Experimenting with a Grid Parabolic Antenna for NOAA GOES Reception

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jjstechino
Posts: 3
Joined: Tue Sep 29, 2020 12:59 pm

Experimenting with a Grid Parabolic Antenna for NOAA GOES Reception

Post by jjstechino » Tue Sep 29, 2020 4:57 pm

I did set up my RTL-SDR with a Sawbird+ and a TP-Link 100x60cm Wifi grid antenna. Regardless of how hard I try, I can't get a Viterbi less than 550. this is sufficient to get some pictures with a clear sky, but the Viterbi goes up when it is cloudy and packets start dropping. The antenna seems to be pointed spot on as even with fine tweaks lead to a higher Viterbi.
I have a clear view of the sky, elevation is 49deg. I should be able to have better reception.
This prompted me to try to understand the construction of this antenna to see if it can be optimized.
The parabola has a diameter (D) of 100cm. The depth (d) is 19cm. The parabola focus calculates at:

D^2/16d = 100^2/16*19 = 33 cm.

The antenna as configured by the manufacturer has a dipole placed at 30cm with a cylindrical reflector placed at 35cm. The focal point lies between the dipole and the reflector. This is a sketch of this antenna to the best of my ability:
Screen Shot 2020-09-29 at 10.56.38 AM.png
Screen Shot 2020-09-29 at 10.56.38 AM.png (281.49 KiB) Viewed 1262 times
And here is a detail of what should be happening at the reflector/dipole level:
Screen Shot 2020-09-29 at 11.08.18 AM.png
Screen Shot 2020-09-29 at 11.08.18 AM.png (135.39 KiB) Viewed 1262 times

The dipole seems to be a doublet 1/2 wave dipole with both arms connected to the coax ground. each arm is made of a 90deg angled strip of copper 10mm wide and 0.5 mm thick. Each arm has a horizontal segment (L1) and a vertical one (L2). L1 = L2 = 32mm. The space between the 2 arms is the width of the coax center insulator or 8mm. One of the arms is connected to the center conductor and this arm correspond to the up pointing direction arrow printed on the case.
IMG_3943 copy.jpg
IMG_3943 copy.jpg (371.47 KiB) Viewed 1262 times
The resonant frequency F of this dipole is c/wl . c = 299,792,458 m/s. wl for this 1/2 wl dipole is 0.032*4 m. F = 2.342 GHz, which I guess it is close enough to the rated 2.4 Ghz.

This describes this antenna the the best extent of my noob knowledge. Now lets see what can be done to optimize this antenna for 1.7 GHz (wl = 176mm).

The first thing I am going to do is to rebuid the dipole. The length of the L1 and L2 segments should be 1/4 wl = 44 mm. This amount need to be reduced a factor of 0.96-0.98 to account for the wave traveling in a conductor and not in free space.

So I am goind to shoot for a dipole arm length of 43mm. Indeed I will need to 3d print a new dipole enclosure as the longer one will not fit in the original enclosure.

The second optimization is optical. I am going to try to move the dipole exactly at the focus point which is 33 cm from the reflector.

Once I have all this complete, I'll report the results.

If anybody more knowledgeable has any suggestion, please feel free to chime in.

radioslicer
Posts: 1
Joined: Wed Apr 07, 2021 9:11 pm

Re: Experimenting with a Grid Parabolic Antenna for NOAA GOES Reception

Post by radioslicer » Wed Apr 07, 2021 9:17 pm

Did you have any luck with this project?

To the future readers:
The dipole gap does matter at this frequency, you need to have a dipole ~ 177/2 ~ 88.5mm end to end, including the gap.
You need to be precise by measuring, but also don't fret too much, as you will not be able to make the antenna resonant on precisely 1697MHz. This dipole will be quite broadband. It's not the resonance you are aiming for it's an impedance match. If you have an antenna analyzer for microwaves - it will help. At the moment of writing, you can buy them of Amazon ~$100 (nanoVNA v2)

The legs of the dipole, marked L2 above, make a L/4 (44mm) matching network to 50Ohm coax cable. You need to adjust them as well to be precisely L/4 length. Note they are soldered to the braid. Keep the solder blob small and precise otherwise, you might ruin the L/4 length. This matching network is needed to compensate for all the imperfection of your cut, solder, and measuring, details of surrounding like capacitative coupling to the trees and antenna mast.
Also, note the "secondary reflector", it's not a reflector in this grid antenna design it's a capacitance hat. It changes your dipole impedance that is why different people have to move it around and bend different ways. If you have nanoVNA you can observe how it changes reflected power (SWR) if you don't - you will have to look at vit(avg) in goestools/goesrecv output and bend/move it until you find the minimum.

The dipole does need to be in the paraboloid dish focus, so you will either need to add a spacer (or 3d print a new radome/enclosure) moving it out of focus is not the end of the world, your signal will drop slightly.
So all the little errors - wrong dipole measurements, wrong cuts, uneven legs, wrong solder job, wrong matching, lossy coax, bent out of shape paraboloid dish, wrong pointing, wrong skew, trees and structure obstructing the view, and finally the wrong focus - they all add up. If you start with a strong signal you have quite a budget for these errors. If you start with a weak one - you might not get enough to decode after all.

There is no way to do everything precisely, so you will end up trying and retrying anyway.

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