The need to know the condition of the power source for any field day / portable station is a major priority. The requirement to know exactly how much current is being drawn by all of the equipment at any given moment during the stations operation is also paramount.
When operating portable, sometimes in the remotest of location, to suddenly loose power due to a battery failure, when that illusive Grid Square is on offer, is nothing short of a disaster.
Coupled with the above, is the disaster of all disasters, a short circuit in the equipment or its cabling that puts a “dead short” across the power source! That pungent smell of melting plastic is not something I wish to experience when operating in a field day / portable location.
Lastly that impossible scenario, when arriving at the appointed location with minimal time to setup and then applying reverse polarity to the equipment, was not something that I ever wanted to happen!
Yes I have been guilty of experiencing all of the above over my 50 plus years of amateur radio operation, but it is something that I was determined not to have happen with my field day / portable operation this time around.
The following examines the Power Distribution System that I have used for my Microwave Project to ensure, as far as practicable, the mistakes of the past will never happen again!
To begin with, I was determined that all cables feeding every piece of equipment would have an in-line fuse in the DC line. Further, every cable would be terminated with a connector. In the case of all the microwave transverters, every power connector would have the same type of connector that is keyed to prevent reverse polarity.
Every Transceiver DC Power feed, while having an in-line fuse, has a different style of connector fitted to ensure that the correct fuses are used for each transceiver: (eg the current drawn by an ICOM IC7000 is greater than that drawn by a YAESU FT 817ND)
Lastly, the power distribution system must accommodate meters for Voltage and Current, an Overload Circuit Breaker, Input polarity sensing and be sufficiently robust in construction, to allow the unit to be placed in any convenient location.
The images show the final DC Power Supply distribution system. The handles on the front of the system are important and are not there for cosmetic effect. They were actually placed to protect the meter faces from accidental damage!
The internal workings of the power distribution system are the essence of simplicity. A number of basic designs were considered and then rejected. In the end, the system has met following design criteria:
A single switch isolates DC Supply from all devices.
Voltage drop between the input and the output is essentially zero (the purists will know that is not true, as if metering is introduced into the line, there will be some voltage drop but in this case it is insignificant).
All connections to the distribution system are via plugs and sockets that are rated at the maximum current that could flow under peak load conditions from the connection.
Interconnection cables are rated at the peak load that could be required by the respective connections
Under “overload conditions” the circuit breaker included will not reset instantly, but will reset after a short period of time, thus preventing the dreaded “on/off/on” condition.
Polarity reversal results in non operation, rather than the destruction of a fusible link or the tripping of a circuit breaker.
Metering is achieved using separate meters for Voltage and Current rather than a ‘multi-meter” style of operation.
Meter Scales are easy to read and not confusing to check the status of the DC Supply System quickly.
A safety warning when using portable generators in the field
As mentioned in another article I use a Portable HONDA 2 KVA generator to charge the batteries via a multi-stage charger when operating for extended periods in the field. I am never ceased to be amazed at the number of amateur radio operators who use a similar system but fail to protect themselves and there second operators from electric shock.
I use a residual-current device (RCD) positioned at the generator just in case. As an Electrician and an Electrical Engineer I have seen, at first hand, what can happen when someone, in most cases through no fault of their own, become connected to 240 volt AC. It is not a funny experience and depending on the situation it can be lethal. These devices can be purchased for a very small cost and add a level of electrical safety that all should heed!
For those who are interested, a residual-current device (RCD) is an electrical wiring device that disconnects a circuit whenever it detects that the current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance is sometimes caused by current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCDs are designed to disconnect quickly enough to prevent injury caused by such shocks.