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J Pole Loft Antenna for 2 Metres


This is just my take on the classic J Pole antenna that's been around forever. There are tons of videos and articles online describing how to make them and so here's another one! They're all pretty much the same electrically (they have to be really) but the way people construct them varies depending on what 'bits & pieces' they already have to hand or how much they want to spend. 15mm copper pipe is quite a popular choice for the elements and that's what I've used here. For my first build I used compression fittings to join everything together, but apart from looking like it belonged to my central heating system, it didn't allow me to easily play around with element lengths to achieve a perfect match, so a different method of construction was required

Apparently, the gap between the longer element and the shorter element isn't critical, so I decided to use plastic double pipe clips not only to set the distance between them but also to hold them together. The coaxial feed line is attached to the elements using
15mm hose clips available from eBay (or Halfords here in the UK). This is the type with an M4 bolt that protrudes slightly from an encapsulated square nut and it's this protruding M4 thread that makes these so useful, as the coax can simply be stripped back, wrapped around the bolts and tightened down with nuts and washers. Incidentally, the coaxial feed cable used is mini RG8 super XX (7mm)

I still needed a way to create the 'J' junction at the bottom and it was the hose clips that were once again put to good use. By bolting a metal strap onto them, the required electrical connection is created between the two elements. The strap was cut from another hose clip as I'd ordered a pack of 10 and had a few spare. Not only are hose clips great for making connections to the copper pipe, they can also be easily re-positioned, not just to find the best feedpoint for the coaxial cable, but also to vary the length of the elements. As for dimensions, there are many J Pole calculator websites out there, but my favourite is M0UKD.com. Although the calculators will get you in the ball park, there is always room for some fine tuning to get the SWR spot on!

Construction is kept simple by using two straight sections of copper pipe so no bending or soldering is required (one length of 3 metre pipe will be enough). The length of the lower section of the longer element is not critical and is purely used for mounting. The lower section of the shorter element (overhang) is 40mm and is there if required for tuning, but after some experimentation the dimensions shown on the right are what worked best for me (measurments are from top to bottom which just seems easier when using a tape measure!). When adjusting for the lowest SWR, start with the dimensions suggested by the calculator and go from there. Move the shorting strap first followed by the coax connection, keeping them about 50mm apart



So there it is, just two cuts with a pipe cutter, no bending, no soldering (except maybe for the plug at the other end), fully adjustable coax feedpoint and fully adjustable wavelength on the 2 metre band. With the dimensions shown, I managed to achieve an SWR of less than 1.1:1 (the needle barely lifts off the stop). Finally, copper stops can be glued onto the ends of the elements to tidy up the appearance but this is entirely optional


       


Footnote: A common thing you'll see on antennas of this type is a choke balun. It usually takes the form of a few turns of coax in a loop or on a former placed near to the feed point. This is to stop stray RF from the antenna travelling back down the coax and getting into the shack (perish the thought!) and upsetting the VSWR. I figured that nothing I added to the feedline could improve the near 1:1 match that I already had, so no choke was used