A tale of misunderstood radio mic systems

[paulears]

Had a hired in rack of 10 Trantec 4000 UHF receivers turn up, a few minutes before tec for an amateur show at our venue.

 

Open the case front, and tripe of 10 cables with XLRs fell out. All numbered, 1 to 10. Plugged them up. All the packs in a nice case, foam inserts, and labelled. 2 chunks of antenna feeder with N types, and two aerials with the proper clips.

 

Then something dawned on me. 10 receivers, just TWO ADUs. So I pulled the back off to see that one feeder cable fed one ADU, the other going to ADU2? The first ADU feeding 5 receivers, the second, the remaining 5 receivers. So no diversity at all - each receiver just fed one RF signal!

 

Next problem, a quick PFL and classic symptoms of intermod, so checking the receiver channels reveals 10 frequencies, apparently all chosen at random - certainly very different from the set recommended by Trantec.

 

The system belongs to a college, put together very neatly, using EMO mains distribution, in a nice case - but all wrong!

 

I guess that people just don't realise that it isn't as simple as buying a load of kit and bunging them in a box!

[Bobbsy]

How many times have I seen this sort of thing? There was even a topic here in the BR a few weeks ago where somebody at least hinted that it was possible to operate that way!

 

For benefit of anyone uncovering this topic in the archive in a few years time, diversity radio mic kits need an antenna connected to each of the two inputs. Preferably each of these two antennae will be as close to the stage as possible and they should be separated as far as possible. The whole idea is to make sure one antenna is providing a strong signal even if the other is obstructed. This means that antenna splitter networks need careful planning, you need to use high quality low loss cable of the proper impedance and (sometimes) you need to place signal boosters near the antennae.

 

Bob

[cedd]

Further to what Bobbsy said, is it the done thing to place an antenna of different sorts (though same impedence obviously) on each input, the idea being that a yagi will be directional and pick up the stage, with a slightly higher gain, but will struggle with anybody in the audience so a dipole on the other input will pick that up?

 

I know Yagi's are good for gain but will struggle with side lobes and don't have an omnidirectional pickup pattern.

[Bobbsy]

Well, speaking for myself, I don't bother with Yagis unless I'm forced into a distant antenna placement. I'm much more prone to use a pair of sharksfin antennae run down to near the front of the auditorium (one either side) and linked with very low loss coax (of a type normally used for links from satellite dish to receiver in commercial installations). Specific to me, I had a Sony antenna splitter which permitted me to use 2 antennae per side and, if I needed to beef up reception in a certain area, I could run another pair of sharksfins.

 

Bob

[mackerr]

There are good reasons for using directional antennae. The fact that they are not omni directional is an advantage, not a disadvantage. Being able to reduce the gain of unwanted RF is a good thing, and a strong reason for using an antenna with gain. While there are "omni" antenna designs with gain, they are not omni in 3 dimensions, but only in the horizontal plane. The coverage narrows in the vertical lane as the gain increases, so they have the advantage of gain, but not the advantage of directional selectivity. The most common gain antennas we see for wireless mics are log periodic like sharkfin, or yagi, and recently helical. The advantage of the helical is the lack of polarity, so the orientation of the RF mic transmit antenna is not as important. The disadvantage is they take up a lot of room in a case. I would almost always use an antenna with gain, but the choice of which one would be driven by where I could place it, and where I need coverage, much the same as I would chose speakers based on coverage needed. A gain antenna also helps recover some of the RF level lost in an antenna distribution system to multiple receivers. A Sennheiser ASP2 antenna distribution system has 10dB-14dB of attenuation from the input to each of the outputs. This loss needs to be made up at the antenna. Antenna designs that have gain, without adding an RF amplifier, are the best way to do this. It is also critical, particularly at UHF frequencies, to use double shielded low loss cable.

 

Mac

[cedd]

Thanks for that, I'd never thought about antenna selection from a rejection point of view rather than a pickup point of view.

 

My problem is that I can't be sure of each and every configuration I will use the things in, it's changeable dependent on the job and venue, so I want to cover my bases, hence the yagi and dipole approach. I suppose two dipoles would do the same job, but I'm quite keen to have the receivers at the mix position, so could do with the gain offered by the yagi.

 

All swings and roundabouts I suppose. I'm more familiar with cable losses at UHF, but how much of a benefit do we think a longer cable run to get an antenna nearer to the transmitter would compare with short cable and a longer radio transmission path? The rule of thumb I use at work is 1dB per 10 metres of cable at UHF, clearly VHF will perform better than this.

[mackerr]

how much of a benefit do we think a longer cable run to get an antenna nearer to the transmitter would compare with short cable and a longer radio transmission path? The rule of thumb I use at work is 1dB per 10 metres of cable at UHF, clearly VHF will perform better than this.

The transmission loss through cable will be less than through air. Through air the transmission loss will follow the inverse square law, through cable it will be the cable loss. The ideal situation would be to have the receivers at the stage, and a good antenna system since there will be much less loss between the stage and FOH at audio frequencies than at RF frequencies.

 

Mac

[Bobbsy]

No argument with anything mackerr has said, but it might be worth expanding on a couple of things.

 

First, the requirement for off-axis rejection in the antennae you choose is far less critical in the UK (and now, for me, in Australia) than it is in New York. Looking at a spectrum analyser in central Manhattan is a sobering moment and it's amazing they get RF mics to work at all there. In the US there is NO legal protection (or even right) for non TV organisations to use radio mics. This, coupled with the very crowded spectrum especially since digital TV started, creates big problems. In the UK, if you're on a licensed frequency, it will already have been calculated that that you should neither cause nor suffer from interference. If you're on the UK shared frequencies, I suppose a pub next door could be an issue but I can honestly say it hasn't happened to me yet.

 

As for the gain issue, this is very real. I have to say that I was very fortunate in that, early on, I purchased a Sony active splitter and some of the Sony powered antennae. I'm not normally a fan of active devices in an RF chain but these were designed specifically for radio mic use and seemed to work very well...phase errors (normally a big problem with cheap active stuff) were practically non existent and the noise floor (when viewed on a borrowed spectrum analyser) was gratifyingly low. The Senneheiser ASP splitters are neither as elegant...but nowhere near as expensive. However, the Sennheiser brand signal boosters, when properly installed at the antenna, seem to work well.

 

However, as mackerr says, if you have a place to hang them and they provide the angular coverage you need, Yagi antennae are a very good way to make up for the splitter losses--certainly cheaper than the electronics I mentioned with far less to go wrong. Certainly, if I'm honest, when I've been called out to deal with radio mic problems, the next most common problem (after operator error/using non-compatible frequencies) is problems with cheap home-TV grade signal boosters and splitters.

 

A note on cable loss: be very certain to read the spec of the cable you buy because there are huge variations. By virtue of working for a company that owned lots of satellite dishes at one time, I've been able to use a cable (I believe Belden 9914 but I don't have my cable cases or catalogues to hand as I write this) which kept the cable loss over 30 metres down to between 3 and 4 dB at the frequencies I'd be using. I've seen some hire companies that should know better providing cheapie RG58 cable which has an attenuation of over 20dB for the same run. I've also seen some using 75ohm cables/connectors though that's an argument I'd rather not revisit just now!

 

Bob

[cedd]

I guess now's a bad time to mention I have a roll of heliax at home! Not particularly flexible but it's the bees knees. Unfortunately it only really lends itself to installation work, which is a shame, but I have some double screened heavy coax on its' way. I'm going to go for the receiver at the stage option wherever possible. I'm not short on multicore sends and there are convenient places to put them at most of my venues, I was just keen to have them at the mix position for a decent view of the meters.

 

I've seen a few poor installations using 75 ohm domestic (white) coax. Not fun and not a lot of use!

 

Cheers all