The lowest risk Kraken DOA bundle for special relativity experiment?

[jabowery]

I’m putting together an experiment to test special relativity.
The BOM for the instrument must qualitatively detect phase change between two antennas.

The requirements:

• 2 antennas*.
• The Kraken DOA software GUI’s DOA graph changes in response to a change in the phase relationship between the 2 antennas.
• Tuned frequency in the 88.0MHz to 108.0MHz range**.
• Set up be simple enough that a typical secondary school physics teacher can assemble and test the DOA system, as such, with about 8 hours investment.

After a good deal of playing with the software and RTL-SDR v3 hardware related to this (not the Kraken DOA hardware itself), I’ve determined that in all likelihood the best way to achieve these requirements is to have the investigator:

1. Purchase the BOM for a complete Kraken DOA system
2. Test the Kraken DOA system as such in the desired frequency range
3. Modify the configuration to be only 2 antennas.

The “2 Antennas” are part of a custom PCB I’ve designed in conjunction with the principle investigator. It presents 2 male SMA connectors. Depending on the requirements of the Kraken DOA system, the PCB may, or may not, include a filter that rolls off outside the desired frequency range.

However, I’m kind of stuck at step 1 because this illustration doesn’t show how the system is powered. After some examination of the RBP4B connectors involved, it appears that the USB-3 block is connected to the Kraken’s USB-C data port.

So I’m thinking the following may be the BOM (this is a link to the Mouser project BOM):

1 KrakenSDR-1 DOA box
1 Embedded Box Computers Pi400GR
1 Kraken DOA MicroSSD card preconfigured for the RPi4B
2 SMA cables (male to female)
2 USB-C power cables (male to male)
2 5V/2.4A+ USB-C power supplies
1 USB-3 to USB-C data cable (male to male)
1 KRAKENANT-01 Matched set of 5 antennas

This gives each component its own power supply via its one USB-C power connector. The data transmission port on the RBP4 is USB-3 – not USB-C because USB-C is being used to provide it power and there appears to be no other power port on the RBP4.

This should make steps #1 and #2 relatively straight forward. The remainder of the “8 hours” would be taken up by manually editing the software configuration to be for 2 antennas only and hooking up the PCB to the corresponding 2 SMA connectors on the Kraken box.

* These are the two antennas – plasma antenna CCFLs mounted on a PCB. In this photo they are hooked up to my failed effort to contain the expense of this project by wiring the clock of one RTL-SDR V3 to the other’s – which unfortunately did not provide adequate coherence for the phase detection as well as requiring a good deal of work to configure the Kraken DOA software system correctly.

** Although the original experiment used the 19kHz FM pilot wave indicating stereo availability, in order to detect a phase shift of on the order of 100ns in that wave, long sample periods and an expensive Picoscope was required. And, yes 100ns is long compared to the 100MHz frequency and, yes, such a phase shift is not predicted by known physics. That’s what we’re trying to nail down in a system cheap and simple enough to replicate that there can be little doubt that this is, in fact, the real phase shift.

[krakenrf_carl]

@jabowery

However, I’m kind of stuck at step 1 because this illustration doesn’t show how the system is powered. After some examination of the RBP4B connectors involved, it appears that the USB-3 block is connected to the Kraken’s USB-C data port.

Yep you will need a USB-A to USB-C data cable to connect between the Kraken and Pi. And a 5V/2.4A supply for the KrakenSDR, and a similar supply for the Raspberry Pi.

Please check out 01. Additional Hardware Required · krakenrf/krakensdr_docs Wiki · GitHub for more information.

One note about coax cables: If you’re using coax to connect to the antennas, please make sure that the coax cables are the same length, and type. It’s possible that mass manufactured cables will have a rather larger length tolerance of 1cm or more.

If you’re only working at 100 MHz, this sort of tolerance is acceptable, but if you go up into the upper UHF you’ll definitely want tighter tolerance cables, or even specifically phase matched cables.

[jabowery]

I’ve now got a KrakenSDR DOA system up and running with the standard hardware and software configuration.

What I would like to do now is disable all but 2 of the antenna ports so that the DOA algorithms are operating in a degraded mode paying attention to only the 2 plasma antennas.

If one opens up the box and sets the 3 DIP switches to off, what must be done in the web interface other than changing it to be ULA?

Bear in mind that what I’m after here isn’t DOA but rather just a simple user interface – like the DOA curve provided by the web interface – that would permit one to by inspection determine whether there has been a qualitative change in the phase relationship between the two signals.

The actual experiment will require that the investigator travel to a very specific radius from an FM broadcast tower (yes I know – better not be too close to a powerful signal) where the theory expects that the plasma antennas will be 180 degrees out of phase compared to when the plasma is turned off.

[Vadim]

Hello. The simplest and cheapest way to measure the phases of two antennas is to use the AD8302 module ( LF–2.7 GHz RF/IF Gain and Phase Detector - AD8302). To work with this module, 5 volts is enough and the result can be observed on a simple multimeter or for this you can use a microcontroller with an ADC with a graphic display. Of course, you will need a very simple program for the microcontroller. Watch my video where I measure the phases of the emitters of a collinear antenna. Watch the experiment at 9 min 20 sec. https://youtu.be/CpfmLMtHfmE

[jabowery]

UPDATE:

I went back to try the AD8302 again because I suspected the reason for my prior failure was due, in part, to the power supply I was using: 2 x CR2032 batteries in series rather than an ordinary 5V plug-in power adapter. I was able to get a reasonable read-out voltage for the 0 degrees phase difference with the 5V plugin supply. I had wanted an integrated power source for the AD8302, which is why I went with the battery supply.

However, this still does not give the highly selective tuner/band-pass – which was a main reason for going the SDR route.

END UPDATE

@Vadim

AD8302

About a month ago I bought a couple of them and tried one out:

With this configuration I can’t get this AD8302 board’s phase/VP+ to register anything but 0.35V. Same for magnitude/VM+. The SMA’s are delivering signal and I even tried it with 30dB gain amplifiers. Tx: 0.5W 91MHz and 10mW 2.4GHz in close proximity in various positions so one would expect at least the magnitude/VM+ voltage to change. The scope clearly shows magnitude change with different positions of the 91MHz Tx.

The plasma antennas are mounted on a PCB:

The principal investigator (Steffen Kuhn) indicated that the antenna PCB needs to include a resonant circuit to select the FM station’s frequency and low noise amplifier to push the signal off the PCB to the AD8302. This would, in turn, require trimming capacitors.

In his opinion this is so tricky that in his experiment he resorted to using standard FM radio tuners which, because the carrier is demodulated out of the signal, outputs only audio frequencies. He filters that to extract the 19kHz pilot (the presence of which indicates stereo FM). He then runs that into a 100MHz PicoScope to digitize at 5MHz and then further interpolates to reconstruct the 19kHz with adequate phase resolution to detect the 100ns phase shift that he expected at his distance (23km) from the FM broadcast tower.

[Vadim]

It is certainly very interesting to conduct an experiment with plasma antennas.
As for the AD8302 module, I have a stable phase difference at an input signal level of about -40 dB and higher. Usually I try to get a signal strength of about -30 dB at the module input. Such a strong signal is usually not found in the lab from other signal sources, and therefore selectivity is not required. Additionally, I can say that I read on the Internet that you cannot use a signal amplifier, since the amplifier introduces distortion into the signal phase. If you use a signal generator, then the signal source must be one, and then this signal must be divided using a passive divider without changing the signal phase. Any attempt at filtering or selectivity using trimming capacitors can introduce unpredictable results for the AD8302 module. I myself cannot confirm these assumptions. As radio enthusiasts say, the best amplifier is an antenna! I wonder what the signal from the plasma antenna looks like on the PC screen in the SDR receiver program. What is the signal level and what does its spectrum look like. I wish you good luck and wish you good results from your plasma antennas.

[jabowery]

@Vadim

Usually I try to get a signal strength of about -30 dB at the module input. Such a strong signal is usually not found in the lab from other signal sources, and therefore selectivity is not required.

This is what I use in my local experiments to get high signal strength:

@Vadim

I wonder what the signal from the plasma antenna looks like on the PC screen in the SDR receiver program. What is the signal level and what does its spectrum look like.

With the 500mW Tx at 108.0MHz

The outputs of the AD8302 are
+VP 0.95V (relative phase)
VREF 1.51V (voltage meaning 0 phase difference)

However, this cannot be taken to mean there is any real phase difference because the same outputs are present when the plasma power is off (and the same signal level in the SDR).

My attempt to run a control experiment using a T connector to a single antenna, which should produce a VP+ voltage = VREF, does not do so unless I hold the antenna as I describe in this video:

AD8302 failure to detect exact in-phase signals

If, as you say, -30dB is required at the AD8302 input, I’m not sure I’ll be able to achieve this without being so close as to be subject near field effects.

For field experiments, I’ll be in about 1km from FM broadcast towers, which may provide all the power I need.

I am a bit concerned about the KrakenSDR hardware though since people continually warn about avoiding nearby powerful transmitters.

[Vadim]

I watched your video. There are several parameters that raise doubts. To clearly determine the phase difference using the ad8302 module, the signal spectrum must be clear and the signal strong (approximately -40 or -30). If you use a transmitter with a frequency of about 100 MHz, the receiving antenna must be much longer to get a good signal with a level of -40 dB. If this is the antenna from the Kraken kit, then a ground plane is needed. You also need to understand that this antenna is not short-circuited by DC and therefore it is quite noisy in urban conditions. To get the expected results, you need to turn on the transmitter. Receive the signal on one antenna, then divide this signal into two with a divider, and then see in the SDR receiver program what signal level you receive on each feeder after the divider. If these levels are approximately -40 or -30 dB, then you can connect these feeders to the module and check the phase difference. It is not clear from your video what signal level comes from the antenna to each of the inputs A and B on the ad8302 module. It is also not clear whether your divider is a real divider with output decoupling or a simple tee. Good luck.

[jabowery]

Thanks for the guidance. Today, I’m going out to a nearby FM transmitter in the middle of the country away from any city or even town noise sources.

https://radio-locator.com/cgi-bin/pat?call=KHLW&service=FM
89.3MHz 50kW Transmitter location:
40° 34’ 33" N, 95° 34’ 25" W

Steffen’s plasma antenna experiment would predict 180degree out of phase at 1288m so I’ll set up the field lab at that distance. I found a road-accessible spot with Google Earth that will give me visual on the tower. There are power lines running along all the roads unfortunately but at least the power lines on this road are behind where I’ll be pointing.

[jabowery]

I drove to within 1km of a 103.1MHz 50kW Tx in the region but the power delivered by the PCB plasma antenna did not reach -30dB – so it looks like the AD8302 is not an option without an external amplifier.

I’m back to trying to figure out why the Heimdall DAQ software doesn’t deliver consistent phase information from the KrakenSDR. This is incredibly frustrating.

[jabowery]

The tests in Nebraska City were misguided because I was relying on Wikipedia for broadcast tower information for the two high power stations nearest me. Since there were still physical towers at both locations it fooled me because I was using only 2 antennas and didn’t know what to expect in terms of signal strength.

The results made no sense until, yesterday, I reconfigured my car to be a complete DOA system and drove a few hundred miles to see what was going on. Of course, since I had no experience setting up the DOA system before I couldn’t believe it was reporting such ridiculous directions – that is until I did a sanity check for my DOA set up using other stations. Only then did I find out that Wikipedia just coincidentally screwed up on the two stations I relied on.

For example, in the map below, KHLW’s tower exists and the station exists but I haven’t found a source for its location – and the signal from that tower is dead.

Wikipedia lists KBIE’s tower at a physical location in Nebraska City, and the tower is still there (there are 2 towers apparently for directional broadcast). There is another tower in Nebraska City north of town as well… but I couldn’t make sense of what I was seeing in the DOA system for another reason: I’d misconfigured the DOA system’s 5 antennas such that a systematic error appeared as I drove around and I figured out that I’d mounted them incorrectly. Then everything started to make sense.