[RFI] Portable Terminated Flag for MF/HF direction finding by WD8DSB

[Don Kirk]

Hi Jim,

It all depends on frequency due to the change in gain of the terminated
flag versus frequency and here is some initial info that might help.

*If using a 20 dB preamp, below is the system gain (antenna and preamp
combined gain) vs. frequency.*
28 MHz = +1 dBi
21 MHz = -3 dBi
14 MHz = -7 dBi
7 MHz = -21 dBi
3.5 MHz = -33 dBi
1.8 MHz = -43 dBi

Note: just subtract -20 dB to the above numbers to determine actual gain of
the 2 foot by 4 foot tuned loop.

I just ran a quick test for you on 160 meters comparing my 160 meter
transmit antenna which is a 68 foot tall base loaded vertical with 55 short
ground radials (2500 feet of radials) versus the portable terminated flag.

Normally my daytime S meter reading is S5.5  It just so happens that today
I have some noise that sounds like powerline noise arriving from the
South.  With my noise blanker on, my S meter reads S5.5 but with it off it
reads S6.3.  (Note: the noise blanker on my 40 year old TS-180s does a
wonderful job making powerline noise disappear as long as there are not
lots of strong signals on the band)

I can just barely hear the powerline noise using the portable flag with 20
dB preamp connected to my DX-440 portable receiver, but I can still
determine the basic direction the signal is coming from (I can hear it when
turned to the South but not turned to the North).  When I switch to my W1FB
preamp set to around 40 dB of gain the signal (powerline noise) is easy to
hear, and easy to determine direction.  While this info is very basic, it
does show that my 2 foot by 4 foot terminated flag using a 40 dB preamp can
easily hear signals that are only 0.8 S units above my normal TX antenna
noise floor on 160 meters, and can just barely hear a signal that's 0.8 S
units above my normal noise floor when using a 20 dB preamp.

While the atmospheric noise goes down as we move up in frequency, the basic
gain of the terminated flag also goes up so hopefully the 2 foot by 4 foot
terminated flag used with an appropriate preamp will work for DFing in all
but the weakest of signal situations.

Note: I can envision situations where I will use a tuned loop in
conjunction with the terminated flag.  Obtain correct direction with
terminated flag, and maybe use sharp null of tuned loop to confirm or fine
tune direction if null of the terminated flag is not as sharp .

Hope this basic information provides a little insight into my 2 foot by 4
foot terminated flag.  How do you think this compares with the Coastal
Navigator receiver?

Don (wd8dsb)










On Thu, Feb 20, 2020 at 11:33 AM AA5CT <jwin95 at yahoo.com> wrote:

> Don,
>
> The 64,000 dollar question: After adding the preamp, what is the
> 'effective'
> sensitivity compared to station receiver?
>
> For instance, using the Coastal Navigator DF receivers (that exhibit a
> very useful unidirectional cardioid pattern that has a sharp null to the
> rear) I can hear powerline noise above S7, in the S8 or stronger range
> as shown on the Icom 756Pro3 station receiver connected to a
> vertically-polarized antenna (a QW, 120 ft circumference, tuned loop).
>
> A noise in the S9 or greater range on the IC-756ProIII is very easy to
> pick up and locate with the Coastal Navigator receivers.
>
> The wideband design of your design is a fantastic asset to tracking this
> stuff down too, and the use with 50 Ohm input receivers (including
> Spectrum Analyzers and lab-type impulse/EMI receivers) is a plus too.
>
> 73, de AA5CT Jim
>
> .
> .
>
>
> On Thursday, February 20, 2020, 9:46:42 AM GMT-6, Don Kirk <
> wd8dsb at gmail.com> wrote:
>
>
>
>
>
> Numerous people asked for information about my portable terminated flag
> that I built for MF/HF direction finding since it's unidirectional and here
> is a link to a simple website I created to show my initial design.
>
> https://sites.google.com/site/portableflagantenna/
>
> My design may look crude, but I needed something quick, simple and cheap
> for help in tracking some noise at the QTH of Mike (W9RE).  A lot of
> thought went into the design based on maximum allowable size for transport
> in my car, minimum acceptable front to back ratio at very low elevation
> angles, frequency coverage, complex impedance, gain (or lack thereof), etc.
>
> The antenna has very negative gain especially on the lower HF and MF bands,
> but with the appropriate preamp its become an extremely valuable tool.
> It's much simpler and robust compared to anything else I could find.  I had
> been considering a sense antenna for use with my tuned loops, but wanted a
> much less complicated system and the portable terminated flag has
> definitely filled my immediate needs.
>
> Flags and Pennants just want to work, and my small design is no exception.
> Hope this initial info satisfies folks that had an immediate need for a
> simple unidirectional antenna for DFing on MF and HF.
>
> In the background I'm working on an article that includes a lot of charts
> related to Front to Back Ratio Vs. Elevation Angle and Frequency, SWR vs.
> Frequency, Gain vs. Frequency, etc.
>
> Please don't laugh at the simplicity of my build.
>
> Enjoy,
> Don (wd8dsb)
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