Hi Tero,
> I'm planning to build T2FD antenna for 160m-10m. But how to
> calculate the dimemensions and the value of terminating resistor?
Let's get this T2FD done with, once and for all.
You can think of the antenna as a terminated transmission line with a
wide place in the middle, or a squashed, shortened rhombic (which is
also terminated).
The terminating resistor is 4/3 of the transmission line impedance.
That is, if you use a 300 ohm line, the resistor is 400 ohms (390 is
the closest standard value). 450 line needs a 600 ohm resistor.
The configuration of the antenna is a wide-spaced, center-fed folded
dipole with the terminating resistor placed in the center of the
'outside' leg of the antenna, opposite the feedpoint.
The total length of wire on each side of the feedpoint to the
resistor is 100,000 / f (in kHz) meters. Mult by 3.28 for feet. The
spreader distance is 3,000 / f.
For 7 MHz (7000 kHz) you should get a wire length of 14.3 meters (46
ft, 10 in) and a spread of .4 meters (1 ft, 5 in).
Neither the actual wire length nor the spread distance is critical
since this is a wide-band antenna (4:1 ratio) due to the
termination -- but the wire should be the same length on each side
of the feedline/terminator. An antenna cut for 7 MHz works fine at
3.5 MHz and 28 MHz (and everything in between).
I'm about to build a portable T2FD out of 28 gauge wire and soda
straws with a 2 watt terminator (cut for 20 meters). I'll find out
how well it works in a week or so when I get the time to try it out
(BTW, I'm using a T2FD now that's cut for 40 meters).
-- Dean -- from Des Moines (KB0ZDF)
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