New Sdrplay and antenna creation

[Mike2459]

In response to Glovisol:

You wrote on May 31, 2019:

"The shape and the size of the surface of the antenna PC board seem to be "magic " in the sense that, as soon as you alter these dimensions, the antenna does not work any more"

I don't know where you got the idea that altering the antenna's dimensions wil cause it to stop working. It will 'work' with a vertical wire or metallic plate mounted in either the horizontal or vertical plane (or any angle in between) equally well. For a wire the sensitivity is primarily determined by it's length. For a rectangular plate, for all practical purposes, by it's perimeter not its area.

"then a bigger capacitor will have a bigger aperture and couple more "E" field, thus more signal to the receiver.....but this does not seem to be the case in practice."

That is exactly the case. I'll quote Bakker on this one:

"A method was developed to compare the signal to noise ratio of active whip antennas. The carrier of
ONO, Ostend, Belgium, is transmitted on 399,5 KHz. With the 100 cm active whip antenna, the
relative signal strength was -19 dBm. This was the signal read at the SPM-3 with preamplifier. The
antenna noise was measured at 399,15 KHz. The measurement was carried out during daytime to
avoid interferences by remote stations through the sky wave. The antenna noise was about -90 dBm.
The measurement was as follows. The receiver was tuned to 399,5 KHz. The signal from the test
antenna was adjusted for -40 dBm using the RF attenuator. Then at 399,15 KHz the antenna noise
level was determined. Table 1 shows the results of the measurements."

SSvSurface.JPG (32.64 KiB) Viewed 61450 times

If the whip is shortened, the signal strength decreases gradually. A whip with a length of 30 cm



gives a signal that is 7 to 8 dB lower than with an antenna of 100 cm. The length of the whip does
not affect the ratio between the received signal and the received noise. This is confirmed by the
practice. In the Faroe Islands there are three aircraft beacons coming out in daytime just above the
noise level. They were received on a 30 cm long whip as well as with the 100 cm one.

http://dl1dbc.net/SAQ/Mwhip/Article_pa0 ... nglish.pdf

[Mike2459]

Bakker got his units wrong in the text. The units in the text should be millimeters (mm) not centimeters (cm). The units used in the table are correct.

[glovisol]

Hi Mike,

I may be wrong and have to do a detailed search in the Forum, but I remember to have read one or more posts by you where you explained the much worse performance of a similar antenna (a copy) because of the altered dimensions (bigger) of the PC board. I apologise if I am wrong in my recollection, but I have no time to check in detail at present and will do so later on.

https://www.sdrplay.com/community/viewt ... f=8&t=4128

Kind regards,

Gianfranco

[Mike2459]

It hasn't been confirmed but it's been suggested that the problem with the RA0SMS is that it uses a MOSFET. The PA0RDT uses a J310 JFET. They are both field effect transistors but MOSFETs generally have much higher input impedances and higher input capacitances. The antenna in question may have been usable if 10-20db input attenuation were added. The symptoms appear to indicate the RSP was overloaded. I have been getting ADC overloads in the evenings when noise levels increase 15-20db at longwave frequencies from all the thunderstorm activity recently here in the midwest US. It has been a nightly occurrence for the past month. There is nothing to hear when this occurs so I just shut it down.

[glovisol]

The pa0rdt-Mini-Whip©
Roelof Bakker, pa0rdt
History.
After several fruitless attempts to make an active loop work in a city environment, it
was found that the electric field from local noise sources was contained within the
house. The magnetic field of local noise sources was not contained inside the house,
making weak signal reception at LF impossible.
Hence an electric field antenna was called for. Tests were performed using an active
whip antenna, designed by G4COL. Results were encouraging and the whip length
could be reduced from 100 cm to 30 cm without loss of performance. It became clear
that at LF an active whip is a capacitance coupled to the electric field.
The pa0rdt-Mini-Whip©.
If it is accepted that a whip is a capacitance coupled to the electric field, shape
becomes irrelevant, as long as the required capacitance is available. In practice the
“whip” can be e.g. a small piece of copper clad printed circuit board. A small die-cast
aluminium box can also be used, with the buffer amplifier mounted inside.
Following this line of thought, tests have been performed to find the optimum
dimension for the pa0rdt-Mini-Whip©. To prevent receiver overload, maximum
output has been set to about – 20 dBm. The buffer amplifier has been optimised for
good strong signal handling performance.
Acknowledgment goes to Steve Ratzlaff, AA7U, who was so kind to conduct IMD
measurements on the buffer amplifier and suggested circuit modifications to improve
strong signal handling performance.

To look at all published material on this antenna would take ages. Enoug to say that, if the above is correct, increasing the SURFACE of the PC board devoted to the "capacitive coupling" should increase the antenna output, so why is it kept so small? I just does not make sense. Now, in another text it is stated that performance does not depend on antenna surìface size. On the contrary, in the further text by Roelof you show that signal vs. size has a (logical) meaning, so it is difficult to understand the reason why using such a small surface is used in the actual antenna.

For a wire the sensitivity is primarily determined by it's length. For a rectangular plate, for all practical purposes, by it's perimeter not its area.

Look Mike, if we talk capacitance, we must talk surface, please let us at least agree on this...However I got the bug now, and sooner or later I am going to prove to myself that this device works in practice even though it does not work in theory. At Philippi we shall meet again!

[Mike2459]

The plate or wire has capacitance but where is the other half of that capacitor's plate?

Plate or whip
Most active electric field antennas do not, like the MiniWhip, use a metal plate, but a whip of about a meter long. This makes no essential difference for the operation. Such a whip will, if it is short compared to the wavelength, assume the average potential of its surroundings, in this case the potential about half a meter above the top of the mast. This half meter extra height will hardly affect the potential difference to ground.

However, there is another important difference, namely the capacitance of the plate or whip. A whip has a capacitance of almost 10 pF per meter of length, slightly dependent on it thickness. A circular metal plate has a capacitance of about 0.35 pF per cm diameter (proportional to the diameter, not to the area, as one might expect). I haven't found a formula for a rectangular plate, but the shape should not matter too much, so a typical MiniWhip has about 2 pF of plate capacitance. That capacitance is important, because together with the amplifier input capacitance it forms a capacitive voltage divider. If the plate or whip's capacitance is smaller, less voltage remains when the amplifier is connected.

http://www.pa3fwm.nl/technotes/tn07.html

Ref: Capacitance of Antenna Elements http://www.pa3fwm.nl/technotes/tn08b.html

[Mike2459]

Enoug to say that, if the above is correct, increasing the SURFACE of the PC board devoted to the "capacitive coupling" should increase the antenna output,

It does increase the output. But the relationship between surface area and output is non-linear. That is, if we double the area the output will not double.
Perhaps it is an exponential relationship.

[glovisol]

Mike,

The earth has a capacitance of 1 Farad and there is ....no other plate. Just look this up...

[Mike2459]

This fellow at MIT says it is smaller and that the other plate is infinity.

Earth.JPG (194.9 KiB) Viewed 61364 times

http://web.mit.edu/sahughes/www/8.022/lec06.pdf

An interesting read that might have some clues.


Mike

[sdrom33]

Hi Mike, perhaps you remember the static electricity machine they showed us at school. The tin on the top, can be a metal ball as well, was the capacitor storing the static charge.