The Double Bazooka seems to have popped up on several fora recently, QRZ, eHam and VKLOGGER.
The Double Bazooka employs a pair of coax stubs as a device for compensation of feed point impedance, extending the VSWR bandwidth, principally to reducing loss in the feed line.
Consider an uncompensated dipole assuming:
- dipole feed impedance 52+j0Ω at resonance (VSWR=1.02);
- resonant frequency 3.6MHz;
- lower VSWR=2 point for uncompensated dipole is 3.41MHz;
- feed line loss in 25m of RG58 with this load is 0.646dB.
These figures are derived from an NEC model as detailed at (Duffy 2007).
Converting this to a Double Bazooka using RG58 stubs:
- feed point Z at 3.6MHz is 50.6+j0.03 (VSWR=1.00);
- lower VSWR=2 point for uncompensated dipole is 3.40MHz;
- loss in stubs is 0.091dB;
- feed line loss in 30m of RG58 with this load is 0.644dB.
So, the addition of the stubs to make a Double Bazooka reduced load end VSWR from 1.02 to 1.00, added 0.091dB of loss, but saved 0.02dB of feed line loss, net cost is 0.071dB of loss.
Though the VSWR improvement is very small, and it comes at a cost of a significant increase in loss, hams get very excited by this antenna and ascribe all sorts of magic properties.
(K3DAV 2014) supported his wild assertion that it has higher gain than a dipole with:
But a dipole does not have a center conductor running end to end and almost doubling the length of the connected line carrying RF.
Sadly, the chap doesn’t understand how a coaxial transmission line with well developed skin effect works, the current on the inner conductor does not directly contribute significantly to fields outside the cable shield. Of course, current flowing through conductors that do not contribute to radiation does contribute to loss. This response questions his credibility on matters of antennas generally. He is a bit of an evangelist for these things and makes a range of design statements and claims that are unsupported and unbelievable.
In fairness, he does recommend RG8 coax, and if we calculate the same scenario but with RG8 stubs and 30m of RG8 feed line, the load end VSWR improves from 2.00 to 1.94 (not much better), and the 0.05dB stub loss is offset by a reduction in feed line loss of 0.01dB, for a net loss of 0.04dB. So, for all that increased cost, weight, and wind resistance, radiated power is down.
Is the Cross Connected Double Bazooka a better idea? No, not really, but that can be a story for another day.
- Duffy, O. 2007. Double Bazooka Antenna performance. vk1od.net (offline) .
- K3DAV. 2014. Double Bazooka antenna issues In eHam forums. http://www.eham.net/ehamforum/smf/index.php/topic,97303.msg763298.html#msg763298 (accessed 14/06/14).