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Observing phase shift due to antenna position

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Chapi

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I have a question re rf: if I am able to display the carrier waveform of a 300 MHz distanced transmitter on two channels of my scope, I will see see around 3 ns gap between peaks in the waveform.

If I’ll move the antenna of one of the receivers in the direction of the transmitter, will the waveform on the scope for this channel move about 1 ns per foot as I move it?

Thanks.
 

JimB

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if I am able to display the carrier waveform of a 300 MHz distanced transmitter on two channels of my scope, I will see see around 3 ns gap between peaks in the waveform.
Yes, the period of a 300MHz signal is 1/300 micro seconds, ie 3.333 nS.

If I’ll move the antenna of one of the receivers in the direction of the transmitter, will the waveform on the scope for this channel move about 1 ns per foot as I move it?
Yes. The velocity of an electromagnetic wave is 300 x 10^6 metres per second, so in 1nS the wave will travel 0.3 metres which is just over one foot.

On a practical note, it would be a very good oscilloscope that can display a 300MHz signal received from an antenna.
Depending on the distance to the transmitter and the transmitter power the received signal strength will be of the order of a few microvolts which is too small for most oscilloscopes to display.

JimB
 

ronsimpson

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it would be a very good oscilloscope that can display a 300MHz
Many hobby o-scopes only work to 20mhz, no more.
Many work o-scopes are use able to 100mhz.
You need a high speed, fast, scope rated at 500mhz. Most of this type are built for speed not small signal.
 

unclejed613

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I have a question re rf: if I am able to display the carrier waveform of a 300 MHz distanced transmitter on two channels of my scope, I will see see around 3 ns gap between peaks in the waveform.

If I’ll move the antenna of one of the receivers in the direction of the transmitter, will the waveform on the scope for this channel move about 1 ns per foot as I move it?

Thanks.
to see the phase shift using an oscope, the oscope and transmitter must share a reference signal, either a cable to the transmitter, or a second antenna that remains in a fixed location that the oscope can trigger from. if the oscope is triggered only from the moving antenna, it will always trigger from the same portion of the waveform, and never appear to be shifting.
 

ronsimpson

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a second antenna
If I’ll move the antenna of one of the receivers
It appears he has two antennas and thus will see the delay between the antennas.

A different idea:
Assume we don't have a 500mhz scope. Assume the signal is strong (0.5 volts or more)
I have some "demodulation probes" for scopes/meters. They turn RF into a dc voltage. In the case of AM-RF it will hand you the audio signal. Now the signal can be seen by a meter or a very slow scope.

Use a TV splitter. Normally they have one input and two outputs. We can use then backwards. Connect a meter or scope to the input and one antenna to one output. You should see the signal (1). Connect the other antenna and the output will double (2). (the signals will add together) Move one antenna closer to the signal and at 1/2 wavelength the signals will add to zero. At 1 wavelength they will add to 2 again.

This idea of signals adding or (at 180 degree = subtraction) is how directional antennas work.
 

MikeMl

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TDOA (Time Difference of Arrival) is a radio direction finding technique which uses two antennas about 1/2 lambda separation, a PIN diode RF switch, one FM receiver. If the antennas are equidistant from the transmitter, no modulation. If one antenna is slightly closer than the other, as the antennas are swiched, the resulting phase modulation is detected by the FM receiver. The polarity of the pulses from the FM detector tells you which antenna is closer to the source.
 

crutschow

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I have some "demodulation probes" for scopes/meters. They turn RF into a dc voltage. In the case of AM-RF it will hand you the audio signal. Now the signal can be seen by a meter or a very slow scope.
But not the phase-shift which is the parameter of interest.
 

Chapi

New Member
Thanks guys for your replies. I am happy you agree with me that I am suppose to get a phase shift.

I have no problem getting those results from a short distance of few meters, The question is how will I be able to display the waveform from a transmitter 100 miles far.

I tried to connect the scope's probe to any possible point in an open radio, but was unable to discover the carrier wave. I opened my baofeng and connected the probe to any possible point but to no avail. I am easily able to watch the baofeng signal on my spectrum analyser or sdr uno from a 20 mile distance, but not able to see the actual carrier wave of 400Mhz.


ronsimpson I tried your interferometer idea in the following way: I connected the two antennas in parallel in the way you described to both my spectrum analyser and my sdr. when I move the antenna I see no difference in the signal strength from far away stations, even though as you mentioned
"Connect the other antenna and the output will double (2). (the signals will add together) Move one antenna closer to the signal and at 1/2 wavelength the signals will add to zero. At 1 wavelength they will add to 2 again".

the interferometer is a good way to go since it does not requires scope or waveform at all, but how will I be able to make it work?

Thanks again.
 

ronsimpson

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But not the phase-shift which is the parameter of interest.
Yes the phase-shift. But you see it by the addition and subtraction of the signal. You don't really see the phase just the results of phase.

the interferometer is a good way to go since it does not requires scope or waveform at all, but how will I be able to make it work?
I have a antenna like this. Signals from left or right of the picture come in strong. Signals from above or below are 180 out of phase and subtract. The gain of the antenna at different angles is close to what the math said it should be.
 

Chapi

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Thanks Ronsimpson. can you move one antenna half wavelength to get a substantial reduction in a distanced signal strength (from a satellite for example) due to destructive interference? can you view it on an sdr? is it possible to do it with a shot antenna?
 

rjenkinsgb

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There is a gadget that is relevant to the question, a "Doppler Direction Finder".

It works by having typically two to four antennas something like half a wavelength apart, switched in turn to a receiver (by eg. PIN diodes) and cycling at an audio frequency.

With a basic two antenna one, if the antennas are different distances from the transmitter, the alternating difference (= phase shift) is heard as an FM signal at the switching (audio) frequency.

By turning the antenna array, the direction of the transmitter can be determined by the null when there is not audio.

A multi-antenna setup plus a bit of signal processing with a microcontroller can indicate the relative bearing of the transmitter.


Some more info:
http://automaticdirectionfinder.blogspot.com/

http://masters.donntu.org/2013/frt/nestrugin/library/article8/db0adf, the automatic direction finder.htm

[Edit - typos].
 
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MikeMl

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unclejed613

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I have some "demodulation probes" for scopes/meters. They turn RF into a dc voltage. In the case of AM-RF it will hand you the audio signal. Now the signal can be seen by a meter or a very slow scope.
but to resolve the time delay still requires a very fast sweep speed 5, 2, or 1nS/cm (10nS/cm might work, but 1nS difference between the signals is 1mm)
 

unclejed613

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pseudo doppler direction finding using a HackRF One. there are also some other ways of doing this, but it requires 4 SDR radios with a single reference clock (and has to do a ton of number crunching on 4 sets of I/Q streams, so you need a very fast computer).
 

unclejed613

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Are you sure?
i must have missed the memo where 0.1cm is not 1mm. the graticule of an oscope CRT is a grid 10cm across with 1cm major divisions, and 2mm minor divisions. with a 10nS/cm sweep speed, a 1nS difference between two signals is going to be one half of a minor division. another way to see the difference in phase is to display the signals from the two antennas in X-Y mode.
 

Nigel Goodwin

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I tried to connect the scope's probe to any possible point in an open radio, but was unable to discover the carrier wave. I opened my baofeng and connected the probe to any possible point but to no avail. I am easily able to watch the baofeng signal on my spectrum analyser or sdr uno from a 20 mile distance, but not able to see the actual carrier wave of 400Mhz.
You've been told repeatedly that scopes aren't capable of that, they have no where near enough sensitivity.
 

Chapi

New Member
So if as Nigel Goodwin says scopes are not sensitive enough to show the waveform (not 100% accurate, I was able to view the carrier wave on a scope with a crystal radio, but I need shorter wave and farther transmitters), then Ronsimpson 2 antennas in parallel makes the most sense. But as I said, I was unable to get reduction of a distanced signal strength in neither Spectrum analyser nor Sdr, though theoretically as he mentioned, the strength should be 0 or twice as we move one antenna.

What am I doing wrong?
 

Nigel Goodwin

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So if as Nigel Goodwin says scopes are not sensitive enough to show the waveform (not 100% accurate, I was able to view the carrier wave on a scope with a crystal radio, but I need shorter wave and farther transmitters).
You've never mentioned crystal receivers before - but they only work on massively strong signals where it's strong enough to directly drive high impedance headphones. Fairly obviously something strong enough to feed headphones is plenty strong enough to see on a scope.
 

ronsimpson

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, I was unable to get reduction of a distanced signal strength in neither Spectrum analyser nor Sdr
Using a Spectrum Analyses allows you to see uV signals. Because you can see it also on the scope the signal must be 50mV or more.
Can you send a picture of your antenna and wires. Some thing is wrong, or my understanding is wrong.
 
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