UltraWideBand antenna design write-up - pedantry required

[dazz1]

Hi

It is possible that the curved lens edge of your board is also reflecting back to the feed at resonance.   In visual optics, lens are coated to improve light transmission.  You could achieve a similar effect by cutting saw tooth edge into the curve.   A bit like an anechoic chamber.   

This should be fairly easy to test (a few minutes with a hacksaw would do the job).  I suspect the effect will be small but no one will know unless someone measures it.

Also, there is obviously a lot going on RF-wise on this antenna.  You could try selectively adding patches of RAM to different parts of the antenna (especially the outer edge) to see what effect individual features have on radiated performance.  Just a thought.

Dazz

[hagster]

Hi
Just reading your blob.  I like it.

Code: [Select]

A further improvement to the Antipodal antenna aims to improve the polarisation purity. The BAVA antenna uses a 3 layer PCB with one element sandwiched between two layers of board. The effect is to make the antenna symmetrical.
What about using a standard 4 layer board with the middle 2 being identical?  The two inner layers might then look electrically the same as a single middle layer of 3.

Dazz

I see no reason why that would not work. It may have an impact of efficiency as more of the fields are within the lossy FR4. Also i'm not clear how I would add the connector.

It is possible that the curved lens edge of your board is also reflecting back to the feed at resonance.   In visual optics, lens are coated to improve light transmission.  You could achieve a similar effect by cutting saw tooth edge into the curve.   A bit like an anechoic chamber.   

This should be fairly easy to test (a few minutes with a hacksaw would do the job).  I suspect the effect will be small but no one will know unless someone measures it.

Also, there is obviously a lot going on RF-wise on this antenna.  You could try selectively adding patches of RAM to different parts of the antenna (especially the outer edge) to see what effect individual features have on radiated performance.  Just a thought.

I don't think there are many reflections heading back into the antenna. This would show up in the SWR measurements.

Your suggestion sounds like creating a soft transition between the Er of the substrate and air. It would be an interesting thing to test if I had more time.

I did look at creating a more aggressive lens using a saw-tooth like pattern(fresnel lens). It does on paper seem like it might work, but becomes a bit ugly.  In fact as the FR4 represents only a thin 2D slice of the propagating field, it seems likely that any changes will only have a very minor effect.

The other thing to try is an ellipsoidal lens. There are a few examples of this sort of thing around and it could be easily tested with some 3D printed shapes. This would allow the beam to be focused in azimuth as well as elevation, but the antenna does become bigger.

[dazz1]

What about using a standard 4 layer board with the middle 2 being identical?  The two inner layers might then look electrically the same as a single middle layer of 3.

I see no reason why that would not work. It may have an impact of efficiency as more of the fields are within the lossy FR4. Also i'm not clear how I would add the connector.

I don't think the FR4 between two middle layers(2,3) would have any effect because if they are identical there should be no voltage between the layers.   It wouldn't be difficult  or expensive to test a prototype.

There could be benefit of using the 2 inner layers. They will add thickness which will add a 3D element to an otherwise planar antenna. This might affect the antenna beam pattern.

Connection to layers 2&3 could be achieved by using a pattern of vias to connect 1&2 ,3&4.    This could be done with blind vias, or more cheaply with full through vias.  The connector could then be soldered to layers 1&4.

I don't think there are many reflections heading back into the antenna. This would show up in the SWR measurements.

I agree.

Your suggestion sounds like creating a soft transition between the Er of the substrate and air. It would be an interesting thing to test if I had more time.

I did look at creating a more aggressive lens using a saw-tooth like pattern(fresnel lens). It does on paper seem like it might work, but becomes a bit ugly.  In fact as the FR4 represents only a thin 2D slice of the propagating field, it seems likely that any changes will only have a very minor effect.

I don't think that the edge shape of the board will have a significant effect because the % of radiated power that reaches the edge of the board (as opposed to the power in free space) will be small.   I don't think a fresnel lens would work at this scale because the lens elements would be too small compared to the wavelengths.  I think it would be more likely to act as a diffraction grating.

Your measured data shows that the antenna works.  Any other features/modifications are likely to provide small incremental improvements.
My old Professor used to say if it isn't measured it isn't true.
The problem with my suggestions is that they are untested, not quantified and therefore largely worthless speculation.

[hagster]

Your measured data shows that the antenna works.  Any other features/modifications are likely to provide small incremental improvements.
My old Professor used to say if it isn't measured it isn't true.
The problem with my suggestions is that they are untested, not quantified and therefore largely worthless speculation.

Your professor was very wise. Measurement is the time consuming bit.

The suggestions are not worthless at all though. They are all good ideas that are worth exploring.

[dazz1]

Hi
Well let me make another suggestion.

I am highly skeptical  about the effects of the palm leaf edge treatment for the simple reason that the palm leaves have no energy dissipation.  Assuming no ohmic losses, any energy that enters those leaves is going to be reflected out again.    If they are radiating energy, you would probably want to know if that has any effect on your antenna performance.   

I suggest the following test sequence:

1.  Make a prototype with straight edges (no palm leaves) and no lens edge. Test and measure. This is the simplest form of your antenna and can be used as a baseline design.

2.   Get some ferrite loaded epoxy or similar and apply along the  outer edges.  Test and measure.   This should be very effective in dissipating any energy that tries to escape down the outer edges where it is not wanted. It should effectively eliminate most parasitic currents.   Adding this material is unlikely to be something you will want to do for production but it should quantify the effect of edge current (radiation) when compared to the untreated PCB edge. 

3.  Take one of the palm leaf antennas and figure out how to add a matched resistive termination at the feedpoint of each leaf. Test and measure. The aim is to absorb any power going into each palm leaf to prevent it being reflected out.    If the termination is a SMD resistor and if this proves effective, that is something that would be easy to added in production. 

These are relatively simple and cheap tests.  If you follow this path, you will end up with a whole lot of data you don't currently have.  When analysed, this data should tell you a lot about the effect (if any) of antenna parasitics and their effect on your antenna performance.

I suspect the greatest potential improvements could be made by refining the main antenna feed point geometry (matching).   Optimising that efficiently probably needs access to a super expensive simulator where multiple geometries can be tested with relative ease. 

Dazz

[hagster]

The following images are from Laboratorio Maxwell where the original creator of the Palm Tree Antenna Dr. Alexandre works.  http://labmax.org/index.php/2016/11/04/labmax-tem-trabalho-de-pesquisa-premiado-no-encom-2016/

You can click on the picture at the bottom of the LABMAX page to see a follow up IEEE article. I can't post the original i'm afraid.





As you say, the cutouts are not absorbing any power as they have no resistive components. Instead they create a wide band high-Z path to reflect the signals back towards the main lobe. I believe the original author arrived at his exact geometry through experimentation. I did the same, but found that in my case(with different substrate and frequencies of interest) I got better results with less leaves.

When I get a chance I will try and do a real world test of this by creating a version without the leaves and do a side by side measurement.

[dazz1]

Hi
Fascinating stuff.  That simulation shows with a minute of study what might take days to find by meticulous measurement.

I can see from the simulation and paper that the palms are being fed rather than just getting parasitic energy escaping from the main antenna feature.

Taking the leaf concept further, I think there would be merit in investigating the antenna conceptually illustrated in the attachment.   This consists of a central radiating feature flanked top and bottom by two secondary directors.  The three are driven by three separate feeds (blue) from the same source.  For the directors to work, they should radiate slightly ahead of the central radiator to focus the main beam.  The central feed needs a small delay line (green) to achieve the correct phase shift compared to the two directors. 

This arrangement would effectively create a passive planar 3 element phased array antenna.  The delay line would need to  be wide band constant angle phase shifter.    There would be enough R&D work here for a solid thesis.

Dazz

[hagster]

Dazz,

It took me a while to get my head round your idea. Your concept is trying to make the emerging field squarer by delaying the phase of the central section. As you say, creating a constant phase delay over a wide-band is difficult.

Hagster

[dazz1]

Dazz,

It took me a while to get my head round your idea. Your concept is trying to make the emerging field squarer by delaying the phase of the central section. As you say, creating a constant phase delay over a wide-band is difficult.

Hagster

Hi
It is not a new problem.  Just search for "Heaviside Condition". 

Dazz