Modify an Analogue Sky TV Offset Fed Dish for AO-40 S-Band
Created 10 June 2001
Updated 11 June 2001 - 'Taste Test' between 30 Turn Wimo Helix & 60cm Dish
Updated 31 July 2001 - Links to: Optimized Helix Turns, Antenna Range Test Results & Audio/Video
Updated 9,10 September 2001 - Page updated with modifications and lots of hints to getting the perfect helix, plus links to alternative antennas: Umbrella Dish and Quadruple Helix
Updated 12 December with a Link to FAQ's
Old offset fed Sky 60cm dishes are available in the UK for nothing. I liberated this one from the roof of my apartment block which had been discarded several months earlier.
This is a tale of how I brought it back to life as an S Band antenna for AO-40. This is all based on James Miller G3RUH's design, and so I take absolutely no credit for any originality, other than to prove that it can be done.
Also I've used a few ideas from Ed Krome's Mode S - The Book 2001 edition.
The article A 60 cm S-Band Dish Antenna where you'll find the information on how the feed was originally designed - with some alterations described below.
Total construction time: 2 hours of which 1 hour was on the feed, and one hour fiddling with mounting the feed and testing it.
Here's a step by step method I use to making a reproducible helix feed first time every time, with a good match and no adjustments or test equipment necessary. Of eight helices (long and short) I've made in this manner, all have presented VSWR's 1.2:1 or under with no adjustment.
How many turns do I need? Check Optimized Helix Turns
o Take 1m of Westflex W-103 or similar single strand core with 100% foil shield cable.
o Extract the 3mm inner copper core for the helix, saving the copper foil shield for later.
o Mark the copper wire every 146mm with a permanent marker like a CD-R pen. (Tnx K9EK)
o Wind the wire opposite to a standard screw thread around a 40mm former. After turning, when you release it it will spring out slightly, with about 7 to 7.25 turns.
o Mark a length of dowel or smaller PVC pipe every 32mm to make a template for stretching the helix to the correct 32mm spacing. The marks made on the helix itself earlier should be made to line up too: (Tnx K9EK)
o The helix is wound left hand circular polarised (LHCP): when it's reflected in the dish it'll be RHCP. LHCP is the opposite way to a screw thread. For LHCP, looking from one end of the helix, it should turn anti-clockwise as the turns move away from you.
o Cut off and discard the first turn which is invariably difficult to make uniform.
o A quarter turn matching section is made out of the first part of the helix by soldering an 8mm wide piece of copper foil from the coax shield. Mark out this quarter turn using the helix itself as a template:
o Expand the 1/4 turn marking to 8mm wide with a ruler and some artistic license, and cut with some sharp scissors.
o Use a high power soldering iron to solder the foil to the first quarter turn of the helix. This can be an interesting test of dexterity and logistics. Bear in mind that when fully assembled, although in a spiral, the foil will be parallel to the reflector, like a carnival slide.
o For the reflector, take a 125mm x 125mm piece of single-sided 1.6mm fibreglass PCB
o Drill as shown (I use sheet metal hole punches for holes over 10mm):
o With a 30mm x 30mm piece of copper foil, use it to through hole plate the 12mm hole in the reflector. Cut a cross in the middle of the foil and from the rear of the reflector push the flaps through to the front copper clad side. Fold the flaps down and solder keeping the surface as smooth as possible.
o Make a 25mm x 25mm spacer out of 1.6mm PCB and drill as shown:
o Take a 30mm x 70mm piece of the copper foil and use it to wrap the spacer, punching through the holes, allowing the foil to through hole plate the 16mm hole.
o Take an N type panel mount socket and carefully cut down the solder pot with a hacksaw down to 1.6mm. Use a spare piece of 1.6mm PCB as your gauge.
o Attach the N socket to the spacer & reflector:
o Solder the helix to the N type, using a spare piece of PCB to maintain the 1.6mm spacing at the start of the helix. Be careful to hold the helix at least two turns away from the base as it gets very hot!
o Snip off the last 3/4 turn or so so that the entire helix including matching section is 5.25 turns.
o If your dish doesn't have a feed arm with LNB, you can find the focal point of the dish using W1GHZ's HDL_ANT program. Select the 'O' option and follow the questions. The dish focal point should correspond to a point 2 to 4cm in from of the reflector at the centre of the helix's axis. The helix should point in the direction of the deepest part of the dish (sometimes, but not always the dish centre). You may well find that empirically there's not much difference as to where the feed is placed to within a couple of centimetres.
o Chop off the collar part of the LNB bracket with a hacksaw.
o Screw the helix reflector to the LNB bracket:
o If you use the dish outside permanently, spray the feed with a couple of light coats of PCB lacquer or colourless spray paint to stop copper corrosion. If you encounter difficulty with the antenna in the wet, cover the helix in a transparent freezer bag (don't completely seal). You can use non-acidic RTV, but be very aware that even a very small amount will mess up the match.
o Because the feed's offset you need to check that you have the correct elevation angle. Most offset fed dishes are about 21º off. Use option 'O' of W1GHZ's HDL_ANT program to calculate it yours.
o More info on dishes? Loads of really good info from Paul W1GHZ Parabolic Dish Antennas, Offset-fed Parabolic Dishes, Common Offset Dishes
Outside on the roof testing on GB3NWK on 2320.850
If the dish looks a bit twisted that's because it is. The tripod and dish keeled over the edge and dropped 7' five minutes before doing the test.
In addition, these tests were done before optimising the feed (documented above) which provide a further 2dB improvement.
Audio of GB3NWK on Wimo 30 turn Helix
Audio of GB3NWK on 60cm Sky dish
Mail Howard, G6LVB