last updated 29 Sept 2004
The "naughty stuff" page...
PMR446 is ideal for UHF radio experiments, without needing an amateur radio license, although it is becoming more and more simple to get licensed (the foundation license just requires a day or two course).
No, you are not allowed to listen on a scanner, however absurd that may seem! The UK's radio regulatory body the Radiocommunications Agency (part of the Dept of Trade and Industry) doesn't permit listening with scanners. This is probably because the bandwidth of a scanner is wider (usually copes with 25kHz spaced systems) and so you may hear the private licensed communications on the frequencies interleaved in between. Also, operation on these PMR446 channels is supposed to be with type approved equipment, and the various radio laws don't permit listening to such a service with any other sort of receiver. A private PMR channel may be just 5kHz away from a PMR446 channel received with a 5kHz step tuned scanner - well inside the 25kHz receive bandwidth.
If you decide to take the law into your own hands ( don't blame me! ) you will probably find that performance would be best on the edge channels 1 and 8 (another reason for using channel 8 for calling - see later) where there is only one adjacent PMR446 channel and the closest adjacent channel is 13.75kHz away. All the other channels 2 to 7 are only 11.25kHz away from a neighbouring channel using the 5kHz steps.
Here are the frequencies for 5kHz step tuning, and the distances from the adjacent channels :
1 (446.00625) 446.005 13.75kHz from 2 2 (446.01875) 446.020 13.75kHz from 1 11.25kHz from 3 3 (446.03125) 446.030 11.25kHz from 2 13.75kHz from 4 4 (446.04375) 446.045 13.75kHz from 3 11.25kHz from 5 5 (446.05625) 446.055 11.25kHz from 4 13.75kHz from 6 6 (446.06875) 446.070 13.75kHz from 5 11.25kHz from 7 7 (446.08125) 446.080 11.25kHz from 6 13.75kHz from 8 8 (446.09375) 446.095 13.75kHz from 7
Note that many amateur 70cm radios can be expanded to cover this band for transmit.
USA models will cover up to 450 anyway, as this is within their amateur allocation.
Use of such equipment is not recommended.
Amateur radios have a maximum FM deviation of 5kHz (to make use of 25kHz spaced
channels) whereas PMR446 is allowed a maximum of just +/-2.5kHz to fit within its
12.5kHz spaced channels. With twice the bandwidth, use of channel 2 with an amateur radio
will cause interference to channels 1 and 3 etc. If you turn the
power down to comparable levels, can operate on the precise frequency (6.25kHz
steps required) and keep modulation levels low you might just get away with it in
some areas but I think most of us would rather you bought the correct radios!
Extra Channels modsThere are 446 radios that can be 'modded' (modified) fairly easily to operate on other sets of channels. With radios manufactured as cheaply as these, and with a number of markets where different nearby bands are used, it makes sense for the radio to be capable of operating on all the various systems but with the country specific system selected at the time of manufacture. This can be built into the firmware, a secret key sequence, or circuit specific with the system determined by links on the circuit board. The cheap Telcom radios for example can be expanded to cover 433 and 444 MHz channels - with various on screen combinations of P,L and K standing for PMR 446, LPD and KDR.
There are far too many systems around the world to list here, but the following have been found available from modified or accidentaly reset 446 radios :
433 MHz 'LPD'
While amateurs moan about low power devices appearing here in recent years (seemingly all of a sudden), the allocation 433.05-434.79 (centre 433.92) has been an ISM (Industrial, Scientific, Medical) band for many years, 433.92 being harmonically related to 6.78, 13.56, 27.12, 40.68 MHz etc. Dividing by 64 we get the 6.765-6.795 band, and by 32 the 13.533-13.587 band.
444 MHz 'KDR'
462/467 MHz 'FRS' (& GMRS)
Extra antennasFor your interest ONLY I present the following item orginally from a mailing list. I do not encourage this in practice :
Date: Wed, 29 Sep 1999 To inductively couple your handy to your base station antenna: Firstly, if you wish to transmit as well as receive, ensure the antenna system is a passive one ie no preamp device in-line. Suitable antennas would include a 70cm amateur band beam, discone or a 70cm colinear. A group A TV antenna of around 18-21 elements is also eminently suitable. Beam antennas are highly directional - although less so if used away from their design freq - so bear this in mind. Take the downlead (that would normally plug into your scanner) and connect this (via a back to back adaptor) to a 70cm rubber-duck type antenna, preferably similar in length to that of the set top antenna on the rig. Turn on your radio and tune to a weak transmission. Bring the two antennas together until they are side by side and move the rubber-duck up and down the length on the transceiver's antenna. The rubber-duck can be upside-down with respect to the rig's antenna - experiment. Cable-tie (or even tape) together the two antennas at maximum received signal strength and there it is. There are all sorts of mis-matches inherent with such a Heath-Robinson system however any resultant losses will be more than offset by use of the superior antenna. Given the low power levels involved with PMR446 the transmitter will not self-destruct despite the mis-match.
... although I would find it fascinating if this caught on!
Imagine... with decent mobile antennas and roof mounted base station ones the ranges would easily be comparable to CB. Then you'd have all the 1970's style spirit of fun of "breaking" against the law, hehe.
I have seen other methods mentioned, including inductive coupling, which is to wind a ten-turn
coil to slip over the antenna, connected to the inner and outer
connections of the coaxial cable. This is not ideal - the coil
will pickup a balanced signal but coaxial cable is designed for
unbalanced transmission. You could try a "choke balun" to prevent
RF currents flowing on the outside of the cable - wind the cable
itself into a coil of a few turns, or clip on a "noise reducer" device
of the sort used on computer leads (contains a ferrite core to
act as a high impedance (resistance) to the passing of radio signals).
In the USA it seems not even coupling is allowed :
If you decide to experiment with these techniques, please let us know how you get on!
Of course, there are those who throw their arms up in terror at the thought of technical violations of the rules...
(alt.radio.family postings) You completely forgot how much RF you will LOSE going thru RG-174 (one of the worst) cable at 467MHz. The antenna on the radio is an electrical 1/4 wave at that frequency and your hand wrapped around the radio is a capacitivly coupled ground-plane. The factory antenna is FAR more efficient than your hodge-podge mess you described in the article. You have to remember that the radio does 500mW at the feedpoint. A 3dB loss makes that 250mW right away. Couple that with feedline loss, connector loss... by the time you get an antenna on the end you're lucky if you have 50mW! Leave the antenna and transmission line design to someone who KNOWS what they are doing. You certainly have not a clue.
(The views expressed are not necessarily those of this page's author)
Of course there's nothing to stop you mounting a unit in a waterproof plastic box clamped to a high mast - with power fed up a cable and a long extension-leaded speaker mic! It could be difficult to change channels though - unless the speaker mic provided it's own display and all the buttons you need. The answer must lie in the equivilant of the Radio Shack #21-1850 mobile FRS unit (FCC ID 'AAO2101850' - 0.499W ERP) which is a radio built into a mobile magnetic mount antenna base. The lead to the mic only carries such signals. All radio signals are generated up on the roof with the efficient antenna. Great! Extend that cable and away you go :o) (although the type approval would be then be void even if it would still pass the test if resubmitted)
This remote-mounting approach could work well for an organisation that needs to cover its local area site, with a central control station towards the bottom of a tall building. The radio could be sited on an upper floor near a suitable window, with a long lead down to the reception area. If a channel/code change is needed it would only need a simple visit to the radio to effect a change. If a long extension cable is not practical, don't forget that 49MHz provides a cheap and simple voice-operated short-range link facility. One 49MHz unit for the operator, and the other 49MHz unit upstairs connected via a short speaker-mic lead to the PMR446! Also useful for those living on the side of a hill facing away from civilisation - place the 446/49 repeater up on the hilltop somewhere secure, with solar-cells charging a battery :o) - sounds daft but could be just the thing if you're miles from anywhere and any activity on the one fixed channel would be welcome!
Types of antennas are discussed on the Antennas page.
|VSWR presented to a transmitter||Percentage power loss||Percentage power transfer||Decibel loss (dB)|
How do Coax. Cables compare at 446MHz?
Diameter Loss per 10m at 1000MHz 50 Ohms RG58 5mm 7.6dB --- =URM76 (4dB 70cm) RG174 2.8mm 9.29dB --- Ugh! RG178 1.9mm 4.6dB RG213 10.3mm 2.33dB --- Good (1.58dB 70cm) RG214 10.8mm 2.66dB RG223 5.4mm 4.39dB URM67 10.3mm 2.52dB (4.2dB 2.4GHz) LMR200 5mm 3.26db (900MHz) H100 9.8mm 1.4dB (1.9dB 2.4GHz) (0.91 70cm) 5D-FB 8.1mm 1.87dB 8D-FB 11.1mm 1.3dB --- Nice! 10D-FB 13.1mm 1.05dB --- big cable! WF103 10.3mm 1.5dB (1300MHz) (2.3dB 2.4GHz) LDF4-50 16mm 0.89dB (1300MHz) - 1/2" Hardline (1.35dB 2.4GHz) LDF5-50 28mm 0.5dB (1300MHz) - 7/8" Hardline 75 Ohms RG11 10.3mm 2.55dB RG59 6mm 4.6dB RG62 6.1mm 2.85dB RG179 2.5mm 2.4dB URM70 5.8mm 5.2dB TV Coax 6.6mm 2.6dB (900MHz) Satellite: FT100 6.6mm 2.1dB FT125 7.8mm 1.874dB
So a run of 10 meters of RG213 50 Ohm cable above will give you a loss
of 2.33dB wheras if you decide to use 75 Ohm FT125 you'll get just 1.87dB
loss apart from mismatch considerations. If you're using a 73 Ohm antenna
like the dipole then you may as well use 75 Ohm cable anyway!
Nevada used to sell a fabulous Japanese coaxial cable manufactured by
Kansai Tsushin Densen. "This family of cables is wonderous for two main reasons,
they offer very low attenuation comparable to that of famous Andrews' Heliax,
(8D-FB is almost half that of standard RG-213 for an equivalent diameter) and
they are very, very, flexible. You can bend the 8D-FB around a 2.54 cm diameter
pipe - try doing that with Heliax!"
The effect of using 75 ohm cable in a true 50 ohm system is fairly minimal. It simply puts a small ripple in the frequency response at various frequencies related to the length of cable in use. As a rule of thumb if the 75 ohm cable has 2dB less loss than the 50 ohm cable then use the 75 ohm stuff. The fact is that the input impedance of most scanners and antennas vary across the frequency range and so the effect of a slight mismatch is usually minimal. I've played with many different arrangements of cable and antennas over the years, and I have been very fortunate to have access to professional measuring equipment, so that I could actually validate some of these observations. Regards, Alan
Some experimentsSee the Experiments page for attempts at coupling.