50Mhz portable vertical antenna
For a whole heap of reasons I want / need a compact set of antennas. My Cobwebb does a fair amount for me but for 6m I am poorly equipped. I used to have a 5 ele beam but the logistics of the antenna and the XYL made for an unhappy union. Because I like to get out and about on the fells for WOTA and SOTA activations and visit family in Scotland regularly something that makes use of my 7m fishing pole would be handy.
The vertical antenna is built using the basic information from G3JVL’s design but in order to make it easily transportable in a back pack for instance I decided to use wire instead of tube. This will adjust the length slightly but as with all these things its easier to remove than to add material so I always cut the antenna on the generous side.
Mike’s design is very simple a coil at the base, 4 radials and the vertical element. What originally stumped me were the dimensions. The vertical element is longer than 5/8 wave (Online calculators like this one give a shorter element of slightly less that 3.5m whereas Mike has suggested nearly 4m for the overall length from the coil to the tip. There must be a reason but it eluded me.
The shopping list for me went as follows:
1. Wire, plenty of it, at least 5m if you follow my method of wire for the vertical element and coil and enough for the radial in a different colour, i just happened to have red about the workshop.
2. Some PVC pipe, the diameter isn’t crucial but around 20mm should do it. I had some 25mm industrial mains air line offcuts from a job which has a 2mm wall thickness. As these were random lengths then a piece around 200mm and they suited me as they could be trimmed.
3. Some acrylic sheet. Again I had some 8mm sheet knocking about
4. 1 off M50 washer. BZP or whatever you have but I’d suggest not a black one, if you’ve got stainless ones then your a lot better stocked than I am! other odds and sods like bolts (I used M5’s) and spades, self amalgamating tape etc.
5. Workshop tools like a drill (pillar is best) taps and a large hole saw or jigsaw. If you have access to a lathe then things will be much easier but don’t worry if you don’t as the design can be adapted based on the materials you have at your disposal.
6. I also had some left over hose ends made from nylon, its very unlikely you’ll have any of these kicking about but they can be substituted.
7. A fishing rod at least. Mine is 7m tall and came from eBay for about £20, it gets used for all sorts of antennas.
Into the workshop we go
I’m lucky in as much as I have access to machine tools. So if you don’t then all is not lost, you may need to adapt to suit what you have at hand. the basic antenna is very simple. I have just used bits I have about the place so the cost is almost nothing.
Wire cutting and forming the coil
First job. Measure out the wire and be generous. As I mentioned I like to ‘trim on installation’ so there is always enough to go about. This first step is to make the coil on the former. Drill into the PVC tube and wrap the wire around it 10 times. mark off the point where you start and finish the turns. The ides is to drill into the tube so the wire can come out from the centre , coil round and go back into the centre of the pipe. remember to leave a long tail on the base end. The picture below should give you the rough idea.
Tapping the coil
Next find the 5th turn and make a tapping point by carefully cutting away the insulation and soldering on the wire that will go to the SO239 outer. Mike mentions moving the coil if you can not get the antenna to be resonant. This method allows you to move the coil up and down the former without having to make a new tapping point.
Radials and base
Cut the radials a bit oversized than Mikes design as they will need a bit of trimming to suit your ground conditions. Put a spade connectors on each of them and put them to one side. The washer is used to connect up the radials but a piece of plate will work just as well. Drill and tap to M5 the washer in 4 off equidistant positions to make the 4 radial connections and the 5th hole between any two of the radial holes for the connection point. The acrylic sheet should also be drilled slightly oversize to M5 to allow the threads to go through. the finished piece should look a bit like this,
Note that the white nylon parts are the bits that were on the scrap pile and not really anything comparative that you can get from a DIY shop. They are used to provide a base for the base section and something to interface with the fishing rod. The rod sits onto the upper one and has been turned to match. This part may need some thinking about and perhaps a better solution can be found with more suitable parts. I tend to favour threaded fitting from places like Yarl hydraulics who can get just about anything and our local trade counter accepts credit card purchases and are very reasonable. I may find something more common there in the future.
Connecting up and tuning
Connect everything together as per Mike’s original design and get ready for the tuning. I used an MFJ-259B analyser for this but any analyser with the right range will be just as good. I borrowed mine from the club and have used it many times. Its about time I bought my own!
Measure out from the coil outwards to 3.950m and trim the wire here. I did this in the garage horizontally before checking it vertically, It was much easier than putting the thing up and down all the time. In practice mine came out more towards the 3.4m length as per the online calculator but I suppose following the original instructions is a good idea if you use different materials.
Measure twice cut once
By folding over the long wire and twisting it together you can avoid cutting off too much wire that you may think is excess. I managed to knacker one length of wire doing this and its best not to follow my lead on this one. Below is an image of the rough tuning with the fishing rod in place.
I wanted to tune mine for an SWR of 1:1 at about 52Mhz but have a bit of flexibility in the antenna by folding over about 100mm of excess wire allowing me to adjust the antenna in accordance with my wishes at the time. Unless you feel the need to keep it on one frequency all the time then I suggest you do the same.
Finishing off tuning and testing on air can be done outside now all the hard work has been completed. The wire is kept in place with a piece of shock cord and end cap, be sure not to make it too tight or you’ll end up collapsing the fishing rod.Here’s the finished off article in all its glory.
The final thing to say is that whilst I had a bountiful heap of miscellaneous parts and the access to a lathe doesn’t mean that you can’t do a similar job with easily bought parts. In fact if I were to do this again I’d probably make a better job of the connection point. Any suggestions would be welcomed. Thanks again to Mike, G3JVL’s more permanent solution for the inspiration.