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Old 7th Nov 2019, 5:49 pm   #1
Martin G7MRV
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Default Building a Radio-Goniometer - inductance values?

I appreciate this is pretty esoteric stuff, but just for fun, I fancy building a radio-goniometer (for those not familiar with the word, its a set of orthogonally fixed coils, which connect to antennas, and an internal movable pick-up coil, used for direction finding)

Now, I can find enough photos or sketches to see how to build one physically, but what I cant seem to find is any info on suitable numbers of turns for the coils, and hence the inductance values. I have seen just one mention of around 40uH.

I intend to use it for HF DF, with either loops or a set of four verticals, so im looking for values that would give 'decent' results (i.e. some results!) between say 160m and 20m, or somewhere sensible inbetween,

Anyone any suggestions?

Martin
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Old 8th Nov 2019, 1:01 am   #2
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Default Re: Building a Radio-Goniometer - inductance values?

Most of the more practical texts I can find are from the 1920's - 30's, but no actual values in some that I browsed.

This text seems to cover various different designs (from 1922) but i have run out of time to go through it a bit more thoroughly: https://archive.org/details/Directio...dingByWireless
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Old 8th Nov 2019, 5:03 am   #3
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Default Re: Building a Radio-Goniometer - inductance values?

Any value will do as far as getting directional sensing goes.

If you don't have enough inductance, it will be a difficult load on whatever is driving it. If you go for too much inductance you'll run into problems with self resonant frequency due to capacitance between the turns. These are exactly the same issues as with designing a simple transformer.

40uH is going to have a reactance of 452 Ohms at 1.8MHz. If you have amplifiers between the two antennae and the goniometer primary coils, this is almost ten times 50 Ohms. In transformer terms it sets the magnetising current and this factor at the LF end is plenty. So the next question is whether you can make a 40uH winding of the physical size required by your mechanism and get the self resonant frequency to be well above your upper frequency goal of 30MHz?

So 40uH is a reasonable figure if the goniometer is driven from 50 Ohm-ish amplifiers or directly from 50 Ohm-ish antennae.

Some (Classic Tektronix) colour TV test equipment had goniometer transformers to allow a front panel knob to rotate the phase of a subcarrier frequency signal through a full 360 degrees. The stator coils were driven with a 90 degree phase shift to make this work.

Marconi made a number of ship's direction finding receivers which worked with Faraday-screened crossed-loop antennae. A motor drove the goniometer sense coil as it looked for the nulls.

Aircraft carry an ADF Automatic Direction Finder. Essentially a square block of ferrite with two coils at right angles to each other... think of it as a 2-dimensional ferrite rod antenna. The goniometer it feeds is simulated electronically. Coverage is LF-MF where the beacon transmitters are plus long and medium wave broadcasters. Pilots are required to know how to use them to get their instrument ratings, but in use they've been supplanted by GPS. They are still officially seen as a fallback. So planes carry multiple GPS receivers, so there are plenty of satellites.... the risk comes from interference problems.

So goniometers are still in use, though the people using them are unaware of the name. They'd probably think it was something quite rude if they heard the word.

David
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Old 8th Nov 2019, 8:27 am   #4
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Default Re: Building a Radio-Goniometer - inductance values?

19 set variometer?
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Old 9th Nov 2019, 10:33 am   #5
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Default Re: Building a Radio-Goniometer - inductance values?

There is a picture of one here but it leaves me completely puzzled.

https://en.wikipedia.org/wiki/Bellin...l_workings.jpg

The two static coils are mounted in parallel to one another - I can't see how that can work.

I thought they needed to sit at right angles with the rotating search coil at the cross point. Odd.
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Old 9th Nov 2019, 10:36 am   #6
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Default Re: Building a Radio-Goniometer - inductance values?

What you see are two halves of one static coil.

There are two halves of the other one wound at right angles to them, all over a square box-shaped frame.

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Old 9th Nov 2019, 10:53 am   #7
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Default Re: Building a Radio-Goniometer - inductance values?

Ah-ha - yes that would solve it.
Many thanks.
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Old 9th Nov 2019, 11:38 am   #8
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Default Re: Building a Radio-Goniometer - inductance values?

S.P Radio of Aalborg Denmark made a beautiful little radio the 66TS, I have one, it contains an RF stage and uses AF127 transistors & others. So they fitted it with a DF antenna input for marine navigation use. I don't have that antenna but I have seen photos of it, it consists of two coils crossed at 90 degrees. It looked like they were probably wound inside PVC or fiberglass tubes to weatherproof them for marine use.

There is some very interesting information about radio direction finding and navigation in the introductory part of the service manual for this radio:

http://www.peel.dk/SP/pdf/Sailor%206...20(Manual).pdf
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Old 9th Nov 2019, 1:29 pm   #9
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Default Re: Building a Radio-Goniometer - inductance values?

The tubes inside which marine crossed-loops are wound are often metal, with a not very visible insulated gap in each tube loop so they don't form a shorted turn. Faraday screening, in other words.... real Faraday screens!

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Old 9th Nov 2019, 1:35 pm   #10
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Default Re: Building a Radio-Goniometer - inductance values?

I assume the coil pairs are set at the Helmholtz distance, 1/2 the diameter apart for an even field, that would, I assume, need a tweak for rectangular coils.
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Old 9th Nov 2019, 1:52 pm   #11
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Default Re: Building a Radio-Goniometer - inductance values?

Quote:
Originally Posted by Radio Wrangler View Post
Any value will do as far as getting directional sensing goes.

If you don't have enough inductance, it will be a difficult load on whatever is driving it. If you go for too much inductance you'll run into problems with self resonant frequency due to capacitance between the turns. These are exactly the same issues as with designing a simple transformer.

40uH is going to have a reactance of 452 Ohms at 1.8MHz. If you have amplifiers between the two antennae and the goniometer primary coils, this is almost ten times 50 Ohms. In transformer terms it sets the magnetising current and this factor at the LF end is plenty. So the next question is whether you can make a 40uH winding of the physical size required by your mechanism and get the self resonant frequency to be well above your upper frequency goal of 30MHz?

So 40uH is a reasonable figure if the goniometer is driven from 50 Ohm-ish amplifiers or directly from 50 Ohm-ish antennae.


David
Cheers David,

That does sound a reasonable value to aim for then. The actual frequency range isnt at all important, so longs as it works well enough to play about with! Its just an experiement for general interest,

when (and if!) I get a working model built, i'll post it up (many of my projects are on the slow boat!)

Martin
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Old 9th Nov 2019, 6:23 pm   #12
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Default Re: Building a Radio-Goniometer - inductance values?

Quote:
Originally Posted by merlinmaxwell View Post
I assume the coil pairs are set at the Helmholtz distance, 1/2 the diameter apart for an even field, that would, I assume, need a tweak for rectangular coils.
You want reasonably uniform fields at least over a length equal to the swept volume of the sense coil, or else you suffer some distortion of the angular scale.

David
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Old 9th Nov 2019, 7:03 pm   #13
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Default Re: Building a Radio-Goniometer - inductance values?

You could get the same result with a four tap 360 degree potentiometer. Modern devices could drive a ring of mercury easily, there would be no contact noise either.
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Old 9th Nov 2019, 9:50 pm   #14
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Default Re: Building a Radio-Goniometer - inductance values?

The ordinary goniometer can have the connections to the rotating coil done via a rotary transformer, away from the main part, and that avoids contact noise too.

David
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Old 9th Nov 2019, 10:14 pm   #15
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Default Re: Building a Radio-Goniometer - inductance values?

Quote:
Originally Posted by Radio Wrangler View Post
The tubes inside which marine crossed-loops are wound are often metal, with a not very visible insulated gap in each tube loop so they don't form a shorted turn. Faraday screening, in other words.... real Faraday screens!

David
Yes electrostatically shielding the loop with a tube or screen, makes it mainly sensitive to only the magnetic component of the field, much like a ferrite rod. In the front section of the 66T manual they also mention a ferrite rod option for RDF applications. And as you noted if metal is used don't forget to put a split/gap in the loop.

One way to easily make a multi-turn shielded loop is to use screened multi-core cable as you already have a good number of conductors that are screened and you can wire them in series to make the coil.
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Old 10th Nov 2019, 9:30 am   #16
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Default Re: Building a Radio-Goniometer - inductance values?

I followed the SP radio trail and it led me here.

Nothing to do with a Goniometer but one day it might be worth trying to make something like this for fun.

http://www.peel.dk/SP/pdf/Sailor%20B...navigator).pdf
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Old 10th Nov 2019, 9:52 am   #17
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Default Re: Building a Radio-Goniometer - inductance values?

I made one (again out of interest) some years back. One interesting application of these is as a variable phase-shifter. Connect one input to the source and another with a simple R/C network to give a 90 deg shift. The output is then continuously variable in phase with reference to the source.
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Old 10th Nov 2019, 12:44 pm   #18
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Default Re: Building a Radio-Goniometer - inductance values?

And if you spin the shaft, you get a frequency shift, one Hertz for every rev/second.
Spin the other way for the opposite direction of shift.

Simulate the goniometer electronically and you get the image-rejecting mixer.

David
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