UPLINK - PATCH VS HELIX - what's better

  • Thanks Lucio, today I bought a cake box that is made from PP5 and is microwave safe. When I can I will test it on receive before I fit it.

    Unfortunately the power amplifier has died for some reason and I cannot transmit at present.

    Mike G4CDF


    Sorry to hear your power amp died. Wish you good luck fixing it!.

    73 Ed PA1EW

  • 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