"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...

"C. J. Clegg" wrote in message
news
On Tue, 21 Nov 2006 22:19:44 +0000, Wayne wrote:

I don't have the technical data you ask about, but I do have experience
in
installing systems such as you describe. We simply used the commercially
available B&W broadband loaded dipole.

Good afternoon, Wayne.

That's certainly an option, but as I said I would rather not spend $200+
unnecessarily, especially since I already have all of the necessary
materials except for the terminating resistor.

Anyway, the BWD-65's lower end is 4 MHz, and some of what I've read about
T2FD antennas seems to indicate that near the low end is where efficiency
is worst (inside of the design range ... outside of the design range it is
MUCH worse).

I suppose I could use the BWD-90 but I have a hard time understanding how
they get all the way from 1.8 to 30 ... that seems very, very wide even
for a T2FD antenna (I suppose it's a moot point, though... I'm sure it
will get to my upper limit of 9 MHz nicely).

As an aside, I see from the latest HRO catalog that the BWD-20, BWD-45,
and BWD-65 are all $220 and the larger / longer BWD-90 is only $200. Why
do you all suppose the bigger one is cheaper than all of the smaller ones?

Yes, as I said, I don't have the technical data. I just wanted to cite
experience that would indicate that you can most likely be successful in
using an antenna such as a T2FD for your application. Good luck.

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?

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?

Dear "C. J. Clegg"

You are looking for an antenna for commercial or government use.
Several on this group are professional engineers (P.E.) who will be happy to
design whatever you wish in the way of an antenna.

Please provide an actual E-mail address and a verifiable name, and I
will be pleased to provide a quotation.

Regards, Mac N8TT and also a P.E.

Apologies for the overlap into an amateur group.
--
J. Mc Laughlin; Michigan U.S.A.
Home:

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 23:10:13 -0500, J. Mc Laughlin wrote:

I will be pleased to provide a quotation.

Thanks. Unfortunately there isn't any money to pay someone to do this.
I'm doing it for free, and in fact I'm paying for the materials out of my
pocket, for whatever design I decide on.

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

On Tue, 21 Nov 2006 22:10:19 -0800, Richard Clark wrote:

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.

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.

Good morning, Richard.

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.

Can a T2FD antenna not be made 50 percent efficient across that restricted
frequency range?

The 50 percent efficiency floor is somewhat arbitrary but is driven by the
fact that some of the users of this antenna are going to be using power
levels as low as 5 watts. That's hard enough to do with a cut NVIS
dipole, without throwing an inefficient antenna into the mix.

The inability to use a tuner is driven by the fact that eventually, these
antennas are going to need to be usable with ALE radios. I don't know
very much about ALE (yet) but I have a hard time imagining an autotuner
that is consistently fast enough to use with ALE. Also, the users of these
antennas are, like me, going to be paying for them out of their pockets.
Few if any of the potential users that I know of today can afford to buy
an autotuner. I know I can't.

If it can't be done, then it can't be done. I'm realistic enough to
accept that and move on to something else, or drop the idea. It just
seems to me that within the limited frequency range, something like that
should be possible with the right choice of design parameters.

An LP near ground also functions as a NVIS antenna, a broadband NVIS antenna.

You will need more space than the B&W for example; but, if space is available a
six or seven element design should work just fine.

Also, any TERMINATED TRAVELING WAVE designs should work. A 1/2 wavelength at 3
MHz terminated in 600 ohms and fed with a good 9:1 balun would do as well.

/s/ DD, W1MCE

BTW: it is proximity to ground and horizontal polarization that causes NVIS

# # #

C. J. Clegg wrote:
On Tue, 21 Nov 2006 17:23:02 -0500, Dave wrote:


A wire based Log Periodic?


Good evening, Dave.

I guess I forgot to mention ... this antenna needs to be omnidirectional
or nearly so. Anyway it needs to be NVIS and I suspect a log periodic
wouldn't work well in that configuration.