21cm Circular Patch Feed Plate Director Yagi Antenna

[Alex P]

This is the result of  months of modeling, fabrication, testing, and development

Hopefully, I can attach the  *.PDF in the email ..

Alex Pettit

[b alex pettit jr]

*.PDF Document Attached

Alex Pettit

[Peter East]

Great work. Well done Alex

[Mathew George]

Excellent work.

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[Sławomir Klimek]

Alex, did you try to simulate a conical reflector?

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[b alex pettit jr]

I tried many variations,  .. you can't make a true conical as the spacing between Refl & Feed

need to be parallel flat surfaces

[Sławomir Klimek]

I though about truncaced cone around existing reflector.

This will cover the driven element, but it will probably also change the impedance.

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[Leif Hille]

Hello all - I finally got my 21cm Circular Disc Yagi + plate director Antenna together and started tuning it. I'm having a heck of a time tuning the antenna's resonance and impedance. I was wondering if anyone had any pointers. 

For testing, I'm using a tiny VNA which I've used for tuning other lower frequency antennas.  I've verified it is calibrated and cross checked against a spectrum analyzer to check that the tracking signal is indeed in the range under test. For folks who've assembled and used these, how much did you have to trim the antennas' spacer between the driven element and the first director element? I've been using combinations of PEX spacers cut in power-of-2 lengths to try to nail down the proper distance  between the driven element and the first director but I don't find my approach repeatable for very long.  The measurements will stay more or less repeatable I have found that I can move the green circled null in the S11 return loss for a few attempts then it goes out to lunch and I can't get it back.  Something is super touchy in this setup that I haven't put my finger on. 

My next approach is to ensure all the directors are reliably parallel with an alignment jig that's drying on my workbench. 

Not sure if folks have run into touchiness with this antenna.     

Thanks,

Leif

[Alex P]

The Instructions include this tuning guide.

" I've been using combinations of PEX spacers " with a threaded steel rod ? 

That will create hit or miss continuity as the elements contact each other through the rod .. Bad Idea

It is required that ALL components be well electrically connected to each... within a few 10ths of an Ohm end to end.

( that is why Aluminum Spacers are used )

If you start altering the components, You are building a 'new' version.

STEP #1  : PLOT the SWR / Return_Loss curves from 1200 to 1600 MHz  not 1410 - 1480

STEP #2 Use All Aluminum Spacers as shown in the design. ( then repeat Step #1 )

Alex

[b alex pettit jr]

The components in the Kit were cut to +- 1mm & better for the 10mm spacer.

They Should Require No alteration .

The only possible required  'adjustment'  = 

Clean the portions of the elements which touch each other with alcohol to ensure good overall continuity.,

and add 1/4-1/2  turn more to the tensioning of the threaded rod .

Alex

[Alex P]

If  you want to verify the primary tuning components, test the first 3 elements alone.

They should ring at 1440 MHz 

The rest of the elements will pull the system down to 1420 MHz

Alex

[Leif Hille]

thanks Alex. 

Here's my lessons learned -

(1) Put the nano VNA as close to the antenna as possible. I put it right on the SMA feed and got rid of all those extra resonances in my first plot.

(2) don't use non-metallic spacers

My plots look much closer to to yours scanning the full 1200-1600 MHz. The initial resonance was -20dB return loss at 1435MHz ish. 

After removing a bit of length off that first 40mm spacer I managed to get the antenna resonating at 1420.3 MHz with a nice deep -34dB return loss. I'm calling good enough. 

I started tuning the impedance at 1420 MHz to be Z=50+0j ohms shortening the 35mm spacer at the last director. 

I seems like I took too much length off the last director's spacing and that threw the resonance frequency out of whack (back up to 1430MHz).

So, I'm going back to the original length spacers on with some replacement identical aluminum cylinder I bought here locally.  

I think I know what I'm doing now though. 

[b alex pettit jr]

NICE... Thanks for the feedback.

FYI, IF you calibrate the VNA Using the length of coax employed for the measurements, you will

normalize the data to it.

It is NOT easy.. I've put in a large number of hours over the past 5 years ..

FYI, you Can use Non-metallic spacers with a Non Metallic core but its build is different

Cheers,

Alex

On Sunday, January 19, 2025 at 04:33:00 PM EST, Leif Hille <leif...@gmail.com> wrote:

thanks Alex. 

Here's my lessons learned -

(1) Put the nano VNA as close to the antenna as possible. I put it right on the SMA feed and got rid of all those extra resonances in my first plot.

(2) don't use non-metallic spacers  < smile >

My plots look much closer to to yours scanning the full 1200-1600 MHz. The initial resonance was -20dB return loss at 1435MHz ish. 

After removing a bit of length off that first 40mm spacer I managed to get the antenna resonating at

1420.3 MHz with a nice deep -34dB return loss. I'm calling good enough.  <Sweet !! >