Modeling and checking 2m Yagi stack with 23cm dish inside

  • Modeling and checking 2m Yagi stack with 23cm dish inside

Possibilities of stacking four 2m Yagi antennas around the 23cm dish.

Discussion started on Moon-net "Moon-net@mailman.pe1itr.com"

Questions:

I have somewhat of a logistics nightmare at my QTH.

I’ve moved my 2.3 mtr dish with septum feed for 23 cm onto a small crank up tower to give me access to feed changes and better moon rise positions.

This will normally be left retracted when not in use.

I have to raise it to approx 6 mtrs above ground to clear neighbouring houses for moonrise and my own house for moonset, it CAN go to 9 mtrs but might blow around in the wind a bit :(.

I have four 7 ele LFA yagis that I’d like to use for 2mtrs eme as well.

I was planning on stacking them approx 2.9 mtrs horizontal and 2.65 mtr vertical but that can be changed. The dish is 2.3 mtrs and is a fine 2mm mesh.

Now has this been done before and how can the results be modelled and checked.

Space is at a premium at home so what I’m thinking of is this.

Using the existing Alfa Spid RAS az/el rotator for the dish, mount the four yagis on a stacking frame spaced around the dish.

Obviously the dish will still be fully illuminated but what effect would it have on the four yagis spaced around it?

VK6EME

Here are the picture of desired configuration:



First we will model the antenna.


Now we will check real performance:

SWR:


Good as promised. 

Elevation  radiation pattern:

Azimuth radiation pattern:

Calculated data with boom on 144.1 MHz, Aluminium losses.
Gain 12.52 dBi
F/B 19.93 dB
It looks that antenna have deficit of claimed gain about 0.3 dB originated from calculation with EZNEC Pro/4 without ability to calculate boom corrections.

Results:

First we will check antennas G/T performance without the dish calculated with new references Tsky=290K and Tearth=5100K.
We will use the same data as on latest VE7BQH table.

Stacking distance 2840/2490 mm



G/T -9.35 dB is real data for the antenna.

Stacking distance 2600/2200 mm



G/T -9.04 dB

Both numbers are worse than predicted in VE7BQH table.

Antennas around the dish.



After modelling the dish and antennas we will check what is the minimal stacking distance for relative small SWR change.

It is 3000 x 3000 mm. With that distances minimal SWR will shift from 144.1 MHz to 144.5 MHz.
Gain 18.1 dBi
F/B 17.81 dB

Elevation  radiation pattern for stacking distances 3000 x 3000 mm:


Azimuth radiation pattern 3000 x 3000 mm:



Calculated G/T:



G/T -11.67 dB

Better gain and G/T could be achieved with 3400 x 3400 mm stacking distances:
Gain 18.45 dBi
F/B 16.98 dB

Elevation  radiation pattern for stacking distances 3400 x 3400 mm:



Azimuth radiation pattern 3400 x 3400 mm:



Calculated G/T:



G/T -11.05 dB

Most of bad G/T originated not only from strong first side lobes but bad F/B.
That F/B is worse on just single antenna without surrounding dish. 
And became extreme bad with dish inside. 
Stack have gain not only in forward but also in back side. It is 1.47 dBi.

Answer to VK6EME question "Is it possible to stack four 2m Yagi antennas around the 23cm dish?"

Yes, if distances are at least 3000 mm or more. 

Main problem is: Not good enough antenna design that produces worse G/T that possible.

Question is: Is it possible to design a much better antenna on the same boom size?

To be continued

  • Product Code: 2050
  • Availability: In Stock
  • 0.00€


Tags: 7element, Yagi, Dish, Inside, 23cm, 2m, Stack, modeling, influence, 144MHz, 1296MHz, EME, Gain