Modified TV-Cassegrain Antenna for QO-100 Uplink and Downlink

  • Hello Folks,

    Last year I got hold of a so-called Cassegrain TV antenna. The Cassegrain principle has the advantage, that by a secondary convex reflector in front of the primary reflector the focal length is extended. Thereby the antenna is very compact.

    The antenna I aquired is made by the company Gibertini and is called Cassegrain CP 60. For some reason it seems that the manufacturing has been discontinued but if you search the net you still can find some companies selling them.

    This antenna has the big advantage that it has a radom covering the entire antenna, thereby protecting the LNB. The diameter of this antenna is 62cm. Due to the radom it can be easily transported and I took it to a few trips , for example to Denmark, where I was QRV as 5P8KW last September . The downlink antenna is the lower of the two antennas.

    At that time I still used a SSB-Electronic 2.4GHz antenna called Airlink 60 (the upper antenna). It has a similar radom but it is not a Cassegrain but a prime focus dish.

    I experimented with both, helical antennas and patch feeds, which led me to be a fan of the helical feed.

    I came accross a dual band design by Bernd, DH1ND, who simply printed a holder for a 3 1/4 turns helix with a 3D printer. The reflector of the helix has a hole where the LNB is peeping through.

    According to Bernd, the loss of the LNB signal by the reduction of the free sight of the LNB is about 1.5dB. I can confirm that the loss is not very high and with the signal strength of the satellitle inside a 62cm antenna, it is affordable.

    I purchased a kit from Bernd to test it in my setup. Note that in a Cassagrain antenna with two reflectors the helical antenna has to be wound right hand wise (RHCP)!

    Here is a look inside the antenna before the modification.

    The secondary reflector has a diameter of 14cm.

    Here is the complete setup with LNB and helix.

    During practical tests with my portable station in my garden showed, that I was unable to detect any attenuation of the received signal from QO-100.

    I tested the the uplink system and compared it to a 60cm prime focus dish with linear vertical polarization (the Airlink 60). The signal of the modified Cassegrain is about 2 to 3dB stronger than the linear polarized antenna (as expected ;) ).

    Now I only need to load one antenna into the trunk of my car for portable operation.

    Vy 73

    Holger 'Geri', DK8KW

  • Hi Geri,

    I like that Cassegrain setup very much. With the radome it is a very compact antenna and should be easy to transport.

    Well done.

    Did you make any measurements of the VSWR of the Helixantenna ? I wonder what is the impact of the secondary reflector
    which is probably very close to the front end of the helix !?

    What is the SNR of the CW beacons ?

    Kind regards


  • Hello Matthias,

    >Well done.


    VSWR of the helix installed in the antenna is around 1,5. I measured this using the voltage outputs of the SG-Labs PA for forward and reflected power.

    The total space between the center of the primary reflector and the center of the secondary reflector is 140mm. The length of the helix is around 80mm, so there is still some headroom to the secondary reflector, thus obviously not influencing the VSWR too much.

    SNR reading was taken with SDR-Console. Without going back into the discussion about the right or wrong way to measure it: the CW beacon had a SNR of 30 „SDR-Console-dB“ ;) .

    Vy 73

    Holger ‚Geri‘, DK8KW

  • Hi Geri,

    thanks for the quick answer.

    I agree not to discuss the presicion of the measurement in SDR-Console but it is good for comparisons.

    In my portable setup am using a 60cm offset dish from Kathrein and measured 34,8dB for the lower CW beacon.

    However that was some time ago and I will doublecheck in the next days ... if it is really almost 5dB more for the same size of dish
    that would indicate that the small cassegrain dish is quite a compromise ... but let me doublecheck my measurements ...

    Kind regards


  • Hello Matthias,

    This morning I made use of the beautiful weather here in Peine to quickly set up the Cassegrain in the garden and to do a quick test on the SNR.

    After carefully adjusting inclination, azimuth and skew (much more carefulyl than I usually do when I quickly set up my portable equipment) I measured a "SDR-Console-SNR" of 31.3dB.

    Provided we use the same bandwidth (I typically use 550kHz to see the entire transponer and a little bit beyond) our two values do not seem to be so far off. Adding the 1.5dB loss that DH1ND claims to have measured due to his "Helix-Reflector-Peekhole" a difference of around 2dB remain. Good enough for me ;).

    I am sure you know that by lowering the window-bandwidth you can almost achieve any measurable SNR with SDR Console (see pictures).

    Widh a bandwidth of 550kHz I get the 31.3dB SNR.

    With 2.7kHz bandwidth I get over 44dB SNR.

    I believe that the 31+dB are plausible. With my fixed setup and the 1.8m offset dish I get an SNR of 41dB (@ 550kHz bandwidt). At 10.489MHz the 1.8m dish has about 10dB more gain than the 0.6m dish, so the SNR difference should also be in the same ballpark figure, isn't it?

    Best 73 and stay healthy!

    Holger 'Geri', DK8KW