Best feed for surplus dish?

Offset feed dishes have different f/D ratios depending on polarization. I just did a quicky measurement of an RCA (Thomson) dish which yields 0.67. Close enough.

Exactly. See:

how it works. Be sure to read chaper 6 on feeds.

Biquad should work.

beamwidth of the biquad is about 60 degrees. Using a protractor to eyeball my pizza dish, it wants a 70 degree angle. Close enough.

See:

fun aiming the dish.

On Fri, 13 May 2005 10:53:14 -0700, Jeff Liebermann wrote (in article ):

Thanks for those numbers, Jeff. I hadn't intuited, nor could I find confirmation of which feed would best illuminate my dish.

Bi-quad, here I come!

The Trevor Marshall BiQuad is the most referenced feed for a dish.

refers to Trevor's site, and has better instructions and pictures.

Biquad , simple , safe ,cheap and you can build it with a minimm of tools.

This might help:

's the 4NEC2 output for Trevor Marshall's biquad design. I'll tweak it to include a VSWR over frequency graph later.

On Sat, 14 May 2005 11:56:51 -0700, Jeff Liebermann wrote (in article ):

Not to look a gift horse in the mouth, but how about a plot of VSWR vs. groundplane-to-element spacing?

Thanks,

If I did everything I promised to do, I'd never get anything done.

See:

've added a VSWR and reflection coefficient graph, main page, and NEC2 file. The 300MHz frequency used is handy because everything is in wavelengths. In this case, to get 2400MHz, just multiply everything by 8. For example, the 288-312MHz frequency plot is actually

2304-2496MHz. If you extract dimensions from the model, it's the same story. Multiple all dimensions that are in wavelengths by 8 to get something useable at 2400Mhz. If you want to use the design for a cell phone antenna or other frequency, just use a different scaling factor.

I think the NEC2 deck I posted is the one that came with the samples in the 4NEC2 program. (Actually, I'm not sure where it came from.) You can download the program and sample antennas from:

The frequency plot was done by changing the FR card to: FR 0 7 0 0 288 4 This results in the plots being slightly off the ideal 300MHz frequency. Therefore, I posted the frequency plot and main page that was generated with the above frequency scan. There are probably better ways to do this, but with 9 minutes per compile, I just used what popped into my foggy brain first.

For construction details and dimensions, see:

antenna to reflector spacing isn't horribly critical. The above construction article uses 15mm. I've been adjusting the height during construction for maximum gain and/or best VSWR and ending up with about 16mm (measured to center line of copper wire).

Dave

Have you considered using a USB wireless adapter as the feed so the line loss might be lower? I have built some illuminators for the off set feed satellite TV dish using a USB wireles adapter. But, I dont yet have any definitive results.

Jerry

There's always time to do it over again after a rush job.

I found a better biquad model at:

that compiles quickly, is much simpler, doesn't require wavelength scaling, is vertically polarized, and is much simpler than what I previously posted. I re-ran the 4NEC2 model and dumped the output at: NEC2 file is at: is about 11dBi. -3dB beamwidth is 60 degrees horizontal and 50 degrees vertical.