Prime Focus dish for QO-100

1st Official Post

    I've got offered a 1.5m prime focus dish, no LNB - just the dish.


    Been doing some searching, and can see a few people are using prime focus for QO-100, but most are using offset.


    If i choose to use a prime focus, what would be the best option, a poty or helix, and what about the LNB ?


    I was hoping for that i could use DX Patrol LNB, since i have the dx patrol down converter, and up converter already on hand, but i know there is some differences when it comes to the LNB between an offset and a prime focus dish.


    Regards

    Ronni

    Hi,

    with a dish that size, the antenna doesn't really matter much unless you plan to run very low power or datv.

    I use a 150cm PFA and got very good results even with the 4 element yagi the manufacturer of my upconverter provided as a free gift, pointed in the dish.

    With a POTY or Helix in the right spot (focuswise) you need to keep the power low (wich is good when you use dx-patrol, because they tend to overdrive everything)


    73, Martin

    Pic 1: Poty in the holder where normally the feedhorn of the original Hirschmann feed is located. The original feedhorn sits snug against the aluminum holder, but because the poty reflector diameter is 100mm, the 3 feed arms prevent it from being placed snug against it. The distance is 3-5cm , i think.


    Pic 2: Reveals the reflector size and why you can't push it closer to the focus position.



    Pic 3: To compensate for the poty not sitting in the focus, i liftet the 3 feed arms a bit above the dish with bolts and nuts. Observe the left side of the picture where the arms are mounted on ( ABOVE , that is) the surface.


    I'm pretty sure this still isn't the best position, but my signal is loud enough even on very low power and i can hear pretty well. Before i lifted the feed arms a bit up, the signal was loud, but afterwards I had to lower power by at least 4-6dB. To this date, i do NOT know my output power, maybe 2-3 Watts , minus the attenuation the 8m long cable from the amp to the antenna introduces.


    73, Martin

    It is always beter to attenuate the INPUT TO THE Upconverter on the radio side , not the output after the upconverter . This will result in a clean signal without nasty sidebands. You should check if that is enough signal strength. If not, you still can kick in the amp. You will need attenuation ahead of the amp, I'm sure.


    73

    Hi Ronni,


    a 1.5m PFD is a nice dish. Go for it. You will then also have a good basis for receiving and eventually transmitting DATV signals via QO-100.

    I am using a 1.8m PFD (old Kathrein dish which I refurbished). My dish is quite deep (f/D=0.29) and therefore a POTY feed is fits well. I do not use a lens in front of the 10GHz waveguide, just leave it open.


    Do you know what the f/D of your dish is?


    Most PFD dishes are between 0.3 and 0.4 and a POTY feed will then still serve you well.


    On my website http://www.dd1us.de you can find some information about my setup incl. the feed-holder I built.


    If you do not find it plase send me an Email and I will give you more details.


    I kindly disagree with Martin: if you setup a 1.5m dish you should try to get out the optimum of it. Otherwise it is a waste of
    time and material ...


    Kind regards


    Matthias


    http://www.dd1us.de

    Quote from dm4im

    It is always beter to attenuate the INPUT TO THE Upconverter on the radio side , not the output after the upconverter . This will result in a clean signal without nasty sidebands. You should check if that is enough signal strength. If not, you still can kick in the amp. You will need attenuation ahead of the amp, I'm sure.


    73

    I will need some attenuation before the upconverter, minimum output on my TS2000 is 5 watt - and the upconverter needs between 1 and 3 watt, output alone should be a max of 200mW, so i need the amplifier, i am counting on being able to turn down the output power directly on the DXpatrol mk4 upconverter using the pwr adj, and in doing that limit the output from the amplifier.

    Hi Matthias.


    Thank you for the reply, i still need to go and pickup the dish, so not much information is known, except its 1.5m, and its made from steel, and its free, so that is always a plus. :)


    I will take a look at your website, and see what i can find, it sounds like a poty will be the way to go - better start looking after one.


    Kind regards

    Ronni

    Quote from OZ2RNH

    I will need some attenuation before the upconverter, minimum output on my TS2000 is 5 watt - and the upconverter needs between 1 and 3 watt, output alone should be a max of 200mW, so i need the amplifier, i am counting on being able to turn down the output power directly on the DXpatrol mk4 upconverter using the pwr adj, and in doing that limit the output from the amplifier.

    I don't buy that. If that is true, you must convert a max. of 2.8W into heat to have 200mW output. Meaning the upconverter has negative conversion gain (in the range of ~ -10/12dB) . If you have measurement gear for 2.4GHz it is of great interest what really is going on.

    Set your TS2000 to a good , strong audio , the audio you like to have when you talk on 2m or shortwave. Do not touch audio settings for the the following measurements. Set TS2000 to minimum available power with it's power potentiometer. Now insert a 20dB attenuator , this should result in 50mW output. Connect your upconverter and measure the output , while you adjust the power adj potentiometer on the dx-patrol board. If you don't get 200mW out, turn pwr adj to it's highest setting , insert about 15 or 13dB instead of 20dB, for ~200-250mW input to the upconverter. Repeat measurements. I'm sure you will reach the maximum output power with less than 250mW input.

    Many dx-patrol users say , the vox circuit will not work properly with low input power, but most of them adjust output power of their radios by reducing mic gain and compression. Which i believe is the wrong way.


    Regards,

    Martin

    P.S. : I may be totally wrong.

    Quote from DD1US

    I kindly disagree with Martin: if you setup a 1.5m dish you should try to get out the optimum of it. Otherwise it is a waste of
    time and material ...

    I agree. I just wanted to point out, that even with a cumbersome antenna, you will have good results with a dish that diameter. My first antenna was the 4 element yagi pointing into the dish , the 2nd was a linear polarised patch feed without waveguide for the lnb and the one i'm using now is a poty . Learning by doing, improving over time. It is more important to have a clean signal, may it be not so strong. The LEILA triggering signals are for later, and even those are clean, if you overdrive NONE of your gear......

    Regards

    Martin

    P.S. : I don't encourage anyone to trigger LEILA.

    Hi Martin.


    Sorry no gear for measuring the output, it is possible that 200mW is a bit of a stretch, but i dont need that much output anyway, had a fellow ham do some measuring on my amplifier, and it will produce 12 watt with a 100mW input, and close to 30 watt with the max of 250mW in, but that is not tested.


    Am i wrong for thinking that with about 100mW of drive and a output of 12w , i will get 4-5 watts at the antenna, there will be some loss in cables, connectors and so on.


    I did read a bit about the TS2000 having ALC overshoot issues, and even at 5 watt setting, it will produce 60 watts for a short time before adjusting down - so it needs quite a attenuator to make sure there is no damage to the up converter.


    Kind regards

    Ronni

    Quote from dm4im

    I agree. I just wanted to point out, that even with a cumbersome antenna, you will have good results with a dish that diameter. My first antenna was the 4 element yagi pointing into the dish , the 2nd was a linear polarised patch feed without waveguide for the lnb and the one i'm using now is a poty . Learning by doing, improving over time. It is more important to have a clean signal, may it be not so strong. The LEILA triggering signals are for later, and even those are clean, if you overdrive NONE of your gear......

    Regards

    Martin

    P.S. : I don't encourage anyone to trigger LEILA.

    Hi Martin,

    yes, we should not overpower.


    Looking at the transmit side for the narrowband transponder a smaller dish is clearly enough.


    The big benefit of such a large dish is on the receive side especially for the wideband transponder and for transmitting
    DATV signals.


    See you soon on QO-100.


    Kind regards


    Matthias


    http://www.dd1us.de

    Hi Matthias.


    I will make sure to add some attenuators to my shopping list, what is the best option for limiting the output ? Attenuators between the output of the dx patrol upconverter to limit the drive to amplifier, or attenuation on the output of the amplifier before the feed ?


    For limiting the output from the TS2000 my shopping list includes a 40db 50w attenuator - 100mW should be enough to trigger the vox on the upconverter.


    Without the possibility to measure the output its a bit hard - plan was not to trigger LEILA. :)


    Kind regards

    Ronni

    Quote from HB9SKA

    Ronni


    With your 1.5m dish you do not need an amplifier. Try first without one, the transponder is very sensitive.

    Hi Thomas.


    Just took a quick look on your QRZ page, im a big fan of the two old Kenwoods for QO-100, i have a TS-711E here, but unfortunately its not working at the moment.


    I will try without the amplifier first, but i doubt it will be enough for a SSB contact.


    Kind regards

    Ronni

    • Official Post
    Quote from OZ2RNH

    I will try without the amplifier first, but i doubt it will be enough for a SSB contact.

    If you point your dish exactly to QO-100 then it should be enough power. My very first signal over QO-100 was in CW with about 500 mW without amp from the upconverter . The result was a very weak but stable signal on Goonhilly WebSDR.

    Quote from HB9SKA

    If you point your dish exactly to QO-100 then it should be enough power. My very first signal over QO-100 was in CW with about 500 mW without amp from the upconverter . The result was a very weak but stable signal on Goonhilly WebSDR.

    Hi Thomas.


    Remember without the amp i will have a max of 200mW, and i am sure the dx patrol up converter will have to be pushed to the limit for that kind of output.


    I got told that the DX Patrol often are making a dirty signal, so i will try and keep my drive as low as possible, it would be nice with a dedicated ptt instead of VOX. :)


    I will try. :)