Handheld geiger-counter

The Eberline ‘Monitor 4’ is an ideal entry level geiger counter for starting off in the hobby.
Here, the first photo shows it clicking away merrily while pointed at a old radium-dial clock i just happened to have laying around, you can see the geiger counter is sensitive. The radium used to make such clock and weatch dials, has not been manufactured since the 1970s –  but they are still as ‘active’ nowadays, as the half-life of radium can be a few thousand years, although the eerie green glow has long since stopped, mostly killed by the radium ironically, (unless you shine UV at them)  but i feel it may be as well to have some way of telling if something’s ‘hot or not’  its interesting, especially if you’re a science geek, and have been studying this kind of thing, like i have (since my teens)

This geiger counter is somewhat handy compared to many you will see on Ebay and here and there, insofar as it also detects Alpha particles – many dont, this counter has a grill on top to protect the sensitive mica window behind, which will allow Alpha’s in to be counted.

Common misconceptions are that geiger counters are radioactive themselves – no.  they’re not. If you’re eating brazil nuts, right now, they are more radioactive than a geiger counter is – they only *measure* things, like a multimeter does. The bright yellow ones you see on the TV and in movies are old, and are generally for high reading areas, and of little use, unless you’re very unlucky.
Just switching the thing on and it will begin clicking,  but no, its not Fukushima silver-foil hat time or time to take the living room door off its hinges, it’s natural, much of the clicks are from space, the sun to be more accurate,  but there are very slight readings all around us, from granite worktops, radon gas in cellars, tiny tiny amounts or radioactivity are even released when even burning coal, and numerous other places you wouldn’t think, it’s nothing to be worried about. Its everywhere. keep a check on it.


Above: The counter, checking an old clock..
below: the mica window, for alpha detection

mica window

All work, text and images © GB7MB

Service interruption.

Week commencing Monday the 9th

There may be some network interruption on the repeater this week, as we swap over equipment and upgrade the internet connection. There may also be some down time on Tuesday afternoon as some electrical work is undertaken at the site.

TG 8 Roaming and a – gloaming..


Ok, I know the photo is of a cellular phone mast, but The news is that Talkgroup 8 (slot 2) is now enabled on GB7MB, enabling users on neighbouring DMR-MARC repeaters to have their own ‘local’ group. This is usually called in your radios’ codeplug, TG 8 Roaming or TG 8 Regional – or variations thereof.  You may hear users from our friends GB7HM repeater in Caergywrie North Wales, or GB7PN Prestatyn  or GB7LP in Liverpool or GB7NM in Manchester.
This new Talkgroup 8 (slot 2) now, for us on GB7MB, takes pressure off the increasingly busier UK wide (TG 235)

In Use..
If the station you wish to communicate with is on either of those semi-local repeaters listed above, instead of using UK Wide, as you had to do before (tying up a slot on all UK repeaters) – you can now call them on the new TG8 roaming and leave UK Wide free for others who are speaking between repeaters who do not have roaming set up yet, and the only way for them to communicate is by using the (rather inefficient, but useful) UK wide talkgroup, which is now on slot 1 remember, almost all DMR-MARC repeaters nationwide from Monday 9th Feb onwards.

Remember that, like on any repeater talkgroup (except TG 9 local)  when you transmit, you are also simultaneously being broadcast through all repeaters that are on that talkgroup, on TG 8 roaming, that means you are being heard on GB7HM, GB7LP, GB7PN and GB7NM all at the same time, a little unusual if you are new to DMR-MARC, but remember this also allows a QSO from any or all these repeaters to listen or join in as they see fit.

Roaming, on compatible sets, is particularly useful for mobile operators wishing to stay in contact throughout their journey, while passing through several repeater coverage areas without switching channels, the radio does this automatically, based on which repeater it sees as strongest in your given location, for example while in range of GB7MB it all works as normal, you can be chatting to your friend on GB7PN (it’s TG8 remember..) and when you leave range of GB7MB, your radio will automatically switch you to the strongest repeater that it has identified (in its roaming list that you programmed in the CPS) which may be in our case, GB7HM or GB7NM or GB7LP – as you pass by those areas.
Programming your CPS to do this is not as tricky as it sounds, and it’s only slightly different between sets, more of which, will be coming in a later article.

All work, text and images © GB7MB

RSSI, what the hell is that!

I did consider the title ‘where have my μV gone’ but hey here we are. If you are rich/lucky enough to have an Impres microphone for your shiny new Motorola DM4600 or have a DP3600/4600 etc you may have played with the RSSI function. Received signal strength indicator, an S meter to you and me. The information is presented in dBm, that is decibel relative to 1mW.

Why move away from the traditional μV? Well using a dB scale makes a lot of things easier. For example if you know from experience that your rig can receive a DMR signal with no artifacts at -122dBm but you nearest repeater hovers around the -127dBm mark what aerial gain do you need to receive the repeater? Easy -122-(-127) = 5 so a 5dB gain aerial will sort things out.

Using similar thinking RSSI function can be used to find good spots to access repeaters, check filter tuning and look for noise sources around the shack. The resolution of the RSSI display is much better that a traditional S meter, and way better than that silly green mobile phone signal indicator.

All work, text and images © GB7MB


Busy, busy, busy!


We all lead a busy life, some more than others. GB7MB also has it’s own busy times, some of these are not due to QSO traffic but are caused by little glitches in the global DMR-MARC network. These manifest themselves as all talk groups and time slots being in use, but no traffic heard. During these times the repeater can be used as normal, you will still be heard. Occasionally when we notice this effect continuing for long periods we will remotely disconnect the repeaters network connection to prevent constant TX of noise. DMR-MARC are aware of this problem and are looking into solutions.

All work, text and images © GB7MB

Norcal audio test server – check your levels..

NorCal audio test

Audio levels between different users, it’s one of the most often heard remarks (or complaints) on DMR  – I didn’t know they had linked the  NorCal DMR Audio Test server onto Talkgroup 13.
but it seems they now have,  (Thanks Bernard) – so if you want to check your audio levels – off you go. You do need Google Chrome though, and that’s the snag. Chromium wont do, neither will Firefox or Internet Exploder.
Google Chrome only.

To use simply transmit on TG 13 slot 1: and keep it ‘in the green’ as they say.
You can download Google Chrome here   Google Chrome Browser download

All work, text and images © GB7MB