Using a TV aerial splitter for wireless mics?

[callumb]

Being budget conscious and looking at ways of making life easier I am looking at using something like this: https://www.screwfix.com/p/labgear-1-way-combiner-splitter/49019 to combine two 863 - 865 MHz radio mics into one pair of standard radio mic receiver aerials or paddles when needed. I would get the proper splitter but seems a huge over kill for two units in the one rack!

Feel free to tell me I'm a total plank for thinking that this is a good idea as I am no RF expert! 🙂

[dmxlights]

I dont think they will work very well. The proper rack mount unit also has a power amp in it to help with signal been split in to different receivers.

Also tv signal has a more power full transmit power than a radio mic (which is about 10mw). Using one of them could give you more problems with interference as it wideband and it could mix it with the radio mic signal and give you more of a headache than it worth

[Brian]

Hmmm, 15dB of insertion loss. So that means only 1/32nd of the power you put in comes out.

[DrV]

12 minutes ago, Brian said:

Hmmm, 15dB of insertion loss. So that means only 1/32nd of the power you put in comes out.

Where did you get that figure Brian? The page I looked at showed a 4.2dB gain (it contains a line powered amp). I'm no expert on this stuff but have an academic interest in whether it would work. It is very wideband so to my mind it should work.

I also don't see the relevance of transmit power as mentioned by dmxlights. Despite the slightly ambiguous wording, the op presumably wants to use this to connect one antenna input to two radio mic receivers. Or have I misunderstood that?

Edited December 9, 2021 by DrV

[david.elsbury]

Don’t use a powered splitter. A correctly selected passive one will perform just as well .

Light reading: https://dbbaudio.com/2013/diy-wireless-microphone-antenna-distribution-part-2/

[sandall]

For many years I used pairs of 4-way Antiference 12dB TV boosters in home-made 4-channel VHF & UHF Trantec receiver racks, using standard receiver aerials for the VHF set & either receiver aerials or cheap set-top yagis for the UHF set. Lost a bit of the 12dB in the feeders & in possible mismatches, but worked fine.

E2A: One of the boosters is now in the roof, as the 4-way Archer (Tandy) amp that fed the house has finally died after 40 years service & I couldn't find a quick alternative.

Edited December 9, 2021 by sandall

[DrV]

Of course, if you're using 2 paddles and diversity receivers you'd need two splitters, whether they're active or passive.

[Brian]

1 hour ago, DrV said:

Where did you get that figure Brian?

By looking at the wrong page on the Labgear website!!!!

The one on the Screwfix site does indeed have gain BUT it'll need line powering to work and it's unlikely to get that from two radio mics. At the very least there's going to need to be a power injector somewhere in the setup.

[cedd]

For the last 6 or 8 years my antenna distribution has been solely TV amplifier based. The ones I'm using aren't made any more, but are a 1 in, 8 out, with 3dB gain. Never had an RF related problem with them.

When we last had some fancy test equipment on hire at work (spectrum analyser with tracking generator and a network analyser) I brought one in and took a look at it. It was pretty much flat across the specified band, with pretty consistent gain. 

Think the main points gave been mentioned already above, but the main one to think about is power. Mine, like yours, are line powered. My receivers have switchable phantom for RF amplifiers and active paddles, which happily power the splitter. One quirk though is that the phantom isn't passed up the coax to the antenna, so if you're using active paddles you'll struggle to power them. Too much gain isn't a good thing in RF though, so I'd always suggest improving cabling losses before adding amplifiers anyway. 

Another tricky one was getting decent quality cabling. F type to BNC/TNC isn't easy to come by. For goodness sake don't by the pro signal adapters from CPC. All of mine fell in 2 over the years, leaving me with dead channels that took a lot of sussing out. In the end I went to a Chinese company through Alibaba and they made cables to my specification. I've got 32 channels of the stuff though, so it was a sizeable order. 

[callumb]

Thanks everyone for their input. Seems its a "your results may very" so I am going to have to just play about and see what happens when the kit arrives. I will try something passive as a starting point and see how I get on avoids the need for powering and extra bits. 

[sunray]

The unit linked to show as 4.2dB gain, however it seems to be a combiner/splitter with a frequency range of 5-1000MHz and power passthrough. I suspect it is actually a passive device. This site: https://everydiy.uk/p/screwfix.com/49019/Labgear-PSC120-S-2-Way-Combiner-Splitter-VHF-UHF shows it as a 3.7dB loss

I'd suggest going for something more like this: https://www.screwfix.com/p/labgear-ldl104rr-4-way-distribution-amplifier/2174x if you can use channel 38, the difficulty these days is finding TV kit spec'd (and not filtered) beyond 790MHz. Keep your eyes open for older secondhand kit, for a long time my only amps were teldis mains powered units circa 1980's which were fairly flat between 50-900MHz and dropped off rapidly each end, they had 20dB gain and Belling Lee sockets with locking rings. I simply used T pieces on the back of the RX's.

So often I see receivers with loads of added gain when it's quite simply not needed, I will ask if you can tollerate some loss and just use passive splitters (50Ω resistors)? (quite common in other RF situations)

 

Edited December 10, 2021 by sunray
Sorry crossed in the post, took an hour or more on and off to write.

[sunray]

35 minutes ago, callumb said:

Thanks everyone for their input. Seems its a "your results may very"

Very much so, a bit like Marmite. Good luck.

Edited December 10, 2021 by sunray

[paulears]

That Screwfix one won't work - it's line powered, so unless you can externally power it and then remove the line power before it gets to the radio receiver no good.

However, decent passive splits with modest insertion loss work pretty well in practice because it's rarely signal level that is the issue, but the signal in the nulls - so two decent antennas with clear paths with different path lengths and sensible separation work rather well. I have two racks with Sennheiser DAs in one and two 4 way passive splitters (RF comms ones) in the other rack, and with side of stage antennas up and separated - at the desk, when you've done some patching - you can't ever tell that chX has to be the passive split and channel Y is obviously an amplified split. 

[sunray]

6 hours ago, paulears said:

That Screwfix one won't work - it's line powered, so unless you can externally power it and then remove the line power before it gets to the radio receiver no good.

However, decent passive splits with modest insertion loss work pretty well in practice because it's rarely signal level that is the issue, but the signal in the nulls - so two decent antennas with clear paths with different path lengths and sensible separation work rather well. I have two racks with Sennheiser DAs in one and two 4 way passive splitters (RF comms ones) in the other rack, and with side of stage antennas up and separated - at the desk, when you've done some patching - you can't ever tell that chX has to be the passive split and channel Y is obviously an amplified split. 

This is basically what I was getting at earlier, I don't habitually add gain into radiomic systems unless it's a long distance twix mic and receiver. However a high proportion of my work was outdoor PA and sometimes the arenas are huge...  Indoors I can't recall ever using pre-amps, other than racked systems with them built in.