10Ghz Dish questions

[VK3XDK]

Gday again everyone,
I have had a dish donated to me (I suspect that it is a hint that i'm not good enough on 10GHz )
anyhow, Ive had a play but are I am not really getting any obvious results with my tuning (the dish was originally on around 9.2GHz (or so))
I have been trying to tune watching return loss but I am not really satisfied that it will give me best results (dummy loads have a good swr!)
I am assuming that the wave guide is shooting into a reversed yagi (see photo)

I wonder if anyone has had experience with these dishes? (element lengths (or thereabouts is what im after)
I wonder if the "yagi" could be modeled (at least for the reflector length) the element spacing is around 12.5mm


cheers, Graham VK3XDK

[VK5LA]

Hi Graham,
Have a look here...

http://books.google.com.au/books?id=Aex ... ed&f=false

This is about the only info I have come across on the Double dipole feed.
They seem to be popular back in the 70's, indeed I have come across a few microwave link systems that use these feeds with 40cm or so dishes.
I have had one of these RXing a Weak Sig Source (50 Mhz Xtal oscillator into a 1N23 diode in a waveguide flange) at 50 metres into a 10 Ghz Kuhne transverter, seemed to work well. The ones I had were on 10.500Ghz

I ended up going with an offset dish and passed on/donated the others...in hindsite I should have kept one as they would be great for doing a /portable or hilltop activation on 10Ghz...I think VK5ZD had a play with one as well...

Cheers
Andy - VK5LA

[VK3XDK]

Thanks Andy, I now know 1000 times more than I did before

[VK4CZ]

Graham

Just so happen to have one Doug VK4OE's 10GHz dishes here following the VK4 Microwave Activity Day, so I took a few pic's to help!



Scott
VK4CZ

Sent from my iPhone using Tapatalk - now Free

[VK5ZD]

Hi

I have a 40cm dish with one of these feeds. I believe it was made by Des Clift (VK5ZO, now SK).

Dipole/Reflector feed

I used the dish to make a plaster mold then used that to make two other fiberglass dishes, one for 10GHz and one for 24GHz. For the 10GHz dish I made a penny feed (actually a 20c feed ) which works better than the dipole/reflector arrangement (and it's easier to make).

20c feed

[VK3XDK]

Thank's heaps scott, I'll have to do some comparisons!

Not sure about your vernier placement in the last photo though?

[VK4CZ]

A bit of parallax error!

Far too cold to return down hill to shed... will re do tomorrow.

[VK5ZD]

If anyone's planning to make one of these feeds you should note that the waveguide is also tapered on the narrower side.

Waveguide taper

I'm not going up on the shed roof to measure it now.

[VK2XSO]

I have a penny feed I made myself, which from memory is a copy of a Des Cliff design too.
I would think that using return loss is a good method for testing the dish.
I used an MTS (microwave test set) to check mine. It's capable of insertion and return loss simultaneously.
The return is good across a range of frequencies.
It's not practical to try and measure return loss (at this stage).

My thoughts on the return loss (SWR) were that if the wave guide as not absorbing energy, and that energy was not being reflected, then it could only be leaving via the dish feed.
This means it's working... though... it doesn't mean that energy is being directed efficiently to the dish surface.

I did two things to verify the feed was working correctly.
First was test it without the dish. Naturally, there was almost no forward energy from the penny feed. (That really wasn't a surprise)
I think this would be a very good test for a dipole kind of reflector.

Then with the feed mounted on the dish I did a 180 degree pattern cut on it with a spectrum analyser.
From that pattern it was obvious that the dish was working well enough. I had made the feed to match the focal length of the original feed.

Another test I just thought of could be done with a TDR. It will confirm a good match the feed to free space and when set up with a reflecting plate will give a good
indication of how much power escapes forward, how much illuminates the reflector and how much escapes out side lobes.
I've never done such a test, it was just a thought on the moment.
I have used TDR measurements on other antennas looking for nearby RF reflections, but never on a dish or above 1.5GHz.

[VK3XDK]

Thanks for all the feedback everyone,
It is difficult (for me anyway) to modify a feed without knowing exactly what is going on! (it's a confidence thing)
Ive converted the dish to a penny feed, things are looking great!

Very sharp pointing and very good return losses, adjustment of the focal length was easy- things behaved as they should!
although a very small dish, I think it will be perfect for general terrestrial stuff. (might be a while till its actually in service though)

I was very inspired by REX and his 10ghz EME setup at the recent" test day " and i have been working on getting towards 10W
have set up a fine azi-ele control (using a "dividing table") and a tripod
looking for a bigger "good dish" (800-1400mm)
about time i used some of the stuff i build!!

[VK3XDK]

here's a photo of the results