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Coaxial Folded Dipole FM Antenna - best kept secret or waste of RF?

Researching a folded dipole for my latest effort, discovered an interesting permutation called a coaxial folded dipole or 'bazooka antenna.'
     Easy to build from readily available materials, compact and, in theory, a good candidate for low power radio.
     Further reading indicated (surprise!) some disagreement on the efficacy of this approach, here are some ideas from around the web.
     According to VE3SQB, software developer and webmaster for his must-visit download site, VE3SQB Antenna Design Programs, "The coaxial dipole is the best keep secret. Made out of coax, it matches closely to 50 ohms and can be set up horizontally or in an inverted V. It is very broadband for a dipole and makes a great field day antenna."
     VE3SQB is so excited about this antenna that the developer created an application to easily calculate antenna dimensions and construction tips - download here: Coaxial Dipole Calculator
     VK2YVA prepared a nice article using screen shots from the Coaxial Dipole Calculator to write a thorough construction article for the antenna entitled Coaxial Folded Dipole Antenna
     Not so fast, says G3TXQ. In this article on Karina.net, our coax-skeptic reports "We conclude that the use of coaxial cable can reduce an element's length by something approaching the Velocity Factor of the cable. However, the penalties are significant power losses and a much reduced performance bandwidth. Finally, in anything other than a simple dipole, the size reduction is likely to be significantly less than that predicted by a simple scaling based on the Velocity Factor."
     To be fair, G3TXQ's testing was in the 75MHz band and broadcast FM will likely perform better.
K3DAV weighs in to report "The Double Bazooka antenna is very popular and the following design is my slightly modified version from the original. The main difference is in the tails.  The original uses an added piece of twinlead or solid copper wire.  My version uses an extended unshielded portion of the center conductor for added strength, and easier SWR tuning.
     As with the dipole, the feedline can be either 50 ohm coax or 400 ohm ladder line.  Every comparison has shown the Bazooka to outperform a standard dipole. Now I am going to show you just how simple this antenna is to build."  And he does, in his article Build A Double Bazooka Antenna (K3DAV Version) and with very nice illustrations.
     Will the Coaxial Folded Dipole work for you?  It seems it will only take about 6 feet of coax and and hour or so to find out!
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2 comments:

E.J.Forwood said...

If you check the dimensions posted by K3DAV against those produced by the software you will see a difference. Which is correct?
I am interested in building for 20M and 6M.
73,
K4EJF

Low Power Radio Guy said...

Howdy K4EJF, great question and thank you for asking. Before looking at which might be correct, I double checked to see where the differences lie.
Of the several dimensions of this antenna, the two with the biggest impact might be overall length and the length of the shielded portion.
From what I could see, the two authors agree on the length of the shielded portion but K3DAV's calculations provide quite a bit more length from the end or the shielding to the outer ends of the core.
This suggest that K3DAV is providing more core radiator to trim during the tuning process but that's only my guess.
The other difference might come from difference velocity factors each author used in setting up their calculations. K3DAV seems to favor RG-8 and VE3SQBR appears to focus on RG-58
Which one is correct? It's likely that both have their advantages but using K3DAV's dimensions would give an antenna that is more forgiving in the tuning process because the unshielded end elements are longer, offering more room for fine tuning.
To reach the authors for more in depth information, use k3dav@msn.com and ve3sqbr@cogeco.ca - VE3SQBR asks that you put "Antenna" in the subject line to get past the spam filter.
Please let us know how your project turns out!

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