Saturday, March 2, 2019

Introducing the bobtail curtain antenna

One of the lesser known antennas is the bobtail. Probably because of the amount of space needed. And they appeal to a niche facet of amateur radio - those who chase DX on lower HF bands like 3.5 and 7 MHz.  Nevertheless, if you have the space the bobtail is simple and cheap to build. And if you're near salt water they can give you a competitive low band DX signal. 

Or, as a novelty, you could experiment with them on the VHF and UHF bands. They would be great if you live on a coast or highway and want good signals in both directions. Just be prepared to explain what it is each time you mention what type of antenna you are using.

HF bobtails are normally set up as wire verticals. Shaped like 'm'. They are a bit over a quarter wavelength high (to allow for some clearance from the ground) and one wavelength long. They need poles at their ends to support. And maybe also a pole in the middle. They are vertically polarised and bidirectional.

How do you feed a bobtail? It depends on where the feedpoint is. If you feed it at the top it's low impedance so you can use coaxial cable. But top feeding can be inconvenient unless you find a way to bring the feedline out away from the antenna. Or you could feed at the bottom. Here it's high impedance. Feeding from coax is normally via a tuned circuit or antenna coupler.

Below are some articles and videos, in some cases comparing the bobtail with other antennas.

* The Half Square, Double Half Square and Bobtain Curtain by WA3AYW
* Bobtail curtain antenna (video by W1GV)
* Bobtails and half squares by K3KY
* A bobtail for 144 MHz (pdf by ZS1AN)
* Testing a bobtail antenna on 432 MHz (video)

PS: Many other wire antennas are covered in Hand-carried QRP Antennas. Available in electronic and paperback form (some countries) this well-reviewed book is a popular read amongst hams who go portable.

 Link to find out more about Hand Carried QRP Antennas

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