UPLINK - PATCH VS HELIX - what's better

  • Hi All,

    Here are some details of my helical feed, which is mounted just above the LNB on a 09m offset dish.

    I'm not sure why the images are blocked on the forum but you can use the link symbol to view them.

    [Blocked Image: http://www.g8jnj.net/LHCP%20Feed.png]

    The Helix was copied from G6LVB's AO40 S-band design "Modify An Anchor Sky TV Dish for AO-40 S-Band"

    I think it would be better to use a larger diameter outer tube than the helix acting as a radome.

    The helix reflector plate has probably made a bit smaller, but it did not seem to degrade the RX S / N by any appreciable amount, so I did not bother chopping it down. I used PCB because I had it to hand, but aluminum, brass or whatever would be OK.

    The underside of the 1/4 wave impedance matching line has the self-adhesive Mylar foil on it, so that it doesn't short against the body of the N connector when it is bent close to the reflector in order to achieve the best Return Loss.

    [Blocked Image: http://www.g8jnj.net/LCP%20side.png]

    The foam at the back is just to keep the LNB short term drift under control When the sun goes behind a cloud.

    More details on my website https://www.g8jnj.net/qo-100


    Martin - G8JNJ

  • Hello Martin, thanks for the info. Good to have you here!.

    I have just finished a similar construction G3RUH/G6LVB's design for the 5.25 turns helix and measured good RL. Only the cover frontpart of the Octagon will protrude the hole in the aluminum reflector plate, so I do not expect much influence from it. Hope to test this one soon.

    Another plan is to use a narrow 23mm rocket-LNB with dielectric lens more closely next to the helix. Already found that LNB attenuation from the helix in this way is very low.

    At this moment my station is separate TX offset dish 110cm + 5,25 helix LHCP and RX offset dish 78cm + modified Octagon OTLSO + GPSDO 27,841846MHz for IF 435MHZ

    73 Ed PA1EW

  • I'm planning my second QO-100 station. This will be in Zimbabwe.

    I'm planning to use the DXPATROL Rx kit with a 1m offset dish for the Downlink with that feeding to an RSPPLAY SDR.

    For the Uplink, I'm still deciding on the upconverter - either the BU-500 or the DXPATROL upconverter.

    I was thinking about the Wimo 40 helix pointing directly at the satellite rather than using a dual feed.

    What's the view on this?

    Tnx de G4NRT

  • I would certainly test it here first. I want to test and setup everything that I need because it is totally impossible to buy anything similar in Zim and even harder to get stuff by mail. One has to hand-carry or freight anything needed. It is also a one-way street - easy enough to get stuff in but nigh on impossible to get to out again!

    Thanks for responding!

  • I removed the (naked) Octagon LNB from my RX dish and started to assemble the Helix-combination version-1. The plastic box houses the mini-GPSDO. Measured Helix Return Loss in feedpoint of a dish is abt 25dB. Just want to find out how much ERP-degradation, if any, the non-optimal positioning of the Helix in this way provides. Also influence on RX LNB to be checked.... To be continued.

    P.s. a weather-proofing solution with a PP-box seems to fit well

    73 Ed PA1EW

  • Experiments with helix element diameter

    I did some sun noise tests recently on a 0.33 f/D prime focus 1.2m dish at 10GHz, using two different diameter wires for the helix. The 10GHz element of the feed is a 22mm diameter water pipe squeezed in a vice to fit inside a WG16 (WR90) flange (to connect to my DU3BC preamp).

    The feed is shown below:

    Sun noise measurements:

    helix element not fitted = 8.5dB

    1mm diameter helix wire = 8.0dB

    2.75mm diameter helix wire = 7.0dB

    These measurements seem to confirm that the reduction in performance on receive is caused by blockage from the helix, and that thinner wire is better.

    The 1mm wire helix was no problem to match to 50ohms using the quarter turn strip per the G3RUH design, and seemed to have a good axial ratio.



  • Experiments with helix element diameter

    The 10GHz element of the feed is a 22mm diameter water pipe squeezed in a vice to fit inside a WG16 (WR90) flange (to connect to my DU3BC preamp).

    I used a MK1 Birmingham screwdriver to get 22mm water pipe into a WG16 flange for a dual mode 10 GHz feed. It worked very well. Your dish is a bit shallow for the patch but it would be interesting to know if it impacts the 10 GHz at all. I think it might a little but nowhere near as much as a helix.

  • My Kuhne up-converter 2424A was returned this week after succesfull modification. I made some contacts that showed improved stability, even with the higher power-levels.

    So, now i could start testing my dual-band helix feed, together with a portable setup with 60cm offset dish on a tripod. Everything experimental and only practical tests done today.

    Several parts are re-used from the home-station. Downlink (Octagon LNB with mini-GPSDO and IF 435MHz ) was first set up and dish heading optimized. Using temporarily RTL-SDR for this test.

    Last test was TX Uplink-only (RX via Amsat-UK webSDR) with the combined dual-feed at a power level of abt 10-12W ( estimated 10W at the feed). FT817 used for driving the upconverter. First results showed strong uplink of some dB above the FSK-beacon, so a nice signal. Real measurements still to be done but this seems to work OK for portable use, as intended.

    Checking the up-and downlink combined (full-duplex) will follow after finishing the (compact) portable equipment construction that will serve demonstration purposes as well. My dualband patch-feed is also waiting to be tested. Hope the weather stays OK...

    73 Ed

  • So, today I managed to test and compare the dualband-patch antenna in the same 60cm offset dish.

    The patch radiator was placed in focus, like where the Octagon LNB was before. I compared, one after the other, the results from the previously shown dualband helix-LNB with the patch-feed and found for both systems similar signal-levels from my uplinked signal. Only a difference of abt 0.5dB for maximum signals with each feed. Measured SWR of the patch in the focal point a bit under 1.3, so that's fine.

    (Pictures of patch-feed were taken after the test setup was removed)

    73 Ed

  • ...

    To illuminate a dish with a helix properly a rule of thumb is that you need one (1) winding per f/D-unit. So f/D = 0.6 needs 6 windings. With smaller f/D's less windings are needed, however, with less windings the axial ratio becomes worse.


    a question to this older post,

    this rule of thumb "1 winding per f/D unit", is this still valid?

    Most people use f/D=0,6 offset dishes, but most use 3.5 windungs and some people 5 windings.

    I am currently building a helix for a 150cm offset TV dish, f/D=0.6 and want to use the best number of turns.

    vy 73, Kurt

  • dj0abr: It is still valid, because it's a matter of goniometry ; -) For a f/D = 0.6 dish you need a -10 dB opening angle ('Offnungswinkel') of 90 degrees, see here: https://twitter.com/remcodenbe…tatus/1091974633111388160 and here: https://twitter.com/remcodenbe…tatus/1073501855869792257

    For lower f/D's the angle has to become bigger, so less turns. It's easy to simulate with e.g. 4NEC2. Doing this you can derive the 1 winding per 0.1 f/D unit (it's not f/D unit, but 0.1 f/D unit, sorry for the typo in my previous post).

    And .. 150cm, is that the offset dish length or the width? With offset dishes one always has to take the width as 'effective surface'.

    So, regardless of the diameter of your dish, you still need a -10 dB 90 degree opening angle for a f/D=0.6 dish --> 6 windings .. because of mathematics/goniometry ; -)

    However, for transmit purposes perfect illumination is not necessary, because it can be compensated with extra 'earth power', so therefore people using less turns are not 'punished' or not knowing that they are 'punished' ; -)

  • Hello group,

    Here is a practical example on how effective a helix can work.

    My antenna is a CAS180 from Kathrein, an offset antenna with a width of 180cm and an f/d ratio of around 0,7.

    My LNB is in the focal point of the antenna. The transmitting antenna is a 5 1/4 turn helical antenna just below the LNB (see pictures).

    To show how effective this setup is, I made the following calculation:

    - according to https://www.everythingrf.com/r…ic-reflector-antenna-gain, the theoretical gain is 31,55 dBi (at 70% effectivity!).

    - according to an information that was posted here on the forum a while ago, the EIRP of DL50AMSAT is around 900 Watt.

    - to be as strong as the CW beacon I need to have 623 mW (27,9 dBm) at the feed of the helical antenna.

    - using that power and the theoretical gain (623mW + 31,55 dBi) I get an EIRP of 890 Watt EIRP.

    Conclusions: obviously my setup is very effective; I don’t think I can do any better!

    Vy 73

    Holger ‘Geri’, DK8KW

  • [user='445']...

    So, regardless of the diameter of your dish, you still need a -10 dB 90 degree opening angle for a f/D=0.6 dish --> 6 windings .. because of mathematics/goniometry ...

    thanks for the answers,

    where does the -10dB at 90deg come from?

    Another "secret" number (at least for me :-) ) is the turn spacing which is usually 0.25, but why 0.25?

    If I play with the turn spacing, a 4 turn helix can get the same opening angle as a 6 turn helix.

    My goal is to make as few turns as possible so as not to shade the LNB.

    Currently I use a poty feed, but want to try a helix, just for comparison.

    vy 73, Kurt, DJ0ABR

  • The idea is to have the feed illuminate the dish so it is -10dB at the edge of the dish. This is sort of compromise between efficiency and sidelobes. Ideally you want the whole dish illuminated evenly and nothing beyond. That's not possible as it violates EM theory, so -10dB is a realisable goal that gives good results doesn't upset physicists. A rule of thumb, not too much power blasting past the dish and doing nothing useful (over-illumination) and not too little of the dish used (under-illumination)

    The good thing about rules of thumb is that like real thumbs you can bend them, but only so far. For the uplink you might want a little less than -10 dB at the edge. It's a trade off which is how the POTY works OK with most dishes 0.4-0.7 f/d - not brilliant but good enough, especially when RF power is plentiful.

    The angle of the edge of the dish from the point of view of the feed depends on the f/d. Higher f/d is a narrower angle. 90 degrees is about right for common satellite dishes but you can work it out with a little geometry.

    Well OK, you need complex geometry to get it properly but approximately is fine. E.g. for 0.6 f/d the make a triangle from dish centre to edge as the opposite and 0.6 x d as the adjacent. So that's 0.5 x d (as the centre to the edge is 1/2 the total dia) and 0.6d. Recall Tan(theta) = opposite/adjacent = 0.5d/0.6d = 0.833. Inverse tangent tells us theta = 40 degrees. That's a half angle so the feed beamwidth needs to be 80 degrees to fully illuminate the dish. I am not sure where 90 came from, except it's the value for a 0.5f/d dish and perhaps also account's for some of the liberties I just took in the calculation.

    Think too much about this and you will tie yourself in knots. The bottom of the dish vs the top, but it's closer, near field, diffraction..oh no, hard sums, panic... At this point invest in some simulation tools or go and have a beer.

    A helix with the same beamwidth should be easy to design but you must pay attention to the phase centre as it needs to be close to the focus. The 10 GHz focus alignment is most critical and with the LNB at the focus there is a good chance 2.4 GHz feed won't be optimum. I don't know where the phase centre of your Helix is but it's worth checking the design you plan to use with one of the reputable online calculators.


    EDIT - following the link I see where the 90 came from. Remco did a more accurate geometry which I think is fine for prime focus but I am not sure it holds for an offset. Anyway, the idea is the same.

    [Blocked Image: https://pbs.twimg.com/media/Dyd5IijXgAILiaP?format=jpg&name=medium]

  • Quote

    My goal is to make as few turns as possible so as not to shade the LNB.

    dj0abr That is -generally speaking- not a good idea because a helix starts to produce CP (with good/sufficient axial ratio) after 4 turns or so. Just model it in antenna modelling software (like 4NEC2) and you'll see.

    G0MJW Yes, the same goniometry / math is also valid for offset dishes. An offset dish is nothing else than a prime focus dish but then another part of the parabola. The higher you get into the parabola, the larger the f/D. Although the illumination may look optically 'squint', the effective surface of an offset dish is equal to a 'prime focus dish' with the diameter of the width of the offset dish, see below.