Short antenna: director is better

[Enthalpy]

Hello, radiocomm tinkerers and everyone!

The Uda-Yagi antenna is the rake you see on roofs to receive television programmes:
http://en.wikipedia.org/wiki/Yagi-Uda_antenna

High-gain versions have over 20 elements, but sometimes the driven element plus a single reflector or director suffice:

I see a reflector almost everytime two elements are used. Though, I believe a director instead has only advantages:

• The radiation resistance drops less;
• The gain is marginally better;
• The backward attenuation improves slightly;
• The passband is wider;
• The elements' width and spacing are less critical;
• The antenna is shorter.

I've built antennas of both types, but not for the same use in order to compare them. The comparison stems from experimental data in hobbyist books. Has someone a different experience? If not, I'll consider that the usual practice can improve a little bit.

Marc Schaefer, aka Enthalpy

[doG]

NASA used such an antenna as the feed for the Arecibo spherical reflector in Puerto Rico in the search for primeval hydrogen. There's a white paper on its design here.

[Externet]

Years ago, I was bothered by television yagi reception antennas and their impedance values along the spectrum from 54MHz to 806MHz, a sustantial frequency span. Because impedance is tightly tied to a frequency. Because impedance is a function of frequency.

Industry standards coaxial feed lines where mainly chosen at 75 ohms, suposedly to match the antenna and tuner impedances.

But such had to be an arbitrary figure. No way an antenna could present such 75 ohm impedance in all the spectrum. A coat hanger can perform equally well.

 

Newer technologies, with ATSC tuners, have carried the same mistake, telling television owners to use 75 ohm feed lines. Only a few TV tuner manufacturers data sheets leave the input impedance value blank as I believe it should be.

 

Modern tuners operate with field effect transistors front ends, presenting very high values of impedance, presenting little load to faint signals; a good thing.

 

So the radiation resistance, gain, back attenuation, bandwidth, are not the only factors to determine the performance of a Yagi antenna into a receiver. Its impedance counts heavily to be able to transfer its energy into the receiver.

 

For fixed and single frequency operation, an antenna can be tailored for optimization in their gain, impedance, bandwidth, parameters. For television reception, no way. But marketing claims go beyond reality and technical limitations with no sense.

 

Try to find a plotted impedance versus frequency on a Yagi antenna, television tuner. Not easy.

Here is one : http://s588.photobucket.com/user/Innernet/media/Screenshot-TVantennapdf.png.html?sort=6&o=41

 

So a Yagi with director instead of reflector may, or may not be better. Too much depends on its impedance, of the feed line, and of the tuner also.

[Enthalpy]

75 ohm give the minimum loss for a coaxial cable with full polyethylene dielectric. The same "e" optimum ratio of conductor diameters gives 93 ohm for coaxial cables containing little dielectric. This impedance was not chosen to match any particular antenna, and without proper adapters generally it doesn't.

 

Fet preamplifiers have an input matching network, but their best match for signal-to-noise ratio does not match source and load, which only gives the strongest signal but not the minimum noise. Bipolar preamplifiers are a bit mismatched, Fet more so. A different cable impedance wouldn't improve: the optimum matching network would always give the same mismatch ratio, because this mismatch equals the mismatch between the best impedance seen by the Fet for signal-to-noise and the input impedance of the Fet.

 

The power lost at mismatch if using 50 ohm instead of 75 ohm is miniscule, and certainly not competing different losses per unit length in the cable.

 

Yes, the feed impedance of an antenna varies a lot over a wide frequency band - except for very special designs like logperiodic, but these are bad in every other aspect. An Uda-Yagi performs well over one octave, say 470-860MHz, which is already quite good and unexpected if one considers it as a group of resonating dipoles. Obviously, the operation of an Uda-Yagi needs a different explanation compatible with its wide band, and a guided wave is a better explanation.

 

Over a reasonable bandwidth, a director gives a more constant radiation impedance than a reflector, and also a bigger one that is easier to match with the cable.