Polarisation of AO-7 signals?

  • Dear Satellite enthusiasts,

    i recently read in cq/dl ham magazine about a circular polarized (CP), omni directional cross-dipole (aka turnstile) covering 12m and 10m and everything in between.

    Looking at the frequency range of 5 MHz, I was very sceptical. But with all the required hardware handy, i gave it a try. No tinkering at all, just built per the article, my antenna analyzer confirmed a good , flat match over the entire range from 24.9 to 29.7 Mhz with a perfect match somewhere around 26.7MHz. This is a contradiction to the statement in ROTHAMMEL antenna book, where turnstile antennas are described as narrow band antennas. Anyway, with the dimensions given in cq/dl, it is not narrow band at all.

    Polarization from sky wave signals (dx) never are pure V or H, but they sure are not CP. Hence the sense of rotation the antenna is constructed for is not important. There should be some advantage over a V or H linear polarised antenna and i can live with the loss i will have from pure H or V signals coming in via ground wave.

    But if i want to receive AO-7, what polarization should i set the antenna to? Is it RHCP or LHCP - just connect the delay line correctly, no big deal. Do i need to switch polarization? Spin modulation comes to mind. Found nothing meaningful (to me) in the ARRL satellite experimenters book, but honestly , i did not dig too deep.


    Now calculating the dimensions for the same antenna, but for 15m - 17m.

    73, Martin

  • Thomas, thanks for the reply, BUT

    i'm not asking if a dipole will do. I know it will, i've been using radio sputniks when they were operational.

    Now that i have this circular polarized antenna, i want to set the proper sense of rotation to benefit from the circular polarization it has by design. If i set the wrong rotation, losses will be higher than with a linear antenna. If i set the correct rotation, performance will be better than with a linear antenna. It's a matter of flipping coax shield and core. I just have to do it right once.

    73, Martin

  • The Satellite Experimenters Handbook, 1984, says we are dealing with Faraday Rotation, where linearly polarized signals from a Mode A satellite (AO-7 IS a Mode A sat) slowly rotate when they pass through the ionosphere, most noticable on 29MHz as fading. The polarization now is elliptical. A circular polarized rx-antenna will compensate this, but will not cure all fading , since the the gain pattern of the antenna in the spacecraft constantly changes as it spins.

    So, i think it is best to switch from RHCP to LHCP and back while observing the signal.

    You may ask why do i care about this? Well, after all it is less an experiment for reception of a Mode A satellite, but more an experiment for diversity receive on 12 & 10m. I will focus on that and put proper reception of AO-7 very low on my to-do list.

    But if polarization was known beforehand, I sure had set it correct. Will i ever bother switching from RHCP to LHCP for AO-7, i don't know.

    Thanks for reading

    73, Martin