OBSERVATIONS FROM SCOTLAND

It's a while since I took a look at the radio spectrum around here, probably about two years. I wondered if there might be anything new and I wasn't too disappointed. What has really taken off here is mobile phones. The mobile phone allocations are certainly busy places these days. In a way that's bad, but in another way, it's good.

It's bad because all that RF adds to the general 'radiofog' but the plus side is that with more masts going up and cells becoming smaller, the power used at the base-stations can be lowered. In fact, many modern phone handsets are now lower power too. That has to be a benefit for the users worried about placing an RF transceiver system against the side of their face. So, rather than more masts being a concern, maybe it should be counted as a blessing.

Digital radio signals are also very obvious in a spectrum sweep in these parts around 220Mhz.

For convenience, I split my sweep into three separate frequency bands, DC-460Mhz, 460-920 and 920-1300+Mhz. I used my trusty ICOM PCR-1000 receiver and an ancient PC running TALKPCR to log the results. The aerial used was a commercially made discone, mounted at 25feet AGL. My site is at the brow of a shallow hill.

The 'discontinuity' in the noise floor at around 345Mhz in the above photo is caused by the receiver switching to a different preamp. That large broad ragged signal around 220Mhz is down to digital radio signals. You can readily see how noisy the RF background is in the DC-50Mhz region while after that, the background noise is at a very low level indeed. From 88-108Mhz there are many very strong VHF FM commercial radio stations. Airband from 110-136Mhz is also a rather busy place. I'm only about two miles from Glasgow International Airport. The airband signals include Instrument Landing System (ILS) air-to-ground comms, ATIS (Automated Terminal Information Service), a VOR beacon (VHF OMNI-RANGE) plus a fair number of Scottish sector voice frequencies, not to mention airline company frequencies. From 136-138 we have the Polar orbiting Weather satellites while above that are pagers (strong!). The 2M band covers 144-146 while on the high side of that until perhaps 180Mhz are various utilities. Digital radio is next (BIG signals) then things get a bit quieter.

On to the next table.

TV carriers galore (460-860Mhz). The Blackhill transmitter plus some fairly local relays, not forgetting of course digital TV. The reason for the abrupt change in noise floor is down to the RX switching to a different preamp circuit.

The TV bands frun rom 460-860Mhz (more big signals) with another preamp switch at 700Mhz. Plenty of strong carriers there plus weaker ones from TV Relay stations. The main multiplexed digital signals from Black Hill in Central Scotland are on 634, 658, 682, 714, 746 & 826Mhz and each multiplex has an ERP of 20KW. Also in this region are the five normal terrestrial analogue signals plus weaker analogue relays. You can read all about UK TV transmitters, masts and relays at MB21

After the TV bands, things go a bit quieter.

Onwards and Upwards.

Cellphone base stations galore, the strongest at around -87DB (10mv into the aerial) ! The large spike around 1280Mhz is local QRM.

The big bundle of carriers you see above are cellphone base stations. Things have certainly changed since I last took a look in this area with a big increase in the quantity of cellphone signals being most notable. Even the strongest signal is only (-87Dbm or 10mv) while most are around -100Dbm or 2.25uv or one tenth of a Picowatt 'in real money' Somehow, I don't think I'll lose any sleep over phone masts! Last time I looked, the GSM carriers seemed somewhat stronger than they are now.

For sure, mobile phone systems are a major contributor to the background RF, in many areas, I suspect including this one, they will be THE major source of RF energy in the environment. Against this must be considered the fact that the levels of RF encountered normally are well within acceptable guidelines and base station aerials are sited to avoid exposing the general public to high field strengths.

The Health Protection Agency (HPA) has this to say of mobile phone installations.

RPD has made many measurements of exposure levels at publicly accessible locations around macrocell base stations and in June 2000 NRPB Report R321 was published containing measurements taken at 118 locations from 17 different base station sites. Average exposures were found to be 0.002% of the ICNIRP public exposure guidelines and at no location was exposure found to exceed 0.2% of the guidelines.

HPA Total Exposure Quotient chart

It has to be said that while the measured signal levels for mobile phone (GSM) transmitters were fairly low, there are many carriers, so this of course increases the actual dose received by an individual. My spectrum scan only covers the lowest of three bands I believe to be in use by the GSM phone systems. So, while the mean signal strength for GSM base stations seems to be low, there are many signal sources which of course changes the overall picture. The NRPB don't seem to have a problem with mobile phones and I can't say that I lose any sleep over them either. There's no doubt that mobile phones are major contributors to the general radiofog background although the total RF dose to the public is very low.

I've never really understood why most people love mobile phones (a 900 or 1800Mhz transceiver clamped against the side of their head!!!) while they object to phone masts on the grounds that they emit 'radiation' .

mobile phones

Phone companies are devising ever more clever ways to hide their base stations. Artificial trees, aerials 'hidden' behind windows inside tall buildings, lamp-standards with a curious 'bulge' on top, does their ingenuity know no bounds? See the link below photos of unusual GSM installations.

The strongest signals I found in the DC-1300Mhz area were easily the commercial VHF-FM Broadcast stations, considerably stronger even than the powerful terrestrial TV signals. Measured against either of these, individual cellphone base-station signals were puny, although collectively, they kick up a 'big din'!

OK, you have to consider frequency-dependent RF absorption by human beings as well, but even so, those VHF commercial broadcast stations are VERY powerful indeed. Because we live in a valley surrounded by hills, many of the broadcasters have their stations on the hills overlooking what is a large chunk of the Central Belt of Scotland. Take walk on the Gleniffer or Brownside Braes and you will see a considerable number of radio relay sites. One particularly larger tower near the Seargeantlaw Road is home to many dozens of aerials, attached to the tower, placed around it and even inside it. I suspect this gynormous tower is the source of that large 'fan' of digital radio signals around 220Mhz. Many other operators have relays on the braes. Taxi services, various utilities and I'd be surprised if the police and MOD don't also have a presence these. I believe the GB3PA repeater was also sited on the slopes of the braes but it has now been re-homed elsewhere.

All in all, a rather interesting radio environment.

For anyone interested, Mini_Circuits do a superb Dbm-Volts-Watts table dbm_volts_watts.pdf for 50R systems.

If you want to see how much ingenuity goes into concealing GSM base stations, have a look here. GSMstuff