"C. J. Clegg" wrote in message
news

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...

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. 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 type of antenna we used was an open wire fed dipole with a rather
hefty remote antenna tuner. It's performance was better than the B&W, but
our customer preferred the B&W for reasons of cost and ease of installation.

A wire based Log Periodic?

Wayne wrote:
"C. J. Clegg" wrote in message
news
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...


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. 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 type of antenna we used was an open wire fed dipole with a rather
hefty remote antenna tuner. It's performance was better than the B&W, but
our customer preferred the B&W for reasons of cost and ease of installation.

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.

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.

Dave wrote in
:

A wire based Log Periodic?

Not too useful unless you need the directivity as well.

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.


--
Dave Oldridge+
ICQ 1800667

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?

"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

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?

"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 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.