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sgcderek / helix-antenna-scaffold

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Helical not receiving well with grid parabolic ant. #2

Open Tim-Deefield opened 1 year ago

Tim-Deefield commented 1 year ago

Hi Derek,

First thanks for your "Beginners Guide to HRPT", it is fantastic! I have been about 90% successful receiving HRPT images because of your info. The best images have been received using the Nooelec grid parabolic and linear feed. I have been trying for several weeks to use your helical feed with the 3D printed support. I want the extra gain it provides as well as the circular polarization.

LHCP Helical_Grid Parabolic

I am working the NOAA 18 & 19 HRPT satellites and no matter what I try the images are noisey and indicate low gain on the antenna, compared to the linear feed. I carefully built the LHCP & RHCP versions of your 3D supports and I believe the helicals themselves are working well. The problem seems to be related to using the helicals with the grid parabolic. Has anyone else used your helicals with the grid parabolic ant? To find the focal point I radiated a 1.7 GHz signal with RHCP helical ant pointed at the grid parabolic /LHCP helical. I moved the LHCP feed in and out and found the strongest signal and then mounted it at that point.

I'm really baffled why it does not perform very well and thought it would really out perform the linear feed.

Any ideas what I might be doing wrong?

If you know anyone that has used the helical with the Nooelec or similar grid parabolic I would be interested in the mounting of the helical feed and the focal point measurements they used.

Thanks in advance for any suggestions.

-Tim Heffield Florida, USA

sgcderek commented 1 year ago

Hello, sorry for not responding earlier, I must have missed the email notification of this issue. Grid dishes usually require a feed with a wider beam, so in case of a helix like this you could try reducing either the spacing or the number of turns. Also, if I understand correctly, you found the focus by setting up your own local 1.7 GHz transmitter, that may not actually give you the "real" focus of the dish because the waves at such a short distance are not parallel, as they would be coming from a satellite. On the other hand, the difference probably would be quite small.

Tim-Deefield commented 1 year ago

Hi Derek, Sorry for my slow response as well, I wanted to get some pics together to send you. I thought maybe it was a beam width issue and thank you for confirming this. I have since had good results using a DIY Biquad feed for the grid parabolic. I am still looking for a dish I can you use with the helical feed, to obtain the additional gain.

Here are some pics of the Biquad feed and an HRPT image I received with this configuration.

Biquad Close Up

Side View LNA Mount NOAA-19_AVHRR_20230511-141935_Compressed

sgcderek commented 1 year ago

I've had good results with a biquad as well, though in my case with an offset dish the helix still works better. Consider giving a different helix a shot, a biquad is a linearly polarized antenna so you will be losing a few dB (but as you can tell it's not really a big deal)