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VHF Radio Microphone / Blue tooth Interference


JohnMac

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Hi,

 

I've been struggling with a Trantec S3500 VHF radio microphone system on 175MHz. I set it up and all is fine only to get called back two weeks because the users are having problems with it. However when I visit to test,everything is fine and can't find anything wrong. (Noise on Audio, transmitter signal not getting through). Even when lurking with a spectrum analyser.

 

There are two adjacent studios using blue-tooth audio transmitters so the instructors can use their music playlists on their phones.

But public can't bear to leave, or turn off, their personal phones, with the result that all their Bluetooth 'search & connect' are left on, all adding to the congestion in the 2.4GHz band.

 

I've been playing with a spreadsheet calculating the 2.4GHz channel differences, and found that channel 0 and channel 35, i.e. every 36 channels the difference between the two channels gives me 175MHz. Given that I believe that there are 80 channels, the difference of 175MHz keeps reappearing.

 

Can anyone advise / confirm / experienced this phenomena of Old & New Technologies clashing. Is this my equivalent of playing 'Russian roulette', two random phones in the same place, generates an intermodulation frequency right on the VHF carrier.

 

Regards

 

John Mac.

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Hi,

 

I've been struggling with a Trantec S3500 VHF radio microphone system on 175MHz. I set it up and all is fine only to get called back two weeks because the users are having problems with it. However when I visit to test,everything is fine and can't find anything wrong. (Noise on Audio, transmitter signal not getting through). Even when lurking with a spectrum analyser.

 

There are two adjacent studios using blue-tooth audio transmitters so the instructors can use their music playlists on their phones.

But public can't bear to leave, or turn off, their personal phones, with the result that all their Bluetooth 'search & connect' are left on, all adding to the congestion in the 2.4GHz band.

 

I've been playing with a spreadsheet calculating the 2.4GHz channel differences, and found that channel 0 and channel 35, i.e. every 36 channels the difference between the two channels gives me 175MHz. Given that I believe that there are 80 channels, the difference of 175MHz keeps reappearing.

 

Can anyone advise / confirm / experienced this phenomena of Old & New Technologies clashing. Is this my equivalent of playing 'Russian roulette', two random phones in the same place, generates an intermodulation frequency right on the VHF carrier.

 

 

I don't know that Bluetooth isn't the cause, but in a couple of years regularly using S3500s with punters in the room it's never been a problem to me (note that the Bluetooth audio gadgets transmit from the phone and receive on the gadget - although the protocol is bi-directional, so both ends transmit to some extent).

 

2.4GHz is a long way away from 175MHz, so I would not expect the Trantec filters (which are pretty good) to have trouble shutting it out at the front end. If it's shut out, then it can't intermodulate ... What is fairly hard is screening these high frequencies out. Now the metal case on a S3500 receiver is nice and solid, assuming that it's still in one piece and the screws are still tight - might worth a check! The final point is that Bluetooth has a pretty low maximum power output (100mW, comparable to the radio mic) compared to the phone radio transmitters (up to 1W of RF in the bands either side of 2.4GHz).

 

I would be more expecting a lower frequency source to be the culprit. It always used to be PMR (mainly badly done taxi systems) but I believe those have mostly gone trunked digital these days - it has certainly never troubled me.

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The test would be what kind of noise? Finding out if it's data, or something with conventional am/fm signals. My guess had you lived near the south or east coast would have been Band III interference from France and Belgium's digital broadcast radio - OFCOM warned radio mic users a few years back who were still using VHF that interference was a real possibility. With the recent hot weather, loads of people here on the norfolk/suffolk border lost their TV channels when the weather did it's tricks. Marine band here on 157 was a bit of a mess because distances suddenly increased and many of the port operations share channels and caused each other some grief.

 

There's no business radio left in the band II section I'm aware of around here, but there still could be somebody using it unaware of the recent years clearcuts?

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Thanks for your replies it's given me food for thought.

 

I'm been called back this evening as instructor had problems again last week-end. Plan is to exchange the Trantec S3500 system receiver, and change the frequency to the opposite end of the VHF range, (173.8MHz).

 

 

The receiver has 'N' type connectors on rear of receiver and I've installed one 'remote' dipole aerial, so it's almost centre stage directly over the instructors head.

Must admit I've used Trantec for years, and this is the first time I've had a proper head scratcher issue with them. It could all come down to basics, battery level, microphone lead wrapped around the aerial acting as a tourniquet on the RF signal. We'll see....

 

Until this kicked off had no idea of the bun-fight that goes on with all the different equipment viaing for space to work in the 2.4GHz spectrum.

Bluetooth, Phone, Wi-Fi, Car alarms, Microwave ovens, video, wire microphones and probably many more.....

 

 

Kind regards

 

John Mac.

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Until this kicked off had no idea of the bun-fight that goes on with all the different equipment viaing for space to work in the 2.4GHz spectrum.

Bluetooth, Phone, Wi-Fi, Car alarms, Microwave ovens, video, wire microphones and probably many more.....

 

 

This is one of the reasons I've always told people to avoid the 2.4Ghz digital radio mics. The band is just to busy.

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I'm remembering the Christmas when Mercury flooded the market with affordable mobile phones, and one panto cast member had a son who was retailing them. So EVERYONE got one. This was also the era when wireless mics were, shall we say, less reliable than they are now. So, every glitch was blamed on the new mobile phones, and they were banned from the building during performances.

 

All a load of b******s of course. Unless one was actually sitting on a receiver the phones caused no problem.

Edited by exaltedwombat
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