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

Thu Feb 20 17:55:23 EST 2020

FB Don.

The Coastal Navigator is late 1960's tech, using Germanium transistors
and the 'usual' tuned ferrite loopstick (characteristic for gear of that vintage)
in conjunction with a telescopic whip to achieve its cardioid pattern.

Given your results on an S6.5 noise signal, the flag with its preamp achieves
a slightly lower noise floor (a little more more sensitivity) than the Coastal 
Navigator at 160 meters. I have seen S7 on the meter for 160 meters here
in the city (after a rain), but presently have two distinct noise sources 
keeping my noise floor more like S9 on the Icom station rig on 160m.

I've located the two sources, one, a residence (neighborhood has all under-
ground utilities), is a first for me on 160 meters. It sounds like a dull thud
and I confirmed the house by bicycling by and seeking a null with the ferrite
loopstick on the Coastal Navigator as I cycled by ... the antenna always 
pointed (using the null) in the direction of one residence, so I'm fairly 
certain of the house. That's a 'tactical' advantage of the Coastal Navigator,
along with the cardioid (unidirectional, when selected) antenna pattern!

73, de AA5CT Jim

.
.

On Thursday, February 20, 2020, 3:36:29 PM GMT-6, Don Kirk <wd8dsb at gmail.com> wrote: 

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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