Dish Modifications

[VK2JDS]

I looked through the board here and couldnt find a section on antennas.
so, i thought this section would be appropriate if you have a larger dish. I have a 4.6m tvro which i have used very successfully for moonbounce on 23cm. to improve this dish and feed setup i would have to add a skirt on the edges and a choke around the septum feed.
how to do this when the feedpoint is mounted on a pole extending from the middle of the hub?
it requires converting it from a centre pole to a feed arm mount.
here bunnings is your friend with 4 3 metre lengths of 20mmX20mm square aluminium tube and 3 metres of aluminium angle. grab some tek screws, roofing bolts and a packet of 20 2 inch long 2.5mm or 3mm diameter machine screws and nuts.
get some builders strap or pryda straps. 40 cm is heaps as you will be cutting these up into small bits for joining the arms to the dish with the tek screws.
a hacksaw and your battery powered drill is great here.
make up a feedpoint cage using the aluminium angle and some rivets or screws, a loose fit around the feed makes it easier to slide in and out for focusing and changing bands to other feeds etc.
leave the septum feed on the pole mount with the cage around it and work around that. start by making up the mountings for the first arm with builders strap either side of the square section. sit it in position and measure out how far it is to the feed point cage. use sticky tape to hold the arm roughly in position at the feed.
make the arms all the same length. drill and screw the arms to the back end of the feed cage then add the offcuts from the arms to the front of the cage back to the arms as stabilisers. the pictures show what i did.
once you have the 4 arms in place and screwed in its time to disconnect the feed from the old pole mount. i used an anglegrinder to cut off the old rusty pole.
its a lot more stable than the pole and you dont loose 5 or 10 % of the dish area due to the shadowing .
you can utilise this type of mounting cage assembly to fit in different types of feeds for terrestrial weak signal work too on various bands
now i can add a choke to the septum and fit different feeds and its lighter than the old mount!
73's

[VK5PM]

Thats good a good article and pics Dave you sure are making good progress...

[VK2JDS]

Now to add a septum fitted with a choke ring assembly (homebrew of course).
the recycled aluminium sheet i bought wasnt quite big enough for the 47.5cm diameter required so an added piece rivetted into place sorted that out, next a ring made from a strip of zincalum about 110 mm wide bent so that 87mm of it becomes the ring and the rest of it becomes fingers of metal bent at right angles to make pieces to rivet to the base of the choke.
rivet the choke to the septum using blind aluminium rivets from the inside of the septum. 4 on each side.
the pics show the feedpoint cage with the top bit lifted open and the new feed ready to drop in, and the septum with choke fitted. this technique works ok at 13cm too but rivets will likely be a significant problem above 5GHz.
make sun noise rise measurements using NFM and peak the focus. i peaked the septum tuning pins using speccy/tracking gen and homebrew return loss bridge, but you can get away with a vswr meter (cringe) and measure both ports.
a slight tweak of the tuning pins inwards to add capacitance was necessary when the feed was moved onto the dish after being tuned aiming into cold sky.
for 23 and 13cm this is a cheap and easy option to make an improved septum feedpoint for moonbounce.

the top of the new dish feed mounting with the 'lid' open

the homebrewed choked septum ready to fit

73 de Dave vk2jds