Focal Point of Parabola (1 Viewer)

Can anyone tell me how to find the Focal Point of a Parabolic Reflector ?

I assume there is a simple way of doing it but I cannot find the relevant info.

Also what type of lead do I need for connecting a line-out (headphone)

socket to Line-in. Thank you.

Regards Charles

Ceejay2000 said:

Can anyone tell me how to find the Focal Point of a Parabolic Reflector ?

I assume there is a simple way of doing it but I cannot find the relevant info.

Click to expand...

Two ways. One is to mount the mic roughly in position. Set up a FM radio tuned to interstation static in the open air, not indoors and away from nearby walls. Line up dish laterally and vertically at radio, adjust position of mic for maximum signal.

The other way, assuming the dish is slightly reflective optically, is to get a piece of tissue paper and slide up and down along the mic mount with the dish pointed at a bright light. Try to avoid using the sun (though I have to admit that's what I've used often enough) - a light bulb indoors about 5 yards away will get you about right. The point where you have the smallest and brightest image is close enough. Your aim is to place the microphone capsule at that point, close inspection of your mic usually shows you where that is.

If you use the sun don't focus on your mic. I have heard of someone melting the foam windshield slightly when taking too long to sort himself out there

There is a third way using maths to calculate f from the diameter of the dish and the depth of the apex but life is too short for that sort of thing unless you have the mind of a planet...

As for the lead, it all depends on what the connectors are on your bits of kit. I take it this is not for your mic? most things come out 3.5mm stereo jacks, so you need a standard male to male 3.5mm minijack to minijack lead. At headphone level you can use the sort of thing you can buy in PC world for wiring computer speakers to sound cards (or you can borrow the one from your PC to speakers). If you're using pro gear it could be anything from 1/4 stereo unbalanced jack to balanced XLRs or whatever you have on your kit.

Hi Emine,

Thank you for the quick reply. Will try out later.

The line-out connecting lead is from a computer 3.5mm mini-jack to a 3.5mm

mini-jack input on a Ipod.

I recorded Simon Elliott in Amble harbour on R4 Play Again

and wanted to transfer to my Ipod.

How would I control volume during transfer ?

If that makes sense.

Regards Charles

I'm unaware that the Ipod is capable of recording without specialist hardware/software. However, if you've recorded this program, and have it on your PC, isn't copying this to your Ipod what Itunes is meant to be able to do for you? As far as I am aware, Itunes doesn't just transfer bought songs, but can transfer any audio file. That would give you the least degradation in quality on transfer, being a digital process.

Hi Ermine,

I can only listen LIVE on internet via Real-Player, dont appear to be able to record from this.
I have a Micro media recorder plugged into base of Ipod, this has a Mic socket and a Line-in socket. However when I try recording with Mic I always
seem to get extraneous noises so I thought I would try Lineout to Line in.

Regards Charles

Ah that old scam eh, when will these media company control freaks learn that if you can hear the sound your can record it.

No problem doing what you propose to do. The other way is to use your audio recording/editing program eg Audacity and in your windows record mixer set the record source to either WAV or "what you hear". Press record then start stream playing, leave computer alone till the end of program and press stop. Voila, one wav recording of the progamme they tried to lock up and 'control' playback. Point Itunes at wav and let it rip.

Hi Ermine,

EXCELLENT. Its not what you know but who !!!!

Please do not leave the Forum.

Thank you

Regards Charles

Ceejay2000 said:

Can anyone tell me how to find the Focal Point of a Parabolic Reflector ?
I assume there is a simple way of doing it but I cannot find the relevant info.

Click to expand...

I recommend you to calculate focal point and then do "fine tuning" if it's necessary. Focal Point is possible calculate by mathematic formula explained here:

http://www.downeastmicrowave.com/PDF/dishfp.PDF

can't open the link above but i presume it's the same as for antenna dishes etc

= D2 / 16d

D is diameter of dish, d = depth (or height if you like)

Crex-Crex,

Thank you for the info.
Even I should be able to get it right !!!

Regards Charles