Lino VK3EI Evolution of a Communication Trailer


I like to build and test antennas of various types but there is usually a problem when it comes to setting them up and using them because of space limitations in the suburban environment and the high level of QRM that invariably exists in the suburbs. As a consequence, I tend to go camping a fair bit with the primary objective being to test antennas and “play radio”.

The biggest annoyance for me is loading (and then unloading) my standard 4×5 trailer at each end of the trip. I decided that a purpose built communications trailer (my Mobile Shack) would be a better solution, so I started mapping out a potential design specification that would suit my specific requirements.

System Requirements

An abbreviated list of the essential elements of what I wanted was:

  1. An enclosed trailer with a good degree of weather proofing.
  2. Load capacity of up to 1.5 tonnes.
  3. Storage for a number of 6m and 9m EMDRC Club masts.
  4. Space for larger antennas.
  5. Built in working/operating surface.
  6. Internal lighting for night operations.
  7. Deployable energy sources including: batteries, generator, solar panel.
  8. Adjustable internal storage.

Getting Started

Over a number of weeks, I kept an eye on the trailers available on eBay until I came across a beautiful looking steel trailer (photo 1) that I managed to get at a fair price. It turned out to be a 7×5 enclosed box trailer with a load capacity of just under two tonnes and, being a tandem axle, is very stable and easy to tow.

Photos 2 and 3 show the right and left sides of the trailer. You can see that both of these have lift-up sides but I chose to permanently lock the right side and only use the left (kerb side) opening. My reasoning for this decision was t
The rear of the trailer has two full doors that allow complete access to the inside. Photo 4 shows the rear view with the doors open; the rack system that evolved over a number of iterations and has ended up it its current configuration seems to work very well with easy access to just about anything that I may need on site.hat if I needed to operate in a mobile environment I could pull off to the side of the road , open the operating position (more on that later), all from the safety of the kerb.Both sides of the trailer have flip-up doors operated by gas struts and the rear has twin doors all of which are fully lockable with no less than eight locking points.

Layout and Operation

As I was aiming to minimise the amount of unpacking, I decided to incorporate the radio operating position into the trailer rather than have to set up a folding table, as I had been doing, on which to put the radio gear. I attached a fold-down table (photo 5), made from a solid core door, to the left side opening and fitted latches to hold it in position when stowed. The table folds down quickly and lowers to the horizontal with the two ends supported by chains. I used chain instead of hinges to allow the table to be adjusted to the horizontal even if the trailer is not on level ground.

This working surface is covered in a very low pile marine-grade carpet. This table makes it very easy to work when you first arrive on site as you quickly have a flat area to put things on instead of putting everything on the ground as you unpack.

The trailer also has a nice rail system around the top that made it easy to fit a couple of custom-built aluminium mast holder tubes in the centre of the front and rear rails. The mast holder tubes (photo 6) can accommodate up to five 9m aluminium masts (plus tent poles, etc) and are securely fixed to the steel rails for safety. The reason for mounting these in the centre is to leave space either side to take other long items needed for camping from time to time or for long antenna sections that I may have to carry in the future. Foam rubber pads were glued to the inside of the tube end-caps (photo 6 inset) to protect the caps should the contents of the tubes slide around under braking/acceleration forces.

Extensive use of plastic storage boxes has been made to categorise the various bits and pieces needed when setting up antennas. These boxes stack easily in the shelving, making it possible to minimise floor space usage, thereby leaving an aisle down the centre of the trailer. The aisle allows access to any part of the trailer without having to take everything out to get to something in the far corner. As there are a number of items that don’t really need to be moved very often, these are located towards the front of the trailer (farthest from the rear doors); these are the fridge, spare antenna/mast accessories, folding table and the solar panel.

Power Supply

The majority of devices requiring power are run from a bank of storage batteries. I originally had the batteries clamped down at the base of one of the racks but access was difficult so a battery box (photo 7) was constructed and bolted to the floor on the left side of the trailer, just below the work table (main radio operating position). The storage batteries have a total capacity of approximately 310Ah (350Ah including auxiliary battery).

The three main batteries (one 150Ah and two 80Ah) are connected in parallel and fitted with a 60A DC circuit breaker that feeds the Powerpole Distribution Board (fused) shown in photo 8. There is also a small auxiliary battery (40Ah) that is used to drive the air compressor for the Clark pneumatic mast (discussed later) and the LED work light. I should note here that I have put Anderson Power Pole connectors on just about everything (including all my radio gear). The Powerpole distribution board has a 1m cable attached that makes it easy to move around to connect to anything that needs power.

There are a number of other power sources available in the trailer; a solar panel with a 120W rating is able to be deployed on sunny days and it is able to deliver a constant charging current of over 8A to keep the batteries charged. On sunny days this has meant that all of the power stored in the batteries is available once the sun goes down and no other external power source is required. Where we need additional power, a small petrol generator is carried on board to provide 240Vac. This is used to power the battery charger that has a charging capacity of 25A and it able to restore the batteries to full charge within a few hours. The generator has a running time of approximately 10 hours from a full tank which means that we are able to keep operating all night if necessary without requiring mains power at any stage. I have never had to run the generator full time as the charging rate is sufficient to only require running it for a couple of hours whenever the battery voltage indicates that a “boost” is required. I have a number of battery monitoring displays at different points in the electrical system.


For night time operation, internal LED strip lighting is fitted to the ceiling. As it turned out, these lights are extremely efficient and I found that they were actually too bright at night. I decided to fit an LED dimmer control which subsequently was found to generate a huge amount of radio interference. I should have done more thorough research on this as I found that these devices use pulse-width modulation to vary the current through the LEDs and therefore generate lots of RF noise. I decided to go back to basics and replace the dimmer with a simple resistor switched into the LED supply line whenever I needed to dim the lights and this solved the RF problem and works to reduce the light intensity to a nice working level. The simple switch arrangement is shown in photo 9; this also has the control switch (red button) that controls the power to the reversing camera.

I said at the outset that the trailer has undergone a number of iterations and during one night time operation I found that the internal lighting made it difficult to see the front panels of the radio equipment because the light was coming from behind the equipment. The solution to this was to add a couple of strips of LED lights to the side lift up panel of the trailer that is directly over the work table. These lights provide even lighting from directly overhead (no reflections on any of the radio front panels). The roof mounted strip lighting is shown in photo 10 and the LED lights over the work table are visible in photo 5.

Storage and Water

To further reduce clutter inside the trailer, a lockable steel toolbox was installed on the towing arm (see photo 11 and refer to photo 1). Here, smaller items, such as: tools, ropes, coax cables, battery charger, straps are stored. The trailer auxiliary battery is also mounted here; this battery feeds power to the rear view reversing camera (refer to photo 6), the Clark mast and the work light as mentioned earlier.

When deployed, the trailer is completely self-contained and it is possible to remain deployed for a few days at a time without reliance on any other external infrastructure. This is important as I am a member of WICEN (Wireless Institute Civil Emergency Network) and I want to be able to deploy the trailer in emergency situations should the need arise. For this kind of deployment, I have catered for the ability to erect a sun/rain shade (photo 12) that makes being out in the open a little more comfortable. This was originally a 4m by 6m heavy duty tarp (as in the photo) but I have since replace this with a 3m by 6m steel framed marquee.

The marquee frame is visible in photo 5 directly behind the blue jerry can on the left side of the trailer. The blue jerry can is also an interesting piece of high-tech equipment; it is called a LIFESAVER jerry and it contains a membrane filtration system that can turn ANY source of water into pure drinking water.

The water produced is essentially sterile and can be used for medical purposes if necessary. The jerry can has the capacity to filter up to 20,000 litres of water before the filter requires replacement. This system therefore caters for our water requirements when the trailer is deployed in the field. The trailer also has cooking facilities and a stock of canned food sufficient for a few days in the field. A small fridge allows perishable items to be stored safely.

Antenna Mast

The free-standing Clark pneumatic mast (photo 13) that I mentioned earlier is a real winner. It has dramatically reduced my use of the aluminium masts that I had come to rely heavily upon for all of my antenna erections. Because the Clark mast is easily and quickly raised or lowered, it is possible to change antennas in a matter of minutes. The mast is approximately 9m tall when fully raised and I have evolved a common quick-change adapter system for the top of the mast that allows rapid swapping of any antenna simply by lowering the mast, removing the installed antenna, and dropping in a different antenna (that uses the same quick-change base adapter).

For even quicker deployment, a FAMPARC whip (manufactured by the FAMPARC Radio Club) is installed on the roof rack and can be raised and locked in the vertical position within minutes. This allows the trailer to be operational on HF at any time with minimal setup.

A small air compressor (mounted in the front toolbox) quickly raises the mast as only 20psi of pressure is required. Once raised to full height, an air valve is closed and the mast remains in position until the valve is opened. Raising and lowering takes about 60 seconds in each direction.

Photo 14 shows the mast in its lowered position with a quarter-wave vertical antenna fitted and ready to be raised into its operating position (photo 13 shows the system in operation).

Off-road Trials

This “mobile shack” has been evolving for almost a year and it is finally at a stage where changes are now very minor – more in the line of “tweaking”.  I don’t expect any dramatic changes to the configuration because it has been operated under a range of conditions and performed flawlessly (or nearly so) on all occasions.

The radio equipment and the associated electrical and antenna systems have also evolved during the evolution of the trailer but I will leave discussion of those for another time.