz, it might be that by mounting them in the lighting rig you are increasing problems caused by that.

 

Hope that helps

 

Michael

[skinda0]

I bought some 30m bnc cables and ran them from rack position up in to the lighting grid so the antenna were above the stage.

The UHF antenna have the little inline amps which form part of the mount for the antenna so they are at the antenna end, they were lit up as expected. The L6 paddles were also lit up.

 

What type of cable are you using?

RG58 has a loss of 14.1dB over 30M at 863MHz. Handy calc

It may be your inline amps are only driving at +10dB giving you -4db at the receivers which may be why you are getting dropouts.

I'd move the rack closer to the stage and use a XLR snake to bring the mic channels back to the rear of the hall, then you don't have to worry about coax loss.

[sandall]

No experience of 2.4ghz mics, but I've found in the past that adding passive Yagis on 5M cables (to get better line-of-sight to stage) made little improvement with Trantec 4.x series receivers. Are your aerials now pointing more-or-less straight down, putting them in the null-zone for belt-packs with vertical aerials, in which case pulling them back to over audience might help?

 

E2A: It sounds as if even with perfect siting the cable lossover 30M would cancel out any gain in signal strength.

 

 

Edited February 7, 2020 by sandall

[J Pearce]

30m is a rather long run for UHF antenna, without going to big fat cable and fancier antennae.

 

A short 3-5m cable taking the antennae above head height is a good compromise, and a fairly standard setup, just get them above the big water filled meat bags (sorry, cast members) that eat all the RF.

[Shez]

A 30m cable that will work well at 2.4G won't be "relatively cheap". Line 6's own offering - a pair of 15m cables for £150! Raw cable to make your own comes in at just shy of £3/m.

I've been meaning to get some longer ones than my 5m cables but have been somewhat put off by cost at the moment. Using P180 paddles mounted side of stage above head height has always worked well for me. Apart from not being able to remotely monitor the receivers of course...

[sleah]

Thanks folks, that's given me something to think about and raised a couple of points I hadn't realised, such as the 'null zone' with antenna pointing straight down and the 2.4ghz being affected by the grid.

The L6 kit is using the omni-directional version of the P180s. The UHF are the stick antenna you'd normally have on a single unit, but attached to the inline amps (as supplied in the "racked'n'ready" Trantec kit - good point about the amp gain too!

 

Every day is a school day, especially when it comes to the dark-art that is RF!!

Edited February 7, 2020 by sleah

[paulears]

This is why racks ALWAYS work best at the stage end. 30m is a ridiculously long run for High Band UHF - lower down the band for comms, a 30m tower is not uncommon - and the cable used from the radio room at the bottom is normally LD-F450 - semi-rigid, corrugated outer copper jacket with foam dielectric. This is the ½" diameter stuff and it needs special tools to fit N type connectors onto it - at the top, and to go to the radios they use more flexible jumpers. LD-F450 is £6-7 a mtr, or five hundred quid for 100m - and the lorry to deliver it is extra. Cell towers, which are closer to our radio mic bands tend to use LD-F550 - which is ⅞" in diameter and twelve quid a metre.

 

6mm diameter cable is just no good for RF at these frequencies. However - satellite CT100 type cable is often talked about as being useful as 30m of this stuff has around 4.5/5dB attenuation, so on paper might work - but uses F type connectors, and using adaptors to go to BNC or TNC can also be a bit lossy. There's a bit of an impedance mismatch - RF cables tend to be 50Ohm as opposed to video cables that are 75Ohm, but this is a small extra loss. If moving the racks is impossible, the satellite cable is cheap enough to try a drum - just the connectors are tricky to bodge properly.

[Simon Lewis]

I have a 12way L6 2.4ghz digital system and an 8way Trantec UHF 4.16 ch68/69/70 system that get used for school productions.

 

Hopefully not treading on toes here, but it's not legal to operate in channel 68 and 69 and I'm pretty sure you won't get 8 channels in the bit of channel 70 we are allowed to use (863 - 865). Trantec show only four frequencies in channel 70. If you are trying to use radios below 863MHz, you may well have problems....

[cedd]

Get your racks under the stage with the antennas pointing across the stage. Then your audience are in your nulls, your actors are nice and close and your cables are nice and short. A balanced audio line will travel hundreds of meters up a multi core, so use it for what it’s good at.

 

I use RG213 cable (thick low loss stuff) and the longest cable I’ll use is 10m.

 

And yes, I thought you could only use 4 S4 series in ch70 so you may be either cramming too much in or be on some frequencies you’re not meant to be (and may be in use). Might be the root of your problems.

[sleah]

I have a 12way L6 2.4ghz digital system and an 8way Trantec UHF 4.16 ch68/69/70 system that get used for school productions.

 

Hopefully not treading on toes here, but it's not legal to operate in channel 68 and 69 and I'm pretty sure you won't get 8 channels in the bit of channel 70 we are allowed to use (863 - 865). Trantec show only four frequencies in channel 70. If you are trying to use radios below 863MHz, you may well have problems....

 

I generally just use up to 4 channels of the Trantec in ch 70. They are quite old now so used when they want a "couple of extra mics"....

Didn't they give ch 69 back? It's hard to keep up with all the different ways OFCOM are trying to sh4ft PMSE users!!!!

[kgallen]

With a shared mic license you get 830-832MHz (so for example with Senny G2 you can get another 4 channels in there).

[ninjadingle]

With a shared mic license you get 830-832MHz (so for example with Senny G2 you can get another 4 channels in there).

 

823 - 832 MHz. So a little more space on a shard licence, with the right kit.

[kgallen]

823 - 832 MHz. So a little more space on a shared licence, with the right kit.

 

Sorry, you're absolutely right. I was thinking of the setup on my G2s.

[Simon Lewis]

However, in this particular case, the Trantec 4.16 only tunes to channels 68, 69 and 70, so the Duplex Gap frequencies can't be used.

 

Sennheiser G2 can make use of part of the 823-832 block, and the newer editions can make use of all of it.