An interesting subject came up on our 70cms voice repeater earlier today following an incident on a local DMR box yesterday and it got me thinking.
There are a lot of 70cms repeaters in the UK with input frequencies between 430 MHz and 431 MHz and many people don’t realise that our licence only allows reduced power in that part of the band. I’ll put my hands up and say that although I’m well aware of the 100km Charing Cross rule, I wasn’t fully up to speed on what I’m about to talk about here. Incidentally, I live within that 100km radius by less than a quarter of a mile!
Specifically, the maximum power for Full and Intermediate licensees is 40W ERP and that goes down to just 10W ERP for Foundation licence holders.
Power limits by ERP are unusual as apart from this small section of 70cms, only the two VLF bands have ERP power limits.
What this actually means is that our Effective Radiated Power is limited when working any of the nearly 150 repeaters on 70cms with an input frequency from 430-431 MHz. These are almost exclusively digital repeaters, either DMR or D-STAR. You can see the most recent list of 70cms repeaters by clicking here (hint: click on the [receive] column to sort by repeater receive frequency).
ERP is calculated by taking into account the output power of your wireless, the loss of your coaxial cable and the gain of your aerial. Remember that aerial manufacturers generally quote the gain of their aerials in dBi which is a theoretical figure and is impossible to achieve. All my calculations below are using dBd which is dBi minus 2.15.
Let’s look at a fairly typical mobile setup first and by coincidence, this will be exactly what I’ve got in the car.
An Icom ID-5100 has three power settings, it runs 50W, 15W or 5W on 70cms.
If I have the radio on the High power setting of 50W and am using a very common type of aerial, a 2 x 5/8 whip aerial with 10ft of RG-58 coax, my actual power going into the aerial will be about 40W. The aerial gain is approximately 3dBd so my ERP will be 80W. That’s illegal. By reducing the power to Medium, I’ll have around 12W going into the aerial and so my ERP will be 24W which is legal. With this setup, a Foundation licence holder can only run Low power of 5W to remain legal as that will give around 4W into the aerial and so be 8W ERP.
I think most people operating DMR use handheld radios – I’m not aware of anyone locally with an actual mobile radio transmitting DMR so if used in a car with an external aerial then the 5W output from a typical DMR handbag will still be legal.
It’s when you get indoors that things start getting a little more tricky because of the wide range of available aerials and radios. I’ve tried to select common equipment.
At home, I have another Icom ID-5100, my aerial is a Diamond X510 and I have around 15m of EcoFlex 15 coax. My coax loss is 0.9dB on 70cms and the aerial gain is 9.55dBd. With the radio set to 5W output I’ll be getting around 4W going into the aerial and the 9.55dBd gain gives me an ERP of 36W. Providing I only use the radio on the Low power setting of 5W, I’m legal. Just.
Many people have smaller aerials such as the Diamond X50 or equivalent. The gain on the X50 on 70cms is a fraction over 5dBd but let’s round it down to 5dBd for ease of calculations.
The MD-380 is a common UHF DMR wireless with 5W output. The Icom ID-51 runs the same amount of power on D-STAR so let’s go with 5W as our power for these calculations.
If you’ve got your dual band aerial up at a decent height, you may have 20m of coaxial cable. I hope you’re using RG-213 as an absolute minimum on UHF. If this is your setup then 5W output fed through 20m of RG-213 gives you 2.4W at the aerial. Take into account the 5dBd gain from the X50 and you’ve got 7.5W ERP. You’re legal, no matter what your licence level is.
The same setup with a Diamond X200 gives you around 9W ERP but if you go to a Diamond X300 (or equivalent) then your ERP goes up to 12W. You’re now transmitting an illegal power level on 70cms if you’re a Foundation licence holder with the 10W ERP limit.
The above examples are just that, examples. You should know how much power you’re running out of your transmitter, you should know how much loss there is in the cable and you should know the gain of your aerial. With those figures, it’s fairly straightforward to calculate your ERP. You could use this as a starting point for working out your coax loss (input cable type, cable length, frequency and power) and then this to calculate your ERP (using the calculated power from the previous link and aerial gain in dBd).
It’s worth being aware of this if you operate on 70cms digital repeaters as I suspect a lot of amateurs won’t have realised this power limit is in place. I know that everyone is supposed to know how much power they can run on what frequency but this ERP limit is quite unusual, we’re more used to the limits of 10W, 50W and 400W that our Foundation/Intermediate and Full licences allow.
If you do transmit on 70cms between 431 and 432 MHz then you really do need to take a close look at your station to make sure you’re transmitting legally. Remember that we are secondary users of the 70cms band and you don’t want to upset the primary users.