The decibel

dB

A three-element Yagi will outperform a dipole by 6 to 8 dB. That's what the books say, give or take a couple of dB.

When I was new to ham radio, life was simple, gear was simple. I used government surplus radios, folded dipoles, and worked CW. Eventually, I graduated to a 100 watt SSB transceiver. I was still using dipoles. In the early sixties, linear amplifiers were dreamed-of accessories, not the necessity they are today. Most of my local friends used equipment similar to mine. A couple of nearby hams had beams. There was even a linear or two around, but there wouldn't be one at my house for several years.

It was during those early years in ham radio, that I observed a very curious phenomena. It seemed as though stations using beams always got great signal reports and they could hear stations that were in the noise or QRM for me. I knew that a beam was supposed to have around 8 dB gain over my dipole, but it wasn't often that my signal reports were within an S-unit or two of the guys with the beams. In those days, there was simply something mysterious about those collections of aluminum that graced the tops of towers in other ham's back yards. I didn't know exactly what the difference was, but I did know that beams had a big advantage.

This lead me to believe that one of two things was happening. Either signal reports were being inflated, or there was actually more than an 8 dB difference between dipoles and beams.

Figures 1 through 4 give us one explanation for the difference in signal strength between my dipole and my neighbor's beam.

First, let me tell you what these figures represent. Figures 1 and 2 are the azimuth and elevation patterns of a 20 meter dipole 50 feet in the air. Figures 3 and 4 represent a 3 element, 20 meter monobander also 50 feet high. The dimensions for the 3 element monobander were taken directly from the book Yagi Antenna Design by Dr. James L. Lawson, W2PV, page 7-2. This Yagi is based on NBS standards. I made no attempt to modify or optimize antenna dimensions, and as you can see, the front-to-back ratio is poor. For this article, it doesn't make any difference. That said, the beam is obviously designed for maximum gain and not maximum front-to-back ratio. The height of both antennas is .75l or about 50 feet, a common height for amateur radio antennas.

Figure 3 shows that the beam's radiation pattern is concentrated at a low takeoff angle of 17 degrees·. This is an excellent takeoff angle for working DX.

The dipole in figure 1 also has a low takeoff angle of 19 degrees, but this lobe is very narrow. A much larger secondary lobe points straight up into the sky and a lot of energy is wasted warming the clouds.

At all other directions, the difference between the dipole and beam would be between -6 dB and -20 dB, perhaps more.

Referring to figures 1 and 3, the difference in signal strength between the two antennas is only about 6 dB under ideal conditions. Ideal conditions? That would mean that with a dipole oriented east-to- west, the DX station's signal had to arrive at around 20 degrees and its direction must be due North or South. If the DX station was located to the East or West, the difference between the dipole and the beam at a low takeoff angle could be as high as 20 dB as seen in figures 2 and 4. At all other directions, the difference would be between -6 dB and -20 dB, perhaps more. These figures are for a takeoff angle of 20 degrees. Choose a less than optimum takeoff angle for the dipole and the results would be far worse than -20 dB.

So, we have part of the answer. The well defined, low angle radiation pattern of the beam combined with rotatability gives the beam a 'real life situation' advantage well over the 8 dB textbook figure. The only time the predicted 8 dB difference is observed is when the takeoff angles and directions are ideal for the dipole. It's a 'hit-or-miss' situation.

The well defined, low angle radiation pattern of the beam combined with rotatability gives the beam a 'real life situation' advantage well over the 8 dB textbook figure.

There are additional factors. First, in most cases, beams are put up with more care than are wire antennas. Beams usually have the advantage of an obstacle free location. Good quality coax is used. Care is taken installing connectors. It all adds up to a further improvement that could amount to several dB. Sure, on occasion our dipoles snag a needed DX station on the first or second call, but most of the time the difference between the stations with the beams and our dipole is a lot more than 8 dB.

Now suppose you have a dipole and a beam in the same back yard and can switch between the two. Most of the time the beam will outperform the dipole by 6 dB, 10 dB, perhaps even 20 dB. It would all depend on the propagation path (incoming angle and direction) of the DX station. Now, suppose you told someone on the air that you had a beam that has 20 dB gain over a dipole. They would call you a nut!

This leads up to the specifications quoted for some antennas made by the RADIO WORKS. When you see specifications in this catalog that claim 10 dB (observed or reported, in the case of the CAROLINA WINDOM®) what we relating is the performance comparisons observed by us and reported from the antenna's users under typical conditions. Of course, there are conditions when there will be little difference between a reference dipole and RADIO WORKS' antenna, just like in the beam and dipole example, but under most normal operating conditions, the numbers we quote are reliable. There's no smoke and mirrors here, I just take advantage of engineering principles that have been ignored by most wire antenna builders.


There's no smoke and mirrors here, just good engineering principles that have been ignored by wire antenna builders.


RADIO WORKS Antennas are -

1. Engineered, not just handbook copies and low cost reproductions

2. Extensively tested

3. Made with the best parts.

4. Hand built and tested

5. Unique and designed for a specific purpose

6. Unequaled value

7. Perform better than competitive products

8. Recommended by our customers

9. The most copied product line.

© The RADIO WORKS, 1992