Radio Microphone Frequencies

[Malcolm Gordon]

Many thanks both for that reassurance.

[sunray]

The issue of running multiple frequencies is they will mix together in 'something', in fact they will mix in pretty much any detector, especially a diode junction including a bad join between 2 bits of metal such as corrosion.

These frequencies will mix in the front end of a receiver and the resulting mix frequencies will be A+B and A-B (or B-A). Practically 173.8 & 174.2 will give 348 & 0.4MHz those 2 are unlikely to be a problem as they are way outside the frequency response of the receiver. Those 4 frequencies will be real and present in the receiver.

Let's now add 174.6, I won't go through all of the 8 new frequencies generated but just working with the 0.4MHz will give us 2 new frequencies of 174.2 and 175.0 - oh dear that's a very real signal on one of the frequencies in use and another on the only unused channel (173.8 & 174.2 & 175.0 will generate 174.6 & 175.4). However all 8 of those new frequencies can also mix with the first 4 to give lots of new, including several the same plus there will be a signal (roughly) every 175MHz and arriving at 860MHz is not uncommon (5x175=875). 

Going back to the original set of 6 DTI frequencies of: 173.8 (0.3) 174.1 0.15) 174,25 (0.25) 174.5 (0.3) 174.8 (0.2) 175.0 in brackets the differences can be seen to be very varied and very quickly 174.8 with a difference of 0.3 had always been classed as rogue however it's no more rogue than 173.8. Actually I think the original band didn't include 173.8.

I used to regularly use 173.8 (S1000), 174.1 (EDC), 175.6 (TOA DO6) & 175 (S1000) together without problems as long as there is at least 10m between mics and receivers

What a lot din't know was interspursed with those 6 frequencies were about about 6 channels for low power mics: 2 or 10mW Vs the regular 10 or 50mW.

When we got our first radiomic (EDC) we contacted all the likely radiomic users in the area and specifically requested (and fought with DTI for) 174.1 as we found it allocated to none other. Back in the 70's the biggest problem was AM taxi radios!

[Malcolm Gordon]

That's a fascinating exposition of intermodulation, which is very helpful.

[richardash1981]

Time for another re-awakening. I'm not sure if I'm getting inter-modulation problems or whether something else is wrong.
I've got four Trantec S3500D receivers (TNC antenna on the back), fed from a Trantec S2 ADU / PSU.

Up to four S3500 beltpacks (options for some handhelds, but leave them out for now). If it's relevant, headsets are all aftermarket, mix of CPC and micronic ones.

So far I have had the systems tuned to 173.8 MHz, 174.1 MHz, 174.5 MHz.

If I only use two transmitters on 173.8MHz and 174.1MHz, the sound is fine, the gain is consistent and all seems well.

174.5 MHz works fine on it's own as well. I have seen a receiver on 175.0 MHz open squelch with noise when nothing was switched on, so I presume there is some source of interference there in at least some of the locations. On the other had it mostly does it when both 173.8MHz and 174.1MHz are

As soon as I have more than two transmitters on, things start to fall apart - gain goes up and down (so the system keeps feeding back), much more noise on the audio.

Whilst the various Tx and Rx have slightly different frequency options, the only one that everything does is what Canford call the 5 old frequencies: 173.8MHz, 174.1MHz,
 174.5MHz, 174.8MHz, 175.0MHz in Bank 1. Some of the kit has Bank 3 programmed with the "new" frequencies at 173.8MHz, 174.0MHz, 174.2MHz, 174.4MHz, 174.6MHz,  174.8MHz, 175.0MHz, but not enough to let me use them (unless I take the kit to Mike at MLEC for a load of re-programming).

From what has been said here, it seems that this should work, so is it possible I've got a transmitter or receiver which is failing and heading "rouge"? Any way I would be likely to find out which one?

[Lamplighter]

Richard

You can choose any 3 from 173.8, 174.1, 174.5 and 174.8 and add 175.0 for intermod free operation. External noise is a separate problem which is not so easily overcome. I haven't looked at the new frequencies as my S3500 kit hasn't got them loaded. A quick look says that they are a disaster waiting to happen with 200KHz spacing all the way.

The old  5 channels give 300, 400, 300, 200 KHz spacings  so you only require to loose 1 channel from the first 4 to be intermod free.

All 5 of the old channels are intermod free of 3rd and 5th order products but only for 2 transmitters in close proximity

The 4 channel scenario works for 3 transmitters, and is generally considered best practice.

Brian

 

[sandall]

For what it's worth - back in the days of fixed-freq crystal txs I remember being able to use a mix of makes on 173.8, 174.1, 174.5 & 175.0, but adding 174.8 caused intermod with at least 1 of the others.

[sunray]

13 hours ago, sandall said:

For what it's worth - back in the days of fixed-freq crystal txs I remember being able to use a mix of makes on 173.8, 174.1, 174.5 & 175.0, but adding 174.8 caused intermod with at least 1 of the others.

174.8 + 173.8 - 174.1 =  ?

For one of our events we booked 208.6, 199.7 for 50mW IEM and 50mW handheld 176.6  having been assured by JFMG and their calculator there would be no problems with those and 'standard' VHF license exempt frequencies. We soon discovered adding 174.8 caused problems to the (and other radio mics and 2 way radios) receivers. The initial calculations took a bit of head scratching until 208.3 + 176.6 - 199.4 - 174.8 or to put it another way (208.3 - 199.4) + (176.6 - 174.8) = 10.7MHz