007: 10GHz Chaparral feed horn

 完成したフィードに受信機(LNA)をつなぐと、ground noiseは5.0dBである。受信機のNFはこの時1.1dBだったのでフィード単体のアンテナ雑音温度は15Kと計算された。この15Kという値は相当良い値だと言って良い。(同じ日に、写真にある16.3dBiの角ホーンで測ったground noiseは4.1dB、アンテナ雑音温度は60Kだった。)
 このフィードをパラボラに取り付け、最適位置に調整した後のサンノイズは7.4dB、ムーンノイズは0.3dB。パラボラアンテナとしてのground noiseは4.3dBであった。これらの値を基に計算すると、アンテナシステムの雑音温度は50K、利得38dBi、放物面効率54.4%と求められた。f/d比の小さな深めのdishに見合う良い放射器だと思う。1994年製作。1995年のVE3ONT 10GHzテストはこのフィードで受信した。

The feed horn for the solid dish of the diameter of 1m and f/d=0.35 that I possessed was showed that the Chaparral feed is suitable in a technical book of ARRL, so I decided to try. (With a chaparral, company name. It became famous for success of an antenna system for TVRO (C band and Ku band) satellite reception.) Its feature is that there is a coaxial chalk ring of 3 to 5 steps around the open end of circular waveguide. This is the same scalar feed as VE4MA, but every step has a ring, so leakage of current of a limb is little. W2IMU is explaining so that it's low noise. A brass stick was processed into a ring with a lathe, and the finished chalk ring was tightened up in a circular waveguide from the outside by a hose clamp. I adjust slightly after it's installed in the antenna, so it's necessary it's variable and to cut a relative position with a ring and a waveguide. A center conductor of a SMA connector was cut a little by little, and moreover adjusted a screw tuner and got more than 20 dB of return-loss.

When a receiver (LNA) was connected with a completed feed, ground noise was 5.0dB above cold sky. The receiver NF was 1.1dB at that time, so the antenna noise temperature of the feed was calculated with 15K. You may say that 15K is a pretty good level. (Ground noise measured with a rectangular horn of 16.3dBi in the picture on the same day was 4.1dB, and Tant was 60K). Sun noise was 7.4dB, and moon noise was 0.3dB. And ground noise as a parabolic antenna was 4.3dB. Therefore, It was calculated based on these result, the noise temperature of the antenna system was 50K, gain 38dBi and efficiency 54.4%. Making was in 1994. VE3ONT 10GHz test in 1995 was received with this feed.



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