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.

  • QO-100 and Astra 19° works fine here, 0,85m Offset centered to QO-100 and 7 degree off I placed the Astra LNB. Before I had only a TechniSat DIGIDISH 45 for Astra and the RX quality is now at least simmilar.

    As the old LNB was overlapping with the reflector of the patch I replaced it with a rocket LNB. See pictures:

    old LNB with BaMaTech duofeed

    and now Rocket LNB with POTY

  • 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