Increase Range

Joined: 25 Apr 2007 Posts: 6

Hello,
I'm fairly new to PMR446 (properly. I've had some for a while, but not really bothered to do much with them). I recently tried to talk to a friend who lives 2.5 miles away. I didn't hear anything. A friend who lives roughly half way in between was barely audible (I briefly heard his voice saying 'testing', but that was it). I was wondering if there was anything I could do to increase the range at least to let me reach 2.5 miles. I live in a built-up area, so this is probably the problem, but there must be a way to increase range...
Thanks for any help,
James
P.S. I have a Binatone MR200, I forget what my friends have.

Joined: 29 Aug 2006 Posts: 211 Location: France

Hi,
In built up area, main problem is signal going through many walls. Increasing output power could help a little but the best way is to get higher and increase line of sight.
The only way i know is to build a roof mounted antena (can be directive for point to point link) connected to the radio with a good coax. I must point out that it's illegal !
Pro UHF users (e.g. emergency services) have at least one uhf relay mounted on top of hight pole and dedicated frequencies but they have to pay a licence for that.

Joined: 29 Oct 2005 Posts: 1373

The suggested realistic maximum range for a (legal) PMR446 radio is 3km. That is just 1.86 miles.

Most cheap 'consumer' level PMR446 radios have a realistic maximum range which is MUCH lower than 1.86 miles.

This does not take into account intervening terrain. So the actual range may be very considerably less than 1.86 miles.

Sure, we all talk about high elevation, uninterrupted line of sight communication using PMR446 radios over many tens of miles, but that is the very special exception, not the norm.

Even the expensive pro-level PMR446 radios, such as the tti TX446, Motorola XTN446, Entel HT446, and others, just aren't capable of much further ranges than this in normal day to day, urban environment use.

Increasing power is one option (an illegal option when using PMR446 radios) but it will not necessarily solve your range problem. For example: Using a hand held radio running 4W, I can reach into an amateur radio repeater located less than 2 miles from me. If I move into a different room in the house, the signal suddenly drops out and I lose communication. Using the same handheld, I can reach another amateur radio repeater that is at least 20 miles away and I can maintain the signal no matter which room I am located in (it varies in signal strength and audio quality, but I never lose it).

The only way to maximise your possible range is to use as effective an antenna as you can get away with, mounted as high as you can and to put as much power into it as you are allowed to. This is the reason that CBers are willing to attach 20ft antennas to the chimney pots of their houses!

When it comes to PMR446, most people just want to be able to use the radio as it comes. What they don't realise is that the professional users they see that have apparently quite extensive range are actually making use of hidden repeaters, mounted at advantageous high points.

I think a more realistic range guide for most 'consumer' level PMR446 radios is to imagine a situation where you are close enough to still see somebody clearly, but not close enough to be understood by shouting at the top of your lungs.

Jon

Joined: 25 Apr 2007 Posts: 6

OK then,
I'm going to try and build a dipole antenna from instructions on Delboy's site.
Would it be possible to increase power output by, say, removing the right resistors (and shorting across the contacts)? I know I'm unlikely to find the right ones in any short space of time, but could it be done?
Thanks

Joined: 29 Oct 2005 Posts: 1373

Do a google search of the internet for your particular radio, if somebody has successfully modified it for higher power you will find info for it.

These things are built to a price point. Some of them are sold in different markets with a slightly different specification so they may have more power available. It all depends on the radio, some are capable of 2-3W, some are capable of 4-5W, some are not even capable of putting out the advertised 0.5W (500mW) so don't expect too much of a radio that costs £11.99 for a twin pack from the local supermarket.

Joined: 25 Apr 2007 Posts: 6

Heh, OK.
Thanks!

Joined: 29 Aug 2006 Posts: 211 Location: France

Some users have built a simple 1/4 wl antena and claim to have improved range over the helical stub.
The dipole could be beter but is not very handy !

Joined: 25 Apr 2007 Posts: 6

I'm going to build a dipole similar to the instructions on Delboy's site. What is the optimum length of the poles?
Delboy's site says 1.4 wave for each pole, but my Dad (works with radio's) says that the longer it is the better it is.
Also, how much effect will a 'bad' coax have? Will the extra height and quality of the dipole make this nominal?
Thanks.

Joined: 29 Aug 2006 Posts: 211 Location: France

Ehhh
If you use a coax then i assume you plan to mount the antenna on a roof pole.
The coax will add signal losses which depend on frequency used. It is usually written on the coax itself. Average tv coax (class 19) has 19dB loss/100m for the tv frequencies, similar for 446MHz.
The single dipole antena will provide about 3Dbi gain (over an isotropic antenna) but probably less over your stub antenna which is not isotropic.
All other things like impedances mismatch and connexions in the way will add losses

Assuming you need 10m coax and impedances perfectly matche, you will gain at most 1dB. In fact you will certainly gain no power at all

BUT your antenna will be best placed, you will not need to go through walls and your range will certainly increase somehow !

The dipole (2*1/4wl) is a good way to experiment but higher gain antennas should be considered later to really improve things.

Joined: 25 Apr 2007 Posts: 6

OK, that explains quite a lot.
Why would you use 2*1/4 wave?
Would 2*1/2 wave give better gain?
Would 2*full wave give even better gain?
Why is 1/4 chosen?
Thanks.

Joined: 14 Apr 2006 Posts: 157 Location: Shetland

Really the best way to increase the range of a UHF handheld is to have a cherry-picker instead of a car, or to have those folding steps that some photographers carrry about, or to stand on the nearest wall just to get that bit of extra height.

Ian
SIBC

Joined: 29 Aug 2006 Posts: 211 Location: France

Antenna theory is not simple: google for specialised sites.
To sum up, the dipole is a resonant antenna for one frequency. It increases the signal for THE frequency at the expense of the others and has a smart omnidirectional pattern on the horizontal plan when vertically hight mounted.
It is commonly used as a reference antenna.
Other sizes are possible, some could be more efficient but most will not !
Remember you are transmitting RF power (not much with pmr), a detuned antenna could destroy your output device.

Joined: 25 Apr 2007 Posts: 6

OK then, I'll go with 2*1/4 wave.
Thanks.

Joined: 30 May 2007 Posts: 29

Even better would be the co-ax colinear mounted in a plastic tube. Much better gain.

http://www.rason.org/Projects/collant/collant.htm

Joined: 10 Jun 2007 Posts: 3

Firstly I cannot condone breaking the law in any way and as such I cannot recommend doing the following modification . If you proceed to do this then it is entirely at your own risk and you need to check if it breaks any laws for your area first.

If you use an Autocom system and a Kenwood PMR radio for bike to bike communications then this one might interest you.

Anyway, this modification will allow you to fit a better performing external aerial to your Kenwood TK 3201 (full 500mw UK spec model only with fixed rubber aerial). The reason behind doing this is to try and increase the performance of the radio without the need to use a non standard ‘high power’ radio such as the TK 3207, and to be able to hide a radio away on the bike where some tealeaf can’t remove it without your permit ion. Problem with hiding the radio is that it inevitably cuts it’s performance by a very big margin, and hence reduces the radio’s effective range (and makes it more prone to interference from the bike electronics). So fitting an external aerial would allow you to hide the radio and keep the best performance.

Sounds like a lot of trouble to go to in order to fit an external aerial however, after doing the change you will have increased the performance of the radio considerably.

The change done is to fit a new ‘SMA’ socket to the radio in place of the original ‘rubber duck’ antenna and that is it. These sockets can be purchased from Kenwood radio dealers (or the internet) and are the same part as used in many other Kenwood radios such as the TK 3202, TK 3207 etc. Unfortunately you will not find an official dealer in the EU that will do the modification for you, as this strictly speaking breaks the PMR specification for a ‘fixed’ aerial and hence cannot be done. However, if you know a local TV-radio repair shop or you are handy with the soldering iron and you are able to get the part then you may want to go for it.

The change is very easy to do, simply strip the battery and belt clip from the radio, remove all the screws that fix the alloy radio chassis to the plastic front face (not the one next to the top battery terminal) and gently lift the chassis at the radio base. Once free from the front (about 0.5cm at the base) slide out the chassis including the coil aerial from the front facia in a downwards direction, be careful not to damage the speaker wires as you go. You will see that the coil aerial is inserted into a rubber tube that appears to be part of the front facia (it isn’t). Once the main radio chassis is free (complete with the little speaker) then push the rubber aerial sheath out from the front facia (it pushes from the outside in), and flick out the rubber grommet that sits at its base.

The next bit is tricky, de-solder the connection on the main radio circuit board where the coil aerial fixes, you will need a solder sucker as this is soldered on both sides of the PCB (printed circuit board). Once you have de soldered the old coil aerial gently pull it out along with the plastic base. This can now be discarded. Now fit the centre wire from your new SMA socket into the same hole in the PCB and fit two small screws into the chassis to secure the new socket (I think they are 2.5mm screws but you need to check, also watch the screw length is correct as long screws will damage the PCB). Next solder the SMA socket centre wire in place ensuring that enough solder is used to make the connection on the back of the PCB as well as the front, it needs to ‘soak’ through.

The only other thing to do is reassemble the radio (and if you have one) fit a new rubber aerial which will now screw into the sma socket (again, only use a Kenwood rubber aerial that is suitable for 446MHz UHF).

Now this modification itself will not give any performance gains as the small rubber type Kenwood aerials all have similar performance, poor. The trick here is to use an SMA to BNC adaptor and couple up the radio to a suitable mobile 1/2 wave aerial. This will give the desired boost in performance. One final point, it is always a good idea to get the SWR checked when you use a new external aerial with a suitable SWR meter.

THE ACID TEST:

After doing the modification and using two radios, one fixed location with a house aerial (base station style) and the other on my bike (with a motorcycle specific aerial, one with built in ground plane), I set off away from the base radio. (bearing in mind that most PMR 446 units fail in built up areas at about half a mile range sometimes less).

At one mile out communication was 100% clear with no problems at all, like being on the phone with a good line, 2-5 miles out some noise but still 100% (now way out of normal PMR range), at 5-9 miles communication starting to fade in built up area’s but when on high clear ground back to 100%. Had a conversation with base using just ¼ watt power (half the normal PMR signal strength) and parked up on very high ground (clear line of sight between both units) and it was like talking to someone next door, very clear. From 9-18 miles (which should be way out of any PMR radio range), communication was intermittent (although there was no clear line of sight on this test so difficult to say whether if we had it then ‘comms’ would be good). Last transmit ion clearly received was at 18.4 miles (my radio to base using ½ watt power), and from base to me was at 21.6 miles (using slightly more power).

This test just goes to show how poor aerials used on PMR radio’s restrict performance and a small change can pay dividends, although this mod cost over £70 (owch) including the new aerials.

Incidentally, the modification done now allows me to use the standard ‘rubber duck’ type aerial used on other Kenwood 446MHz radio’s (cost £4.99 on ebay), so I have also have the option to use it as a ‘walki talki’ as well as on the bike you could say best of both worlds