Observing phase shift due to antenna position

[unclejed613]

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.

using an SDR to measure the signal may not appear to change the strength of the signal very much, especially with the default settings in the software where the graph is done in 10dB increments (which is a logarithmic scale, each 10db is a tenfold increase or decrease in power). to get a better idea what's going on, try using the software on a linear scale instead. some SDR software might not have that option, so second best would be to use a scale with 1db increments. a 3db change in signal is doubling or halving of the power. with a crude measuring setup, such as wiring the two antennas in parallel, will never give you a complete null in the signal, one reason being that the two antennas are in parallel with very low resistance, and one antenna, in addition to receiving a signal from the transmitter, may also re-radiate some of the signal fed through the connection from the other antenna. you could isolate the antennas by using a resistive combiner (basically a "Y" adapter with 25 ohm resistors splitting off the antennas, and the input to the radio being the junction between the two resistors.

 

[Chapi]

Sorry for the low quality of the pictures, but the idea is clear I hope..

 

[rjenkinsgb]

Without the antennas being properly tuned and reasonably well impedance matched, the whole cable will be acting as an antenna to some extent and reducing the effect.

The simplest approximation is likely a simple dipole with a crude "gamma match" feed.
Concept, not in proportion:
http://fucimin.altervista.org/pmr446/gamma.gif

Cut a stiff wire or rod to the 5% less than the calculated half wavelength at the frequency you are working with.
Connect the coax cable braid to the exact centre point and the inner to a point roughly one fifth of ether way from the centre to one end.
(It should ideally be adjustable and with a capacitor to compensate inductance, but like I say it's a crude approximation - but better than just a bare wire).

Make two of them.

Then try to get the two antennas parallel and half a wavelength apart. Attaching them to the ends of a piece of wood or a brush handle etc. allows you to keep the spacing while turning the whole thing.

 

[ronsimpson]

SDR to measure the signal may not appear to change the strength of the signal very much

I want one of those radios! There are videos showing how they work.

The SDR is reading in db. So a 3db change in signal seems small on a meter that reads 100+ of db.

the whole cable will be acting as an antenna

I agree.

 

[Chapi]

Thanks guys, your advices are professional and helpful.

Will this be too much to ask for a short youtube showing exactly how the trick is being done with all the details, and the effect of the antenna moving on the signal strength in the sdr?

 

[ronsimpson]

too much math:

more math but he now draws patterns for the antenna

More antenna theory

 

[Chapi]

I saw those videos in the past and also read this article:

https://umich.edu/~lowbrows/reflections/2007/jabshier.1.html

I still need the technical details for how exactly the trick is done, and how much the signal strength is decreasing due to interference.

 

[unclejed613]

I want one of those radios! There are videos showing how they work.

they are a little pricey for just starting out, but they cost less than a Grundig SWL receiver, and can do a whole lot more.

 

[ronsimpson]

they are a little pricey for just starting out, but they cost less than a Grundig SWL receiver, and can do a whole lot more.

I have a very old spectrum analyzer that needs a update. I can see this radio would do a nice job.

I made a network analyzer year ago. The signal was made using DDS and worked well but the receiver was not good. I can see how I can make a signal and them look at the very same signal with a narrow band receiver. (lock the transmitter and receiver's frequency together)

 

[unclejed613]

depending on the portion of the spectrum you want to use, there are a few different solutions for a tracking RF generator that are not expensive:
HackRF 1Mhz-6Ghz
fl2k 0 to about 150Mhz using a USB-VGA adapter

the USB-VGA adapter can be used for DDS (direct digital synthesis) or as I/Q baseband (using two of the three DACs to feed a modulator). this idea has been around for a while, but the recent development of code that eliminates the blanking intervals now makes this a very inexpensive source of RF. a tracking generator would require no modulation.

 

[Chapi]

I ordered this antennas kit

https://www.ebay.com/itm/400824562771

for a RADIO DIRECTION FINDER, based on the principle of TDOA. I hope I'll be able to make it work..

unclejed613

Do you think the antennas in this kit could solve the problem with the sdr you mentioned? I am not sure how to connect the 25 ohm resistors, simply in series? wouldn't it block the signal coming from the antennas?

I did not find the linear scale on my SDR uno. Do you know any sdr with a linear scale I can download?

Thanks.

 

[unclejed613]

I ordered this antennas kit

https://www.ebay.com/itm/400824562771

for a RADIO DIRECTION FINDER, based on the principle of TDOA. I hope I'll be able to make it work..

unclejed613

Do you think the antennas in this kit could solve the problem with the sdr you mentioned? I am not sure how to connect the 25 ohm resistors, simply in series? wouldn't it block the signal coming from the antennas?

I did not find the linear scale on my SDR uno. Do you know any sdr with a linear scale I can download?

Thanks.

the 25 ohm resistors are connected in a "Y" configuration... the common point is connected to the receiver, and the other ends of the resistors go to the antennas. so you get a summing point of the two antennas, and the 25 ohm resistors gives the antennas some electrical isolation from each other (it isn't perfect, but it's better than connecting the two antennas directly together).

as far as changing the scale on the spectrum display, you may have to build an application in Gnuradio to get a linear scale. Gnuradio gives you the tools to build your own SDR applications, and has the spectrum and waterfall displays where you can set the type of scaling, etc... with Gnuradio, you can also build tools to extract information from the received waveform that can give directional cues.

most of the direction finding applications use multiple radios running from a common timing reference such as a 10Mhz timebase generator. the radios used aren't usually the $20.00 dongles, unless they've been modified to use a common clock signal. without a common source of timing information, the random differences in clock rates of multiple radio dongles will make the collected data useless because they can't be compared in any predictable way. radios such as SDRPlay have input and output connectors for syncing clocks between the radios, which is a must for direction finding. you could have two dipoles a half wave apart, and watch for a sharp null in the signal, which is the purpose of the antenna you ordered, but it will null twice, once when pointed at the source, and once when pointed away form the source. this requires two measurements at different locations, and plotting on a map for triangulation. the measurement takes a while to perform, and your body's proximity to the antenna distorts the antenna pattern.

 

[unclejed613]

just saw this today, maybe something like this is what you need...
coherent SDR radio

 

[Chapi]

Thanks. How about this loop antenna:

Is it also based on destructive interference of the wave front due to TDOA?

 

[Nigel Goodwin]

Thanks. How about this loop antenna:

Is it also based on destructive interference of the wave front due to TDOA?

It's just a directional aerial - similar to a ferrite rod aerial.

 

[Chapi]

"you could have two dipoles a half wave apart, and watch for a sharp null in the signal, which is the purpose of the antenna you ordered"

The pictures are from the antennas I got with the device I ordered from ebay. So how do you recommend to connect the coaxes in the first picture to the sdr input in order to get the sharp null? Simply connect both in parallel to the sdr coax? use the 25 ohms in between the 2 coaxes in the first picture?

Thanks.