1.2m dish and 1700MHz patch feed

[fylb...@gmail.com]

Gentlemen,

This is not quite related to RA but I'm pretty stumped right now and could use some assistance if people are willing.

My project is to receive GOES and NOAA imagery from geostationary and LEO satellites. I have a 1.2m grid dish from RF Ham Designs and a home built patch feed based on the G3RUH design found at http://sat.cc.ua/page3.html. When I have the feed separated from the dish pointing skyward, I can easily tune it to 1700MHz (with a NanoVNA) with response similar to that found on the feed webpage. However, When I attach it to the feedpoint at the dish the response changes quite a bit and is no longer tunable to 1700MHz.

Obviously the electrical relationship between the dish and feed is affecting the feed response.

My question is: what has changed and how can I get the feed to tune in correctly?

Thanks very much,

Phil, N7FWL

[Jeff Kruth]

A picture of the mounting arrangement would help, because it is detuning the patch.

J. Kruth

 

 

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[Lester Veenstra]

 

Looks rather rotten about 1.7 GHz  (3.5 Grid Square from bottom)(1 Grid square = 200 MHz)

 

 

Lester B Veenstra  K1YCM  MØYCM  W8YCM   6Y6Y

les...@veenstras.com

 

452 Stable Ln (HC84 RFD USPS Mail)

Keyser WV 26726

 

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GPS: 39.33682 N  78.9823741 W (GPSDO)

 

 

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[Lester Veenstra]

I should have said;

This data was for: Premiertek Outdoor 2.4GHz 24DBI Directional High-Gain N-Type Female Aluminum Die Cast Grid Parabolic Antenna (ANT-GRID-24DBI)

 

https://www.amazon.com/gp/product/B005M8KU3W/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

 

Obviously not your patch feed.    No real difference between bare dipole feed, Feed with “reflector” and Feed and “reflector” and dish (real reflector)

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[fylb...@gmail.com]

Here's picture. The mount is a tripod mount with a clamp for the feed.

[Marko Cebokli]

Hello Phil!

This is a normal behavior on electrically small dishes. It is not a
fault of the feed or the dish. On an electrically small dish, the feed
is close (in wavelengths) to the dish, and receives quite a bit of
reflected signal from the center of the dish.
It should be possible to (narrowband) cancel this reflection with a
reactive element at the feed.

Another solution was to use a "vertex plate", a small disk at the vertex
of the dish, about lambda/4 in front of the dish, to cancel the
reflection. You have to experimentally find the size of the disk, to get
good cancellation. In your case probably around 5...15 cm diameter.
However, a vertex plate will increase your sidelobes (noise).

Marko Cebokli

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