Combining Radio Mic Aeriels

[Nicktaylor]

I have my nice new Radio mics and have boxed them in fours. I have actually discovered that the reception is very good even with the aeriels shielded partially by the flight case sides. I am thinking of extending to the backed of the case about 20cm and then mounting the aeriels on the back of the case. Can I combine maybe two to one aeriel, rather than having eight ariels for four units

[Jivemaster]

Work out what works for you, but some systems have two aerials and then use a distribution amp to get the signal where it's needed. Some people put the rack by the desk and the antennas raised above them, some keep it all compact. It really is what suits your needs.

[pete10uk]

What Systems have you purchased?

 

Extending the aerials or distributing would be simple enough but the cost may surprise you.

[Nicktaylor]

What Systems have you purchased?

 

Extending the aerials or distributing would be simple enough but the cost may surprise you.

I have ordered the parts to do a non amp distributed system. the parts about £50 for the two racks. I will try with the cabling before committing to any real hardware work, drilling etc!

 

What Systems have you purchased?

 

Extending the aerials or distributing would be simple enough but the cost may surprise you.

G3 EW100 kit

[mackerr]

I have ordered the parts to do a non amp distributed system. the parts about £50 for the two racks. I will try with the cabling before committing to any real hardware work, drilling etc!

 

Does this mean you have purchased 2 1x4 high quality passive splitters, like Mini Circuits? These are usually about $75US each, and each output will be about 6dB down from the input, plus about .7dB insertion loss. This loss in the splitter is usually made up by using an antenna design that has passive gain, like a LPDA, or with an active gain stage.

 

While building your own RF splitter is not rocket surgery, it is not as simple as a "Y" cable.

 

Mac

[Jivemaster]

The antenna system is critical to system function. The cable losses have to be minimised and using a signal twice loses lots of power, which has to be replaced, SO Active splits are needed and the correct low loss cable is needed.

 

Some antenna splitters get their power through the coax some need DC volts and some need mains.

[paulears]

We shouldn't knock passive splitters they're a common feature in industrial radio systems and can be rather expensive. Plenty of devices use passive RF components - like circulators and cavity resonators which all exhibit loss. In our type of radio systems the problem is rarely RF level, it's the absence of RF when people step into null areas, or mask the line of sight path with their bodies when the RF level momentarily drops to zero - hopefully, the other aerial channel will have a good signal at this moment and operation continues. Watching the RF level of a distant static transmitter often shows full strength on the meters - and usually they show signal levels either close to full, or close to nothing. I have two UHF racks, one with manufacturer supplied active splits, and the other using passive splitters removed from a piece of gas industry telemetry equipment. With two separate aerials connected - moved apart a useful amount - the performance in terms of noise free reception is identical.

 

Aerial systems can be designed to combine two aerials into one output to add gain, and can be split, reducing the RF level. Probably not sensible to promote doing a straight split when ADUs are so easily available - but if your system is going to be used closer in, rather than at a distance, then passive splits may well work perfectly well. If you need to run from the front of house position, then I'd avoid them to save every dB you can of RF level.

[pumphouse]

An antenna distribution unit built in to the flight case is the right way to do it and guarantee negligible losses, but as others have said, and as in many situations, whilst a passive split arrangement is not the best, it may wel work for your application.

 

Jason

[Nicktaylor]

I have ordered the parts to do a non amp distributed system. the parts about £50 for the two racks. I will try with the cabling before committing to any real hardware work, drilling etc!

 

Does this mean you have purchased 2 1x4 high quality passive splitters, like Mini Circuits? These are usually about $75US each, and each output will be about 6dB down from the input, plus about .7dB insertion loss. This loss in the splitter is usually made up by using an antenna design that has passive gain, like a LPDA, or with an active gain stage.

 

While building your own RF splitter is not rocket surgery, it is not as simple as a "Y" cable.

 

Mac

 

No nothing as complex as that. In fact I think what I will do is just do a short extension to a plate on the back of the flight case. If in the future I have requirement for proper antennas I will buy the proper box as Pumphouse suggests, HNY Jason by the way!. As it is with these new G3s the gain seems far bigger than the old Evolutions

[parky58]

I have used 4 sennheiser G2 receivers in a rack and simply used passive splitting via a few BNC "T" peices. This works great in our band environment where the receiver rack is on stage with us, say up to 10 metres away with good line of sight to the musicians.

 

For jobs where the receivers are required to be located at FOH position on hire jobs, I have a rack fitted with 2 of these . These are fed by a couple of 850MHz yagis (now redundant due to change to CH38). This system is usable with up to 8 receivers and (touch wood) have had no interference or intermod problems.

[paulears]

TV distibution amps can work pretty well - the only real snag is that they are too wide band to offer rejection to powerful sources in the area. The description implies a filter applied above 900MHz and below 470MHz, passing just ch21-68, but that's actually quite a big chunk, and of course there will be some pretty chunky local digital transmitters all over the place soon. So removing the ch69 aerial and replacing with an A group one works. So the result in practice of using these sideband splitters is simply if a local transmitter saturates it, which will have implications on the weaker signals you're trying to use. It could be a workable situation, but move to another town and it fails miserably. A specially produced device notched to cover a small band is better at out of band rejection. As you say - short paths, close people usually = success.

[parky58]

TV distibution amps can work pretty well - the only real snag is that they are too wide band to offer rejection to powerful sources in the area. The description implies a filter applied above 900MHz and below 470MHz, passing just ch21-68, but that's actually quite a big chunk, and of course there will be some pretty chunky local digital transmitters all over the place soon. So removing the ch69 aerial and replacing with an A group one works. So the result in practice of using these sideband splitters is simply if a local transmitter saturates it, which will have implications on the weaker signals you're trying to use. It could be a workable situation, but move to another town and it fails miserably. A specially produced device notched to cover a small band is better at out of band rejection. As you say - short paths, close people usually = success.

 

Most Digital TV transmitters are located outside urban environments on hills to obtain coverage. A typical household receiver will require a directional antanna at 7m plus - usually 10 elements or more. This gives approx 40-50dBuV at the receiver. In the case of a radio mic distribution amp, it will usually be placed at 2 metres height, often indoors and without the benefit of a high gain antenna which will mean that any TV signal going into the distribution amplifier is likely to be considerably less than 50dBuV. The distribution amp will work up to 80dBuV which means it should not overload before this level , hence little chance of saturation. There is more of a chance that existing analogue transmitters may cause overload, as their carrier peaks are significantly higher than DTV. The filtering for TETRA and cellular transmitters is essential as their relatively narrow band transmitters are often on buildings in urban environments. The other important factor is of course the selectivity of the receiver, which I think is quite good on the Sennheiser series.

 

As I stated earlier, I have not had any problems with these amplifiers to date, and I will at some point borrow a sennheiser distribution amp and compare it to the TV amp by way of spectrum analyser plots.

 

No system is perfect and there will always be circumstances when radio equipment may be subject to interference - I have experienced interference from thermostats, computers, fire alarm systems, energy saving lightbulbs etc, but with correct frequency planning to prevent intermodulation products and careful siting of equipment we can significantly reduce these instances.

[paulears]

At home, from my bedroom window I can see the new relay on the local college - and there's another just up the coast, slap bang in the middle of a busy area - surrounded by housing. The main transmitters are a long way away. In the case near me, there is a performing arts building about 50m away from the transmitter aerials. I wonder how good their filtering really is?

[ramdram]

Our theatre is close to Caradon mast (all digital TV here); if it fell over the candle would be in our car park.

 

We never have any probs at all with the rms and they are quite cheapos up to Trantecs. Something works OK then, ** laughs out loud **.

[cedd]

That's just the problem for those of us who move around a bit. A fixed venue is fine and means you can get away with a few things that "work for us". I use the Proception DA's linked above (out of necessity - cost - rather than choice). I managed to rack my last set of 6 S5.3's for under £150, and that included the rack! Because mine are in 6's, it would mean two of the trantec splitters per rack, at an awful lot of cost. 8 outputs for 20 or so quid really isn't too bad!

 

I have had problems with moving venues though. One local high school has no end of RF garbage floating round just out of the band. My spectrum analyser (size and weight of a small bungalow) shows up some particular nasties right between ch38 and ch70 at really quite high levels.

As Paul says, my system is quite wideband. I use Log Periodic paddles which cover ch38 and 70. This is in some ways, by design. It means one pair of antennae chain through my ch38 and ch70 racks. In some of the theatres I "do" that saves an awful lot of work only having to mount 2 antennae front of house. It does however mean that I'm pretty open to interference and receiver desense.

I've also found that 2 racks is the limit on chaining these things back to back (with the proception amps). I have a loop through (N type T piece off of the RF in) on every rack, as well as an amp out (+3dB). I always prefer to use the loop through as it's one less stage of gain for intermods and noise. 3 racks and the system just seems a little more unstable than I'd like. No audible dropout, but the RF meters are just flickering a little more than I'd like. I've yet to find myself the space (and time) to set up a test at range with the 3 racks in use. This instance happened (as usual) when I was in a bit of a rush. I just put up a pair of ch70 Yagi's and fed that rack from them. I want to experiment with different arrangements of amplifiers and racks to see if I can come up with a better solution.

I do think there's a market out there for somebody to make some in-line bandpass filters for ch38 and 70. I have a feeling they'd need to be powered. Getting a decent slope on a passive filter is quite hard work.

 

If people are having trouble with interference on a specific frequency that's well away from your own, then you might have some success with the age old quarter wave stub. It's literally a T piece with a length of coax open circuit coming off of one side of the T. Trim it to a quarter of the wavelength of the offending frequency and you've got a really dirty notch filter. They're far easier to set up with a rectum spanalyser spectrum analyser and tracking generator though!