Finding your @ss with both hands - Direction finding

[OSFG]

I am starting this to serve as a technical thread for non technical people to discuss the art/science of direction finding. Not the North/South/East /West kind but the to a Radio signal kind.

I will not put anything that is currently not publicly available into this thread, nor answer any questions related to current capability of the military so please don't ask.


Now what does Direction finding (DF) mean and how does someone do it?

Direction finding is simply to determine the direction from which a signal is coming. This can be done on multiple planes.
a one dimension, where I have only angle measurements such as degrees or North, south, etc, relative to another spot...Like on a map.
a Two dimension, which can give me not only angular measurements but also elevation or declination measurements...example...North relative to a spot, with an elevation or declination of "x" degrees.
or a three dimensional one, with angular, declination, and distance

Confused yet? I will slow it down...and start with the basics...and keep this focused on how could someone find YOUR transmissions

To get a Direction to a radio transmission, you need a receiver and an antenna at a minimum. plus either some really groovy-ass mathematical algorithms, or with some specific types of antennas.

The best way to easily locate you is with fixed site based receivers that are spaced apart where they can all hear your transmission at the same time. This then allows them (the bad cats) to do some simple Boolean Algebra and they got you pegged to a specific area. This can be a really big area or fairly accurate.

This is where it can become important to you or not!!!! If your operating in a city...narrowing your location down do a Mile square area doesn't mean a lot...But if you own 20 acres and that Mile square they find contains just you and your family...Well....Bob's your Uncle.

The downside is that if your operating in a city, they can have more an better receive antennas placed around to triangulate you more accurately than they can out in the rural areas.

I'm gonna take a pause and continue with Directional antennas!!! Yeah!...My favorite!! cause nobody likes simple pole's except for strippers and those who watch them...

[OSFG]

For mobile reciever direction finding, its typical to use a directional antenna. These can either be simple flat panels or high gain Yagi antennas etc.

The idea is to point the antenna in the direction where the signal comes in the strongest or clearest. Now this is hard because signals can bounce and reflect so its more like trying to determine the location of a gun shot...the more you hear the shots the easier it becomes.

There have been some fairly advanced antennes developed since world war II. I'll Start with Commutative DF antennas. The idea here that instead of point a single antenna around you while the transmission is occurring... You use a multi-omni antenna head....With 6 to 8 or so Omni (pole) antennas spaced a specific wave length apart and these elements are turned on and off in a circular pattern very quickly.
How quick? Like as quick as I can piss off my wife quick!!!! Go read up on it if you want. (unh...the antenna thing that is....not the pissed off wife thing...I'm pretty sure I deleted all the stuff....)

Now these antennas do that pointing in the right direction stuff, a helluva lot faster...so even some short duration transmission can be direction located.

With modern GPS and communications now multiple locations can share their location information, timing, and directional information so it makes it easier for them to close in..But....

Be not afraid...They still need two or more locations and some significant chatter.....

[OSFG]

For mobile reciever direction finding, its typical to use a directional antenna. These can either be simple flat panels or high gain Yagi antennas etc.

The idea is to point the antenna in the direction where the signal comes in the strongest or clearest. Now this is hard because signals can bounce and reflect so its more like trying to determine the location of a gun shot...the more you hear the shots the easier it becomes.

There have been some fairly advanced antennes developed since world war II. I'll Start with Commutative DF antennas. The idea here that instead of point a single antenna around you while the transmission is occurring... You use a multi-omni antenna head....With 6 to 8 or so Omni (pole) antennas spaced a specific wave length apart and these elements are turned on and off in a circular pattern very quickly.
How quick? Like how quick I can piss my wife off quick!!!! Go read up on it if you want. (unh...the antenna thing that is....not the pissed off wife thing...I'm pretty sure I deleted all the stuff....)

Now these antennas do that pointing in the right direction stuff, a helluva lot faster...so even some short duration transmission can be direction located.

With modern GPS and communications now multiple locations can share their location information, timing, and directional information so it makes it easier for them to close in..But....

Be not afraid...There still needs two or more locations and some significant chatter.....unless they are a super power nation....

[OSFG]

I'm gonna change the focus point for a minute and talk about the signals....
Radio frequencies are no different than visible light or sound waves, etc.... The waves can be bent, reflected, blocked, distorted, etc. The simple rule is, The higher the frequency is, the greater the loss will be, as it applies to absorption.

Whats that mean? Lower frequency waves will pass right through you, me, buildings, the atmosphere, even the earth and water. Higher frequency waves will be absorbed or blocked.

Light from the sun.. is very High in the RF spectrum, as is Xray, and Microwave, so these things get absorbed or blocked very quickly.

The same hold true with communications radio frequencies... The higher the frequency the more it gets absorbed into the earth, buildings etc.

Another significant fact is that the earth is not Flat...so at a certain point an RF wave just isn't being seen from a certain distance.

as a reference point...a person at sea level seas the horizon at about 3 Miles. Thats where the earth behind that point seems to drop below what you can see. Obviously the higher in elevation you are up or the distant lands. the further you can see it...

But at a certain point you cant see land and thats where the RF energy is going to....Out and up!!! It's going to either be absorbed into the earth or hit the atmosphere.

Now the atmosphere has several layers to it I will focus on 3 layers only. The E layer, the F1 layer, and the F2 layer. These are layers that either block and reflect signals from exiting the atmosphere or don't. There is also a D layer and others.... but I haven't dealt with them....They are strangers....and I do not trust them.....

Now the E layer typically reflects many Medium band frequencies and allows higher frequencies to pass thru...but it is sporadic...we shall call it...Timmay!!!..Nope...JK, its the Sporadic E layer.
The F1 and F2 (at night they blend to become one layer). reflect more reliably and allow frequencies from around 2MHz to 30MHz to be reflected back.

Just about everything above HF will pass through and make it to Mars...Venus....even Uranus...if you let it.

There is more to it all, but...you get the point.....It ain't coming back down to earth..so it ain't getting heard from beyond the horizon without being rebroadcasted from another location.


So nobody can DF from beyond the horizon those waves that are not being reflected.

[OSFG]

This leads me to another point...just because something gets bounced off the atmosphere, doesn't mean it can't be located back to source...The azimuth direction remains consistent.

But obviously with a bounce off of the sky that directional line gets blurred/distorted, because the earth is round and so is the atmosphere. This is where we go from one dimensional DF to two dimensions.... SHAKA KHAN!!!!


With a technique called interferometry and looking at not only the electrical phase of the RF energy but also the magnetic, (damn...now I'm just making stuff up right?)...they can determine the declination that a signal has been bounced off the atmosphere... this is most accurate if atmospheric readings are taken at the point of the suspected bounce and they can account for the shot angle that hit the atmosphere and compute the start point...Lots more stuff.....but in a nut shell ....2 and 3 dimensional locating.


good news....This is not highly accurate....bad news....lots of satellites....Just saying.


I'm gonna stop here and call it a night...thanks ya'll.

[A Watchman]

Excellent thread OSFG and well explained. Just for the record though ..... I like simple poles, for all the right reasons.

[hawgrider]

I need a drink.....

[TJC44]

Whats that mean? Lower frequency waves will pass right through you, me, buildings, the atmosphere, even the earth and water. Higher frequency waves will be absorbed or blocked.

Light from the sun.. is very High in the RF spectrum, as is Xray, and Microwave, so these things get absorbed or blocked very quickly.

The same hold true with communications radio frequencies... The higher the frequency the more it gets absorbed into the earth, buildings etc.

Another significant fact is that the earth is not Flat...so at a certain point an RF wave just isn't being seen from a certain distance.

But at a certain point you cant see land and thats where the RF energy is going to....Out and up!!! It's going to either be absorbed into the earth or hit the atmosphere.

Now the atmosphere has several layers to it I will focus on 3 layers only. The E layer, the F1 layer, and the F2 layer. These are layers that either block and reflect signals from exiting the atmosphere or don't. There is also a D layer and others.... but I haven't dealt with them....They are strangers....and I do not trust them.....

Now the E layer typically reflects many Medium band frequencies and allows higher frequencies to pass thru...but it is sporadic...we shall call it...Timmay!!!..Nope...JK, its the Sporadic E layer.
The F1 and F2 (at night they blend to become one layer). reflect more reliably and allow frequencies from around 2MHz to 30MHz to be reflected back.

Just about everything above HF will pass through and make it to Mars...Venus....even Uranus...if you let it.

There is more to it all, but...you get the point.....It ain't coming back down to earth..so it ain't getting heard from beyond the horizon without being rebroadcasted from another location.


So nobody can DF from beyond the horizon those waves that are not being reflected.

WHATT?? The Earth isn't Flat?? OMG!!

Great thread, OSFG.
If I may add 1 or 2 comments.
The skywave bounce also needs a couple of ingredients for flavoring. That comes from the sun.
The sun has an 11 year sunspot cycle. We are now on the downside of the latest cycle. When there are lots of sunspots, we are being bombarded with solar plasma. it does crazy things with the Sporadic-E, F1 and F2 layers, allowing a radio operator to bounce higher frequency signals. It is a radio operator's wet dream. When the cycle is down, as we approach what is called "solar minimum", atmospheric bounce, and range drop.
There is another factor involved called a solar wind. A magnetic "hole" will open up on the sun, and will stream plasma out. If that "wind" is pointed at the Earth, it dampens radio signals further.

On a closing note: Being in solar minimum, we are at much lower risk of a solar generated EMP.
http://www.spaceweather.com/

[OSFG]

WHATT?? The Earth isn't Flat?? OMG!!

Great thread, OSFG.
If I may add 1 or 2 comments.
The skywave bounce also needs a couple of ingredients for flavoring. That comes from the sun.
The sun has an 11 year sunspot cycle. We are now on the downside of the latest cycle. When there are lots of sunspots, we are being bombarded with solar plasma. it does crazy things with the Sporadic-E, F1 and F2 layers, allowing a radio operator to bounce higher frequency signals. It is a radio operator's wet dream. When the cycle is down, as we approach what is called "solar minimum", atmospheric bounce, and range drop.
There is another factor involved called a solar wind. A magnetic "hole" will open up on the sun, and will stream plasma out. If that "wind" is pointed at the Earth, it dampens radio signals further.

On a closing note: Being in solar minimum, we are at much lower risk of a solar generated EMP.
http://www.spaceweather.com/

Right you are...the dredded sun spot/Solar flare!.....Like the assault rifle of RF...It's blamed for the loss of communications way more than actual causes that loss.... at least among SF communicators.

During my final exam we had tremendous sunspot activity that affected comms all over the mid-west and east coast. On my team 4 out of 5 failed their shot that day... I'm not bragging, but I was the 5th one who passed.

Why? well we all got the solar report from the broadcast channel...we all knew it was gonna be ugly.

They all made their best antenna type and put them up in the tree. I did something unorthodoxed ... Instead of making 1/4 wave antennas or full wave length long wires, or terminate sloping V's...I made a two wave length L shaped antenna to transmit on and a standard omni to receive...but put the omni at a slant away from the emitter point (which was 1,000 miles away.

It worked.....don't ask me why...guess I got lucky...sometimes it pays to be lucky...
But solar activity can play hell with your comms and it will make you have to use different frequencies and antennas.

[MountainGirl]

I'm gonna stop here and call it a night...thanks ya'll.

As hawgrider says... DAYUM!
Wow. Thank you.
I have some questions - not about what you've typed so far, which has been incredibly clear and easy to follow - but a bit about the availability of hardware used for DF... I'd like to wait to ask, though, in case there's more you want to add... not sure if you were done or just tired for the night. Thanks again, OSFG.