A Helix Feed System for the PrimeStar Dish and Other Small
Offset Fed Dishes
A
PrimeStar dish comes in several shapes, but one of the more common, shown at
the right and below, has an oval shape, measuring 40" wide by 30"
high. It is much heavier than the usual barbecue grill dish, and being solid
plastic, it has much higher wind loading. But being slightly larger, and with
circular polarization of a helix feed system, I found it to be 4 to 6 db stronger
than my 3' by 2' barbecue grill dish. I use a PrimeStar dish on top of a 50'
tower for the European AO-40 passes as my house and trees block my ground mounted
10' dish from 90 degrees (East) to about 145 degrees azimuth. Many AO-40 users
have found that all their AO-40 communication needs are not met by a single
dish system.
If
you decide to replace your barbecue grill dish on a Yaesu rotator with a PrimeStar
dish, be sure to mount the PrimeStar as close as possible to the rotator to
lessen the chance that you will tear up your azimuth gears. Also mount it as
close to the horizontal boom as possible to minimize the strain on the elevation
rotator. The picture at the left with the tower tilted over shows how I mounted
mine. In the picture above with the yagis, notice that the dish is rotated so
that the off center feed arm comes off the top. This produces 2 desirable effects.
The feed arm angle is about 10 degrees above the horizon when the dish is at
0 degrees elevation, so with the arm at top, you can get the dish several inches
closer to the dish without hitting the rotator at 0 degrees elevation. The second
reason is that the SSB UEK-3000 downconverter case must be oriented so that
water doesn't run inside the case, and with the top side arm and the tower back
to vertical the downconverter will not be a water collector.
Take
off and discard the LNA that came with dish. Save the 2 bolts that held it to
the arm. Make a small plate that will attach to the arm and hold the UEK-3000
if that is your downconverter. On another PrimeStar dish, I made a small 1/16"
thick plate to which I could bolt a California Amplifier downconverter from
WJ6T, and if using the AIDC 3731 from K5GNA, you can actually make a small metal
clamping piece to hold the Type N connector, Helix, and downconverter to the
arm without any special plate. I made a few special cuts to the UEK-3000 plate
to provide clearance for the double male and right angle type N connectors.
These pictures were taken with the tower folded over, of course.
The
Helix itself takes about a 1/2 hour to make. The 4" in diameter 2401 MHz
reflector disk is a piece of aluminum or copper or even a peach can lid. Depending
on the downconverter's front end geometry, sometimes I mount a male type N chassis
connector and sometimes I use a female type N with a double male connector between
the downconverter and the helix. In the case of the UEK-3000, to keep the case
upright and dry, I used a double male type N, from a female type N on the reflector,
to a right angle type N to the antenna input of the UEK-3000. The chassis connector
is placed about 7/8" out from the center of the reflector. The coil is
5 1/2 turns of #6 bare copper wire that is wound on the outside of a short length
of 1 1/4" PVC pipe. The turns are spaced out to about 1 turn per inch.
Make sure the coil is wound counterclockwise outward when looking from the reflector
(LHCP). The dish reflection will reverse this to desired RHCP to match the RHCP
of the antennae aboard AO-40. Flatten the first 1/2" of the coil and drill
a 1/8" hole in the coil where it is flattened. Shape the 1st 1/2 turn of
the Helix so that it can run just above the surface of the reflector. The natural
impedance of a Helix is about 130 ohms and this lowers the impedance so it can
better match the 50 ohm input impedance of most downconverters. Take out your
trusty old Weller gun, and solder the Helix to the type N chassis connector,
making sure the 1st 1/2 turn of the helix runs close to the reflector but does
not touch the reflector.
Offset fed dishes are difficult to aim in elevation because the
arm angle does not run in line with the radiation angle. In this Primestar case,
the arm is about 10 degrees higher than the radiation angle if the arm comes
off the topside of the PrimeStar dish - to set the dish to 0 degrees elevation,
set the arm at 10 degrees elevation.
I find the 32" round version of the PrimeStar dishes and
even the 18" DirecTV dishes, all offset fed dishes, also work best with
a 5 1/2 turn helix feed systems.
A
PrimeStar dish comes in several shapes, but one of the more common, shown at
the right and below, has an oval shape, measuring 40" wide by 30"
high. It is much heavier than the usual barbecue grill dish, and being solid
plastic, it has much higher wind loading. But being slightly larger, and with
circular polarization of a helix feed system, I found it to be 4 to 6 db stronger
than my 3' by 2' barbecue grill dish. I use a PrimeStar dish on top of a 50'
tower for the European AO-40 passes as my house and trees block my ground mounted
10' dish from 90 degrees (East) to about 145 degrees azimuth. Many AO-40 users
have found that all their AO-40 communication needs are not met by a single
dish system.
