Beverage Design and Beverage Basics by N4DJ
There is a lot of information around about Beverage antennas. It is not all easy to understand and apply to a practical antenna installation. I thought maybe I could help somewhat with this page. Many of the numbers that are found on various websites do not apply in all cases. There is frequently disagreement on a lot of them, for a multitude of reasons. I will try and be specific as I can, but also include enough information to cover most cases. I have had different degrees of success with beverages myself. I have learned a good bit by trial and error as well as going back to the theory. One of the things that I always like to do with any electronics or antenna problem is to go back to the basics and build up to be sure I did not overlook some detail that would lead me to a wrong answer. Regardless of how hard anyone tries, there will be mistakes. I do not expect to make any, but it will be inevitable that I do.
My interest in Beverages came about because of a love for the Topband, or 160 meters. Early on, it becomes obvious that hearing DX is the real challenge. I had for many years an ideal transmitting location. The salt water would rise and flood the area under my 108 foot vertical or later my Inverted L. I could hear fair. Not well enough. I tried a couple of Beverages. I may never be sure why they did not work as expected. In any event, a move to a new location lead me to try again with the beverage. Still no luck. I now attribute it to possibly my transformer.
In 2012 I tried a BOG or Beverage on the ground. For a variety of reasons it worked. It worked surprisingly well! Now the Beverage had my attention. One difference was I used a different style transformer. Same core but a different style of winding. Next came the new binocular cores. I wound some of them and they worked great. As a result I set off to really investigate the Beverage antenna, especially the Beverage on the Ground or BOG. The Beverage is a directional antenna. Signals build up along the wire as the space wave sweeps down the wire. The induced current or signal moves slower down the wire than does the space wave. The ratio of the speed of the signal in the wire to the space wave is the velocity factor. It seemed to me that the velocity factor was usually guessed at by those constructing the Beverage antenna. In the past I certainly did. Maybe that was part of my problem? There is a point when the current or signal induced on the antenna wire drops so far behind the space wave that the two signals no longer add in phase. That point is when the induced signal is 90 degrees out of phase with the space wave. The length of the wire at this critical point is somewhat significant. It is the length when the induced signal will be the strongest. I will at this point not venture to say that is the length that any particular antenna should be, just that it is the length where the induced signal is the strongest. That length varies depending on the elevation angle. That maximum length is not the same for a signal arriving from a zero degree elevation angle and one arriving from some other angle such as 45 degrees. That length may not be the one for the best front to back ratio. At the moment I will not complicate the issue with any discussion of directivity or any of the ways of measuring such parameters. Many of the antenna books have a good explanation of directivity. In the original documentation of the Beverage, H.H. Beverage gave a formula for the length of the antenna. It was based on a zero degree arrival angle. As in ground wave and not skywave. When the arrival angle of a skywave is considered, due to the geometry of the situation, an additional factor is introduced which is the cosine of the arrival angle. In general this causes the effective length we are considering to be somewhat less the higher the desired angle. Depending on the arrival angle you are interested in, you might want to choose your particular length to suit. If you want to work DX you want your antenna to have a better response to lower angles, and thus it needs to be closer to the "maximum length". My primary focus will be on working DX, so if there are any choices to be made, I will tend to make the choice that favors working DX. There are maximum lengths for various velocity factors and angles of arrival. Note that these are for maximum signal strength, not front to back, directivity or any particular null placement. There are other parameters to investigate, such as placing the nulls in a specific location to reduce a particular noise or QRM problem. In my particular case (in 2012) I had a BOG about 225 feet long. That is the maximum length for a BOG to receive best at an elevation angle of 20 degrees and assuming a velocity factor of 65%. If the velocity factor is not as low as 65% then the antenna will have a best response at a higher elevation angle. One good thing is that the Beverage seems to have a rather broad response, both in azimuth and in elevation. (Note: the velocity factor of this particular antenna has subsequently been determined to be 75% and not 65 %) At 160 meters, data seems to indicate that the velocity factor is in the neighborhood of 90% for an antenna about 9 or 10 feet above the ground. Maybe between 85 and 90% for a wire 2 meters high. As a check on the above numbers, I compared to the charts in the ON4UN Low band DXing Book. He shows plots for Beverages 2 meters high on 160 meters. Note that the one for a length of 353 meters (1158 feet) has a peak response of very close to 20 degrees. The calculations above indicate that you can expect a peak at 20 degrees if the length is 1194 feet and the velocity factor is 95%. These numbers are pretty close considering that we do not know the ground under the antennas in the plots, nor the exact velocity factor. I certainly think both the calculations and the charts in the ON4UN book are not that far off. The calculations are also based on formulas that are well established in both amateur publications and in university text books. Besides that, I think they pass the " logical thinking" test. Think about a space wave with a wavelength of 540 feet, sweeping along a wire at 100% velocity inducing a current in that wire that travels only 80% as fast. By the time the induced signal reaches the end of the 540 foot wire, how far ahead of the induced signal will it be? The space wave has traveled 675 feet. 80% of 675 is 540. The distance that the space wave is ahead of the induced signal is 675 - 540 = 135 feet. Remembering that 360 electrical degrees is 1 wave or 540 feet, the 135 feet is 1/4 of 540 feet which is 90 electrical degrees. So by logical reasoning we came up with 540 feet as the length for maximum signal if the velocity factor was 80%. Looking at the chart above, we see that the formula gave us 540 feet if the velocity factor was 80% and the arrival angle was zero degrees. It takes a little more effort to visualize the answer when the arrival angle is other than zero. You have to visualize the angle the arriving signal makes with the wire. Because the wave is at an angle the speed of the wave front sweeping the wire moves faster than when it is perpendicular to the wire. Thus the space wave completes the sweep before the induced currents have had time to travel as far. The 90 degree phase shift point comes in a shorter length of wire. That is exactly what the charts show. The literature (and experience) tells us that the closer the Beverage wire is to the ground the lower the velocity factor. In a transmission line the velocity factor is determined to a great extent by what is between the conductors. Open wire line with only air as the dielectric, has one of the highest velocity factors. The velocity factor is related to the dielectric constant as the inverse of the square root. For example, the dielectric constant of free space is 1and the VF in free space is 1 or 100%. If the dielectric constant increased by a factor of 4 then the Velocity factor would decrease by a factor of 2. The Beverage looks a lot like a two wire transmission line, with the ground being the second wire. The dielectric is composed of both air and the ground as well as whatever is in the ground. That could be conductive material as well as non-conductive material. It could change over a range of just a few feet, as well a vary with moisture content. For a Beverage 6 to 10 feet high, the velocity factor seems to level off in the range of 85 to 90 %. Below 4 or 5 feet the velocity factor usually falls off at a much more rapid rate. The ground characteristics becoming the determining factor. For a high Beverage a larger percent of the dielectric is air. For a BOG most of the dielectric becomes whatever is below the antenna, such as dirt, leaves, grass, etc. The surge impedance of the beverage also varies to a great extent on the height above ground. As it gets closer to the ground, the composition of the ground tends to become more and more important. I think an ideal height is just above the ground a few inches but that is not ususlly convenient. Lots of beverages are installed 6 to 8 feet above the ground. In this range the variation of impedance does not change very rapidly with small changes in height. A range of from 400 to 600 ohms is common for a Beverage made of #14 wire. Smaller wire tending to be have slightly higher impedance and higher antennas having slightly higher impedance also, all other things being equal. The surge impedance also shows a tendancy to be higher over poor ground, than good ground. In general the terminating resistor should be close to the surge impedance of the Beverage. This can be determined with an SWR sweep of the antenna with an instrument such as the MFJ-259B. We are looking for a rather constant low SWR. The variables here are the ratio of the impedance transformer and the terminating resistance. Naturally a good ground rod should be installed at both ends. The quality of the ground will have a significant effect on the value of the terminating resistor. The ground resistance appearing in series with the terminating resistor. The better the ground, the closer the terminating resistor will be to the surge impedance. I do not think it necessary to get the absolute best ground posssible as you would need to do for a transmitting vertical. I am using 5 foot ground rods at the present time and am very happy with the improvement the BOG is affording me over any of my other antennas. This is not to say things could not be improved. I just think my efforts are better directed on other issues at the present time, such another similar BOG for another direction. With the BOG or Beverage on the Ground, the velocity factor is quite low. In many cases it has been measured, modeled and estimated to be in the range of 50 to 60%. The impedance has been determined to be usually between 200 and 300 Ohms. It could be even lower depending on the actual ground conditions. My test results have indicated that the velocity factor on my BOG is lower than I had first thought. More testing needs to be done on my own antenna. If the velocity factor is indeed in the 50 to 60% range for a particular location, that means that significantly less space in required to have a BOG than a conventional Beverage. Without any other effort, it seems that it would be hard to be too far off with a BOG only 200 feet long, fed with anything from a 4:1impedance transformer to a 9:1 transformer and terminated with 160 to 300 ohms as a starting point. Variation in the Velocity factor would simply change the optimum angle.The antenna should work. For example, if you designed for a Velocity factor of 60% and put down a 198 foot wire expecting an optimum angle of 10 degree, and the velocity factor was really 55% what would the result be? Looking at the chart above, we would see the wire was a little long for optimum. At the desired 10 degree angle we would not have the absolute max signal. We probably could not even tell that by listening! It would not have any significant effect on out pattern. The nulls may change in elevation and azimuth, but we were not really designing with them in mind at this point anyway. If the Velocity factor was higher than our design assumption of 60%, what would that effect be? The wire would be a little short. That would raise the peak angle from 10 to maybe 19 degrees. Again, I am not sure if you could even tell that on the air. Nulls would again change in azimuth and elevation. It would be hard to tell in most cases. A good thing to do is to try and measure the velocity factor of your Beverage or BOG on the actual ground where it is to be used. There are several ways to do that. Some easy and some not so easy. I think that any of them will get you close enough. Measuring Velocity Factor with a Dipole on the Ground My quick method of determining the velocity factor was to use an existing 40 meter dipole and my MFJ 259B antenna analyzer. I took a 40 meter dipole that was 65 feet 4 inches long. I will use the number 65.33 in my calculations. This dipole is normally resonant somewhere in the 40 meter bandwhen up in the air. By putting it on the ground just like my Beverage on the Ground (BOG ) the resonant frequency will go down. Note: do not try and tune a dipole on the ground, because when you raise it to a normal height it will be resonant at a much higher frequency. By measuring the resonant frequency of the dipole on the ground with my MFJ 259B analyzer I read about 5.7 MHz. I moved the dipole around and took several measurements. In the woods area where I have my NE BOG, I got readings all very close to 5.7 MHz. To calculate or estimate the velocity factor I determined the free space wavelength of a half wavelength at 5.6 MHz to be 86.43 feet. (491.8 / 5.69 = 86.43 feet) Since the length of the wire was 65.33 feet, the ratio of 65.33 to 86.43 is the velocity factor. 65.33/86.43=.756 So the velocity factor is .756 or 75.6%. It would have been good to have the dipole a full 220 feet so I would only need to take one measurement. This would have meant the antenna analyzer would have to measure a reasonant frequency in the neighborhood of 1.2 MHz. My MFJ will not read that low. The best that can be done with the MFJ is to have a dipole about 204 feet long. In that case with a 75% velocity factor you would measure a resonant frequency of about 1.808 MHz. The calculation than would be something like 491.8/1.808= 272 then 204/272=.75 I think this method will give you a good idea of the velocity factor you have at your specific location. One good thing is the Beverage lengths are not all that critical. As additional information I am adding a chart of the measurments I made on my 40 meter dipole at various other locations and will update the chart as I take more measurements. Based on a measured velocity factor that varies from 76 to 77 percent for my NE BOG. I now can be pretty sure my 220 foot BOG has a maximum response to signals arriving from a 40 to 45 degree angle. Maybe it could be a little longer to hear DX better. However, we must not forget that simple gain in the desired direction is not the goal of the Beverage. Eliminating noise and unwanted signals from all but the desired direction is the real objective. We need to look at the complete picture. Directivity is the most important parameter, not pure gain. It is true that, in general, the longer wire will receive a stronger signal and will have better directivity. In my particular case, with an average Velocity factor of 76.5 and designing for a 10 degree angle, that length is 418 feet.
In the 2013 CQ 160 contest the beverages made a big difference. Switching to the right beverage meant the difference in making a contact or not. I owe most of my European QSOs to the NE Beverage. There were many stations that called me that I would never have heard on the transmitting antenna. They only could be copied on the NE BOG. Several times as I switched form NE to W or S I heard stations calling me only on one of the Beverages. If a station was weak, I would switch the beverages and it would make a 2 or 3 S unit difference on my K3 meter.
Heard real well in the 2013 CQ 160 Phone contest. Many times a W1, 2 or 3 would call and I had trouble getting their call listening on the NE Beverage. Turns out they were in Florida and switching to the south Beverage brought them up to Q5. Worked 29 DX stations on SSB in 19 countries. I worked 530 QSOs overall in the contest. All of my receiving was done with the three Beverages.
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