Re: Elecraft Digest, Vol 160, Issue 18 - More on the TTFD

[Joel R. Hallas]

I agree with most of the statements about the TTFD, or B&W FD, if you
prefer. While not as efficient as some, it meets a number of particular
needs, especially that it has a similar directional pattern over a 5:1
frequency range -- especially useful for point-to-point links using multiple
HF frequencies, either manually or by ALE.

I did an assessment of this antenna and wrote a QST article on it that I
recommend for those interested. The one point I disagree with is the
statement that the terminating resistor dissipates half the power. The power
dissipated is a function of frequency and, while it is not far from 3 dB at
the high end of the frequency range, it absorbs much more at the low end --
about 90%! Not only is that 10 dB less signal on transmit, but the resistor
power rating needs to be 90% of the transmit output, if high duty-cycle
modes are used. Note that it is not as big a problem on receive, since the
receive S/N on the lower bands is sufficiently atmospheric noise limited,
that the receive S/N is not greatly impacted, even though the s-meter will
read lower by 10 dB.

My article is available to ARRL members on the QST print archive on the ARRL
website.

Sep 2010 - QST (Pg. 51)
A Close Look at the Terminated Folded Dipole Antenna

Regards, Joel Hallas, W1ZR
Westport, CT




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[k6dgw]

True.  For any given total length, the loss to the termination increases
as the TX frequency goes down, and the loss vs freq curve rises rapidly
near the lower end.  The solution of course is to make the total length
of the FD, end-to-end long enough that the termination loss at the
chosen TX freq(s) is close to 3 dB.  This works well for the military
and government installations who often have a small number of assigned
frequencies, and the ubiquitous "B&W Over Armories" version capitalizes
on this.  ALE complicates that strategy somewhat of course.

As has been noted before, antenna choice is a basket full of trade-offs
and not every application for an antenna is to make Honor Roll in a
week.  Paraphrasing Rowdy, my #1 while in the military, "We sometimes
find ourselves trying to pick fly poop out of the pepper."  In that
vein, I suspect that 95%+ of the time, anything I can work with 10 W on
20 m from my K2 I can also work with 5 W on 20 m from the same K2.  Of
course, 87.49% of people make up their own statistics too. [:-)

73,

Fred ("Skip") K6DGW
Sparks NV USA
Washoe County DM09dn

On 8/14/2017 8:54 AM, Joel Hallas wrote:

> The one point I disagree with is the
> statement that the terminating resistor dissipates half the power. The power
> dissipated is a function of frequency and, while it is not far from 3 dB at
> the high end of the frequency range, it absorbs much more at the low end --
> about 90%! Not only is that 10 dB less signal on transmit, but the resistor
> power rating needs to be 90% of the transmit output, if high duty-cycle
> modes are used. Note that it is not as big a problem on receive, since the
> receive S/N on the lower bands is sufficiently atmospheric noise limited,
> that the receive S/N is not greatly impacted, even though the s-meter will
> read lower by 10 dB.
>

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