40m Coaxial Dipole Antenna aka Double Bazooka
The first HF antenna I put up was a coaxial dipole, the concept was developed by MIT. Electrically it is very different to a conventional dipole; it incorporates a balun and is almost like having two quarter-wave dipoles connected side by side. Also, unlike a dipole, you have 0 ohms between the core and the braid because they are connected together over the centre 50% of the antenna. I suspect that this is all about balancing but have yet to understand it properly.
It’s worth saying that I wouldn’t plan an antenna with so many compromises due to proximity of buildings again but I would certainly consider another coaxial-dipole. Coax is thicker and more noticeable to XYLs of course and unless you can support the feed-point there’s more weight in the air and on the solder joints – one of mine broke after 2 weeks!
At max 6m and average 4.5m above the ground, it isn’t as high as I would like it but I have had to compromise to maintain XYL relations! The house wall runs about 1′ away from the antenna for some of its length and on the other end it has to dive down to 2m with a 230 degree bend in it! Consequently, although being designed for 40m and having a reputation for low SWR and being broad-banded, I found I couldn’t get a reasonable SWR from it. The Z-match ATU, however, has allowed me to tune up this antenna and get good results from my 10w – eg. PY2VA Brazil on 15m, ZP2MAL Paraguay on 12m. Tuning with the Z-match is easy and I’d advise everyone to build one, they’re so easy to build and use – I can get a SWR down to practically 1:1 on all HF bands except 160m. 12 and 17m are very easy to tune, 15 and 40 are easy, 20 and 80m are more difficult because as you retune the radio across these bands you have to keep adjusting the ATU. However this isn’t all the story, ease of tuning on the ATU is all for the radio’s benefit, it doesn’t necessarily give you a good balance on the antenna feeder. Checking for stray radiation, I found that 12-20m are fine yet 40 and 80m radiate badly all the way down the feeder and in the shack. This was a real surprise on 40m, the band it was designed for. 17m though is perfect which is interesting when you look at my log, the higher frequencies also cope better with the relatively low height of the antenna giving a lower take-off angle helping with DX of course!
My last words…probably worth the effort if you’re looking for an all-purpose, performing dipole – it is certainly more broadbanded!
Design by VE3SQB on site http://www.brainz90.karoo.net/coaxdpl.htm
Coax end section: allow 1.5″ extra coax for soldering to end wire
Coax end piece
End support is near where the coax section is soldered to antenna wire. There is a lot of strain!