This weekend, I set myself a little project. Construct a dipole for use on 2m and an appropriate stand.
The project is pretty much complete apart from some minor adjustments and pictures are available on my Flickr page (see link in 'Related Links', or visit www.flickr.com/photos/m0gky/sets/72157625058766415/) showing the results.
The Aerial
The dipole, being a simple aerial, was very simple to construct. I decided to use my favourite of materials - 20mm PVC conduit tubing (available from B&Q), of which I had a number of offcuts from my Magnetic Loop project. All I needed for the dipole structure were two pieces of tubing, a T joint and a couple of straight-through joiners which I used to make the ends nice and tidy.
A BNC socket was fixed to the T joint allowing for the easy connection of RG58. To the BNC socket I soldered two lengths of wire, each a quarter-wavelength long (in approximation), resulting in a half-wavelength dipole. Once the soldering and drilling had been completed, it was simply a case of bolting the BNC socket in place, threading each wire into it's own piece of tubing and connecting the tubing to the T connector.
With the dipole constructed, I connected a length of RG58 between it and my 2m rig, balance the dipole between two curtain rails and gave it a try. Using an SWR meter and taking measurements at 144.025MHz, 145.000MHz and 145.975MHz I attempted to optimise the length of the dipole by trimming about 1cm of each piece of wire and then re-testing. I continued with this until I reached a SWR reading I was happy with.
The Mast and Stand
The next stage was to create a suitable stand permitting orientation of the dipole in either a horizontal or vertical polarisation and at any rotation I required. My solution was to create a small indoor mast with an H stand and guy strings.
The stand, like the dipole is constructed from 20mm PVC conduit tubing. Each leg has 1 meter of tubing, terminating in straight-through joiners with a hole drilled to accommodate the guy string. Half way along the leg, a T joiner is placed to allow a cross bar to be fixed, joining both legs together.
For the cross bar I used an offcut totalling about 40cm in length. Like the legs, this accommodates a T joiner at half it's length. This is where the central mast fits.
The mast is two pieces of tubing, each about 77cm long and connected with a straight through joiner. The split is so that the setup can be collapsed into a small easily portable package. At the top of the mast sits another joiner to which sits a short (20cm) piece of conduit with four holes drilled around it's circumference. This is where each of the four guy strings (one from each end of each leg) terminates. These guy strings are important to ensure the stability of the structure. 20mm tubing is not strong enough to support the aerial on it's own. It will bend out of shape without guys to keep it upright.
Next, the dipole can be connected directly to the mast using it's T joiner. This will allow it to be oriented in a horizontal polarisation only. Instead, I have attached a right-angle joiner. A very short piece of conduit extends horizontally from the joiner to which the dipole can then be connected. This allows orientation in either horizontal or vertical polarisation, but does put a bit of stress on the mast as it moves the centre of gravity. It's for this reason that the conduit extending between the right-angle joiner and the dipole must be as short as possible.
Still to do
The T joiner at the base of the mast, and the right-angle joiner both have removable covers. This means I can run a length of RG58 up through the centre of the mast and out of the top before connecting to the aerial. This will reduce strain on the mast as at the moment, the weight of the cable pulls down on the aerial which already moves the centre of gravity away from the mast as mentioned above. I need to order a BNC plug this run of cable but once that arrives, I can add this in.
As for the aerial, I'm considering the possibility of adding a director and/or reflector for improved directionality. I'm not sure if that will make it just too big for indoor use, but the advantage of using the tubing is that I can try it and if it doesn't work, I can just re-configure it back to a dipole.
The project is pretty much complete apart from some minor adjustments and pictures are available on my Flickr page (see link in 'Related Links', or visit www.flickr.com/photos/m0gky/sets/72157625058766415/) showing the results.
The Aerial
The dipole, being a simple aerial, was very simple to construct. I decided to use my favourite of materials - 20mm PVC conduit tubing (available from B&Q), of which I had a number of offcuts from my Magnetic Loop project. All I needed for the dipole structure were two pieces of tubing, a T joint and a couple of straight-through joiners which I used to make the ends nice and tidy.
A BNC socket was fixed to the T joint allowing for the easy connection of RG58. To the BNC socket I soldered two lengths of wire, each a quarter-wavelength long (in approximation), resulting in a half-wavelength dipole. Once the soldering and drilling had been completed, it was simply a case of bolting the BNC socket in place, threading each wire into it's own piece of tubing and connecting the tubing to the T connector.
With the dipole constructed, I connected a length of RG58 between it and my 2m rig, balance the dipole between two curtain rails and gave it a try. Using an SWR meter and taking measurements at 144.025MHz, 145.000MHz and 145.975MHz I attempted to optimise the length of the dipole by trimming about 1cm of each piece of wire and then re-testing. I continued with this until I reached a SWR reading I was happy with.
The Mast and Stand
The next stage was to create a suitable stand permitting orientation of the dipole in either a horizontal or vertical polarisation and at any rotation I required. My solution was to create a small indoor mast with an H stand and guy strings.
The stand, like the dipole is constructed from 20mm PVC conduit tubing. Each leg has 1 meter of tubing, terminating in straight-through joiners with a hole drilled to accommodate the guy string. Half way along the leg, a T joiner is placed to allow a cross bar to be fixed, joining both legs together.
For the cross bar I used an offcut totalling about 40cm in length. Like the legs, this accommodates a T joiner at half it's length. This is where the central mast fits.
The mast is two pieces of tubing, each about 77cm long and connected with a straight through joiner. The split is so that the setup can be collapsed into a small easily portable package. At the top of the mast sits another joiner to which sits a short (20cm) piece of conduit with four holes drilled around it's circumference. This is where each of the four guy strings (one from each end of each leg) terminates. These guy strings are important to ensure the stability of the structure. 20mm tubing is not strong enough to support the aerial on it's own. It will bend out of shape without guys to keep it upright.
Next, the dipole can be connected directly to the mast using it's T joiner. This will allow it to be oriented in a horizontal polarisation only. Instead, I have attached a right-angle joiner. A very short piece of conduit extends horizontally from the joiner to which the dipole can then be connected. This allows orientation in either horizontal or vertical polarisation, but does put a bit of stress on the mast as it moves the centre of gravity. It's for this reason that the conduit extending between the right-angle joiner and the dipole must be as short as possible.
Still to do
The T joiner at the base of the mast, and the right-angle joiner both have removable covers. This means I can run a length of RG58 up through the centre of the mast and out of the top before connecting to the aerial. This will reduce strain on the mast as at the moment, the weight of the cable pulls down on the aerial which already moves the centre of gravity away from the mast as mentioned above. I need to order a BNC plug this run of cable but once that arrives, I can add this in.
As for the aerial, I'm considering the possibility of adding a director and/or reflector for improved directionality. I'm not sure if that will make it just too big for indoor use, but the advantage of using the tubing is that I can try it and if it doesn't work, I can just re-configure it back to a dipole.