PA8W's Radio Direction Finding Technology

Adding an Elevation Indicator to a Radio Direction Finder

The following feature seemed to be a first in amateur radio.
I found some notes referring to the same technique in a few patents,
but I didn't find a single amateur or commercial pseudo doppler 
Radio Direction Finder offering the following feature...
Hunting airborne radio sources like weather balloons (radiosondes) using the doppler 
Direction Finder,
the following question will be unavoidable:
How high is the darned thing flying?...

Well, we don't have a simple or straight answer to that questiion, but there is a simple way to extract some elevation information from your doppler
Direction Finder:

In a normal setup, the "doppler" tone has its maximum amplitude for signals originating from the earth surface.
For an elevated source, like an airplane or balloon, the doppler amplitude will be slightly smaller,
because for that signal the antenna separation distance seems to be a little bit smaller.
If we would increase the elevation of the source, the doppler amplitude would reduce further until it would drop to zero when the source is right on top of your antenna.
This doppler amplitude reduction simply follows a sine function.

I used this effect to add a simple elevation indicator to my Version 1 doppler:

Adding an Elevation Indicator to a Radio Direction Finder

The add-on box on the right side of the RDF has a 10 LED indicator to give an approximate of the elevation.
I had to take out the ALC of course, otherwise it would cancel any amplitude differences.
Then I take the filtered doppler tone right before it is fed into the zero crossing detector, and feed it into a LED VU meter.
This VU meter is upside down, so maximum level lights the bottom LED.
This bottom LED is the point to calibrate the input level to on a low elevation signal.
Any reduction of the doppler amplitude will now cause the LED VU meter to climb up until it reaches the "almost zero" point: Zenith!

Remember, this is not about the RF signal strength, this about the doppler signal strength.
Reduced antenna separation distance means reduced phase jump magnitudes in the FM receiver and thus reduced doppler tone amplitude.

Obviously, there are a lot of parameters that may have an impact on the doppler amplitude.
Therefore, the elevation indicator will be reliable only on strong, undisturbed signals.
But that is exactly what we have when driving below a radiosonde.

In field tests the accuracy turned out to be fair, but far less than the azimuth accuracy, which makes sense of course.
However it does add a bit of useful information to the balloon hunter.
That's why I added this feature to my RDF42 and RDF43
as well, in the left bottom corner:

Adding an Elevation Indicator to a Radio Direction Finder

Cheers, PA8W.