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Old November 21st 06, 10:57 PM posted to rec.radio.amateur.antenna
C. J. Clegg C. J. Clegg is offline
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First recorded activity by RadioBanter: Oct 2006
Posts: 33
Default Questions on broadband antenna design (e.g. T2FD)


I have a need to design and erect a single antenna that covers a number of
government frequencies from around 4 MHz to around 9 MHz. It needs to
cover a radius of around 300 miles, give or take, and so an NVIS
installation seems indicated. It cannot use any sort of antenna tuner and
must be fed with a single coax of 50 or 75 ohms. The frequencies to be
used are scattered here and there within the 4 to 9 MHz range and so I
can't just put up a trap dipole or multiple-legged dipole with legs cut
for different frequencies (well, I probably could, but it would be
impractical).

I've been looking at the T2FD (tilted terminated folded dipole) design or
some variation thereof. I realize that these antennas are a compromise at
best, and suffer from varying degrees of inefficiency at various
frequencies within their design range. However, it seems they are the
only antennas I can find (so far, at least) that claim to cover the
desired frequency range with a single coax feedline and no tuner.

I don't need 3 to 30 and so I'm not inclined to spend $200+ on the B&W
variant.

I think I have ample space and trees located at reasonable endpoints to
put up a model that's at least 150 feet long, perhaps longer. On the
other hand, if shorter is just as good given the desired frequency range,
that will be easier to do.

Every indication I've found on the web seems to say that 75-ohm coax, a
4:1 balun, and a 390-ohm noninductive terminating resistor will give a
reasonable match across the design range of the antenna.

So, my questions are...

1. How do I determine the overall antenna length that will give me the
most efficient (which is to say, the least inefficient) performance across
the 4 to 9 MHz range?

2. How do I determine the minimum power rating for the terminating
resistor for an antenna that will be driven by 100 watts maximum?

3. Where can I buy a few noninductive terminating resistors that meet the
power rating determined in (2)?

4. I would rather feed it with 50 ohm than 75 ohm, since I think the
transmitter would be happier with that, and I'm told that for best results
I should use as much as a 10:1 balun for that, and a 470-ohm terminating
resistor. Would I be better off doing that? Where can I buy a 10:1 (or
other oddball ratio) balun?

5. Are there other antenna types I should consider? A discone might work
over the frequency range but it's vertically polarized with a relatively
low radiation angle so I suspect it isn't much good for NVIS. Anything
else?

Thanks...