70CM EME system

Rebuilding my satellite system for 70 CM EME operation

 In 2022 I designed, built and installed my satellite system (for LEO/MEO orbits). I have been on EME before (2M and 23CM), but never (seriously) on 70CM.

Early 2024 I decided to rebuild the satellite system with a little more emphasis on the EME capability. I wanted to do the conversion while at the same time be able to operate LEO/MEO satellites, so I needed to still have an 2M antenna in the system.

I had a lot of conversations with Goran YU1FC from Antennas&Amplifiers about this. Goran has an invaluable amount of knowledge when it comes to antennas and stuff around these!

As I wanted this new system to replace my "old" satellite system, and use the same tower and rotorsystem, I had to deal with a couple of limitations. The biggest one being the low height of the tower. The top of the rotor (where the horisontal boom rests) is only at 2.5 meters above ground. This puts some limits on the length of the antennas in order for them not to touch the ground when elevated (I needed to be able to elevate to 90 degrees because of the satellites, this is not needed for "normal" EME as the moon does not get that high on the sky here in Denmark)


Goran did some modifications of his 70 CM 23 element antenna. He changed the attachment point for the vertical boom so that it no longer was in the center of the boom of the antenna, the antennas are 5.7 meter long, and I needed the attachment point to be a little further back on the antenna compared to the "normal" position. This ensures that the back of the antennas does not touch the ground when elevated. Goran also modifed two of the antennas dipoles so that they could be mounted at the top and bottom and still have the elements on the top and bottom of the booms. The feedpoint was also done so it is mounted on the opposite side as the elements. All of this helped me make a neat and tidy installation of the antennas and routing of the coax cables.

Now, in an EME system, you need to keep the losses BEFORE the preamp at an absolute minimum. Every tenth of a dB counts ! So ideally, I should have extended a boom in the center of the array all the way out to the end of the antennas, mounted the 4 port combiner and preamp there. However, due to aesthetics, I selected to mount the combiner and preamp on the central boom.
I could have saved approx. 2.2 meter cable (and the associated loss) if I had done it the optimal way, instead I compensated with some heavier (less loss) LMR-600 coax cable. The extra loss introduced by this approach is approximately 0.08 dB for the 3.7 m LMR-600 (total loss 0.21 dB) compared to 1.5 m LMR-400 (total loss 0.13 dB). Had I instead used 1.5 m LMR-600 and the "boom to end of array" solution, the 1.5 meter cable (LMR-600) would have saved me 0.12 dB compared to running the cables along the antenna boom/H.


0.12 or 0.08 dB is not "nothing", but I feel it is still relatively low numbers in order to get rid of the "boom" going to the back of the array. Time will tell....


Thanks for input from Conrad PA5Y, Spiros SV8CS and Goran YU1CF



Equipment list for the system

Here is a partial list of stuff that I have used in this system.


  1. The tower, rotor and rotorcontroller from my satellite system
  2. 4 pcs 70cm23DX6A antennas from Antennas&Amplifiers (Called 70cm23DXD in my case because of modif.)
  3. 1 pcs 2M8Cross 2M antenna
  4. 1 pcs 4 port 70 cm combiner 
  5. 1 pcs EME3-432 Preamp with seperate TX/RX lines and sense output (for sequencer)
  6. Multiple brackets, boom jointers etc, also from Antennas&Amplifiers
  7. 2 pcs 60 x 60 x 4 mm Aluminium square tubes, from Amazon. For horisontal booms.
  8. 2 pcs 40 x 40 x 3 mm Aluminium square tubes, from Amazon. For vertical booms. 
  9. LMR-600 cables from antennas to combiner from McGill Microwave. (N male/N male right angle). More details in coax section below.

Radiation diagram of 70CM antennas

Specification of the 70CM antenna:


Number of elements: 23

Boom length: 5.7 meter

Antenna gain is 18.9 dBi

F/B 40 dB

G/T for Tsky=27K, Tearth=1800K: +8.22 dB

Above is the radiation pattern of a single antenna in the azimuth plane.

Above is the radiation pattern of a single antenna in the elevation plane.

Overview of system

This shows the system mounted on the tower/rotor system of the original satellite system. The antenna in the center of the "H" is the 2M8Cross antenna for VHF (typically uplink) to LEO satellites.


Details

The 4 x 70CM antennas are stacked 175 cm vertically and 185 cm horisontal. This was verified by Goran from Antennas&Amplifiers


There are two horisontal booms, the one that is attached to the rotor is offset around 185 mm from the "true" center of the "H", this is so that the 2M antenna is mounted in the center of the "H" and so that routing of cables etc. will able to be done without problems.

All hardware, plates the connects the booms with each other, plates the attach the antennas to the vertical booms etc., are all made of stainless steel. 

Instead of using square "U" bolts, Goran suggested using stainless steel flatbar and stainless steel bolts. This makes the connection very strong and easy to assemble. All bolts are M10.

The total weight of the H frame including joiner plates but excluding bolts and flatbars are 23.5 Kg

The total weight of the H frame including antennas, joiner plates and flatbars are 51 Kg
(minus bolts, combiner, preamps and cables)

Cables from antennas to combiner

The 4 cables between antennas and the 4 port 70 cm combiner are made of LMR-600-UF cable. The LMR-600-UF cable has a loss at 450 MHz of 0.056 dB/meter. 

The cables have a N male connector at the "antenna" end, and a right angle N male connector at the "combiner" end.
I checked with McGill in the UK about insertion loss of their right angle and straight N male connectors, and they are both less than 0.05 dB loss. By using a right angle connector at the combiner end of the cable, I can save close to 30 cm cable (as I would have to curve the cable, radius >= 100 mm).
The cable was drawn as a 3D sketch in the CAD program and fitted on the complete assembly of the system to check for lengths etc. I also designed some 3D printed (ASA) bracket to hold the cables, whether these will be use in real life is yet to be determined.

The cable is LMR-600-UF
Straight N male: TC-600-NMH-X

Right angle N male: TC-600-NMH-RA-D

Instead of just fixing the LMR-600-UF cables to the H/antennas using cable ties, I designed some holders/spacers for the cables. The holders were printed in ASA (UV safe) with 100% infill to give them some strength.

All the holders (26 in total) are attached to the booms/antennas using 4 mm pop-rivets.


I used a total of 4 different sizes/designs (STL files in the links for each):

  1. 30 mm wide, low height for the 30mm part of the antenna boom (3 pcs each antenna)
  2. 30 mm wide, "higher" type for the 30mm part of antenna boom just before 40 mm section (1 pcs each antenna)
  3. 40 mm wide for the vertical booms of the H frame, 2 on each leg
  4. 60 mm wide double holder, for the center horizontal boom (2 pcs in total)

Measurements of cables

I chose to get readymade cables from McGill Microwave Systems in the UK (I was a bit in a hurry, and I do not have the right crimp tools for LMR-600).

McGill Microwave Systems can customize any cable for you, professionally assembled with connectors. I supplied them with the exact measurement of the cable (specified where the distance was from/to), they adapted the length to their normal "pin to pin" length. Delivery was super fast, cables was shipped the day after I ordered them!


The cables used is LMR-600-UF (LMR-600-UF datasheet)

Connectors used:

Straight N male: TC-600-NMH-X (Datasheet)

Right angle N male: TC-600-NMH-RA-D (Datasheet)

The length specified was 357 cm (-0/+2cm).


The LMR-600-UF cable has a specified loss of 0.056 dB/M, so 357 cm cable has 0.199 dB loss at 450 MHz.

Both connectors was specified as having less than 0.05 dB loss each.

I tested all 4 cables on my R&S ZNLE3 VNA (in calibration. Setup calibrated with ZN-Z150).

Measured was both the insertion loss and the phase.


Cable #1 has a insertion loss of -0.253 dB and phase is measured at -37.96 degrees.

Cable #2 has a insertion loss of -0.251 dB and phase is measured at -36.60 degrees.

Cable #3 has a insertion loss of -0.254 dB and phase is measured at -37.86 degrees.

Cable #4 has a insertion loss of -0.263 dB and phase is measured at -37.50 degrees.

Comparing cables

Data for the 4 cables in short form:


Cable #1     Loss: -0.253     Phase: -37.96

Cable #2     Loss: -0.251     Phase: -36.60

Cable #3     Loss: -0.254     Phase: -37.86

Cable #4     Loss: -0.263     Phase: -37.50


Largest difference in loss is 0.263 - 0.251 = 0.012 dB

Largest difference in phase angle is 37.96 - 36.60 = 1.36 degrees


The cables are VERY close to each other and are well within specs. If the 0.056 dB/M is correct, the loss for the cable itself should be 0.199 dB, this leaves only 0.064 dB in loss for both the N connectors!


Joiner plates etc

Below some pictures of the boom joiner plates as well as DXF files for these.

If you click on the images, you will download the dxf files for each of the components. 
The "U" brackets are made of 2 x 10mm bolts (stainless steel) and the small flatbars shown.

Main boom joiner, 60/40 mm joiner, 5 mm stainless (4 pcs)

Antenna to vertical boom joiner, 40/40 mm joiner, 5 mm stainless (4 pcs)

Boom 2M8 joiner, 60/30 mm joiner, 4 mm stainless (1 pcs)

Elevator plate to main boom, 6 mm stainless (1 pcs)

30, 40 and 60 mm bracket plates (for "U" bolts), 4/5/5 mm stainless

30 mm (2 pcs)

40 mm (24 pcs)

60 mm (14 pcs + 2 spares)

Pictures of antennas

Below some pictures received from Goran (Antennas&Amplifiers) during manufacturing/testing of the antennas.
Looks really nice, and same high quality mechanically as the antennas I had in my "old" satellite system.

Assembling of antennas etc

Antennas and joiner plates etc. arrived from Antennas&Amplifiers. Everything was packed very nicely and safe, double boxes etc. The good thing about the antennas is that EVERYTHING is very clearly marked with small stickers, there is nothing to go wrong with. Just match the numbers/letters of the elements, boom segments etc., and all will be perfect.
All the hardware are stainless steel (AISI 316 A4) and very nicely done.
First task was to assemble the booms, next up is mounting the 23 elements on each boom.
Also started on the "H" frame.

Alu tubes for the H frame. 2 pcs 200 cm long 60x60x4 mm for the two horizontal booms, and 2 x 170 cm 40x40x3 mm for the vertical booms. This is VERY strong and could maybe have been "dialed down" a bit. But, better safe than sorrow as they say :)

To be continued...