The thread isn't the only thing that has gotten convoluted. I didn't
write that, N3OX did.
Jim Brown wrote:
>This thread has gotten a bit convoluted, so I'm changing the subject
>to reflect the content.
>>A small amount of lengthening would put the current max in the middle
>>of the vertical section, but enough to get you to 50 ohms gets a lot
>>of current flowing in the horizontal section.
>That depends on how tall the vertical section is. It also depends on
>how much lengthening is required to hit 50 ohms resistance, which
>depends the loss in the circuit -- the wire and the radial system. My
>vertical section is 86 ft, with horizontal legs on the order of 60 ft
>(the ground slopes under this antenna, so my top loading is
>asymetrical). For a base current of 1A, the current at the top of the
>vertical section is 1.07A and the current maxima (1.17A) is at
>roughly 50 ft. So in my vertical, the current in each of the
>horizontal wires is just slightly half of the base current. I have 60
>radials, average length about 80 ft.
>NEC shows a very small effect on field strength or vertical pattern as
>a result of the lengthening. As I see it, the principal advantage is
>simplicity of tuning and easy matching to 50 ohm coax.
>As I recall, the cap in my system is about 330pF. This value is NOT
>difficult to obtain -- 10-15 ft or so of coax would do it, and would
>certainly handle the voltage. I'm using some 3kV ceramic discs from a
>local electronics surplus store. I've determined experimentally that
>their loss is quite low -- I put 1.5kW into the antenna for a while
>and then go out and feel them to see if they've gotten hot. You can't
>use just any caps here, of course, and some I tried got quite hot.
>To summarize: In my antenna, at least, the current maxima has been
>moved up about 50 ft from the feedpoint, current in the vertical
>section is quite uniform, varying by less than 2 dB, there is a slight
>improvement in the vertical pattern, and a slight increase in computed
>field strength. It matches 50 ohm coax, doesn't require a tuner
>between 1800 kHz and 1850 kHz, and is easily tuned by my Ten Tec 238
>above 1850. Yes, there is signficant current near the center of the
>toploading wires, but they're #10, so loss is small. I would certainly
>recommend this technique for antennas as short as 1/8 wavelength (70
>ft on 160M), and wouldn't rule it out for shorter ones.
>Jim Brown K9YC
>TowerTalk mailing list
TowerTalk mailing list