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Originally Posted by daverfw
I have a feedhorn that does not have markings on the throat to adjust the focal length, instead it has threads and youi spin it. how in the heck am i supposed to adjust to the correct focal distance? thanks!
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I'm not familiar with a feedhorn that does that, however if I understand what you're saying, don't forget that if you "spin" it, that you're also changing the polarity. Because of that, you can't really do real time "peaking", but rather have to make a change, then change the polarity via the receiver, etc, ie a cumbersome process.
Also, none of the feedhorns I've ever had have had markings on the throat that were for "adjusting to the correct focal distance". The markings that you see on the throat of a feedhorn are typically for adjusting for the F/D of your dish. Ie the feed sits deeper or shallower inside the rings depending upon the F/D of the dish. The manufacturer of the feedhorn really can't put markings on the throat for FL, because they don't know what kind of dish or mount you're going to use.
Basically, you generally have to set the focal distance by measuring it, and adjusting it is often complicated, depending on whether you have a 4 arm support or one of those "J" shaped supports that comes from the center of the dish. Adjusting the FL is supposed to involve moving the whole assembly, throat AND rings, not just the throat. With the 4 arm support, you have to adjust each arm where it connects to the dish, which is something you don't generally do with real time peaking. Of course, you are changing the FL by moving the throat in and out, but you are also changing the illumination of the dish, ie the F/D adjustment.
What I "tried" doing with my 4 arm support, was to twist the whole assembly so that the support arms ended up approaching being tangents to the outside of the rings instead of perpindicular to the rings. Twisting like this also changed the polarity, so I'd have to loosen the screw that held the throat, and rotate that to bring the polarity back to where it was before, before checking the signal.
Not an easy process, no matter what you do. I eventually ended up just setting the FL by measurement and leaving it alone. I'm still a bit unhappy that I never managed to figure out a way to peak it, but it works well enough that I think I'd only make it worse by messing with it more.
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