Radio observation in hydrogen line (1420MHz)

Advice on how to progress with my interferometer based on AD8302 amplitude phase comparator board 8/1/2024

Correspondence with Jan Lustrup:

Hi Andy….

You want to test changes of phase between the ports, not frequency!

Use the same frequency generator to a 2 way power splitter. One splitter output to cannel A and the other splitter output to a different coax cable lengths to cannel B and you should see a phase difference as a voltage level change..

Try 420MHz and the use 2 or more different coax cables lengths and you will see the phase voltage output will change for each cable used!

Or use a small loop on the splitter output and a loop on B port. Now move the loops together or away from each other. The voltage swing will follow the distance moved between the two loops! This will be easily seen  if you use a 1420MHz signal as only 21cm will show a full wavelength of voltage swing!

Make sure you have enough power output so the AD8302 will work….


Hi Jan,

Does that mean the phase output of the AD8302 is a constant voltage, the strength of which changes according to phase difference, rather than varying fringes? I have been expecting wobbling trace go up and down to show fringes!


No not constant voltage output…..

This is how I understand it:

For a given phase angle there is a specsefic voltage (or swing rate is phase is changing) over or under zero volts.

For example: At phase  0 degree = zero volt, 90 degree max voltage (for the given RF input power), 180degree zero volt again, 270degrees you get max negative voltage (for the given RF input power) and when 360 degrees back to zero…and so on.

For a given RF power input there is a specsefic voltage amplitude.

So more power the lager the fringe voltage  (or amplitude)…..the faster the phase changes the higher the fringe rate etc..

So you have two factors that affect the output voltage & fringe rate..

Try this using two signal generators—-1000Hz on input A and 1001Hz on input B. You will see a voltage swing moving up and down once every second.

Or 100.000000MHz on A and 100.000001MHz on B and you get the same result.

Now how high the voltage peaks are dependent on the input power level to each A & B ports.

You won’t see any real minus voltage value your AD8302 output (it only outputs 0 to + 5Volts) must be connected to an OP amp with + and – voltage supply. Otherwise any “negative” voltage swing is referred to the  Pset input Voltage setting (you may use the onboard Vref for that)!

Sorry if I made any confusion.


Here is a simple way to amplify and zero adjust your AD8302 output. Here you get x10 amplification, You use the potensiometer to “zero adjust” your output. You may use a variable potensiometer lets say 100K  then you could adjust the amplification from 0 to 100 times!


My own note – compare above with Jan’s previously sent schematic and op amp circuit:

Correspondence with Melvyn:

Hi Andrew.

Those results look good.

To understand the final test (different frequencies in the two arms of the phase detector) it is better to think of a phase detector as an RF mixer. The device will output the sum and difference frequencies, these will be 2MHz and 2842MHz (plus many other harmonically related frequencies).   However because of the intended use of a phase detector these high frequencies are filtered out so you don’t see any output in your test.

To work it will expect to see the same frequency (1420MHz) on both arms. If you put in two signals differing by 1Hz you would see a 1Hz sine wave on the output, but this is very difficult to achieve with the stability of typical amateur signal sources.

In use, the output from the two antennas is at the same frequency and as the relative phase of the signals changes you see a change in the DC output. So to test this connect the same source to both inputs and record the dc output. Then change the length of the cable to one arm This will change the relative phase of the two signals and you should see a change in the output. 
Let us know how you get on.


Correspondence with Peter:


To check your phase comparator

From your splitter, feed one output directly to the phase comparator input and the other output to the second comparator input via a 10m length of cable.

There will now be a phase difference between the comparator inputs equal to 360 x 10L x f /c  degrees.  

10L is electrical length of the 10m cable in m, f is the frequency in Hz and c is the speed of EM signal in space in m/s. 

When you tune the RF generator frequency you will notice the comparator outout varies sinusoidally.


Correspondence with Mike:

Hi Andrew,

Changes in phase because of different lengths of cable will just be a solid voltage.

For the Sun in transit past your dishes will be a slowly changing  sine type wave.

Fringe rate = Baseline / wavelength *angular rotation of earth * cos declination

Invert this to get the fringe period.  

so the sun is at  -22 Declination  and I am guessing your dishes at 3 meters apart (baseline) =1034 sec period

If dishes are 10 meters apart = 310 sec  or 5 minutes

Fringe period = (3 M / .21M * 7.3 x 10 -5 * cos -22)-1 = 1034 sec



Thanks Mike.

Does change in phase present as change in voltage level or fringes on AD8302 when signals are presented into RF inputs from generator like this?

And for interest similar question for when sun passes through field of 2 dishes?

It is basic questions like this that are causing me to stumble!



Sounds like it is working now.  good!

To check the phase, I would  use one generator hooked to the splitter and two different lengths of cables  from splitter to each of the inputs.  It could be just fitting to cause a difference in length. This should give you a phase output other than  0.9vdc which would be the phase . The range of the phase  is 0 to 1.8 vdc   for 0 to 180 degrees.   .9v is 90 deg.       1/2wavelength is 180deg so you won’t see a a difference.  1/4 wavelength will give a 90deg change

If you try to calculate the phase remember that cables have velocity factors depending on the type/model/manufacturer of the cable.

Have fun


Hi Mike,

I think I’m a step towards sorting this out – new AD8302 phase comparator board. Taken much of stuck off old layout and starting as simply as possible. Found I had stabilised 5V power supply in shack – very convenient.

Labjack U3 and Radio Sky Pipe II software used for these tests and my Dell Inspiron laptop.

Started with nothing attached to either RF input not even dummy load, as per advice from Jan Lustrup (thanks Jan for your advice!) Also didn’t bother with any case for these first tests.

So, firstly confirmed that both signal outputs (phase and total power) are 0.96 V or thereabouts compared to ground, consistent with the bias voltage known to be on these lines.

I then added a RF signal generator to the RF lines – used a power Splitter so same signal to both RF inputs to AD8302. Total power went up, phase stayed same.

Added Attenuator between power Splitter and AD8302 – changing attenuation changes power detected on PC.

Changing frequency from 1420MHz to 420MHz on RF signal generator made no difference. 

Now took out power Splitter and added 2nd RF signal generator to 2nd RF input on AD8302. Turning one or other of the signal generators changes total power and power on phase lines in ways I don’t understand. Changing RF frequency on one of RF signal generators eg by 2MHz seems to make little or no difference. No interference fringes seen.

I don’t understand the last set of findings. If anyone can help me it would be greatly appreciated.


Thanks Mike – I do have a controlled power supply but not sure it is 5V but 12V – is it easy to build simply circuit to transform this to 5V?

Otherwise can you recommend a power supply that would do trick on


Dear Andrew,

After  our zoom meeting, I was thinking about what we saw and have a few thoughts to share.

I attached a picture you shared on the SARA forum. this was very similar to what we saw.

The outputs did not respond to changes on the inputs, so definitely it was not working.

I don’t know what the vertical scale units were.  but both outputs had the same/similar traces. This says the same source. So I would suspect the power supply or ground loops.  I think the first. If it is a dynamic power supply, we are seeing the charging of the output capacitors maintaining the voltage. It looks like 3 or 4 pulses a seconds so it don’t look like power line frequency(50 or 60 hz). Maybe usb bursts? but they are mhz.  Find a solid power supply with a transformer, regulators and a big capacitor.  

Also when you build your op amp board use capacitors (.1uf) on all supplies [COMMENT FROM ANDREW: Op amp is part of voltage regulation to correct voltage to stable 5V recommended by Mike – however, I ended up not making this as I discovered I had stabilised 5V power supply in my shack.]

I tested a AD8302 board on the bench.  Both inputs of the board have 50 resistors to ground and an isolating capacitor to the chip.  No external 50 ohms load is really necessary, so the screw on load is extra. I put 5vdc on from the laptop usb comms converter.  

I told you that the outputs were biased to 0.900 volts no signal and that is where they are.

Mag output no signal is .912 vdc

Phase output no siganl is 1.076 vdc

Mag output with finger touch to a input was 0.930vdc

Phase output with finger touch was 0.907vdc

It would bounce about a few mv.

Touch with a finger is an old technician’s signal generator from radio days.

Just some notes to help you along.



Mike Otte W9YS

Correspondence with Marcus:

Dear Andrew,

Two signal generators that aren’t phase-locked to each other will produce random phase measurements in anything trying to
  measure phase between them.

A single signal generator, with a split going to each side of your phase measurement will produce a more-or-less constant output,
  even when changing frequency, because at least at gross scales the phase relationship between the two sides stays constant.

You only get fringes when the phase relationship between the two sides change over time — as when a source moves across the
  sky, and you have two widely-separated antennas (a dozen or two lambda will do) feeding the two inputs of your phase
  measurement scheme.  Then the phase relationship between the two sides will change over time, as the wavefront takes
  slightly longer to reach on antenna compared to the other….


Thanks Marcus.

With regards to your comment below, what in practice would I see on RSPII with this?

“Two signal generators that aren’t phase-locked to each other will produce random phase measurements in anything trying to measure phase between them.”

Andy It kind of depends, and I’m not that familiar with RSP.

Two non-phase-locked sources tuned to the same frequency will generally “dance around” each other over timescales ranging
  from fractions-of-a-second to several seconds.

It’s easiest to relate to on an oscilloscope display.  Put up two sinusoids from two different signal generators, notionally
  set to the same frequency.   Let them free-run rather than triggering on one channel.

In the oscilloscope scenario, we have three players that aren’t phase-locked to one another, making the “dance” more

Depending on the nature of the random phase measurements, and over what timescale, a low-pass filter might even
  smooth these out so that you’ll end up seeing a near constant value, depending on the distribution (in a statistical sense
  of the random phase variations between the two sides).

The essence of fringes is that they’re caused by the physical relative motion between your baseline (the distance between
  your two antennas) and the source.  This necessarily causes a slightly longer path between the source and the two antennas,
  except precisely at the moment of transit, where the path-lengths (ignoring fine-scale trop-effects, etc) will be the same.  Since
  this distance will change fairly slowly over time, the relative phase will also change rather slowly, unless you have a baseline
  that is thousands of lambda wide.

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