Radio Microphone Frequencies

[Barreller]

Canford say that it is typical for three VHF frequencies to be used together. http://www.canford.co.uk/Technical/Article/UKLegalRadioMicFrequencies

 

Can someone explain why four frequencies cannot be used in the VHF band? If I use 173.8, 174.2 and 175.0MHz then why not 174.6MHz as well? Sorry in advance if I'm missing something obvious.

[GR1]

You need to follow the advice of the radio mic manufacturer who will know how many frequencies and what they are that their product can successfully operate in any band. Find this information and follow it.

[Shez]

Coordinating frequencies is vastly more complicated than just leaving 0.4MHz between them. You can either do some hefty number crunching and still potentially get it wrong or just rely on the data supplied by those who designed the hardware.

[paulears]

If you google for intermodulation you'll start to see what happens - not just the frequency spacing, but the deviation too - which is often different between makes, and sometimes even within makes. The big manufacturers even try to make it easy with the popular UHF lines by publishing on their web sites groups of channels that play well together - so if you need say 12, there will be a set of recommended frequencies - but if you need 14, then many of them will be different. If you need less channels, then you can miss some of them out, but you can't just divide the beginning and end frequencies by the number you need and space them evenly. RF wise, it causes all manner of weird problems. They all appear to be interference free, until one transmitter gets a bit to close to another piece of kit and the two of them mix together, and the maths of the sums and differences between them go horribly wrong - and weird warbling and burbling noises suddenly appear, only to vanish when one of the actors moves. You can use clever software to try to do the productions if you have the data on the full specs, which most people don't have - or just use the ones somebody has used before.

 

In the 175MHz band 3 way pretty normal, but there were extra channels going right up to over 200MHz, where DAB is now, and you could then get around 6. The VHF band is just too small for more channels than this.

[Bobbsy]

Years ago I did achieve 4 VHF channels...but that was by using Audio Ltd. equipment with excellent frequency filters and a 2000 pount per channel price tag. (No, it wasn't my money--I borrowed them from my TV studio work for a few nights.

[sunray]

I have Trantec S1000's on 173.8 & 175 and Toa 3000's on which stay on 174.25 & 174.5 and never noticed any problem but if I add in 176 or 177 all hell erupts.

[Barreller]

Thanks for the replies guys. For information I have been involved in radio communications for fifty years or so both professionally and as an hobbyist. In the amateur radio world, 12.5kHz channel spacing for NBFM operation is commonplace in the 144MHz band so having 400kHz spacing is a luxury!! What I don't understand is that if any four frequencies are available, which is what Canford suggest, why can we only use any three? This cannot be an intermodulation issue, nor a deviation problem. The VHF band is exclusive to radio microphones so it can't be a sharing issue. I am using Trantec VHF radio microphones and can indeed use all four frequencies without a problem. I haven't actually used all four together but thought that I would ask opinions before acquiring a fourth unit. Edited June 9, 2017 by Barreller

[sandall]

Trantec off-the-shelf deregulated frequencies for their S.1, S.101 & S.2 ranges were 173.8, 174.1, 174.5, 174.8 & 175, the only problem being that 174.8 caused severe intermod with 174.5 & 175.0, so you couldn't run a set of 5, but I've run a set of 4 (+ a couple higher up) for years without problems. 173.8 & 175.0 appear to be standard across most manufacturers, but they all seem to use a different pair of frequencies for 174.x

[Jivemaster]

The previous deregulated VHF frequencies have apparently been replaced, there were five spot frequencies, and one gave intermodulation with the others so four were usable together. Recently the allocated spot frequencies were revised and are now different numbers in the same band, but only four spot frequencies. It's very unlikely that a mix of mics from the old and new spot frequencies will play nicely together.

[sunray]

The previous deregulated VHF frequencies have apparently been replaced, there were five spot frequencies, and one gave intermodulation with the others so four were usable together. Recently the allocated spot frequencies were revised and are now different numbers in the same band, but only four spot frequencies. It's very unlikely that a mix of mics from the old and new spot frequencies will play nicely together.

 

Indeed they have been changed several times over the years. Back in the depths of the 1970's the six LICENSED channels at 10/50mW (handheld/body) were 174.0, 174.1, 174.25, 174.5, 174.8 & 175.0. One paid a fee for each channel required.

 

Between 173.8 and 175.2 there were some other frequencies limited to 2/5mW and 50KHz bandwidth for which a free license was issued showing your allocated frequencies.

 

IIRC, at deregulation the spot frequencies were dissolved but manufacturers tended to stick with them, following on from there spot frequencies were recommended with variations (ie 173.8) and then more recently the 200KHZ spacing recommendations. I believe there are currently no hard and fast rules within the 173.8 - 175.0 band.

 

Above these and up to around 230MHz are another 30 or so spot frequencies (now significantly reduced by DAB) including high power 100/1000mW but obtaining the license can be very hard work.

 

The biggest problem I find with VHF is the digital interference, particularly in hotels.

[Simon Lewis]

Ofcom say that in addition to the VHF licence exempt range which runs from 173.700-175.10 MHz there is "UK wide shared access to 15 spot frequencies in the range 175.250 to 209.800 MHz internally or externally and licensed on a non-protected basis for one year only"...

[Barreller]

In case anybody else is interested, I now have Trantec hand microphones on all four of the new frequencies of 173.8, 174.2, 174.6 and 175 MHz. With all receivers close to the mixer and testing each microphone within 3 feet of the rx, I can use any combination of two and the two combinations of three excluding 173.8, 174.2 and 174.6. Obviously I cannot use all four together. Interestingly, even though the combinations above are useable, an output from intermodulation can always be seen on the unused channel when other combinations are in use. I will experiment again with increased distance between tx and rx and add to this thread. I only have two VHF belt packs so cannot test these where the results may well be different because of the increased power.

[Malcolm Gordon]

Apologies for resurrecting this old thread, but I didn't find a relevant more recent one.

For a very undemanding simple PA I have a Pulse Megamouth radio microphone system (successor to the Carlsbro Speakezee). It is on the exempt legacy frequency of 173.8 MHz. I now have the opportunity to purchase one of two Megamouth Mk3 units, unused but in stock for years. The frequencies available are the legacy 174.1 and 175 MHz. Would it be advisable to go for the 175, in case drift brings 173.8 and 174.1 too close together, or is it wiser to test if possible anyway?

[sandall]

Either should be fine (old mics used crystals, newer one use frequency division, but neither will drift). The "rogue" frequency in the "old" set was 174.8, which did cause intermod with at least 1 other freq (I forget which).

[Lamplighter]

There should be no drift problem as they should be crystal controlled. As a rule crystal controlled equipment creeps very slightly downward with age though the rate of creep decreases after the first few years. The three frequencies you indicate work well together.

Edited February 15, 2022 by Lamplighter
Typo