Designing/establishing viable comms. links

Hello Derek!

Who has told you that site SCADA cannot be reached? Was that a radio systems engineer?

If the distance to the site is similar to the links using 460 MHz, then that might very well be true. Point-to-point microwave links use various frequencies, and can be up in the high double-figures of GHz.

The propagation conditions are different up here, compared to 460 MHz.

Firstly, the path loss of a link is frequency dependent, and there will be a 20 log F1/F2 difference in path loss between 460 MHz and X GHz. Let's say it is 4.6 GHz (for argument's sake); the path loss with be 20 dB worse. That means that either the link budget is 20 dB less, or you need 20 dB more power to operate a link at the same distance. 

Then, there are other conditions such as atmospheric absorption, weather effects, scattering losses and diffraction to consider, which all vary with frequency. Propagation is almost a science in itself, but there are good references online. Foliage and buildings 'in the way' are particularly good at eating-up microwave link margin!

Anyway, if a competent radio systems engineer has told you it can't be reached, I'd have no reason to doubt it. 

The telecoms/internet based solution seems to make sense (e.g. a leased line), but you could also consider a Satcom solution, using a commercial modem. Again, you'd have to rent airtime (or spectrum), but if your average data rates are low, I suspect this might end up being decent value for money. You'll have to consider avaialbility as well. A satcom link is unlikely to match the availability and link quality of a T1/E1 line, for example.

Good luck, I hope that adds some context.

Regards,

Gav

Although 20dB (100 times) more RF power on a microwave link sounds like a high price for using the higher frequency, it is not as the antenna gain and directivity  can be much higher - in effect the transmit antenna is only beaming it's energy into a narrower cone, and the receiving antenna is presenting a pro-rata larger capture area - in the direction of maximum antenna gain only, so you can trade off an angular alignment problem for a   smaller transmitter power.

(A telescope with a large lens is an example of this effect, collecting far more of the light than a smaller aperture, but having a far narrower field of view - and that is often very desirable.

The other advantage of narrower beams is frequency re-use without cross- talk, so long as the beam directions are sufficiently different,

It may be that 460 MHz can reach a given distance but XXGHz cannot, but in other cases the reverse will be true - it rather depends on what is in the way (longer wavelengths are better at penetrating obstacles like woodland and buildings and do better on a part obscured line of sight.) and how much space/ planning permission there is for towers and big dishes, and of course the permitted transmitter powers on the two frequencies.

Mike.

Some great points Mike. 

I would add a modicum of caution that a 20 dBi antenna will have a very narrow beam, and typically require some pretty accurate alignment. 

Also, RF power is harder to come by at GHz freuqencies, so although the PAs may have comparable power (with the high gain antenna making up for the addiitonal link loss at GHz), the GHz PA will be bigger, heavier and more expensive. Transmission losses in cables will also add burden.

A complex and multi-faceted problem as you've alluded to...which is where good RF engineers earn their crust!

Well for me radio is both a hobby (G7VZY) and also a part of the day job, and I am aware and agree with  the points you allude to, but it is always tricky to know at what level to pitch replies to these sort of questions, in the sense of what best to leave out to not over complicate matters on a forum that is mostly read by folks who do not specialize in the electronics and radio matters. I'm always happy to come back and discuss things more deeply if it looks like folk have the appetite to do so.

More seriously, I fear that between us we have lost the original poster from the conversion, which would  be  a shame.

Mike

Indeed. But I think you might mean the OP Derek Eccles ?

oops. Editing that now. thanks.

i

oops. Editing that now. thanks.

i