Questions on broadband antenna design (e.g. T2FD)
 

On Wed, 22 Nov 2006 18:59:31 -0500, "C. J. Clegg"
wrote:

On Wed, 22 Nov 2006 15:15:35 -0800, Richard Clark wrote:

Perhaps you would care
to elaborate how the simplicity of two extra wires has been trumped.

I'll try, though from the tone of your message it sounds like your mind is
made up. :-)

My mind is made up? I've repeatedly wondered why you have approached
this with a defeatist attitude.

We will be operating on many different frequencies across the range of 4
to 9.

Again, this has been apparent from the beginning.

I don't even know (yet) how many different frequencies will be in
use (they won't tell me).

That doesn't matter all that much, except to anticipate failure.

So I can envision many pairs of dipole elements, each cut for a certain
frequency in the range, and laid out like the spokes of a wheel.

If you re-read my posting, I've done nothing more complex than to add
TWO more wires. ALE may easily jump between 200 frequencies, but
there is absolutely nothing about that which demands a resonant
frequency for each of them.

I have plenty of land here but I don't have ready supports for that kind
of an array.

That's what I mean by "impractical".

Then the solution is not impractical by any definition, you are simply
over embroidering the problem with a slavish interpretation of
necessity. One pair of wires cut to a low end, one pair of wires to a
high end, both pairs fed at the same point. It may take as many as
four pairs (I doubt it), but to abstract this wildly to 200 goes
beyond the pale when a skeleton biconical could easily accomplish this
with flat response (over a much larger bandwidth) with only 16 pairs
of equal sized wires.

This cage monopole:
http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm
is flat over 5 Ham bands.

This discone:
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm
operates flat over more than two octaves of bandwidth. With scaling,
I can count at least 55 discrete frequencies that would fall into the
2:1 mismatch region - and this say nothing of those frequencies
between them, nor of their end points which could be made to span 4 to
9 MHz.

Yes, a lot of wire, but use less wire for a rougher approximation. If
you are looking at an arbitrary 50% efficiency, a 5:1 circle
encompasses a lot more points for less wire.

73's
Richard Clark, KB7QHC

Questions on broadband antenna design (e.g. T2FD)
 

On Wed, 22 Nov 2006 22:35:39 -0800, Richard Clark wrote:

This cage monopole:
http://home.comcast.net/~kb7qhc/ante.../Cage/cage.htm
is flat over 5 Ham bands.

This discone:
http://www.qsl.net/kb7qhc/antenna/Discone/discone.htm
operates flat over more than two octaves of bandwidth.

Thanks. I have said earlier that I considered a discone (I also
considered a cage monopole but didn't mention it here) but those are
vertically polarized antennas and I'm pretty sure they're not good for
NVIS. Am I wrong?

I am also considering an array of inverted vees cut for selected
frequencies within the range, Something like that is likely to be much
better for NVIS.

Sorry if I seemed to fit all those names you called me... you say I'm
defeatist, one or two others here say I'm too optimistic. Can't please
everybody I guess. For the most part I do very much appreciate all the
help everyone has offered here, in a thread that has grown way beyond
anything I envisioned when I started it ... you guys are great. :-)

Questions on broadband antenna design (e.g. T2FD)
 

"Dave Platt" wrote in message
...
In article ,
C. J. Clegg wrote:

So, my questions are...

Take my answers as best-guesstimates, please, rather than as gospel!

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?

A range greater than 2:1 means that you're almost certainly going to
run into at least one operating frequency at which the antenna itself
is an extremely difficult load (very low, very high, and/or very
reactive) and that most of your power is going to end up in the
terminating resistor.

Since his range is just slightly more than 2:1, I wonder if he can pick the
resonance so that the thing is not full wave at any frequency of interest.

Either Mouser or Digikey had 50 W non inductive resistors.

Tam/WB2TT

Questions on broadband antenna design (e.g. T2FD)
 

On Thu, 23 Nov 2006 08:22:25 -0500, "C. J. Clegg"
wrote:

but those are
vertically polarized antennas and I'm pretty sure they're not good for
NVIS. Am I wrong?

1. Build the mirror elements;
2. Combine;
3. Turn 90 degrees;
4. Elevate.

73's
Richard Clark, KB7QHC