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.
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.

Let us know how you do.

Dave WD9BDZ

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

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

On Thu, 09 Aug 2007 16:13:28 -0700, Jim Lux wrote:

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

snip
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.


I used one of those resistors (sold as motor shunt resistors) for my
antenna and constructed a small 5" by 5" heat sink that flys with the
antenna. The commercial balun, ($80USD) which was a bit heavy to mount on
the antenna was mounted on a fence post underneath the antenna. I
constructed some 450ohm balanced line to go between the balun and antenna.

pics:
http://vambo.org/pictures/antenna/

This antenna has been the greatest thing I've ever done to improve my
Hobie. This antenna is about 8 years old and has survived a move to a new
QTH.

David G. Nagel wrote:
The resistors need to be carbon non inductive high wattage.

I have some 50 watt, 600 ohm, non-inductive
resistors removed from a military surplus
antenna tuner.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:

...
I have some 50 watt, 600 ohm, non-inductive
resistors removed from a military surplus
antenna tuner.

Cecil:

Welcome back.

In your absence, posters to pick on have been sparse. ;-) Those here
for jest and ridicule have been hard pressed ...

Regards,
JS