Computer analysis of dual band dish feed (G0MJW, PA3FYM, M0EYT). Part 2 –S Band

  • 105mm outer diameter of the reflector will fit in my DN125 tube radom perfectly.

    73 Mike (DL1GNMike)

    Can we move discussions about what it will fit into into a new thread please. This is supposed to be about computer simulation results but the technical discussions are getting swamped and people who are interested in the weatherproofing won't think to look at a thread on computer simulation.


  • I tried 64.5 mm - the axial ratio is excellent, 1 dB, but the VSWR rises to 1.3:1.

    Hi there Mike,

    happy to see the two resonances in the SWR plot - and symmetrically about the CF of interest too, essential for good axial ratio in this type of design. And yes, at the minor cost of slightly increased SWR at the sum impedance spot frequency:…ment/2149-newpatch-7-jpg/

    Cheers - Michael, oh2aue


    "If you have data, you have something, if you do not, you have nothing." (Bengt Hultqvist, SK 24.02.2019)

  • Still overthinking the process Mike and I went through when 'fiddling' with the design in relation to Rasto's analysis about the somewhat squinted lobe (not that is may be of much importance, but it's nice to think about it)

    If I remember correctly, the first try G0MJW did was to model the K3TZ patch and the subsequently added the protruding waveguide ('copper pipe'). Of course problems with the match occured. After that I suggested to insert a short in the middle of the patch.

    Using an M3 bolt as short (the middle of the patch is Z = 0 + j0 Ohm), no matching problems were encountered. Increasing the diameter of the 'short' revealed worsening of the RL. We could match the real part (R) (Z = R + jX Ohm) to 50 Ohm but there was a significant part of X which could not be eliminated. The approach was then to find another feedpoint where the RL was enough.

    This led to the first prototype made by M0EYT. Indeed, it had a good RL, but was it also circular (enough) ? From the Smith Chart it appeared that the 1st prototype was circular, but not at 2400 MHz. The 'balloon' (my words) was at the wrong place and the challenge now was to iteratively find a feed point where the center of the Smith Chart (Z = 50 + j0) lies in the middle of the balloon. This appeared with feed point coordinates X,Y = 8,28 mm and I built the 2nd prototype accordingly.

    Several simulations followed, but it still converged to 8,28 (could have been we found a local optimum?) so that was the end value of the feed point which was published subsequently.

    The patch can be seen as two antennas. One somewhat too short (-j) , the other somewhat too large (+j) to such an extent that their individual phase offset amounts 45 degrees relative to the design frequency, so that the total phase shift is 45 + 45 = 90 degrees, necessary for circular polarization.

    Having a 'short' in the middle the matching method can be illustrated as a 'gamma match'. The center-feedpoint distance (so along the Y-axis, with X = 0) defines the real part (R) of the impedance and the X-axis (Y = 0) the imaginary (j) part.

    With the M3-bolt in the middle (with neglectable influence) there is one variable, being the Y-value (hence X = 0). With the copper pipe there are two variables.

    Because the feed point is now 'off centered', hence introducing a frequency dependent component by itself -in this case (iirc) some inductance (+j)- the whole thing gets some 'imbalance' and -apparently- some small squint.

  • My memory is I had arrived at the design around Christmas, I was board as I was up with the family away from the shack. John, G4BAO commented my home made F5XG designed patch antenna wasn't circular and it would need two feed-points. DJ7GP has developed a design that apparently is, but I didn't have the dimensions. I thought I would have a go at simulating one. I struggled with this in Octave until Prof. Robert Watson suggested I try the student version of CST and to check the "Axial Ratio". A measure of the circularness.

    It took me an evening to learn how to use CST and a few more to learn to use it properly. The student version of CST is limited to 10000 polygons and that turned out to be a challenge. Much time was spent getting the simulation to fit and I was (and still am) doubtful of the fidelity of the results. I am sure the full version with mesh optimisation will be better, but it's not cheap.

    The CST simulations implied John was right about the F5XG design so I started again, with an idea based on the old K3TZ patch for AO40, and then put copper water pipe in the middle. The result was awful but after some fettling I could make it well matched with a poor axial ratio or I could make it not very well matched and circularly polarised.

    Speaking to Paul M0EYT about this he offered to make a prototype so we could see if it really did work and put to be the doubts about the accuracy of the simulations. This was a heroic effort by Paul as he had no copper sheet in stock. He did have some copper pipe and somehow turned it back into sheet to make the prototype. Paul ended up making lots of prototypes.

    Meanwhile, I looked at other solutions, the elliptical patch, the notched patch, the tabbed patch, patch with horn, patch with choke ring. etc. All these patches are approximations of the ellipse. The idea is to have two distinct resonances. You could probably make one rectangular if you fed in in the right place. I understand you can also do the same thing with a tuning capacitor if you get it just right, but I couldn't.

    To address the match I experimented in CST with tuning screws, dielectric loading and so on. The best result was with an ellipse and a small coaxial PTFE tuning disc.

    Paul made one later on and I am using that now with fair success. The problem is, ellipses are not easy to make, at least not intentionally.

    The square patch with the corners cut off and an offset feed was easiest to make with simple tools so that design was settled on. I had assumed most people would be making these in sheds, not with laser cutting and CNC milling Paul made some more prototypes and tested them which verified we were on the right track.

    After Christmas, with Remco's help back to basics on the tuning. I was getting better at using the student version of CST's variables which allow you to paramaterise a design and then do some hill climbing optimisations. The correct resonances show up as a loop on a Smith chart, tricky to optimise but the ability to adjust things in 0.1mm steps while doing something else or overnight was extremely useful. Even with the 10000 polygon restriction and no mesh optimisation possible because of that limitation, each run takes several minutes, but far fewer minutes on my i7 desktop than my i5 laptop. I think the feedpoint only moved a couple of mm from the pre-Christmas design.

    Paul made another prototype with the revised dimensions and it worked. Remco made a batch for his local club and others reproduced with fair but sometimes variable results, an indication the dimensions are critical.

    Remco suggested writing it up. So far the work had carried on on Twitter and IRC. He produced a nice paper with a few tweaks from Paul and myself and it can be found at

    Even now, nobody has actually tested one on an antenna range but there are quite a few in use and they seem to work well enough for the simple design. The compromises are:


    I have always been aware of it, but as the patch over illuminates a standard dish it doesn't really matter all that much. It is caused by the asymmetrical fields.

    Over Illumination

    The over-illumination loss is the main failing of the feed. I simulates additional directors, but the gain wasn't worth the complexity and I quickly ran into simulation size issues.

    Poor Match

    For the return loss (VSWR). Low enough is good enough. This is not HF where resonance matters, if the return loss is 10dB (VSWR 3:1) and patch is not getting hot it must be radiating. The poor match means 10% of the power is reflected and lost, but almost 90% of the energy is radiated or absorbed in the copper.

    Axial Ratio

    Polarisation mismatch loss between CP and LP as a function of axial ratio.

    The patch axial ratio is not perfect and get worse if the resonances are not right. The loss with the fees properly made should be small, under a dB. A well matched but linear rather than circularly polarised patch loses 50% of the power to cross polarisation.

    10 GHz

    This is all about 2.4 GHz performance. 10 GHz performance depends on your lens. and how well it matches the dish.


  • I can confirm that this is a good compromise. With 2 Watt + 1m RG142 and then into my 1.2m (f/d ~0.68) offset dish using POTY I'm a bit above the CW beacon level. No need for a big PA.

    It works even in my small 60cm (55cm x60cm) offset dish with nearly the same f/d. It works even out of focus quite well. The feed holder clamp did not let me mount the POTY at the right position.

    For offset dishes like these I recommend POTY as feed.

    You did a good job, thank you.

    PS: Next challenge for POTY: one dish for ASTRA TV on 19 deg plus QO-100. I'll try this.

  • Using a slim quad LNB will be a solution for this. This is what I want to try as well.

    What kind of koax do you use for 2.4GHz uplink there? I have to use ECOFLEX 15 because of indoor uplink transverter. (The solution is for an OM who is a friend of mine.)

    Thank you.

  • Don't want to go to much off-topic here.

    Anyway, I use a short peace of RG402 with SMA plug and N-Socket to adapt from SMA to N on the Antenna side, followed by about 2m Ecoflex10. On the other side in the temporarry Transverter/PA side I have a N-Socket to SMA-Socket adapter and a short peace of RG142 or RG400 to connect to Isolator infront of PA. Pictures see my page

  • Consider the cropped corners as a 'too short' antenna (so Fres > 2400 MHz) and the square corners as 'too long' (so Fres < 2400 MHz).

    Also imagine that the phase shift of every antenna is either -45 or +45 degrees , so the combination is 90 degrees --> delivers CP at the center frequency (Fcenter = 2400 MHz)

    (This is a very simple explanation, but hopefully enough to get a feeling how -and why- this antenna works/produces CP)

  • OK thanks for that, not sure if any clearer to me. I accept what you say, that is not an issue, I would guess that the different lengths from the feedpoint to each 'element' would also add a phase change as one would have a shorter path from the feed therefore the other lags behind, but that suggests to me they are not 90 degrees out. Not to worry I think if I get chance i will search for some simple theory on patch antennas. Guess I am only used to 1.4 and 1/2 wave style dipoles.


  • OK, now we get into the tricky point (the real secret of the POTY) ... the feed point .., which is the holy grail ; -)

    First .. I go 'back to basics' ... (forget about the 'patch', just let's simplify it down to earth) ... consider one of the 'antennas' (let's take the 'too long' one) ... as a 'dipole' .. or a 'radiator' grounded in the middle, like many 2m Yagi's and imagine a 'gamma match' (or 'shunt feed'). We know that Z = 0 in the grounded middle and that Z = (theoretically) infinite at the end of the radiator.

    Thus .. there is a 'tap' where R = 50 Ω (leave the jX for a while.. come back to it later).

    So, imagine an infinite small diameter 'waveguide' (call it a short) in the middle of the patch (because Z = 0, there can be an electrical short). The distance from this middle towards the vertical 'end' of the patch determines R (the real part of the impedance).

    This is (with the infinite small waveguide, say a M3-bolt ; -) along the vertical axis. Say that the 50Ω point will be at 28mm from the (grounded) center of the patch.

    However, we don't have an infinite small diameter waveguide (or M3 bolt), we have a 22mm (OD) copper tube in the center, which 'disturbes'.

    We go back to the infinite small wave guide in the center (or M3 bolt ; -) .. given the 28mm vertical 'tap' ... if we go (in the picture, see above) to the left we introduce a -jX (we run to the shorter side) .. if we go to the right .. we introduce a +jX. For a M3-bolt the jX = 0 is along the vertical axis.

    However, with the 22mm copper tube we do not have a M3 bolt, so there is 'some place' where Z = (almost) 50 + |j0| <-- absolute) Ω

    At the desired frequency (2400.175 MHz) jX is not 0 but a compromise .. such that the return loss (RL) is acceptable.

    Therefore the RL @2400 MHz is less than the two resonances (too short and too long antennas) but the phase differences are 90 degrees apart .. creating CP (with a good axial ratio)

    The 'horizontal' 'pick' (8mm in the POTY) is in fact a 'match' to eliminate the 'disturbance' of the 22mm OD copper tube (wave guide) and the 28mm 'pick' is the R = 50Ω point (in conjunction with the 22mm waveguide)

  • Thank you for your response, it is appreciated. One thing that still puzzles me is the length of the 'dipoles'

    I get one leg of the dipole to be 35.3155mm for the too long and 28.013mm for the too short. these seem to be a lot longer and a lot shorter then 1/4 wave at 2.4GHz, I accept that there could be a change to the electrical length of the dipoles as they are close to 'ground' as it where. But even if I consider no shortening, I get the short length would be resonant at around 2.675GHz and 2.12Ghz for the long length. When I looked at the instructions for the POTY it showed theoretical resonances less than 35MHz above and below 2.4GHz.

    Put me down as being a pain in the A**, I like to understand what is happening rather then just take it for granted, so I can ask a lot of questions.


  • Like I said .. forget about the (working of the) patch. That is a different story but in the explanation above I tried to simplify it as much as possible (perhaps too much) in order to explain the CP effect of the antenna and how the feedpoint was iterated.

    Google on articles where the precise working of patch antennas is explained, like in this Czech doc where a picture below summarizes what I wrote above: