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Friday, August 1, 2008

6 Meter Half Wave Vertical

At the time I decided to construct this antenna I was using a quarter wave vertical and decided that something with a bit more gain would yield better results as it was winter time here and we were right in the lull of the sun spot cycle.
The only real propagation was the odd sporadic E opening allowing the occasional contact of between 1200km and 2400km.
Also some time prior to this I had been given an old Station Master 27MHz CB antenna.
A Station Master is basically a half wave vertical with a air core inductor at the bottom tapped to provide an impedance match to 50 ohms.
The particular antenna that was given to me was quite old and the rivet connecting the coil to the radiator had let go due to corrosion and also the plastic conduit that secured the vertical radiator to the mounting bracket had cracked and completely let go.
I decided that I should be able to convert the Station Master to the 50MHz Amateur Band by shortening the radiator and taking some turns of the inductor at the base.
Since the antenna was smaller I also decided to shorten up the mounting bracket that held the coil and radiator in place and provided a manner in which it could be mounted.

Converting the Radiator to a 6m Half Wave

This was and is a very easy process as the old antenna is in telescoping sections of T1601 tubing.
If the Plastic Conduit at the base had been intact I may have used the largest diameter tubing but I had nothing to replace the conduit in that size so I decided to go with 25mm orange electrical conduit of which I already had.
It just so happened that the third section from the bottom fits perfectly into the 25mm conduit.
So I used from the third section up and adjust the final length to 2950mm by cutting the tubing in the middle to length.
2950mm is around 60mm too long for the centre of the band but because around 60mm of the tubing is running down into the bracket some adjustment has been made to the radiator.
The Impedance Matching CoilNow I wanted to use up the coil that came with the original antenna and I had to find a way to work out how many turns would have to be removed for it to function correctly.

Now having tinkered with antennas for some time now I had come across a property that half wave verticals end fed with a tapped inductor for matching seem to have.
If one does a SWR sweep on one of these half wave verticals they seem to to have a low SWR at around 1/3 of the operating frequency. It seems that at 1/3 the design frequency this design behaves like a base loaded quarter wave. Bearing this in mind I used a helical quarter wave design program to design a vertical for 17.3MHz with the dimensions of the coil I had and a vertical section 2950mm long. It was found that removing two full turns from the coil would make the system resonant at 17.3MHz.

I figured that tapping the coil to operate this system as a half wave at 52MHz should not be a problem as there were 7 turns remaining on the coil that could be tapped.
At this point it is a good idea to make sure the brass terminal block is on the windings of the coil.
After removing two full turns the end of the coil was flattened out with the use of a vice in the appropriate orientation to allow it to be drilled and fixed to the radiator.

The Bracket

Sizing every thing up it was found that 160mm was a good over all length for the bracket between the 90 degree bends so cutting off the excess from one end and bending at the 160mm mark soon happened after a 25mm hole was drilled o that it would align with the existing 32mm hole in the other end (easier said than done).
Cutting the 25mm conduit to length allowing for two threaded end caps was next on the agenda. After that I worked out where the coil would attach to the radiator and drilled a 12mm hole in the conduit in the appropriate place.
Next I slid the bottom section of the radiator into the conduit section so it went around 3mm past the hole for the coil attachment. At this point I drilled a 3mm hole all the way through the aluminium tubing and through the corresponding side of the conduit at the centre of the 12mm coil mounting hole in the conduit. Placing a bolt through the hole in the end of the inductor and through the end of the radiator and conduit holds that part of the assembly together. Prior to this one could smear a silicone based sealant over the end of the radiator where it slides into the conduit to seal it against weather and provide adhesion to make for a more permanent construction.

Connect the coil to the plate at the bottom by drilling and screwing. Also drill a 14mm hole for the placement of the SO239 socket and fix the socket in place and solder 50mm length of hook up wire to the centre pin of the SO239 and move the terminal block to the forth coil from the top and tighten and solder the other end of the hook up wire to the top of the terminal block.
Tune Up of the 6M Half Wave
Now slide the remaining sections of the radiator into place and secure either by drilling and screwing or cutting slots and using hose clamps. Check that the length of the radiator is correct and erect the antenna in a location that is easy to get to but clear of metallic obstructions.
Connect a coaxial cable via the SO239 and proceed to check the SWR at low power or check resonance by some other reliable method.
If the tuning is not satisfactory simply try another tapping point. If it is found that the antenna will not tune to the 6M band check continuity of all joints within the antenna and also the feed line.

Building the 6M Half Wave Vertical from Scratch.

Building this antenna from scratch would not be too difficult. The only critical point would be finding a tight fit between some PVC tubing and some aluminium tubing with the PVC tubing on the outside. There is little point in having a parts list as supplies vary every where. That said I will give the critical measurements and dimensions.
Basically the 2950mm section of tubing could be a single length of any nominal diameter of tubing within a few millimetres of 21mm.
In fact a single length would possibly give less trouble because there are no joints.
The coil dimensions are as follows:

Wire diameter: 5mm

Wire length: 1100mm

Diameter of coil inner: 45mm

Coil height: 80mm

Trim off excess at the coil ends so that the coil sits around 10mm from the conduit when fixed in place.
Mounting plate dimensions:
320mm x 80mm x 2mm Aluminium plate.
Mark out 80mm from each end and fold at 90 degrees after marking and drilling holes for conduit and SO239. Folding at these points should leave somewhere close to 155mm section between folds for coil and mounting.
Tune up procedure should be as above.
After tune up seal all joints with a silicone based sealant to prevent corrosion. Also seal up the top of the SO239 connector and the PL259 on the coax.
This antenna should give years of service and can easily be serviced, repaired and retuned. Enjoy!


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