Homebrew Vertical Antenna
Homebrew Vertical
Are you like me and most other Hams, where your needs far exceed your budget? Where your have-to-haves stretches far further than your pocket money? And on top of that, if you are inquisitive and like experimenting like I do, the shortage of money really hurts.
Now please understand what I am saying: I have a good solid radio with a fantastic antenna that works very well and gives me great pleasure. When I operate. Which is quite often. Nope. What drives me is this insatiable curiosity to know how things work, and to try to replicate the mechanisms until I understand them. The “second” radio projects. I’m not touching my main radio, no, but my second radio is used for all sorts of experiments. My second radio is often lying in pieces on the operating table with all sorts of measuring tools sticking out of it.
Thus the story starts with me looking for a new vertical antenna. The piggy bank is standing on zero, and the desire is standing at boiling point. Nothing that is advertised seems to come close to my budget. What is a man to do? However, the Comet CHA-250BXII that is advertised for around R4995 seems interesting from a variety of points:
- It is a multibander. Essential for me.
- It seems to be a clean design with no coils and traps. I like that.
- It can be assembled in minutes
- They say it doesn’t need radials. We will see.
- It stands only 7 meters tall. Ehh, what did you say?
Reviews vary from good to bad. Most people think it is a good listening antenna but not a good transmitting antenna on the lower bands. The really attractive thing is the easy installation. It consists of a 7 meter high radiator, with a tin can at the bottom that holds whatever black magic it does. Looking further it appears that Iain Crawford VK5ZD had some trouble with his Comet CHA250 (the link has lots of pictures and information). Great article. Apparently he got some water in his tin can, so he decided to open it and see whats inside. My kind of man!
And once the can of worms have been opened, the design seems to be fairly simple 6:1 transformer feeding a NON_resonant element. Searching around I could find any number of people that have tried to build a similar vertical antenna. One of the best is Broadband Vertical [click here] by Martin Ehrenfried G8JNJ where he writes about his experiments with a CHA-250 style vertical. He compares various commercial verticals. His measurements indicate that no tuner is required for this CHA250-style antenna (just like the commercial version) and that it only needs a RF transformer. He follows that with a second article where he improves the design significantly – a must-read.
Look, this small little 7 meter high baby antenna is not going to perform like a huge tri-bander on a tower 40 meters above the ground. This is to be expected. But it works good enough and is very easy to construct. So I set out to make one from parts I have lying around. You can follow the slide show on my RigPics page. Firstly I decided to use 4x RFI Snap-On Chokes from MFJ that was intended to be used as RG-213 coax shields. They have inners around 10mm, and thus a few centimeters of 10mm copper gas pipe was scrounged. Cut the two copper pipes to stick out 2mm on both ends. For the inner cable I wanted a high voltage insulation, so I stripped the black PVC from a few centimeters of an electric fence high voltage feeder cable. This was fed through the two copper tubes. The one side only has two windings, the other 4 windings. The hardest part was soldering a piece of 1.5mmsq wire onto the copper tubes (they conduct heat very quickly away from the joints). Finally I connected it all up to a piece of 7 meter long electrical wire thrown up into a tree.
The SWR readings were reasonable and I could hear stations all over the band. As expected, on the lower bands the remote stations gave me lower S-readings although I could hear them well. This is exactly what Martin predicted.
- 7.060MHz 1.4
- 14.250MHz 1.1
- 21.250MHz 1.3
- 28.850MHz 1.5
This is an excellent constructional antenna, supported by lots of information on the internet. The fact that it tunes across all the ham bands without a tuner is a big plus for me. It is a good listener. The only downside is that it is not a particularly good transmitter on 80 and is marginal on 40. It is definitely in my backpack for my next weekend away. Enjoy the slides on my “RigPics” page.
