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Hello,
Has anyone tried using a 11.1V battery on the QMX?? Fully charged the batter has12.7 volts.??
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Thank you,
Brian K?BWH?
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I use 3S lithiums exclusively. I had to set the minimum voltage warning as low as possible because the QMX works fine with as little as 10 volts. JS6TMW
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As stated in the manual (big and in red(, QMX shall not be operated with voltages, exceeding 12V. The output power depends on the input voltage. You may get away with 12.7 V on CW, but components may get hot. I would not use overvoltages on FT8. Keep in mind: 12.7 is clearly out-of-spec.
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Will the 9V QMX+ receiver accept 14.75V or will it break?
I ask this question in another thread "QMX+ dev-board" in which I explain that I want to install a bank of 4 LiFePO4 batteries on the dev-board and they receive 14.75V from the charger. Obviously the PA will be protected by the settings "5.17 Protection menu" - "Protection voltage" set to "Prevent Tx", "Min Voltage" set to 7 and "Max voltage" set to 10. Usually the QMX+ will be "OFF" when the 14.75V charger is plugged in the DC socket in order to avoid applying this 14.75V to the components but I wonder if in case the QMX+ is "ON" there might be a danger for the receiver section?
73 - Pierre - FK8IH
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Thank you, all for your replies.?
K0BWH
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Pierre,
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I think 14.75v would be fine except for the SMBJ13A TVS protection diode at D110.
D110 is on the bottom left of the first schematic page, near the power connector.
It works something like a Zener diode.
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The datasheet is here:
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It has a "Reverse Standoff Voltage" of 13.0 Volts.
That's the point at which it might start conducting some current, and the maximum recommended voltage across the part in normal use.
It has a "Minimum Breakdown Voltage" of 14.4 Volts, at which point it could start conducting several Amps, you definitely don't want to exceed that.
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I suppose you could replace that diode with an SMBJ15A, which has a Reverse Standoff Voltage of 15 Volts,
but may not start protecting the rig till the power supply reaches the Maximum Breakdown Voltage of 18.5 Volts.
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You could move D110 to the output of Q105, so after the "Power On" switch of the QMX.
Accidentally pressing Power On during charging would then blow Q103 and Q105, but the rest of the radio would survive.
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You could add a diode such as a 1n4007 in series with your 14.7 Volt supply to drop it to around 14.0 Volts.
That would probably work ok for charging your QMX battery.
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Or you could add an extra power jack to the back panel for charging, making it obviously different than the current power jack.
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The only difference between a 9v build and a 12v QMX build is in the turns ratio of T501.
That turns ratio doesn't matter unless you were to transmit.
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Jerry, KE7ER
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On Fri, Mar 14, 2025 at 12:37 AM, Pierre - FK8IH wrote:
Will the 9V QMX+ receiver accept 14.75V or will it break?
I ask this question in another thread "QMX+ dev-board" in which I explain that I want to install a bank of 4 LiFePO4 batteries on the dev-board and they receive 14.75V from the charger. Obviously the PA will be protected by the settings "5.17 Protection menu" - "Protection voltage" set to "Prevent Tx", "Min Voltage" set to 7 and "Max voltage" set to 10. Usually the QMX+ will be "OFF" when the 14.75V charger is plugged in the DC socket in order to avoid applying this 14.75V to the components but I wonder if in case the QMX+ is "ON" there might be a danger for the receiver section?
73 - Pierre - FK8IH
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Adding a diode to your charger as Jerry suggests would likely work well.? It may not charge the battery to 100% with the reduced voltage, but it will get most of the way there, giving you almost as much runtime (which will be many hours), and make your battery last longer.
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When I read the manual and use the chart showing the 9V & 12V expectations I get a tad confused.? The manual indicates a "little over" the wound target.? How much is a "little" ??
Since it's not easy to find a perfect 9 or 12 volt supply and the chart shows power levels exceeding what I would think "little" might be what is the MAX voltage for a 9 volt build ??
John KK4ITX
for Zephyrhills Area Amateur Radio Club Many of life's failures are people who did not realize how close they were to success when they gave up. ? ? ? ?Thomas A. Edison? ? ?
On Friday, March 14, 2025 at 09:30:54 AM EDT, Jerry Gaffke via groups.io <jgaffke@...> wrote:
Pierre,
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I think 14.75v would be fine except for the SMBJ13A TVS protection diode at D110.
D110 is on the bottom left of the first schematic page, near the power connector.
It works something like a Zener diode.
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The datasheet is here:
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It has a "Reverse Standoff Voltage" of 13.0 Volts.
That's the point at which it might start conducting some current, and the maximum recommended voltage across the part in normal use.
It has a "Minimum Breakdown Voltage" of 14.4 Volts, at which point it could start conducting several Amps, you definitely don't want to exceed that.
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I suppose you could replace that diode with an SMBJ15A, which has a Reverse Standoff Voltage of 15 Volts,
but may not start protecting the rig till the power supply reaches the Maximum Breakdown Voltage of 18.5 Volts.
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You could move D110 to the output of Q105, so after the "Power On" switch of the QMX.
Accidentally pressing Power On during charging would then blow Q103 and Q105, but the rest of the radio would survive.
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You could add a diode such as a 1n4007 in series with your 14.7 Volt supply to drop it to around 14.0 Volts.
That would probably work ok for charging your QMX battery.
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Or you could add an extra power jack to the back panel for charging, making it obviously different than the current power jack.
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The only difference between a 9v build and a 12v QMX build is in the turns ratio of T501.
That turns ratio doesn't matter unless you were to transmit.
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Jerry, KE7ER
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On Fri, Mar 14, 2025 at 12:37 AM, Pierre - FK8IH wrote:
Will the 9V QMX+ receiver accept 14.75V or will it break?
I ask this question in another thread "QMX+ dev-board" in which I explain that I want to install a bank of 4 LiFePO4 batteries on the dev-board and they receive 14.75V from the charger. Obviously the PA will be protected by the settings "5.17 Protection menu" - "Protection voltage" set to "Prevent Tx", "Min Voltage" set to 7 and "Max voltage" set to 10. Usually the QMX+ will be "OFF" when the 14.75V charger is plugged in the DC socket in order to avoid applying this 14.75V to the components but I wonder if in case the QMX+ is "ON" there might be a danger for the receiver section?
73 - Pierre - FK8IH
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Another option is a Very Low Drop Out regulator like G4COL described in the latest SPRAT (issue 201) page 12. More efficient than a buck regulator, all analog, and capable of huge currents.
I built mine for a 12 volt output and when it's running off of a "12.8" volt 4S LiFePO4 battery anything above 12.0 volts coming into the regulator from the battery (including its 14.6 volt initial charge) results in a 12.0 volts available to the transceiver. When the battery discharges below 12 volts then the output is just millivolts below the input. . . essentially the same coming out as going in! All the way down to 8 volts (which is below the level where you begin ruining the battery). My adaptation uses less than 1 mA for itself.
I used what I had in my junque box: Substituted an IRF4905 (74 amp when sufficiently heat-sinked!) MOSFET, a couple of plastic 2N2222 generic switches, and an LP2950-5 LDO regulator for the reference. I also chose a 100K resistor for the gate bias for better operation at high current when below the regulator output voltage, and a higher emitter-to-ground resistor value as well as my own fixed values for the divider network. It's all very non-critical and could be built for less than $5 USD for new parts.
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73, Don
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Don, thanks for posting!
Since your jpg will disappear in a few days, I have given it a permanent home at:
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Looks like a very good solution to the problem of limiting voltage to the QMX at 12v.
I believe that circuit will have a quiescent current of around 1ma.
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Could change the resistor divider to give 9v out, for example? ?R3=40k and R4=50k,
or any resistors of the same 4/5 ratio.
Given that the LM2950-5 is a 5v reference, we need that ratio to give
? ? ?9v*50k/(40k+50k) = 5v on the base of Q2.
If the 9v output exceeds that value, the base of Q2 rises above the base of Q1?
and raises the voltage at both emitters, resulting in no current through Q1
and a rise in voltage at the Q3 gate.
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Q1, Q2 could be any small NPN's, such as a 2n3904 or the QMX's BC817.
Q3 is an enhancement mode PFET, it must be able to dissipate any excess voltage?
at the expected current.? For example, with 17v in, 12v out, and a 2a current,?
it will be dissipating (16-12)*2 = 8 Watts.
May need to be bolted to the back cover to dissipate the heat.
The LP2950-5 is a 5v voltage reference, could be replaced by a 5v zener and a 10k series resistor
at slightly increased quiescent current and somewhat poorer regulation.
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Perhaps we could have a PC board with pads for all the parts.
Gets mounted to the back panel by the tab of Q3.
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I doubt many of us have signed up for SPRAT, perhaps we should.
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Jerry, KE7ER
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On Fri, Mar 14, 2025 at 02:09 PM, Don, ND6T wrote:
Another option is a Very Low Drop Out regulator like G4COL described in the latest SPRAT (issue 201) page 12. More efficient than a buck regulator, all analog, and capable of huge currents.
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So instead of using fixed R3/R4, if we use a potentiometer, we can effectively make QMX’s output power adjustable, right? Maybe not very accurate in terms of the absolute output power, but should be pretty accurate in terms of percentage. Like defining 12V to be 100%, then at 12V x 0.7=8.48V would be 50%, something like that.?
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On Fri, Mar 14, 2025 at 05:07 PM, Zhenxing Han N6HAN wrote:
if we use a potentiometer, we can effectively make QMX’s output power adjustable
I'm not so sure about that.
The QMX does not like the supply voltage changing while it's running.
Mine rebooted when I tried it.
And the Rev 1 models would blow up (before Hans swapped the tantalums for ones with higher voltage rating).
The SMPS have to track the changing input voltage and adapt quickly to produce steady supply voltages for the microcontroller, etc.
There is a long thread on this topic from the QMX rev 1 era.
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73, Mike KK7ER
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I had a 7812A regulator in the junk box, and added electrolitic capacitors on the input and output plus a 1n4007 diode in series to drop output voltage to 11.4VDC.?
Please your advise, what are the disadvantages of this regulator vs the Sprat regulator?
I considered the Aliexpress BUC converters but fear eventual rf noise.
Thank you,
Luis
CU2IJ
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Luis,
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Here's a datasheet for the 7812A:
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On page 8, it says that Vdrop when supplying 1 Amp out is typically 2 Volts, could be somewhat more on your device.
So if you put the 7812A on a 13 Volt battery, the you will typically only get 11 Volts out when the transmitter is drawing 1 Amp.
It is only when the battery is above 14 Volts that the output is a well regulated 12 volts when drawing one Amp.
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The IRF4905 that Don showed in that schematic has an RDSon of 0.02 Ohms, so when fully on it looks like a 0.02 Ohm resistor.
The two NPN transistors are very sensitive, they slam that IRF4905 fully on if the output voltage is a little bit below 12 Volts.
So with a 12 Volt battery at 1 Amp, the output would be 12v - 0.02*1 = 11.98 Volts, not the 10 Volts you would get out of the 7812A.
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Again, that's a typical drop of 2 Volts for the 7812A, they don't spec a worst case, it could be only 9 Volts.
And your diode would reduce it by another 0.7 volts, to 8.3 Volts when transmitting and drawing 1 Amp from the 7812A.
So with the 7812A you might be wasting a quarter of the power coming out of a 12.0V battery, with Don's PFET you lose almost nothing.
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Also, you can adjust the value of R3 and R4 to give exactly the voltage you want, instead of using a diode.
If you really want the 11.3 Volts that you might get with a diode following a 7812A (when the battery is at 15 Volts),
then adjust the resistors such that there is 5 Volts across R4 and (11.3 - 5.0) = 6.7 Volts across R3.
The ratio you need is then R3/R4 = 6.7/5.? ? ?Having R3=6.7k and R4=5k would work.? Or R3=67k and R4=50k.
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For exactly 12.0 Volts out, that ratio should be (12-5)/5 = 7/5 = 1.4
Don's schematic shows a ratio of 30k/22k = 1.36, not exact because he is using standard resistor values.
The current through R4 will be? 5v/22k = 0.2273 ma, and the voltage across R3 will be 0.2273ma*30k = 6.819 Volts.
So the output voltage is 5 + 6.819 = 11.819 Volts, slightly below the target of 12.0 Volts.
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Jerry, KE7ER
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On Sat, Mar 15, 2025 at 06:10 AM, Luis Pacheco wrote:
I had a 7812A regulator in the junk box, and added electrolitic capacitors on the input and output plus a 1n4007 diode in series to drop output voltage to 11.4VDC.?
Please your advise, what are the disadvantages of this regulator vs the Sprat regulator?
I considered the Aliexpress BUC converters but fear eventual rf noise.
Thank you,
Luis
CU2IJ
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Luis,
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Also, know that a buck converter will also have a dropout voltage of perhaps 2 Volts, similar to the 7812A.
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Jerry
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Luis,
Although much simpler, your circuit would induce about 2.5 volts of loss and a higher quiescent drain (5 mA). When the voltage from the battery drops to 14 volts then the output of the regulator begins to drop from 11.4 volts. When the battery drops to 12 volts then the radio is only seeing 9.5 volts.
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With the VLDO the only loss is when the battery is over 12 volts. When it discharges to 12 volts then the output is still 12 volts. When it discharges to 11.5 volts the output is about 11.5 volts. If the battery were to drop to 10 volts then the output to the radio would be about 10 volts (well, maybe 9.95 volts). Also the VLDO has a lower quiescent drain. Mine measures 750 microamps maximum under any load. When my IRF4905 is turned on (when the battery feed is 12 volts or below), its series resistance is around 0.02 ohms. Negligible!
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Depending upon which buck regulator you use, you will have high quiescent current and usually efficiency of 80% or so. Fed with a 12 volt battery you won't get a stable 12 volts out when the battery charge gets down near 12 volts, much less below. Check the specifications for your particular device.
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Buck converters are useful when you need to use a high voltage input to create a low voltage output. If you wish to run a 5 volt device from 12 volts then they are the answer. Otherwise, scale your battery to your device for best efficiency.
73, Don
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Jerry, you are far faster at responding than I am! Looks like I repeated most of what you said :-)
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Actually, my output voltage was 12.012 volts. I used 1% surface mount resistors and checked that they were, indeed, within a few ohms. That made me wonder about the LP2950-5 regulator that I used for reference; So I pulled out one of my boards and checked. Yep! 5.0047 volts. So close 'nuff. Not to worry. It's a curiosity sometimes when there are small discrepancies betwixt design and prototype results. Perhaps someday I will investigate further.
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Oh, by the way...I did try zener reference for the circuit. All of them proved quite sloppy without buffering. Usually .2 to .3 volt change from 12 volt to 20 volt input. That and the higher current overhead led me to my old favorite regulator.
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Thanks for posting the schematic!
73, Don
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Don,
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Good stuff!
You are drawing zero current from that LP2950-5, they probably set it a bit high so that
when drawing max current it is not too low.? The last tenth of a volt does not matter here.
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I have in the past recommended the use of the LM2940CT-12 low droput regulator,
has a 0.5v droput and over 10ma of quiescent current, gives 1 Amp max
Don's circuit from SPRAT is better for this application, as it has almost zero droput and
far less quiescent current.? Don's circuit is also useable for a 9v build where the 1 Amp
limitation of an LM2940CT-9 is not quite enough.
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Advantages of the LM2940CT-12 are that it is a single piece, easily mounted to the back panel
for a heat sink.? The 1 Amp limit can be a feature if your board has a short, the LM2940CT-12 will
limit the current to something reasonable.? It also shuts down gracefully if it gets too hot.
The QMX has reverse power supply polarity protection in Q105, for projects which do not have
this feature the LM2940CT-12 would provide it.? There are linear regulators with lower dropout
and higher current limits than the LM2940, but they tend to be much more expensive.
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I think we should have a small board for Don's circuit.? The PFET has the pins bent 90 degrees
up away from the surface of the heat sink and soldered to 3 holes in the edge of that PC board.
The PFET is then mounted with a bolt to a hole in the QMX back panel, the board
being held well away from that panel.? Quiescent current is very low at around 1ma,
but the battery should be disconnected when not in use to avoid damage from over discharge.
One might add a fuse of some sort to the input of Don's circuit to limit current, being careful
not to introduce significant voltage drop.
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Jerry, KE7ER
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On Sat, Mar 15, 2025 at 08:11 AM, Don, ND6T wrote:
Jerry, you are far faster at responding than I am! Looks like I repeated most of what you said :-)
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Actually, my output voltage was 12.012 volts. I used 1% surface mount resistors and checked that they were, indeed, within a few ohms. That made me wonder about the LP2950-5 regulator that I used for reference; So I pulled out one of my boards and checked. Yep! 5.0047 volts. So close 'nuff. Not to worry. It's a curiosity sometimes when there are small discrepancies betwixt design and prototype results. Perhaps someday I will investigate further.
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Oh, by the way...I did try zener reference for the circuit. All of them proved quite sloppy without buffering. Usually .2 to .3 volt change from 12 volt to 20 volt input. That and the higher current overhead led me to my old favorite regulator.
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Thanks for posting the schematic!
73, Don
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I run my 12v @10.5 using a buck/boost it's happy!
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I have been using these-?
On Wed, Mar 12, 2025 at 7:37?PM K0BWH. Brian via <brianhowe14= [email protected]> wrote: Hello,
Has anyone tried using a 11.1V battery on the QMX?? Fully charged the batter has12.7 volts.??
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Thank you,
Brian K?BWH?
-- Anthony Luscre
K8ZT Ohio Section Section Youth Coordinator & Education Outreach ARRL - The National Association For Amateur Radio? ?
K8ZT Radio Website-
Amateur Radio Resources for Students/Youth -
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Pierre - FK8IH
KK4ITX John
Don, ND6T
Richard Dyer
Anthony Luscre