Circularly Polarised Yagi Antenna

Antenna Requirements

The Vela pulsar signal is almost completely polarised, with linear polarisation being the dominant mode.  This means there is the potential for the pulsar signal and a linearly-polarised receiving antenna to be cross-polarised, resulting in the received signal dropping to zero - possibly for the whole observation period.  In the worse case this condition could last for a numbers of days - fortunately unlikely.  An extra complication is that the polarisation angle swings about 45° during the pulse on phase.

Note that even though the PA (position angle) of the polarisation at the peak of the pulse of Vela is fixed in space, at 436 MHz the last stage of the signal's journey through space (Earth's ionosphere) introduces a variable amount of polarisation twist due to Faraday Rotation.

Therefore, an alignment of the polarisation of a linear-polarised antenna to be coincident to the fixed orientation in space of the Vela pulsar signal is made ineffective by this last "twist".

To avoid the effects of an unknown polarisation orientation for the incoming Vela signal it has been decided to utilise a circularly-polarised antenna - which responds to linearly-polarised signals from all orientations.

Construction and Initial Configuration

Construction details of the Yagi antennas and the initial configuration.

The boom length is about 5.7 m long and all up has 42 elements.  The circular polarisation is produced by introducing a 1/4λ physical offset between two crossed yagis with the notional vertical polarised yagi placed forward w.r.t. the horizontal polarised yagi.

The options to get the antenna here in Australia were to order from M2 in the USA directly (where non-arrival or damage would be my responsibility) or through a local dealer (who would assume those risks).  Although adding a "middle-man" added to the cost, I chose the second option.  Another penalty (in addition to the dealer's margin) attached to purchasing through a local dealer is that they import in aggregated batches.  That is, they wait until enough items have been ordered to fill a large shipment.  This means the delay in despatching an item can vary widely depending on where in the aggregation cycle your order lands,  Can be up to 3 months before it is despatched.  Then it is loaded on a slow ship.  Overall, it can take months for the item to arrive.  To circumvent this I paid for air shipment of just my antenna.  All this added up to a tidy sum - the size of which I will not reveal to avoid embarrassment...

Assembly

The contents of the package were laid out in an organised fashion.

The first assembly task was to convert all the relevant dimensions in the dimension sheet from decimal inches to fractional inches via the small table left-of-page.

Accurate measurements of the components of the antenna is required as they are not identified by any markings.  The five sections of the boom needed to be identified by their length from the assembly diagram in the dimension sheet.  While the boom lengths were easily differentiated, the 42 elements were sometimes different in length by only a 1/16" and required careful measurement.

The elements were each individually identified by holding them in a bunch and tapping one end of the bunch on the table to progressively identify the longest elements.  Each element was marked with an identification (e.g., HD1 = horizontal yagi director 1) by permanent pen.  Care had to be exercised as there was not a strict ordinal progression in decreasing length for the elements, i.e., director 16 is the shortest, with directors 17 to 19 being longer.

Having sorted and identified each item of the element set, the next task is to assemble them to the boom.  The style of the elements is thru-the-boom-insulated.  Assembly entails pushing on one insulating bush, passing the element through the relevant hole in the boom, and then pushing a second insulating bush onto the other side.

It must be said that the attachment of the 42 elements and the slow process of ensuring they were all centred was a mind-numbing task.  It was a chore to maintain the concentration needed to ensure all the elements were correctly installed.

While these measurements were performed with antenna lying on the trestles - and so would return values different to those produced by the final installation configuration, they at least give a rough indication that nothing terrible had happened during the assembly.

Mounting the Antenna

Now that the antenna itself has been assembled the next task was to build a mount for the antenna.

At this location, due to the small patch of sky visible through the trees, there is little point in providing a pointing mechanism for any low-frequency antenna - the beamwidth at low frequencies for the small aperture antennas used (due to space restrictions) is not much narrower than the sky 'window'. For the antenna beamwidth of approximately 20° the exercise becomes one of centralising the beam of the antenna in the sky 'window' in a fixed configuration.  Observation sessions are carried out in drift-scan mode.

For drift-scan observations of the Vela Pulsar (declination 45°S) at the latitude of this location (33.6°S) the antenna needs to be pointed almost straight up.  Specifically, the pointing should be azimuth 180°, altitude 78.4°.

There are two sets of mounting holes for the mast bracket.  I have chosen to use the set that places the mast bracket major dimension the furthest from an element in the plane, i.e., the rear-most set. Of course, when pointed straight up at cosmic objects, the concept of vertical and horizontal polarisation is irrelevant.  The choice of orientation then becomes a matter of mechanical convenience.

The manufacturer's documentation stresses that metal objects near to the elements will degrade the performance of the antenna.  Specifically, it states that the mast itself must be non-conductive.   A minimum clearance of 30 cm is recommended for any conductive object.  Fibreglass is recommended for the mast.  Given these specifications it was decided to construct the antenna mounting hardware from plastic pipe and wood.  I have constructed wooden masts in the past and have found them adequate against the effects of the weather encountered here.

Two designs first came to mind.  The first is a "chin-up bar" design, with two vertical posts and a horizontal bar between them.  The antenna is affixed to the horizontal bar at the mast bracket and declination can be varied by rotation of that bar.  The advantage of this configuration is its simplicity.  The disadvantages are that the vertical posts have to be long enough to ensure the rear of the antenna is clear of the ground (at least 600mm).  That would mean two poles of 3.6m length with a buried length of 600mm - separated by at least 1.5m to keep the poles clear of the antenna.  Also the pivot point is 3m above ground which makes access not without danger.

The second was a tilting "flagpole" design, with two short vertical posts spaced by the width of a third, central, pole.  The two short poles should be about 1.5m out of the ground.  The centre post is designed to tilt to the ground.  The disadvantage of this design is that the attachment of the antenna itself to the centre pole will need to be more complicated as it will need to be approximately parallel to, but spaced from, the vertical centre pole.  The advantage is that the highest mounting point which needs access is only 1.5 m above ground - a much safer option.  Also, all parts of the antenna can be accessed in the tilted position.