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Author Topic: T2FD  (Read 3325 times)
AK0B
Member

Posts: 37




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« on: August 23, 2008, 08:01:34 PM »

Has anyone every used a T2FD to make a contact?  How about on QRP?

I know it is supposed to be a good receiving antenna but not sure if anyone uses it for transmitting.

I have been thinking about trying one for 50 years.  So guess if I am ever going to do it now is the time. Smiley

Know where I can get some 2 watt carbon resistors or better yet non-inductive one 390 ohms?

Stan ak0b

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W5ESE
Member

Posts: 550


WWW

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« Reply #1 on: August 25, 2008, 06:08:24 AM »

> Has anyone every used a T2FD to make a contact?
> How about on QRP?

> I know it is supposed to be a good receiving
> antenna but not sure if anyone uses it for
> transmitting.

I'll be honest up front; I have no experience with
this antenna.

The T2FD has a poor reputation as a transmitting
antenna, particularly on the lower bands.

In this article on NVIS propagation from QST, the
author mentions that amateurs serving in the
aftermath of Hurricane Katrina initially tried
using T2FD antennas, with poor results. They
obtained much better signal reports when they
switched to full length fan dipoles. (This is
described in the sidebar on the last page, labeled
'Hurricane Katrina').

http://www.arrl.org/files/qst-binaries/Straw1205.pdf

73
Scott
W5ESE
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WB6BYU
Member

Posts: 13567




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« Reply #2 on: August 25, 2008, 09:35:16 AM »

There are a lot of T2FD antennas around - most local
National Guard installations have one (though not necessarily
installed at a 30 degree slope as called for in the original
description.)  Yes, many ham QSOs have been made using them,
too.

The pattern is the same as for a center-fed wire of the
same overall length.  Antenna efficiency varies, but when
the overall antenna is longer than about 0.6 wavelengths
the losses compared to a center-fed wire are around 5 dB.
Efficiency drops quickly as the antenna is made shorter,
so a 90' antenna may be down 10 to 15 dB or so on 80m.

They are considered a good receive antenna because they
are broadband and will operate over a wide frequency range
without any tuning.  Since most HF receivers have plenty
of gain and the signal to noise ratio is set by the
background noise picked up on the antenna, the poor
efficiency isn't a problem on receive.  (Both the signal
and the noise are attenuated by the losses, so the ratio
between them stays the same.)

For transmit, the losses reduce the radiated signal.
The advantage, of course, is that they give a low SWR
over a wide frequency range without tuning.  With 100W
input you'd get about 30 watts radiated - those of us
who operate QRP know that is often plenty of power, but
when conditions are difficult you might do better if you
could radiate the full power out of your transmitter.

While a loss of 5dB may be tolerable in a lot of circumstances,
I suspect that such antennas are often used below their
critical cutoff frequency - the 90' antenna on 80m as
an example.  The losses are even higher in that case.
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AA4PB
Member

Posts: 13032




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« Reply #3 on: August 25, 2008, 12:18:27 PM »

I've done some testing of the B&W version of the T2FD. The T2FD converts some of the transmit power to heat in the load resistor. The amount radiated vs the amount converted to heat depends on how long the antenna is in terms of wavelength. The lower the frequency and the shorter the antenna, the more of your power is converted to heat.

The gov't makes use of the T2FD primarily because it covers a wide frequency range without a tuner. My opinion is that since hams are limited to bands in selected frequency ranges the T2FD is NOT a good antenna choice. Because we are limited in frequencies anyways, we have more efficient options for antennas (fan dipole, trapped dipole, etc).
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W8ZNX
Member

Posts: 1




Ignore
« Reply #4 on: August 27, 2008, 01:54:22 AM »

have been thinking
about trying one
for
50 blinkin years

don't ask here
JUST DO IT

thats right
make one
put it up

if it works for you
GREAT

if it does not work
try something else

mac
dit dit
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NJ2E
Member

Posts: 35




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« Reply #5 on: September 10, 2008, 03:08:38 PM »

Go to the following link for a detailed technical review of this antenna. There is another link for another article. It is a compromise antenna at best, especially QRP. Don't be concerned about having to register. The late W4RNL was a true leader on antenna modeling. 73s Don

http://www.cebik.com/content/a10/wire/t2fd.html
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KQ6Q
Member

Posts: 993




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« Reply #6 on: September 23, 2008, 09:26:45 PM »

The only WORSE antenna for QRP would be a dummy load. The T2FD achieves low SWR by having a big resistor in it. You don't have power to waste with QRP. Use ANYTHING else - the more wire the better - use a tuner if you need to, but a resonant wire is the most efficient antenna, and with QRP, efficiency is key.
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ON4ARJ
Member

Posts: 3




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« Reply #7 on: October 31, 2008, 01:00:47 PM »

I suggest to wait another 50 years.... I have built one and the result was very poor, more or less OK on RX, terrible on TX. I have removed the resistor and balun and have fed it with open feeder, the result was fantastic. But then I actually had a sort of doublet antenna with a lot of plastic spreaders.....If you have the space for a wire antenna just hang a doublet with open wire feeder to the shack, if you don't like open feeders to enter the house, put an ATU like SGC or CG 3000 outside and coax to the shack. Good luck!

Pat


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