PMR446: Technical

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 mods

There 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'
69 channels for Low Power Devices (10mW) from 433.075 to 434.775 MHz (25kHz spaced). This band can be used in most European countries for remote control and telemetry, and in some also for voice. Voice modes not allowed in Denmark, Finland, Hungary, Latvia, Luxembourg, UK. This falls within the amateur 70cms band (in all regions), in the UK's repeater outputs and simplex FM segment, and is also commonly used for car locks and weather sensors, etc. It's a welcome bonus for a licensed amateur to find that his 446 can also be used on the amateur band, but I wouldn't advise anyone else to use these channels. Amateurs can be fiercely protective, although in reality the amount of use this band doesn't get... they would probably never notice you! That is not the end of the story however, it is a 'secondary' band for the amateurs. The band is used by all sorts of 'government' users, with amateurs permitted by agreement so long as they cause no interference. In theory, if an amateur hears non-amateur use of a channel they are supposed to tune away and forget all about it ;o) It could be the local scouts/cadets, for example.

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'
6 channels of 'Kort Distanse Radio'/SRBR from 444.6 to 444.975 MHz (1W), as used in Sweden and Norway. In the UK these channels are within an area of the radio spectrum used by the military - so you never know who might be listening! Best avoided, really.

462/467 MHz 'FRS' (& GMRS)
14 channels, the USA's 500mW Family Radio Service, at 462.5625-462.7125 and 467.5625-467.7125 MHz (shares a band with higher powered GMRS - radios with 22 channels or more, and 2W) The Tsuen Shing WT-401 is well known for a 'bug' of suddenly having 14 channels instead of 8! The radio suffers a glitch and defaults to being an FRS radio instead of a PMR 446 - no known cure. FRS radios are also the most likely to be brought back to Blighty from abroad, or sold in dodgy markets. Beware, the channels are used in the UK by the 'Home Office For The Emergency Services', as 'on board' systems on large ships, and by TV/Radio broadcasters & programme makers. They will not appreciate interference!

Extra antennas

For 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).
An apparently better method is the capacitive coupling techniqe involving wrapping foil around the antenna. This technique was initially thought to require an "earth" connection from the coax braid to a suitable point on the radio either internally (why bother with the foil if the radio is open) or from a circuit ground point such as an earpiece socket outer connection or power supply (the most negative voltage point) - i.e. run a lead out from the spring contact of the first battery in the chain. This is not ideal though considering that just 6 inches of lead is a quarter wavelength. However, practical experiments have shown no difference between such a connection or none.
In the USA it seems not even coupling is allowed :
">So, what if the inductive coil is simply slipped over the antenna, not attached to it in any way?"
"Is it touching? Then it's attached. That's the response I got from the FCC 8 years ago when I first brought up this possibility. The FCC's intent should be clear. The unit must stand alone, nothing attached, nothing connected that was not part of the original design and certification (such as an earphone)." - Corwin Moore (PRSG), 2003
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 antenna on the radio is an electrical 1/4 wave
No - some are just random wire lengths!
 
> by the time ... you're lucky if you have 50mW!
50mW would be 10dB loss - sounds a bit much to me! Then
again this can be more than compensated for by the better
antenna location. Even 10mW from a great location will
outperform 5W from a bad one. 
 
> Leave the antenna and transmission line design to someone
> who KNOWS what they are doing.
He **HAS** already done it and is happy with the results!
 
Would you object to the guy sitting on his roof and operating
from there? Of course not, so what difference does it make if
he puts an antenna up there? I'd much rather he did that then
use a ham HT with twice the bandwidth.

...

He's not going to cause any more interference than he would
if he was up on the roof or slightly closer to you.
 
The modulation hasn't changed in any way. The spurii are
unlikely to be any worse. Think about it.
 
Instead of screaming "You can't do that! It's not allowed!"
ask yourself "is this actually hurting anyone?". Common sense.
So you never break any speed limits when it's safe to?

(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)
1 : 1 1 : 1.1 1 : 1.2 1 : 1.3 1 : 1.4 1 : 1.5 (i.e. 33 or 75 Ohms : 50) 1 : 1.6 1 : 1.7 1 : 1.8 1 : 1.9 1 : 2 (i.e. 25 or 100 Ohms : 50) 1 : 2.2 1 : 2.4 1 : 2.6 1 : 2.8 1 : 3 (i.e. 16.6 or 150 Ohms : 50) 1 : 3.5 1 : 4 1 : 5 1 : 7 1 : 10 1 : 20 1 : 50 1 : infinity	0 0.23 0.83 1.70 2.78 4.00 5.33 6.72 8.16 9.63 11.1 14.1 17.0 19.8 22.4 25.0 30.9 36.0 44.4 56.3 67.0 81.9 92.3 100	100 99.8 99.2 98.3 97.2 96.0 94.5 93.7 91.8 90.4 88.9 85.9 83.0 80.2 77.6 75.0 69.1 64.0 55.6 43.7 33.0 18.1 7.7 0	0 -0.01 -0.04 -0.08 -0.12 -0.18 -0.25 -0.28 -0.37 -0.44 -0.51 -0.66 -0.81 -0.96 -1.10 -1.25 -1.61 -1.94 -2.55 -3.60 -4.81 -7.42 -11.1 -infinity

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!"
Now seems to be made by http://www.hansen.com.cn or http://www.hanqin.com/fy3.htm ??

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



Don't listen to the 'experts' who blindly repeat what they've heard
without checking for themselves. For reception purposes, a lot of the
snobby holier-than-thou attitudes regarding VSWR can be safely ignored
in favour of an empirical suck-it-n-see approach. If it works, why worry?

75 Ohm coax. versus 50?!! Duh! VSWR is the ratio of one impedance to
another, so lets see... 75/50 is a "SWR" of 1.5 - wow, not exactly
huge is it?

And what is the main reason to avoid high VSWRs? Loss.
So given the choice between 5dB of loss in a long run of properly
matched coax, and 2dB loss in a run of not-quite-optimum but lower loss
coax - I know which one I'd choose!

I use the very low loss satellite cable too, it works very well indeed.
And I'd quite happily make ham transmissions over any system that
provided adequate reception and gave a VSWR reading that was 1.5 to 1
although to be honest I'd keep overs short when near the red!

Some experiments
See the Experiments page for attempts at coupling.