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

Michael Coslo wrote:
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.

Pah! I should have said wavelengths, not harmonics!!

- 73 de Mike KB3EIA -

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

Looks like a very useful page. And, the section down at the bottom
labelled "Terminator Resistor Losses" tells the sad tale. With both
of the designs Cebik modelled, the power loss into the termination
resistor is only rarely less than 50%. For the longer of the two
models (165', wide spacing) the losses between 4 MHz and 9 MHz range
from 4 dB to over 6 dB, with an average loss of around 5 dB.

It looks as if it might be possible to model/design a T2FD with an
intermediate length and spacing, which might result in average losses
in the 4 - 9 MHz range which are lower than either the 100' or 165'
models. However, from the shape of these curves I doubt that you'll
get the average loss down much below 5 dB. Even getting the average
loss down below 3 dB looks as if it won't happen, and I don't think
you're going to ge the worst-case loss down below 3 dB at all.

You might want to go back and reconsider the possibility of using a
feedpoint-mounted autotuner - in particular, one which has a
frequency-based tuning memory capability and fast-retune feature, and
which does not require a large amount of RF power to initiate a tune.
Some such tuners require only a couple of watts of RF in order to
tune, and can re-tune to a previously-memorized frequency and
match-set within a couple of hundred milliseconds. I believe that an
autotuner of this sort would be compatible with low-power ALE, as long
as you had "pre-programmed" it by doing a full tuning cycle at each
frequency in the range you're interested in, and given it a chance to
search for and "memorize" a suitable match.

The lady who has demo'ed ALE at a number of hamclubs in my area
(regrettably I don't recall her callsign) uses an ALE-equipped
military radio, and a Motorola (or Harris?) longwire autocoupler
hooked to a random length of wire and a counterpoise.

--
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 Wed, 22 Nov 2006 19:29:39 +0000, Dave Platt wrote:

the section down at the bottom
labelled "Terminator Resistor Losses" tells the sad tale.

Yes, it does.

My heartfelt thanks to all of you for all of your assistance and advice.
I think I have enough info now to guide my experimentation and come to a
conclusion.

Doesn't look promising...

On Wed, 22 Nov 2006 07:10:55 -0500, "C. J. Clegg"
wrote:

I was hoping to mitigate all of that by restricting the frequency range.
I'm not asking for 1.8-30 or even 4-30, but 4-9.

Hi OM,

That has been evident from the beginning. Still and all:
The 50 percent efficiency floor is somewhat arbitrary
Exactly. It is just such arbitrariness along with competing,
conflicting restrictions that leads to self-defined failure tacitly
accepted with:
If it can't be done, then it can't be done.
However, it has been done, and without the unnecessary complications
of forcing ANY design to span an octave without a tuner. Hoping that
a resistor will solve this is dope-slapped with the expectation of
efficiency.

some of the users of this antenna are going to be using power
levels as low as 5 watts
So what is all this angst about efficiency? Pour 100W into any hank
of wire and you will achieve at least that. Or do you mean that the
design you are looking for will be used as a model for others? If
that is the case, more the pity that the requirement of having a $25
tuner is an unreasonable expectation of Amateur radio operators. I
cannot imagine they will spend less on those resistors....

73's
Richard Clark, KB7QHC

On Wed, 22 Nov 2006 12:37:20 -0800, Richard Clark wrote:

So what is all this angst about efficiency? Pour 100W into any hank
of wire and you will achieve at least that. Or do you mean that the
design you are looking for will be used as a model for others?

Good afternoon, Richard.

Actually, what I mean is that some, including me, will be using 5-watt
radios (Yaesu FT-817). If the option of pouring 100W into any hank of
wire was available to all, the issue wouldn't be critical. It's not,
so it is. :-)

more the pity that the requirement of having a $25
tuner is an unreasonable expectation

Remember the ALE requirement. I am only just now becoming convinced that
some autotuners might work for ALE. I'm quite certain manual tuners won't.

In article ,
C. J. Clegg wrote:

Actually, what I mean is that some, including me, will be using 5-watt
radios (Yaesu FT-817). If the option of pouring 100W into any hank of
wire was available to all, the issue wouldn't be critical. It's not,
so it is. :-)

There's another approach to the antenna that you might want to
consider, _if_ the ALE ranges are limited to a few (say, no more than
four or five) and aren't too wide (say, no more than perhaps 5% of the
center frequency).

If your usage pattern fits this model, then you could construct a
multi-wire "fan" dipole array... simply a set of individual wire
half-wavelength dipoles, each cut for the center frequency of a given
ALE range, spread apart physically, and fed from a single coax at a
single feedpoint. They can be spread vertically (e.g. hang the
longest one as a flat-top from a convenient set of trees, suspend the
next-shortest beneath it on 6" spreaders, hang the next-shortest on
another set of spreaders, etc.) or horizontally (run one north/south,
another east/west, etc.).

There will be some amount of coupling/loading between them, so you'll
need to trim them for lowest SWR once they're installed, and you may
find the SWR bandwidth less than you'd get from individual dipoles.
Coupling/loading is greater for the vertically-stacked case, and less
for a star-like horizontal pattern (which takes more space and more
trees or etc., of course).

The nice thing about this approach, if it's suitable for your needs,
is that the losses are quite low. The wire which is resonant on the
band in question loads up and radiates, and the other wires (which
will typically have high, reactive impedances) accept very little
current. No lossy termination resistor is needed.

I've been using a three-wire fan dipole (5" vertical spacing) cut for
40/20/10 for several years, with very satisfactory results.

This approach won't give you continuous coverage of your 4:9 range...
unfortunately I can't suggest any which would, which don't require an
agile tuner/transmatch and which don't introduce high losses in some
portions of the band.

--
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 Wed, 22 Nov 2006 15:56:45 -0500, "C. J. Clegg"
wrote:

more the pity that the requirement of having a $25
tuner is an unreasonable expectation

Remember the ALE requirement. I am only just now becoming convinced that
some autotuners might work for ALE.

I seriously doubt that. What you are describing is a sacrifice in
Link Quality for the sake of not using something like a fan dipole
which would immediately satisfy every requirement and fulfill mission.
How this is justified with
multiple-legged dipole with legs cut
for different frequencies (well, I probably could, but it would be
impractical).
forces a new meaning to the word impractical. Perhaps you would care
to elaborate how the simplicity of two extra wires has been trumped.

73's
Richard Clark, KB7QHC

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. :-)

We will be operating on many different frequencies across the range of 4
to 9. I don't even know (yet) how many different frequencies will be in
use (they won't tell me).

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.

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".

I've learned a lot from you guys the last few days, though, and maybe
something like that isn't so impractical ... I'll have to experiment.
Perhaps an array of inverted vees on a single feedline...

"C. J. Clegg" wrote in
news
On Wed, 22 Nov 2006 01:01:56 +0000, Dave Oldridge wrote:

The loaded folded dipole idea is actually quite efficient if you stay
above the "knee" frequency. This is about .5 wave at the lowest
frequency.

Good evening, Dave.

But, isn't there another frequency, around 1 full wavelength at the
operating frequency, above which efficiency starts to go to pot again?

I didn't see this. But as the size increases, the pattern tends to start
to come apart.

125 ft (which you suggested in another message) is a full wave at
something a shade under 8 MHz, well within my upper limit of 9 MHz.
So, how will a 125-foot T2FD operate in the range of, say, 7 to 9?

Actually, I just modelled it. The 125-foot version is very good between
7 and 9, but the SWR (with a 650 ohm terminator) jumps to 4 to 1 at 5.25
mhz. Like I suggested, you need to model your actual situation, then
build to the model.



--
Dave Oldridge+
ICQ 1800667