Centre Fed Dipole

This would have to be the simplest Centre Fed Antenna to contruct and is the basis for many broadside and yagi arrays. Quite simply the antenna is a half wave of the operating frequency and fed in the middle where current is of the maximum value and voltage is of the lowest value.Thus the feed point impedance is low in comparison to an end fed half wave because the voltage is at maximum at the ends of a half wave radiator.The characteristic impedance of a centre fed half wave dipole is a little over 70 Ohms. The same length antenna fed from one end would have an impedance of 4000 Ohms.So centre feeding a half wave definately has its pro's as 75 Ohm coaxial cable will provide a very close match.The length of a half wave antenna for a particular frequency can be found by:468/Frequency(Mhz)= length in feet.It is a good idea to cut the antenna on the long side and using a tuning device trim the antenna to resonance.Take care as to cut equal amounts from both ends as this will cause the feed point to shift to a diferent impedance point and also cause a feed point imbalance encouraging feed line radiation.These type of antenna can be built for almost any band and out of a mutitude of materials.Diploes of this kind built for 40m and 80m and sometimes the higher bands are usually made from wire.On the higher bands they can be made from aluminium tubing and are often not directly fed but are coupled to the antenna through simple matching networks.

Unbalanced Fed Quad

The Quad is traditionally regarded as a centre fed antenna due to the way it is normally fed.
The normal feed for a quad is to feed it directly to the feed line with a 2 to 1 balun and 50 ohm coax or maybe with a quarter wave matching section of 75 ohm coax. In this instance the feed point impedance is normally around 100 ohms for a radiator on its own.
But there is an alternative to this that allows vertical and horizontal polarisation with only one radiator and also allows tuning adjustment without having to vary the length of the loop.
It is called a voltage feed and the feed point impedance is around 1000 to 1500 ohms.
To feed a quad in this way you must have a matching circuit much like that of a end fed half wave and feed the quad from one corner. This has the effect of having two half waves in parallel in the way the current and voltages are distributed.
Feeding the side of a loop will give horizontal polarisation and feeding the top or bottom will give vertical, this is the opposite to the traditional current fed quad.
The interesting thing is one can have a quad with both vertical and horizontal feeds with no negative effects.
To understand why we must look where the voltage is at minimum throughout the radiator.
At our feed point the impedance is quite high, high impedance equals minimum current flow, minimum current flow equals maximum voltage. One quarter wave length around the loop we should end up at the next corner and at his point our current is at maximum and voltage is at minimum so this is a low impedance point. At the low impedance point the current is flowing in one direction with in the wire. Now the interesting thing is that if a conductor is connected to this point current will not flow in this conductor providing it is at right angles to our radiator.
The reason for this is quite simple, currents flowing towards our conductor will flow into our conductor, on the other side currents are flowing away which would make current flow in the opposite direction in our conductor. Equal currents in opposite directions in a conductor will cancel out thus ensuring that our conductor does not become part of the antenna.
Examples of this can be seen every where and all yagis built in plumbers delight fashion where by the elements have a conductive connection to the boom are one example of this.
So when we feed our quad loop at the vertical feed point our horizontal feed point will be ignored providing currents are equal in amplitude either side of our horizontal feed point.
So what matching network do we use on our quad when fed in this way?
Methods similar to end fed half wave antennas will work good with some adjustment.
The simpler the better possibly a series-parallel LC circuit would be best.
I have seen quarter wave matching sections used that were some what similar to the way a J Pole or Slim Jim are fed.
When constructing ensure that your loop is tuned to the operating frequency.
This can easily be done by cutting your wire a little more than 300/frequency and temporarily stringing the wire around the spreaders, then connect to a transmitter on low power with a SWR bridge and trim for minimum SWR mid band. An SWR of no less than 2/1 can be achieved with a 50 ohm feed.
Then construct for permanent fixture making the join in the loop at one of your feed points.
Also ensure that your other feed point is one quarter wave away from the first if duel polarity is desired.

The J Pole Antenna

The J Pole Antenna is basically a half wave vertical with a quarter wave balanced line matching stub at the feed point.

J Poles are commonly built for the 2 metre band by Amateur Radio Operators due to the convenient size and ease of construction.Normally fed with coaxial cable but really could be fed with any impedance feed line, balance or unbalance with some adjustment to the point of feed on the stub.

The Matching Stub
Near to the bottom of the J Pole the iconic J section is actually a quarter wave balanced closed stub.Quarter wave stubs are quite common in antenna construction for a number of reasons.One reason is that a closed stub that is a quarter wave long has a circuit length of a half wave.
This means that currents on the two conductors at the end of the stub will be 180 degrees out of phase to each other. This alone forms the basis of coaxial 1/1 baluns.
Another property of quarter wave stub is that the closed end normally has a low impedance point at the short circuit end and a high impedance at the open end and any impedance between the two can be found some where between by tapping off.
Yet another quality of closed quarter wave stubs is that they tend to balance unbalanced currents.
Once one is aware of what these stubs can do it is no surprise that they are used to feed an end fed half wave to form the J pole.

Tuning J Poles

J Poles can be tuned to any kind of feed line at any impedance. Its just a matter of finding a matching impedance feed point on the quarter wave stub. It has been suggested for operation with coaxial cable to connect the centre conductor to the side of the stub without the radiator attached. Careful tuning can achieve a near perfect match and there are three points at which tuning can take place. Temporary connection of the feed line conductors to either side of the matching section moving them generally together up or down to provide the best match. Some independent movement may yield further improvements. Also adjusting the length of the open end of the quarter wave matching section can improve tuning.
One thing to watch is near by metallic objects. They can detune a J Pole quite a bit. Also In some cases the attachment point at the bottom may need to be insulated as some detuning can take place with a conductive masting. Its a bit unusual but it has happened.

Construction
I have heard of and seen a few different construction techniques applied to J Poles. I have heard of them being constructed from balanced feed line, copper tubing, Aluminium tubing and Printed Circuit Board at UHF.
There doesn't seem to be too many hard and fast rules just that the quarter wave stub be a quarter wave at the operating frequency and the radiator be a full half wave long from the end of the stub.
Some say that the open side of the stub needs to be of a smaller diameter tubing but this is not necessary. The spacing of the conductors in the stub is not overly critical either but a good rule of thumb would be around four times the diameter of the tubing that it is constructed from.
One notable experience I had was with creating a chock from the coaxial feed line near the antenna feed point. I found that the SWR was adversely affected by the introduction of a choke of more than a few turns.
Once tuning is complete the feed line can either be screwed or clamped to the feed point that gave best results and some water proofing is recommended around all connections and joints.

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!

Multi Banding a Station Master CB Antenna

The Antenna that I have converted to a multi band vertical is the Station Master.A Station Master is a half wave vertical for 27MHz and can easily be converted to 28MHz (10m) band by simply moving the tap on the matching inductor up one turn.I found that once this was done that the antenna also tuned quite well on 10.1MHz (30m) band with a SWR of around 1.3/1 across the band.I also found that moving the tap to a point close to where the inductor attaches to the radiator the antenna tunes to the 14MHz (20m) band with a SWR of between 1.3/1 and 1.5/1 across the band.Just as a temporary experiment I attached a small relay to allow switching between tapping's.It was noted that the 28MHz tap had to be moved up one more turn to secure a good SWR.The additional capacitance from the relay tunes the antenna to a lower frequency thus the need for the adjustment.Apart from the retuning there seems to be no detrimental affects of placing the relay into the system and it switches between bands without a hiccup.It must be said that the small relay that I have used I would not recommend for high power use and I would recommend using a weather proof unit or weather proofing the unit that is pressed into service.At this point I have not tried to tune the Station Master to any other Amateur Bands.