Antennas side by side: Ground Plane versus Delta Loop

03/05/2012

At the moment I am using two antennas for 27MHz operation: a 1/4 wave Ground Plane and a Delta Loop, and I had the chance to do a little comparison between them. For who’s not familiar with how these two antennas look, with my inexistent Windows paint skills I have quickly illustrated how mine look and how they’re placed:

The one on the left is the Ground Plane and has 5 elements of 268cm each: the vertical one is connected to the central wire of the 50 ohm coaxial feeder cable, the rest (“radials”) are connected to the outer shield of the cable. Typical impedance of a 1/4 wave Ground Plane antenna depends very much on on the angle between the vertical element and the radials, it’s around 25 ohm for 90 degrees and at 180 degrees should be around 70 ohm; i have used around 130-140 degrees wich should bring the impedance to aproximately 50 ohm, but it didn’t as we’ll see later. It is a omnidirectional antenna, has vertical radiation polarisation and it’s takeoff angle is around 20 degrees.

The Delta Loop on the right is basically a full wave loop in a triangle shape, and fed on the right corner as I did makes it’s polarisation inbetween vertical and horizontal, wich shouldn’t matter too much for DX but it will either be better or worse for line of sight contacts depending on what type ¬†of polarisation your partner is using. To obtain vertical polarisation the feedpoint should be at a level where a line parallel to the ground trough that point would split the antenna in two equal perimeters; for horizontal polarisation, a vertical imaginary line trough the feedpoint should split the antenna in 2 equal perimeters as well. The impedance is around 100 ohm for a triangle with equal sides so it shouldn’t adapt perfectly to 50 ohm feedline, but closing one of the angles like I have done should bring it close to 50 ohm; if instead of a triangle we would make a square from the same perimeter, it would have 50 ohm feed impedance. Because it’s balanced it will need a 1:1 BalUn to work correctly with coaxial (unbalanced) cable, and it can be done either by winding a bifilar wire on a properly chosen toroid (i have used Amidon T80-6, enough for about 100W RF power), or by winding 15-20 laps of 50 ohm RG-58U coaxial cable on a PVC 10cm diameter pipe (enough for much more power). Length of the loop should be a full wave loop, but experiments proved that it properly tunes with a loop slightly longer than full wave: in my case, almost 10% more, 11.7 meters. This really depends on the type of wire used to build it, the height above ground level and what other objects are nearby. The takeoff angle is around 25 degrees but it has about 2dB gain over the Ground Plane so in some situations it might perform better; also the closed loop should offer protection against static noise and improve reception in some cases. It is also slightly directional, favoring the direction perpendicular on the triangle’s plane by 3dB.

Enough theory, now to the real life testing on the next page.


How to build a SWR meter

01/10/2009

In case of an imperfect match between the transmitter’s output impedance and the antenna’s impedance (both of them wich should be 50 ohm), some of the power transmitted will not be radiated and will return in the transmitter’s final amplifier stage to overload it, first by overheating and in serious cases even by damaging it.

To check the correct adaptation of the antenna, a device called SWR meter needs to be used; the achronim comes from Standing Wave Ratio, and it will determine the ratio between the transmitted power and the radiated power. In ideal cases this ratio should be 1:1, but in usual cases values of up to 1.5:1 are OK.

A SWR meter is very simple to build, you just need a type of transformer that will pick up the waves circulating in both ways, rectify the picked up voltage and measure it with a voltmeter. The schematic is below:

The transformer is made of 3 copper lines on a PCB, each of them 65mm long. The center line, that carries the power to the antenna, is 8mm wide, and the “secondaries” that pick up the signal for measurement are 4mm wide. The 68 ohm resistors are used to adapt the secondary lines to the 50 ohm impedance of the line and for large power measurements should be able to hold at least 0.5W of thermal dissipation. D1 and D2 are high-frequency rectifing diodes, for large power 1N4148 is OK but for precision and lower powers germanium-junction diodes like EFD 109 are a must.

Here is how this looks, mounted in the case of my old President TOS-01 SWR meter, that had burned-out PCB:

P1110557

I combined more resistors to get a value as close to 68ohm as possible, this is important in getting the correct reflected power; replacing these with 100 ohm will make the SWR meter compatible with 75 ohm feed lines.

“DIR” will be a voltage proportional to the output power’s voltage, and “REF” will be a voltage proportional to the returning power’s voltage ( the voltage is proportional with the square of the power = you double the voltage, the power grows 4 times). Generally you want “REF” to be as close to zero as possible in the frequency band you are usually working, or at least in the middle of it. Mine is almost zero from 27.1Mhz up to 27.7Mhz, but this depends alot on the type of the antenna you are using.