Resolving the 180° ambiguity in the Watson-Watt method with Adcock antennas

[NickFi]

I read on this website: An Introduction to Radio Direction Finding - Cojot that adding an omnidirectional antenna to an Adcock antenna would help resolve the 180° ambiguity.

"A Watson-Watt antenna cannot determine if a signal comes from the front or the back without the use of a third omni directional antenna to resolve the 180° ambiguity."

Is this statement true? Because if it is, I cannot figure out how.

 

[mmckenna]

I have an old marine RDF radio and it has the whip for doing the 180º thing. It does work, but its been years since I've done it.

 

[ScannerSK]

I have noticed when tracking down weak signals they are stronger in signal strength when I travel in the direction of the signal and weaker in signal strength when I travel away from the signal source. This is noticeable when using a whip antenna mounted on the roof of my Chevy S-10 pickup. I just thought I would mention it. It's an interesting phenomenon I have noticed on a number of different occasions.

 

[NickFi]

Thank you for confirming the whip works to solve the ambiguity. I came across the document presented at EA4FSI-28T1 :: Adcock/Watson-Watt Radio Direction Finding and it describes how the signal from the sense monopole (the antenna located at the centre) is used to eliminate half of the segment visible on the scope. The system is using sort of half rectification to display only half of the segment on the scope display (in the presented case, from origin to first quadrant). In fig 10, I consider the phase shift has to be any value that causes a common zero-crossing of the sense (reference) signal and the combined signal from N-S elements.

 

[NickFi]

I have noticed when tracking down weak signals they are stronger in signal strength when I travel in the direction of the signal and weaker in signal strength when I travel away from the signal source. This is noticeable when using a whip antenna mounted on the roof of my Chevy S-10 pickup. I just thought I would mention it. It's an interesting phenomenon I have noticed on a number of different occasions.

Thanks for comment ScannerSK. It can be expected that the signal gets stronger when you travel toward the signal source, although probably not noticeable on very short distances travelled. Do you have other setup when chasing signals, apart for the whip antenna?

 

[mmckenna]

I have noticed when tracking down weak signals they are stronger in signal strength when I travel in the direction of the signal and weaker in signal strength when I travel away from the signal source. This is noticeable when using a whip antenna mounted on the roof of my Chevy S-10 pickup. I just thought I would mention it. It's an interesting phenomenon I have noticed on a number of different occasions.

Where is your antenna mounted, and what bands?

 

[ScannerSK]

When a signal is barely detectable, I can turn around and head the opposite direction at times to determine which direction it is coming from. When traveling up and down the same stretch of road, I will at times detect the signal only when heading in the direction of the signal source. I'm not into direction finding too much so I just have a VHF whip antenna mounted in the center of the roof of my Chevy S-10. I use to have a Ramsey direction finding kit however sold it.

 

[NickFi]

Where is your antenna mounted, and what bands?

Good question - if the antenna is mounted at the front of the car, when travelling away from the signal, it may get shielded by the car itself

 

[mmckenna]

Good question - if the antenna is mounted at the front of the car, when travelling away from the signal, it may get shielded by the car itself

It'll make it directional away from the ground plane, also. So, antenna on the front of the car will perform better in the direction the car is going, and less well behind you.

At least you'll know if you are heading in the right direction.