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The VK5BUG Black Stick under the microscope

AR Magazine Jan 2006 carries an antenna article entitled "The VK5BUG Black Stick". The article describes the antenna that is "a cocktail of 75 ohm transmitting twin-lead and a multi-band HF vertical".

The article opens with the warning "Purists of the RF-world please be patient and accommodating; some of us want to build an aerial that works well rather than discuss why it might not!".

When all said and done, it is nothing more or less than a 10.08m vertical, ground mounted with a shallow buried radial system, and fed with about 7m of Belden 8222 70 ohm transmitting twin lead. "No base loading - no capacity hats - no - traps - and it works really well!" says the author.

Well, does the Black Stick stand theoretical explanation?

Consider its performance on 80m where it is a one eighth wave radiator fed with a short length of lossy feedline. I modelled a 10.08m vertical over perfect ground and feedline configuration in NEC, and obtained a feedpoint impedance of 6-j350. A real ground system such as that described would probably add at least 10 ohms of earth system resistance giving a feedpoint impedance of 16-j350 and an earth loss of 4.3dB. The loss in 7m of 8222 with a load of 16-j350 is 5.5dB, and a load of 3.4-j52 ohms is presented to the ATU which will have a loss of around 3dB. Total loss in the antenna, feedline and ATU is probably about 13dB, ie 5% of the transmitter output power is radiated.

Does the Black Stick depend on magic qualities of the Belden 8222 feedline? The author seems to think so, he states "Of course there must be a mismatch between the feedline and the radiator in my Black Stick, but RF loss from such a line, even if it showed an SWR of 25:1 up at 28MHz, would still be less than the loss in RG58 coax when the latter is matched."

Well, the loss in 7m of Belden 8222 at 28MHz with a 2.8+j0 ohm load (SWR=25) is 5.5dB, whereas the loss in 7m of matched RG58C/U at 28MHz is 0.54dB. The 8222 loss isn't better, it is ten times worse in that scenario. It is a bit academic anyway because 8222 is obsolete and not readily obtainable in case you had in mind building the Black Stick. (The unavailability of the "magic" component questions the practicality of the article for would be constructors!)

The author makes a point of delivering 40W to the ATU from a CW transmitter with 70W DC input, but he doesn't tell you that it is likely that just 2W is radiated.

Says something of the meaning of "works really well", doesn't it!


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