This is a phase difference difference not a true Doppler.
For more interferometer info look for WB2HOL TDOA or K6BMG Super DF'er.
I have had a Super DF'er for +25yrs - it works but is difficult to just pick up and DF with. Since it nulls at the perpendicular (equal phase) angles to the TX --- is the TX in front Zero-degrees or behind 180-degrees? It takes practice and more practice to insure you have the DF'er setup the "same" way each time you use it or it will lead you off on frustrating tangents, i.e are you sure your dipoles are placed identically each time (hot element up on both sides) or could one be flipped - guess what that will do to your left-right?
Then there is the TX signal; these TDOA/Phase DF'ers do not work well (if at all) with weak signals (=<S3) and cross-polarity TX signal. Are you sure the TX signal is vertical polarized, how do you determine the TX signal polarity (diagonal or horizontal); what will different phase angles/polarities do to your DF'er? Could you be seeing a multi-path signal, how do you sort out the bounce from the real bearing? Is the signal knife-edging?
I'm not saying that these devices do not or will not RDF a signal. But DO NOT expect to just put the kit together and magically become a super hunter - it is not a magical device! It will take many hunts to become proficient and learn/know/discover the DF'er boundaries/limitations and for you to create methods/techniques in how to use the device effectively (or you're going to walk lots of extra miles becoming more frustrated at each mile)!
Build the kit THEN GET A 2m QRP (<100mW) Hidden T - have a friend/XYL hide it somewhere (in a local park) and do your own mini-training hunts to get proficient (XYL's love to laugh/taunt us when we get frustrated, especially since they already have the answer/location). I have 100mW 2m TX in a cammy-taped band-aide box with a cammy 1/4wave whip as my practice T --- photos/info can be found on RDF-USA Yahoo User group, that has many good files/info in it, plus many of the top USA Hunters are Group members - they can/will answer intelligent questions.
The bottom line is I have a TDOA/Super-DF'er but rarely us it, as I do not use it often enough to stay on top of it.
It is much simpler/easier/faster to use my version of the famous WB2HOL tape-measure 3el Sniffer Yagi and an HT (TH-F6a w/S meter). On my mobile I sport my home built/design special 2m 3el T-Hunting Quad and specially modified mobile radios. If/when I mobile RDF 70cm I'll use a 6el Quagi, that can easily double as a portable sniffer antenna. Mobile RDF 12/10/6m the antennas get big - here is where the use of a Loop antenna is needed.
If you seek a challenge then RDF >70cm, the higher the frequency the more exponentially difficult the RDF due to severe to horrendous TX signal multi-path'ing. Add intermittent TX signals and you'll tear your hair out!