Verticals for 30, 40, 80m

[VK4ADC]

Well, just to show my ignorance of antenna theory, I ask:

In the circuit diagram it shows that both 'sides' of the feed from the transmitter attach to the right-hand tube, which appears to be metal in the photo.

Why does this not simply present a short-circuit to the transmitter?

The thing that the wiring diagram does not show is the fact that the two metal tubes are inside ferrite rings/sleeves which will make the impedance from one end to the other a relatively high impedance, even at HF.  What is not disclosed by the photos is the grade of ferrite involved so it becomes difficult to figure out the actual values of the design. Look at the detail within the red rectangle as it reveals most.

The twin tubes within the two rows of ferrites is a well known technique  and described in many articles on the web, and even one that appeared in AR magazine within the last few years.  Solid state PA output stages have been using it for many years (>20) particularly for push-pull outputs where the transistor output impedance is so low and is essentially the multiple turns through the sleeves are just a wide-band impedance step up transformer.   The one in the Iain's diagram looks to be basically a 1:3 turns ratio so about 1:9 impedance transformer, ie 50 : 450 ohms.   The top of the transformer connects to the vertical radiator hoping that at any applied frequency, the feed impedance will be somewhere approximating 400-500 ohms - which will only occur at a relatively few frequencies within the HF range.

The overall antenna design still requires a ground plane to be used if it is going to be an effective antenna.

73 Doug

See also :
http://www.g8jnj.net/cometcha250b.htm

http://www.vk5zd.com/CHA250/CHA250.aspx