RadioBanter - Building a T2FD antenna

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Building a T2FD antenna

I've pretty much decided to go ahead and build a T2FD "radiating dummy
load", hopefully for use from 2 to 24 MHz.

This will be almost (but not quite) exclusively for receiving, wherein my
limiting factor is usually noise anyway, rather than antenna efficiency.
If somebody appears on one of the frequencies I scan that I want to talk
to, if he's not S9 or better I'll likely switch over to a "real" antenna.
But, the occasional use for transmitting means I'll need the appropriate
power resistor on the load end (100 watts or so).

You guys have suggested googling for "T2FD" and "TTFD" and I have done
that, but I haven't yet found anything that really lays out how to design
and build one of these things, what the formulas are, etc.

Some of the Google hits suggest using baluns and resistor values all the
way from 11:1 balun, 550 ohm resistor, and 50-ohm coax to 4:1 balun,
300-ohm resistor, and 75-ohm coax. Some seem to imply that one should use
a resistor value equal to the coax impedance times the balun ratio, and
others say that the resistor value should be some percent higher than the
coax impedance times the balun ratio (one site suggested 75 ohm coax, 4:1
balun, and 390-ohm resistor).

Obviously I'd like to do this a cheaply as possible and I do have a W2AU
4:1 balun sitting on the shelf here, so would 50 or 75 ohm coax, 4:1
balun, and anywhere from 300 to 390 ohms for the terminating resistor work
as well as a higher resistance value and a higher ratio, scarce, and more
expensive balun?

I have seen formulas, which I can no longer find, that say what the length
should be for a given minimum frequency, and what the spacing between the
conductors should be. I assume that the spacing of the conductors has a
lot to do with the balun ratio and terminating resistor.

I guess if I can find the length, conductor spacing, and terminating
resistor value needed for 2 MHz minimum and 4:1 W2AU balun I should be all
set.

Thanks...

Building a T2FD antenna

Rick (W-A-one-R-K-T) wrote:
You guys have suggested googling for "T2FD" and "TTFD" and I have done
that, but I haven't yet found anything that really lays out how to design
and build one of these things, what the formulas are, etc.

Actually I suggested you look at Heys book, or the second edition.
The original one had several pages complete with measurments, how
to build resistors from cheap low power ones and so on.

Obviously I'd like to do this a cheaply as possible and I do have a W2AU
4:1 balun sitting on the shelf here, so would 50 or 75 ohm coax, 4:1
balun, and anywhere from 300 to 390 ohms for the terminating resistor work
as well as a higher resistance value and a higher ratio, scarce, and more
expensive balun?

You can wind a 9:1 balun pretty easily, I found that for HF, you could
make one from torroids found in PC power supplies. I tested the ones
I made with a dummy load and ran some power into them. If they got
hot, I did not use them. :-)

However Heys does not suggest you use a 9:1 balun, he suggests a 4:1.

I have seen formulas, which I can no longer find, that say what the length
should be for a given minimum frequency, and what the spacing between the
conductors should be. I assume that the spacing of the conductors has a
lot to do with the balun ratio and terminating resistor.

While they are not extremely critical, they need to be pretty close.
If you change the geometry of the antenna, you change how it acts.
Too narrow and it becomes a folded dipole with a center load and
too wide it becomes a rhombic or rectangular loop.

Note that the antenna needs to be mounted at specific angle with a
a specific height from the ground. Changing them changes the way the
antenna works.

Captain G. L. Countryman, USN(W3HH*), who developed it, placed it over
conductive ground (the sea shore), but it seems to work well over
regular ground too.

I guess if I can find the length, conductor spacing, and terminating
resistor value needed for 2 MHz minimum and 4:1 W2AU balun I should be all
set.

That would be an awfully big antenna. However, I think you could
cut one for 3.5mHz and it would still work at 2. They do work
well at lower frequencies than they are cut.

From Heys' book:

For 1.8 mHz top length 182' 2" spacing between top and bottom 5' 5"".
For 3.6 mHz top length 91' 1" spacing 2'8"

He lists the height as being 35' above ground sloping to 5 feet for a
7mHz antenna and says it would be proportional, so a 3.5 mHz antenna
would need to be 70' up. The angle of the antenna is 30 degrees, from
the mast to the center of the spacer.

You probably could get away with a 7mHz version, (half of a 3.5
version).

Note that he also suggests using 75 Ohm coax,a 4:1 balun and a resistor
made of four rows of six 270 ohm 2w resistors for a 100 watt CW or
300 watt SSB transmitter.

I suggest that before you go any further you get a copy of the book.

Geoff.

* He was W3HH, his call has since been reassigned.

--
Geoffrey S. Mendelson, Jerusalem, Israel N3OWJ/4X1GM
IL Voice: (07)-7424-1667 U.S. Voice: 1-215-821-1838
Visit my 'blog at http://geoffstechno.livejournal.com/

Building a T2FD antenna

On Thu, 09 Aug 2007 12:55:33 -0400, "Rick (W-A-one-R-K-T)"
wrote:

I've pretty much decided to go ahead and build a T2FD "radiating dummy
load", hopefully for use from 2 to 24 MHz.

Hi Rick,

I've seen discussion about cutting the length of this air-cooled
resistor (what do you mean almost exclusively receive? Use a 1/4 watt
resistor then!). A halfwave design at 2 MHz is going to be 6
wavelengths long at 24 MHz. The resistor/BalUn may match well, but
the resulting lobes may not point where you expect them to.

Very small antennas (let's say halfwave for 20M) will receive 2 MHz
quite nicely (or as well as the resistor will allow); and point in the
direction (broadside) you expect for any of these frequencies.

If somebody appears on one of the frequencies I scan that I want to talk
to, if he's not S9 or better I'll likely switch over to a "real" antenna.

You don't seem very committed to any particular "general" solution; so
what is your real goal?

73's
Richard Clark, KB7QHC

Building a T2FD antenna

On Thu, 09 Aug 2007 11:39:55 -0700, Richard Clark wrote:

I've seen discussion about cutting the length of this air-cooled
resistor (what do you mean almost exclusively receive? Use a 1/4 watt
resistor then!).

Because the one time I need to transmit on it, I'll blow the resistor.

You don't seem very committed to any particular "general" solution; so
what is your real goal?

I'm active in CAP and MARS on frequencies from 2 to 24 MHz. I need to be
able to program all of the frequencies of interest into my FT-1000 and put
it on scan, then if someone I need to talk to shows up (and is strong) on
one of the channels in my scan, I can just pick up the mike and call him.

I have two inverted vees fed with ladder line and tuners that I normally
use, but they're not suitable for scanning because one of the tuners is
manual and the other one can't keep up with scanning different channels
even if I had a computer interface for it so I could tell it what
frequency I'm listening on.

I could just throw out a random length wire, without a tuner, and use that
for receiving but I wouldn't be able to transmit on it when someone I need
to talk to shows up on one of the channels.

Hence, right now at least, the T2FD looks like the only viable short-term
solution for the scanner... a longer term solution will be an array of cut
dipoles whose fundamental or odd harmonic is near one of the frequencies
of interest, but that's going to be a while coming.

Building a T2FD antenna

On Thu, 09 Aug 2007 12:55:33 -0400, "Rick (W-A-one-R-K-T)"
wrote:

I've pretty much decided to go ahead and build a T2FD "radiating dummy
load", hopefully for use from 2 to 24 MHz.

This will be almost (but not quite) exclusively for receiving, wherein my
limiting factor is usually noise anyway, rather than antenna efficiency.
If somebody appears on one of the frequencies I scan that I want to talk
to, if he's not S9 or better I'll likely switch over to a "real" antenna.
But, the occasional use for transmitting means I'll need the appropriate
power resistor on the load end (100 watts or so).

You guys have suggested googling for "T2FD" and "TTFD" and I have done
that, but I haven't yet found anything that really lays out how to design
and build one of these things, what the formulas are, etc.

Did you see the Cebik page? http://www.cebik.com/radio.html He lays
out dimensions & resistor values for two t2fd's in your frequency
range.

bob
k5qwg

Some of the Google hits suggest using baluns and resistor values all the
way from 11:1 balun, 550 ohm resistor, and 50-ohm coax to 4:1 balun,
300-ohm resistor, and 75-ohm coax. Some seem to imply that one should use
a resistor value equal to the coax impedance times the balun ratio, and
others say that the resistor value should be some percent higher than the
coax impedance times the balun ratio (one site suggested 75 ohm coax, 4:1
balun, and 390-ohm resistor).

Obviously I'd like to do this a cheaply as possible and I do have a W2AU
4:1 balun sitting on the shelf here, so would 50 or 75 ohm coax, 4:1
balun, and anywhere from 300 to 390 ohms for the terminating resistor work
as well as a higher resistance value and a higher ratio, scarce, and more
expensive balun?

I have seen formulas, which I can no longer find, that say what the length
should be for a given minimum frequency, and what the spacing between the
conductors should be. I assume that the spacing of the conductors has a
lot to do with the balun ratio and terminating resistor.

I guess if I can find the length, conductor spacing, and terminating
resistor value needed for 2 MHz minimum and 4:1 W2AU balun I should be all
set.

Thanks...

Building a T2FD antenna

The terminating resistor of a B&W antenna is 600 ohms resistive i.e.
there is a 600 ohm resistor connected to the ends of the folded dipole
antenna. It's purpose is to swamp the current leftover when the standing
wave is formed. The balun is of necessity a 12 to 1 ratio balun. Other
values may be used to other effects.
The biggest problem is the balun. I haven't seen any specs for making
one. The resistors need to be carbon non inductive high wattage.
==================================
A 12:1 ratio balun is decribed by Jerry Sevick , W2FMI in his book :
"Building and using Baluns and Ununs" , pages 54-55
It is in fact a combination of a 1:1.33 Unun and a 1:9 (type Guanella)
Balun , hence for an impedence transformation of 50 Ohms unbalanced to
600 ohms balanced. The unit has 4 toroids.

The above book : Library of Congress Catalogue Card Number 94-69520
ISBN 0-943016-09-6

Frank GM0CSZ / KN6WH

Building a T2FD antenna

David G. Nagel wrote:
Rick (W-A-one-R-K-T) wrote:

I've pretty much decided to go ahead and build a T2FD "radiating dummy
load", hopefully for use from 2 to 24 MHz.

This will be almost (but not quite) exclusively for receiving, wherein my
limiting factor is usually noise anyway, rather than antenna
efficiency. If somebody appears on one of the frequencies I scan that
I want to talk
to, if he's not S9 or better I'll likely switch over to a "real"
antenna. But, the occasional use for transmitting means I'll need the
appropriate
power resistor on the load end (100 watts or so).

You guys have suggested googling for "T2FD" and "TTFD" and I have done
that, but I haven't yet found anything that really lays out how to design
and build one of these things, what the formulas are, etc.

Some of the Google hits suggest using baluns and resistor values all the
way from 11:1 balun, 550 ohm resistor, and 50-ohm coax to 4:1 balun,
300-ohm resistor, and 75-ohm coax. Some seem to imply that one should
use
a resistor value equal to the coax impedance times the balun ratio, and
others say that the resistor value should be some percent higher than the
coax impedance times the balun ratio (one site suggested 75 ohm coax, 4:1
balun, and 390-ohm resistor).

Obviously I'd like to do this a cheaply as possible and I do have a W2AU
4:1 balun sitting on the shelf here, so would 50 or 75 ohm coax, 4:1
balun, and anywhere from 300 to 390 ohms for the terminating resistor
work
as well as a higher resistance value and a higher ratio, scarce, and more
expensive balun?

I have seen formulas, which I can no longer find, that say what the
length
should be for a given minimum frequency, and what the spacing between the
conductors should be. I assume that the spacing of the conductors has a
lot to do with the balun ratio and terminating resistor.

I guess if I can find the length, conductor spacing, and terminating
resistor value needed for 2 MHz minimum and 4:1 W2AU balun I should be
all
set.

Thanks...

The terminating resistor of a B&W antenna is 600 ohms resistive i.e.
there is a 600 ohm resistor connected to the ends of the folded dipole
antenna. It's purpose is to swamp the current leftover when the standing
wave is formed. The balun is of necessity a 12 to 1 ratio balun. Other
values may be used to other effects.

I don't know that the balun ratio corresponds to the resistor value
necessarily, although it's true that B&W's patent essentially says that.
One might spend a few minutes modeling various combinations to find out.

Looking at W4RNL's page:
http://www.cebik.com/wire/t2fd.html
says
"(The general recommendation is to use a terminating resistor that is
about 5% to 10% higher in value than the feedline characteristic
impedance.)"

One might also look at B&W's patent (4,423,423)
http://patft.uspto.gov/netahtml/PTO/srchnum.htm

"The non-inductive resistive network 78 is preferably a single wire
wound resistor having an impedance dependent upon the selected frequency
range of the antenna. This impedence must be totally non-inductive for
the antenna to function properly. The resistive network 78 must also
have a power dissipation capability which should be at least one quarter
of the power transferred through the antenna matching means 46 to the
antenna from the radio frequency generator. Of course, in order that the
environment not affect the operation of the resistive network 78 the
casing 80 should be weather proof.

The antenna balancing or load impedance is chosen to give the antenna
its broad band characteristics while retaining its shorter overall
length. It has been found that a load impedance in the range of 100-900
ohms will work satisfactorily with an antenna having a frequency range
of 3.5-30 MHz and an overall length between 50 and 130 feet. A load
impedance of 600 ohms is preferred because it increases the bandwidth of
the antenna while decreasing the voltage standing wave ratio associated
with each frequency in the 3.5-30 MHz range. Since the output impedance
of the radio frequency generator is most often 50 ohms, the ratio
between the two coils 74, 76 of the antenna matching means 46 will be
12:1. The antenna load impedance of 600 ohms works equally well with a
frequency range of 7.0-30 MHz. However, for bandwidths which include
lower frequencies, such as 1.8 MHz, and for other frequency ranges of
smaller bandwidths, other impedances in the range of 100-900 ohms may be
preferred in order to decrease the voltage standing wave ratio for those
frequencies while permitting transmission and reception over a
continuous bandwidth. With the use of a smaller impedance in the antenna
balancing means 44, the ratio of the coils 74, 76 in the antenna
matching means 46 and the length of the antenna will vary to reflect
such changes.
"

"An antenna balancing or load impedance of 800 ohms has been found to
cause the antenna 90 to perform quite well where said antenna has a
frequency range of 1.8-22 MHz and an overall length averaging 185 feet.
The 800-ohm load impedance is preferred because it increases the
bandwidth of the antenna while decreasing the voltage standing wave
ratio associated with each frequency in the 1.8-22 MHz range. This
impedance increases the ratio between the antenna matching means and
output impedance of the radio frequency generator to 16:1.
"

The biggest problem is the balun. I haven't seen any specs for making
one. The resistors need to be carbon non inductive high wattage.

Or those nice inexpensive Caddock non-inductive resistors available from
Mouser for about $2 each for the low power versions, up to $10 each for
the higher power ones.

http://www.mouser.com/catalog/631/552.pdf

http://www.caddock.com/Online_catalo...000_Series.pdf

available up to 100W rating (assuming bolted to an appropriate
heatsink). I'd string together a bunch of an appropriate resistance,
either series or parallel, your preference.

Let us know how you do.

Dave WD9BDZ

Building a T2FD antenna

Rick (W-A-one-R-K-T) wrote:

I've pretty much decided to go ahead and build a T2FD "radiating dummy
load", hopefully for use from 2 to 24 MHz.

This will be almost (but not quite) exclusively for receiving, wherein my
limiting factor is usually noise anyway, rather than antenna efficiency.
If somebody appears on one of the frequencies I scan that I want to talk
to, if he's not S9 or better I'll likely switch over to a "real" antenna.
But, the occasional use for transmitting means I'll need the appropriate
power resistor on the load end (100 watts or so).

You guys have suggested googling for "T2FD" and "TTFD" and I have done
that, but I haven't yet found anything that really lays out how to design
and build one of these things, what the formulas are, etc.

Some of the Google hits suggest using baluns and resistor values all the
way from 11:1 balun, 550 ohm resistor, and 50-ohm coax to 4:1 balun,
300-ohm resistor, and 75-ohm coax. Some seem to imply that one should use
a resistor value equal to the coax impedance times the balun ratio, and
others say that the resistor value should be some percent higher than the
coax impedance times the balun ratio (one site suggested 75 ohm coax, 4:1
balun, and 390-ohm resistor).

Obviously I'd like to do this a cheaply as possible and I do have a W2AU
4:1 balun sitting on the shelf here, so would 50 or 75 ohm coax, 4:1
balun, and anywhere from 300 to 390 ohms for the terminating resistor work
as well as a higher resistance value and a higher ratio, scarce, and more
expensive balun?

I have seen formulas, which I can no longer find, that say what the length
should be for a given minimum frequency, and what the spacing between the
conductors should be. I assume that the spacing of the conductors has a
lot to do with the balun ratio and terminating resistor.

I guess if I can find the length, conductor spacing, and terminating
resistor value needed for 2 MHz minimum and 4:1 W2AU balun I should be all
set.

Thanks...

Rick, pretty much the only good treatment of the terminated tilted folded
dipole, (T2FD) I've found in Practical Wire Antennas by John Heys, an RSGB
book published in 1989.

Apparently there are also articles in June 1949 QST, and CQ Antenna Roundup
1963.

While some describe the T2FD as a broadband resistor, it's most salient
characteristics are it's broadband coverage over about a 4:1 ratio, and
with the 20 - 40 degree tilt, an aperiodic vertically polarized radiator.
It can be fed with 300 ohm line.

The optimum tilt of the antenna is about 30 degrees.

My skills with ascii art aren't that good, but I'll try to describe the
antenna.

Non Inductive resistor
B | C
____________________________________________ _____________________
| | | |
| | | E |
______________________________________________ _____________________
A | | D
| |
300 - 600 ohm feedline

Of course it must be tilted at 30 deg from vertical.

The length of each leg when measured from the centre of the wires across the
spreaders to the feedpoint or terminating resistorshould be
50,000/f(kHz) X 3.28 feet. The total top length and the lengths AB and CD
will be twice this calculated length. Frequency f is the lowest operating
frequency of the antenna, although it will work at half this frequency with
reduced efficiency. The spacing between the two radiator wires in feet is
found by dividing 3000 by the frequency in kHz and multiplying this by
3.28.

The terminating resistor must be non-inductive if you want the antenna to
work over a large frequency range. It will still work in an inductive
resistor is used but it will become resonant on one or more frequencies and
the feeder must be used as a tuned line.

The terminating resistor is to some extent determined by the impedance of
the feedline. With 300 ohm twin lead, the optimum resistor value is about
400 ohms. With 450 ohm line, a 500 ohm resistor is fine and 600 ohm line
needs 650 ohms. Keep in mind that 50% duty cycle of CW or even lower of SSB
wattage needed in the resistor. Of course RTTY or FSK will dissipate even
more power.

The article suggests that 24 270 ohm 2W carbon film resistors, wired into
four lines of six resistors in series, and then paralleled would produce a
final resistance value of 405 ohms, and would allow 300 watts on SSB.

Hope this adds to the discussion.

Bob, VE7HS

Building a T2FD antenna

Robert Smits wrote:

...
The article suggests that 24 270 ohm 2W carbon film resistors, wired into
four lines of six resistors in series, and then paralleled would produce a
final resistance value of 405 ohms, and would allow 300 watts on SSB.

Hope this adds to the discussion.

Bob, VE7HS

Hmmmm ...

Any suggestions for a method for dumping the heat from the resistors
back into the shack in the wintertime? :-)

And heck, during the winter, wouldn't it be nice to have the resistors
overall rating able to dissipate a KW and provide more heat?

Regards,
JS