Antenna things

Antennas

Yagi antennas

When you read the data about that antenna you have just bought, how much of it can you believe?

An HF Yagi antenna usually quoted as giving 7 or 8 dB gain is probably meaningless. Firstly the gain must be quoted against a known reference source. This is usually a dipole but is sometimes an ‘Isotropic’ source. (Just a big word for a point source.) Now a dipole has a gain of 3dB over an isotropic source, so take off 3 dB to start with. Let’s look at an example.

A ‘Supadinga 3’ has an advert stating an 8.5 dB gain. OK, first there is no mention of the reference. We can safely assume that it means they don’t want to say that the reference is an isotropic source. Which means that the actual gain is more likely to be in the order of 5.5 dB over a dipole. This is more like it. Although the theoretical gain of a three element mono band Yagi is in the order of 7.5 to 8.5 dBd (the d means referenced to a dipole) in practice, 6dBd is the best you would get in actual home ideal conditions. Remember we are talking full size MONO band antennas, not tri-banders or other shortened compromise antennas. These will be much lower. There is no substitute for “metal in the air” as my mentor used to say.

Adding extra elements to the 3 element will only give an extra 1dB of gain. To get more gain it is better to ‘stack’ one antenna above another of similar construction. This will give in the order of an extra 2 to 3 dB gain, totalling up to 8 dBd.

Yagi antennas should be tuned for maximum forward gain rather than best front to back ratio.

Gains

A power gain of 3 dB is like doubling your power output. A 6dB-power gain is like increasing your power output by four times. This means either 100watts into a 3 element monobander or 400watts into a dipole. Your choice. An increase in power from 100 watts to 400 watts gives an increase of only one S point at the receiving station but the extra gain on receive will give you and edge of about an S point over a dipole.

Yagi  versa Quad

 A two-element Quad antenna will have a practical gain of 6 dBd, similar to a three element Yagi. The quad works as two phased two element antennas. This is where the gain comes from.

The parasitic element should be tuned as a reflector, and set to give maximum forward gain.

Height of wire antennas

There is no optimum height for a horizontally polarized antenna. The more height the better. However, for optimum local working, use can be made of NVIS (near vertical incident sky) radiation. This can be used to good effect during ‘local’ contests like the NZ field day Jock White contest, where on 80 metres particularly, it is hoped to work the whole of ZL from Auckland to Invercargill, without a ‘skip zone’. Similarly, for AREC and rally radio working.

By mounting an 80m dipole at under an eighth wavelength, 10m (30 feet) the radiation pattern changes from the usual, to a vertical/ 30 degree angle of radiation. The SWR will rise due to the feed point impedance rising. This gives a good NVIS radiator and good local coverage without gaps. The mismatch is tuned out with an ATU. The radiation pattern will be Omni-directional. (All directions)

Unless an 80 mtrs wire dipole is above 20 metres (60 feet) in height, it will behave along the lines of the above NVIS radiator, to some degree or other. That being the case, there seems little point in attempting to get the ends up at the same height as the centre. Consequently, the ends may be dropped down to form what is commonly known as an ‘inverted V ’, although that name refers to something else.

NVIS only works for frequencies below the vertical incident critical frequency. IE the lowest frequency that will not pass through the ionosphere but will be reflected back. The best ones are 80 and 160 metres, but 40 can be used during the day sometimes.

Feed point impedance

For Yagi antennas, the feed point of the driven element changes from a dipole at 75 ohms downwards as additional elements are added. This is why often VHF beams of more than 4 elements use a folded driven element. A folded dipole impedance is 300 ohms, and the addition of the other elements brings this down closer to 50-75 ohms.

For wire dipoles, the theoretical impedance of the feed point is 75 ohms. However, if the angle at the centre is changed from a straight 180 degrees, as happens in practice, the impedance falls. At about 120 degrees the impedance is close to 50 ohms. This assumes a correct height of course. So by drooping the dipole ends down in an inverted V configuration, a better match can be made to the rig.

Multiple band wire dipoles

An 80 metre wire dipole can have a 40 metre dipole connected to the same feed point without affecting the 80 metre antenna. This is because at 40 metres the 80 antenna impedance is so high compared with the proper 40 metre feed point that almost all the power will go into the correct antenna. The antennas can be set at 90 degrees to one another or hung off each other with spacers of about 300 mm.

Think about these things.

Mike ZL2CC