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Antenna No 65


A pair of stacked, phased vertical dipoles for 4m


I have since long been fascinated by collinear antennas.  A pair of stacked, phased dipoles in vertical polarisation is a textbook example of such an antenna.  This type of stack can often be seen on community FM broadcast station masts in 2-stack or 4-stack version. 


I had a couple of matching 4m dipoles and decided to use these for an experiment.  To avoid unwanted/unknown influence from nearby metal objects I bought a 38mm fiber/polyester tube to insert into the aluminium mast on the gable wall.  I also mounted the dipoles before-the-mast to minimise influence of the coax cable(s) running to the shack.


This experiment called for 2 'matching' dipoles and a 2:1 splitter/combiner.  The latter I had made some time ago to experiment with phased Yagis.  All splitter/combiner ports are 50 Ohms meaning I could run equal length coax cables to the dipoles.  For this experiment I used Ecoflex-10, L/2 * VF in length.


I used MMANA-GAL to simulate a vertical stack distance of 0.50 to 0.75 lambda between the dipole centres.  A spacing of 0.50 lambda distance provided maximal suppression of (unwanted) higher angle lobes.  I could have used 0.50 to 0.65 lambda to fit the available mast, which would have been interesting for long term observation of signals.  Unfortunately because the whole structure was a bit top-heavy I could not erect it for too long.



I decided on a 0.60 lambda stacking distance for the experiment.  Coax was Ecoflex-10 with N-connectors.



I used MMANA-GAL to simulate the effect of a 2-stack phased vertical dipoles.  Compared to a single dipole the difference in gain was about 2.4dB thus improving surface based communications.  Note the suppression of the higher lobes (i.e. wasted energy for ground communications) on the far field diagram.



Higher angle lobes are suppressed by the 2-dipole stack yielding a 2.4dB advantage at low angle.



Some photo's of the assembly:



A thick walled 38mm fiber/polyester tube.

Two matching 4m dipoles.  Open dipole style, 10x1.5mm aluminium tube, ABS box, N-connectors.

Chunky saddle clamps are needed for the 38mm tube.

The 2:1 splitter/combiner for 4m and equal length coax sections to connect to the dipoles.

The top dipole mounted and connected.

The bottom dipole mounted and connected.

The complete assembly resting on the patio.



The result looks quite striking.



Blue sky world ... 2 stacked, phased vertical dipoles for the 4m band.





I was unable to source locally 40mm or 50mm fiber/polyester tubing in 6m industrial lengths.  More luck if you live in Germany or the U.K. where these seem to be readily availably.  It proved horribly expensive to have them shipped to Belgium.  I had to settle for an 38mm tube diameter which unfortunately is too small for its 6m length, the tube is thick walled so quite heavy making it flex and difficult to handle.


If considering a more permanent set-up it is advisable to run some nylon guy ropes to the fiber/polyester mast.  The whole structure being top-heavy I was not sure it would withstand even a moderate storm (especially using support brackets on a gable wall like I do).


When positioning the splitter/combiner under the dipole stack a coax length of "Lambda * VF" to the dipoles would have been a better choice;  the splitter/combiner would have been fully clear of the lower dipole.


If you are lucky enough to own a fixed height tower (perhaps for your HF antennas) consider mounting a 2-stack or a 3-stack dipole assembly to the side of the tower.  It would make a formidable antenna for 4m FM contacts.


I observed a signal improvement with a fellow ham about 45km away, a path over ondulating terrain with two hill crest obstacles of +- 50m.  With both stations using dipoles in VP at about 8m agl, the signal is around S3 and quite hissy on FM.  The signal strength was measured up 2dB and FM was less hissy.




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