Modeling and checking 2m Yagi stack with dish inside

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

Discussion started on Moon-Net “” Questions:

I have somewhat of a logistics nightmare at my QTH. I’ve moved my 2.3 meter 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 meters above ground to clear neighboring houses for moonrise and my own house for moonset, it CAN go to 9 meters but might blow around in the wind a bit :(. I have four 7 elements of LFA yagis that I’d like to use for 2 meters EME as well. I was planning on stacking them approximately 2.9 meters horizontal and 2.65m vertical but that can be changed. The dish is 2.3 meters and is a fine 2 mm 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?


Here is the picture of the desired configuration:

2m Yagi antennas around the 23cm dish


First, we will model the antenna.

LFA 7el


Now we will check real performance:



Good as promised.


Elevation  radiation pattern:



Azimuth radiation pattern:



Calculated data with a boom on 144.1 MHz, Aluminium losses. Gain 12.52 dB and F/B 19.93 dB
It looks like the antenna has a deficit of claimed gain of about 0.3 dB originating from calculation with EZNEC Pro/4 without the ability to calculate boom corrections.

First, we will check the antenna’s G/T performance without the dish calculated with new references Tsky=290K and Tearth=5100K.
We will use the same data as on the latest VE7BQH table. Stacking distance 2840/2490 mm

G/T -9.35 dB is real data for the antenna. Stacking distance 2600/2200mm
G/T -9.04 dB Both numbers are worse than predicted in the VE7BQH table.


Antennas around the dish.

144MHz Yagi antennas around 23cm dish


After modeling the dish and antennas we will check what is the minimal stacking distance for relatively small SWR change. It is 3000 x 3000 mm.
With that distance, minimal SWR will shift from 144.1 MHz to 144.5 MHz. Gain 18.1 dB and F/B 17.81 dB


The elevation radiation pattern for stacking distances 3000 x 3000 mm:

Elevation 3000x3000mm


Azimuth radiation pattern 3000 x 3000 mm:

Azimuth 3000x3000mm

Calculated G/T:

23mDish inside 3000 x 3000mm KSC Stack 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 dB and F/B 16.98 dB


The elevation radiation pattern for stacking distances 3400 x 3400 mm:

Elevation 3400 x 3400mm


Azimuth radiation pattern 3400 x 3400 mm:

Azimuth 3400 x 3400mm


Calculated G/T:

230cm Dish inside 3400x3400mm KSC Stack G/T


G/T -11.05 dB Most bad G/T originated not only from strong first side lobes but bad F/B. That F/B is worse on just a single antenna without a surrounding dish. And became extremely bad with the dish inside. Stack has gain not only in the forward but also in the backside. 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. The main problem is Not a good enough antenna design that produces worse G/T than possible. Good antenna SWR means nothing when looking at gain, F/B, and G/T performance

The question is: Is it possible to design a much better antenna on the same boom length?