ZL1RS - Antennas

With several portable VHF operations planned for the coming years I needed a high gain antenna system that was easy to erect and light weight. Here are a couple of experiments to find the "ideal" antenna for my portable VHF and 2m EME operations!



The first experimental lightweight portable yagi.
A YU7EF 13 element yagi on a 8 meter long boom. Tests showed an SWR curve as per the YU7EF website ... but too long and floppy with the lightweight materials used. Antenna data : 14dBD gain, 50 ohm feedpoint with a split dipole.



The second experimental lightweight portable yagi.
Four 5 element DK7ZB yagis on a compact stacking frame. This array is similar in weight to the long yagi, but easier to handle. Antenna data : 14.8dBD array gain, 50 ohm feedpoints with split dipoles. Stacking distances : 2M x 2M, phasing harness all 50 ohm coax cable. DK7ZB has many other great designs on his website.



The completed portable yagi array with phasing lines.
SWR looks OK ... 1.2:1 at 144, 1:1 at 145, 2:1 at 146 The feedline is 8 meters of 8D-FB coax (similar to LMR-400) This array was used at A35RS in the Kingdom of Tonga.


Although the 4 x 5 element array worked OK at A35RS ... one is always on the look out for just a little more gain!


G0KSC offers some interesting wide band yagis on his website. These yagi are a "50 ohm" design with very good efficiency and a wide bandwidth that would result in good tolerance to minor construction errors and rough installation while on a dxpedition.



I chose the 7 element model on a 3.2 meter long boom. This seemed to fit my requirements for an array with 17dBd of gain but not be too heavy a package for an EME dxpedition.

In order to save weight I re-modeled the yagi in 4nec2 using 3/16" diameter elements and added boom correction factors to allow the elements to pass through an aluminium boom ... OK, one could argue that this is no longer the same antenna, but the principles are still there ... and the resulting yagi parameters displayed in 4nec2 antenna modelling were still quite close to the originals.



Here is how I temporarily fix the parasitic elements in place.
The 3/16" rod element is insulated at the center with a short length of PVC tubing which passes through the boom hole and is fixed in place with a small cable tie either side. A permanent installation requires a better fixing method ... such as glue! The elements in the 4 x 12 array at ZL1IU are mounted the same way and glued in place with hot-melt glue. They have survived at Nick's exposed hill top location since we built the array in 2003.

A decision to go with folded dipole driven elements was motivated by the chance to eliminate several kg of coax phasing line from the dxpedition package. A 200ohm feedpoint would allow use of "open wire" feedlines ... actually TV ribbon!! I used this phasing system on my first EME array in 1984 and it seemed to work fine at modest power levels.



However, TV ribbon is relatively fragile stuff ... the end connections can break easily as the line flaps in the wind. To prevent this breakage from happening, simple strain relief insulators were fashioned from perspex and the lines were secured to these with nothing more complicated than pvc tape!



The TV ribbon phasing lines were cut to multiples of a 1/2 wavelength so the 200ohm folded dipole impedance would be presented at the other end of each phasing line. The four phasing lines were simply connected in parallel at the array center, thus a 50ohm impedance is available via a 1:1 balun to match the main coax feedline going to the masthead box or to the station.

Rather than a 1/4 wave sleeve balun, my balun consisted of a few "ferrites" passed over a short length of RG-213 forming an effective choke balun. In a permanent installation the clamp-on ferrites should be well sealed to prevent water getting in between the ferrite halves. Alternately, solid ferrite cores of the appropriate mix type (31 or 43 for VHF) could be used.



Antenna data : 17.0dBD array gain, 200 ohm feedpoints with folded dipoles. Stacking distances : 2.8M x 2.8M, phasing harness from 300 ohm TV ribbon. The overall array SWR looks OK ... 1.2:1 at 144, 1:1 at 145, 1.5:1 at 146. The old 8D-FB feedline has since been replaced with light weight LMR-400 coax cable.

Initial testing from the home QTH was very satisfactory. Eleven sations were worked in 1 hour via EME during a mini pile-up! Single yagi and two-yagi stations have been worked, and I get excellent reports from the "big guns" ... we often exchange lines of random text.


As part of my "keep it simple and light weight" dxpediton package I needed basic azimuth and elevation systems to point the array at the moon and know the array was correctly aimed in cloudy conditions.


Here is the azimuth system.
The whole mast rotates in a PVC plumbing fitting and an aluminium rod through the mast indicates the direction as marked on the plywood base plate. Pegs or bricks on the other half of the rod stop the array turning in the wind. Very simple, very light!

... no, I do not take the bricks away on a dxpedition



Here is the elevation system.
An aluminium tube from the horizontal cross boom and elevation hinge is connected to a rod. The rod has holes drilled every 5cm or so that "hook" onto a bolt through the mast. The rod pushes the lever out to elevate the array and the holes are marked in degrees to indicate the array's elevation. Very simple, very light!



Dispite a relatively short feedline with very low loss, a mast head preamp box has been added to the portable package to get the most out of the receive side. The main relay is a high isolation item with a 12V coil obtained surplus from China (thanks Wong). These relays have been tested to 2kW and have 90dB isolation, so no problem to handle my carry-on sized PA.

The preamp is a homebrew unit using an ATF-21186 GaAsFET. Another small coax relay with a 12V coil switches the PA input and Preamp output back at the radio. Having all components running from 12V meant the old 28V relay power supply could be removed from the package ... another small weight saving!