On Tue, 21 Nov 2006 22:19:44 +0000, Wayne wrote:

We simply used the commercially available B&W broadband loaded dipole.
Our customer typically ran 1KW into the antenna, and was very happy with
the price/performance. In spite of the negatives often cited about the
antenna, it is a very effective compromise for some situations.

The other reason I don't necessarily want to use the B&W is that with a
little freedom of design parameters and my limited frequency range of
interest (don't need 1.8 to 30 or even 4 to 30, just need 4 to 9), I was
hoping I could adjust the design characteristics to minimize the
inefficiency and mitigate the negatives.

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.

One way to evaluate different lengths would be to model out such an
antenna using NEC2/NEC4 or the like. Vary the antenna-length-to-
operating-frequency-wavelength ratio, and evaluate the amount of power
radiated vs. the amount of power dissipated in the termination
resistance at each. Then, given the specific frequencies at which you
actually want to spend most of your time operating, figure out which
length gives you the best overall efficiency for your own operating
conditions.

At a guess - and it's just a guess - I think you might get reasonably
satisfactory results by choosing a length which would give you a
folded dipole that's resonant right in the middle of your 4-to-9
range, or perhaps a bit longer than that. You'd avoid the "too short
to load up efficiently" problem at 4 MHz, as well as the "a full
wavelength long, and thus presenting a high feedpoint Z" problem at 9
MHz.

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

Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

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

You could probably stick a bunch of Caddock MP9100 power film
resistors, wired in series, on a chunk of aluminum heatsink and make
it work OK. Caddock sells direct, I believe, and if I recall
correctly Mouser carries many of their parts.

You might ask B&W if they'd be willing to sell a "replacement" for
their termination resistor assembly.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote:

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

Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

Yeah, this is exactly what I'm trying to avoid, by careful choice of
design parameters like length, resistor value, balun type. Feeding the
antenna with 100 watts and having all 100 of those watts dissipated in the
resistor, at any frequency within my range of 4 to 9, isn't going to work.
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.

On Tue, 21 Nov 2006 23:15:44 -0500, "C. J. Clegg"
wrote:

Yeah, this is exactly what I'm trying to avoid, by careful choice of
design parameters like length, resistor value, balun type. Feeding the
antenna with 100 watts and having all 100 of those watts dissipated in the
resistor, at any frequency within my range of 4 to 9, isn't going to work.

Hi OM,

By turns, you've painted yourself into a corner when we add up this
wish list. Worse yet is the complaint you anticipate with:
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.

3dB is hardly they abyss of performance, and, in fact, you would
probably be hard pressed to notice it. If you could, your dream
antenna (for the price you are willing to pay) would automatically
qualify for the dung-heap.

When you lead with your chin with:
It cannot use any sort of antenna tuner
this simply breaks the camel's back. Also, come to terms with there
is also no such thing as a miracle BalUn.

In traditional engineering, there is the adage that a well defined
problem contains its own answer. You can reconcile facing abject
failure by falling back and building a cost/benefit analysis of all
the characteristics of your desired antenna. It should take more than
half an hour to come up with the price tag. You may also discover
that you can live without some of those restrictions and you might be
able to live with a tuner - generations have survived and flourished
under similar circumstances.

73's
Richard Clark, KB7QHC

C. J. Clegg wrote:
On Tue, 21 Nov 2006 23:24:59 +0000, Dave Platt wrote:

2. How do I determine the minimum power rating for the terminating
resistor for an antenna that will be driven by 100 watts maximum?
Ummm... I'd say that you'd need a resistor capable of dissipating 100
watts, continuous, when used under ambient-free-air conditions in the
highest operating temperature you'll encounter. I'd probably de-rate
it by at least 50% (200 watt resistor) just to be sure, especially if
you're going to be operating RTTY or any other high-duty-cycle mode.

One of the unfortunate things about a T2FD is that there are going to
be frequencies where most of your power warms up the feet of the birds
perching on the termination resistor :-(

Yeah, this is exactly what I'm trying to avoid, by careful choice of
design parameters like length, resistor value, balun type. Feeding the
antenna with 100 watts and having all 100 of those watts dissipated in the
resistor, at any frequency within my range of 4 to 9, isn't going to work.
If I can't keep the efficiency above 50 percent across the range, then it
probably isn't going to be worth doing.



Narrowing the frequency range isn't going to help much with the design.
I'm no expert, but if I read my signs correctly, the T2FD has the same
ups and downs in SWR as does most other multiband antennas. IOW, it
relies on frequency harmonic relationships.

Cebik has a good web page at:

http://www.cebik.com/wire/t2fd.html

"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

C. J. Clegg wrote:
It cannot use any sort of antenna tuner and
must be fed with a single coax of 50 or 75 ohms.

Why not an autotuner at the antenna feedpoint?
--
73, Cecil http://www.w5dxp.com

On Wed, 22 Nov 2006 00:13:48 +0000, Cecil Moore wrote:

Why not an autotuner at the antenna feedpoint?

Too much money.

Anyway, eventually (not right away) this antenna is going to be used in an
ALE network and I rather doubt that autotuners work very well in that kind
of an environment.

"C. J. Clegg" wrote in
news
On Wed, 22 Nov 2006 00:13:48 +0000, Cecil Moore wrote:

Why not an autotuner at the antenna feedpoint?

Too much money.

Anyway, eventually (not right away) this antenna is going to be used
in an ALE network and I rather doubt that autotuners work very well in
that kind of an environment.

Anything over about 90 feet should work. 125 would be just about perfect.
And no, they DON'T have bad spots....usually they stay under 2 to 1 across
a fairly wide band and efficiency stays good above the knee frequency.

Model it, if you want exact performance figures.


--
Dave Oldridge+
ICQ 1800667

I may be able to help. Drop me a line at ...

-Spencer

C. J. Clegg wrote:
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...