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Feedhorn for 1296 mc
Category: EME Equipment,
Created: 2003-11-18 20:55:20
The feedhorn described below is a combination of W2IMU and VE4MA. It was intended for use in a f(D Dish of 2.4m. WA6PY (ex SM0PYP) has helped me a lot when I built my first feedhorn in 1993. Below you find some of the bench drawings of our sessions: Please note that the correct diameter of the feedhorn should read 160mm, not 165.
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Here comes a little story on building the feedhorn. Please click on the icons to enlarge them.
The length of coax line from feed to LNA is to be as short as physically possible.
Chris GW4DGU writes: As Peter G3LTF (a very, very experienced and highly respected engineer) said, it's very important to minimise losses ahead of the preamp. In particular, at UHF and above, that means ruthlessly minimising cable lengths. It gets more difficult to do as you go up in frequency, but it can be done.
What matters is that a mismatched input LNA sees a source impedance very close to what ever the design Zo is. Providing your feedpoint is, for example, very close to 50ohms, then the impedance seen by the preamplifier at the end of a short length of any reasonable 50ohm cable or proper professional adaptor will still be 50ohms. The resistive losses in the cable will add (in dB)
to the noise figure of the preamp, but there is no extra loss due to the mismatch at the preamp input. Providing the source impedance is still close to 50ohms, the phase of the source is irrelevant.
For more technical information on matching preamplifiers to the antenna you may refer to the article Noise Figure of this website.
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