Does an Rx antenna's Zout differ using near- & far- field "beacon" Tx?

[Swamp Thing]

I want to do comparative experiments with different ways of coupling a wire antenna to a portable shortwave receiver, including a $\pi$-type tuning circuit and other experiments.

It's hard to find a shortwave station that is free from fading, so I am considering using one of those programmable frequency sources like the Si5351 clock source. If I put this as far away from the antenna as possible within my apartment, will the comparative experiments give the same relative/comparative results in the receiver as an actual plane wave coming from a far field source?

(If the answer is no, I will have to put the "beacon" in my car and park it say 100 m. from my apartment building, but that wouldn't be convenient at all).

I do realize that this kind of thing is usually done using an antenna tester / vswr bridge, but I want to experiment with this method if it is a valid one theoretically.

 

[berkeman]

I am considering using one of those programmable frequency sources like the Si5351 clock source. If I put this as far away from the antenna as possible within my apartment

Please do not broadcast illegal signals like that -- the interference you cause could hamper valid emergency communications and get you in trouble. If you were in my geographic area, I'm one of the people who could track you down and knock on your door...

Such experiments belong in shielded rooms and anechoic chambers, not in free space. Or use legal signals in bands that you are licensed for, and broadcast your license number at the required intervals.

 

[Swamp Thing]

If a near field test is technically ok, then the source would be within my apartment with just a couple of centimetres of transmitting antenna. I would adjust the level so that it was just above the noise in my receiver. This would radiate much less than a milliwatt, I think?

Edit: My desktop computer probably radiates various discrete frequencies that can be picked up from across the apartment - I could use one of those as a beacon

 

[berkeman]

Check what level of EM is allowed in the band you are targeting -- usually if the level is low enough it is allowed. But also be careful about the fidelity of your Tx waveform. If you have harmonics or other mixed products that can put you into other bands with different limits.

 

[Swamp Thing]

The Si5351 produces a square wave so I would have to add an LPF. A DDS might not need one. I have both in my junk box.

 

[Baluncore]

I do realize that this kind of thing is usually done using an antenna tester / vswr bridge, but I want to experiment with this method if it is a valid one theoretically.

An antenna analyser only looks at the reflection of energy from your chosen antenna feed-point. It does not reveal where the forward energy is going. The antenna analyser is being used to protect your transmitter from unexpected reflected energy.

An antenna is a network or array of coupled resonators and scatterers. The impedance will depend on frequency, and how the signal energy being received enters the array. That rules out test signals in the near field, or off-axis in the far field. It also rules out real antennas near buildings, wires, railways or underground pipelines.

For wire antennas, you need an expensive HF antenna test range, or a numerical model.

I would model the antenna with available simulation software, to generate S parameters for the antenna over the frequency range of interest. I would then model the possible antenna matching networks using SPICE. My aim would be to flatten the response of the matched antenna across the band. For best results, that may require detuning the antenna before matching it to the feedline. A Smith chart, may well become your most valued friend and colleague.

 

[Swamp Thing]

That rules out test signals in the near field, or off-axis in the far field.

I had no idea that Zout would depend on the direction as well

 

[Baluncore]

I had no idea that Zout would depend on the direction as well

In theory, reciprocity says the antenna impedance is the same in transmit and receive.
https://en.wikipedia.org/wiki/Antenna_(radio)#Reciprocity

An antenna analyser measures the "bulk" impedance of the antenna beyond that point. There are many paths taken by energy in the antenna, as that energy is being coupled into space.

As you change the wavelength of a test signal to a long wire antenna, the nulls in the radiation pattern sweep in angular position.

If you judge your antenna matching network by signal power into the receiver, then you must eliminate the changing antenna pattern from the analysis. That is why you must place any reference generator in the far field, on the main axis of the beam.

 

[Swamp Thing]

....

As you change the wavelength of a test signal to a long wire antenna, the nulls in the radiation pattern sweep in angular position.

If you judge your antenna matching network by signal power into the receiver, then you must eliminate the changing antenna pattern from the analysis. That is why you must place any reference generator in the far field, on the main axis of the beam.

I'm not sure I understand. What if we are only comparing A and B at a single frequency, where A and B involve changes only to a lumped circuit between the antenna and the receiver? Will the relative delta_AB in signal strength be a function of the source direction -- which is what I understand from your earlier post?

------
Edit: Int the 1980s I used to own a couple of WW2 era vacuum tube receivers. (AR-88 and BC-454). They had a little capacitor connected to the input tuned circuit, labelled "ANT TRIM" or similar. One would tweak this control whenever one changed frequency by a significant amount. But now it seems it may have helped to tweak it after tuning in a station, and potentially it may have allowed one to boost one station at the expense of an interfering signal based on direction.

 

[Baluncore]

What if we are only comparing A and B at a single frequency, where A and B involve changes only to a lumped circuit between the antenna and the receiver? Will the relative delta_AB in signal strength be a function of the source direction

That can only happen on the rare occasion when the lump distorts the radiation pattern of a complex antenna. That can occur when there is a mismatch of the feedline, because then the feedline and receiver become part of the antenna element network.

But now it seems it may have helped to tweak it after tuning in a station, and potentially it may have allowed one to boost one station at the expense of an interfering signal based on direction.

Antennas change impedance with frequency. That was a reactance matching capacitor, used conceptually to peak the signal, but really used to avoid a deep null at the wanted signal frequency.

Only antenna pattern, or polarization, can be used to separate interfering signals. Usually, that must be done at the antenna, before the signal enters the feedline.

If you do your antenna matching research numerically, you will gain understanding, and turn the unpredictable black-art of impedance matching, into a science.
Get a copy of "RF Circuit Design" by Chris Bowick.

During the next lockdown, or a long winter, take a deep dive into;
https://www.qsl.net/va3iul/Files/Old_Radio_Frequency_Books.htm

 

[tech99]

I want to do comparative experiments with different ways of coupling a wire antenna to a portable shortwave receiver, including a $\pi$-type tuning circuit and other experiments.

It's hard to find a shortwave station that is free from fading, so I am considering using one of those programmable frequency sources like the Si5351 clock source. If I put this as far away from the antenna as possible within my apartment, will the comparative experiments give the same relative/comparative results in the receiver as an actual plane wave coming from a far field source?

(If the answer is no, I will have to put the "beacon" in my car and park it say 100 m. from my apartment building, but that wouldn't be convenient at all).

I do realize that this kind of thing is usually done using an antenna tester / vswr bridge, but I want to experiment with this method if it is a valid one theoretically.

For experiments involving the method of coupling the antenna to the receiver it is possible to do this by coupling a small signal into the antenna. In principle the test signal might be coupled into the antenna in a way that generates a different current distribution, so it would be best to space the test source by, say, lambda/8 if possible and to locate it in the main lobe. It should also be satisfactory to couple a very weak signal source into the antenna wire at the feedpoint if preferred, say by using a small wire loop.