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Friday, August 1, 2008

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.

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