The Perfect Layout (My Fervent Wish) For A Vertical Antenna
Having fun with the world using 5/8 wavelength end-fired phased ground-mounted verticals!
By Tom Sowden W6KAN (Formerly K0GKD)
Hams have been building and playing with verticals from the beginning of the hobby, or back to the Marconi era when verticals became his namesake. If one were to do a survey among the thousands of us that have or are using vertical antennas it would be a mixed bag you either love them or not. Those with verticals mounted over salt water are not inclined to consider alternatives they just perform exceptionally well! The recent popularity of the 43 ft. ground mounted vertical has opened a lot of eyes based on their proven performance. I get on the air with my friend (and famous fisherman) Orrin (K9KEJ) from time to time. Orrin's signal, whether on 40M or 20M, always amazes me it is consistently strong! He runs a Pro-III and amplifier into a ground mounted 43 ft. vertical antenna (by Zero-Five Antennas) out of a Chicago suburb. He routinely gets 5/9 reports all over the world. It is just an amazing fact especially considering he has no ground radials. I tease him that his property sits on top of a copper deposit.
What is going on with these 43 ft. verticals that are playing so well for long distant communications? The idea is certainly not new but needs a better understanding as to the reasons behind their DX prowess. Intrigued with the overall idea of verticals higher then the traditional ¼ wavelength I started researching the idea. Everything I read pointed to the fact that as the length starts to exceed ¼ wavelengths the results for long distance communications improve dramatically. This is simply due to the decreasing angle of radiation, and additional gain as the antenna is stretched out. The only problem is the added inductance of the antenna at the feed point. In all of my research on verticals no one seems to have a better handle on the fundamentals then Lee Barrett K7NM. Lee is a former broadcast antenna engineer and has detailed his considerable experience in his book "The C Match Method of Phasing Vertical Antennas"(available from Universal Radio). Lee quantifies this is in his book with the following field strength intensity measured at one mile driven with one kilowatt over perfect ground:
Isotropic Source Over Ground Plane ………… 152.1 mv/meter
¼ Wavelength (90 degree) Vertical .……… …. 194.9 mv/meter
½ Wavelength (180 degree) Vertical …………. 236.5 mv/meter
Based on these measurements the extended antenna (up to a point) has an advantage over a traditional ¼ wavelength vertical. This, of course, bodes well for those looking for DX contacts. This concept was so intriguing to me that I purchased a 5/8-wavelength 20 meter vertical from Zero Five Antennas (wwwzerofive-antennas.com). I particularly liked their model as it had an adjustable network at the base to tune out the added inductance from the lengthier antenna. Tom N9ZV dropped me an email about the matching network:
5/8 Wavelength 20M Vertical Tuned Circuit
The 20M 5/8 wavelength vertical has a parallel tuned network at its base. It matches ultra high impedance to a low one. The shunt capacitor sets the resonance point on the band the vertical is designed for. For example if you slide the cap out it will tune the vertical higher on the band if you increase its overlap buy sliding it in it will tune the vertical lower in the band. The shunt tapped inductor sets the feed point impedance. You can keep it tapped at 50 ohms and slide the cap in and out to move the resonance point up and down the band.
I put the new 5/8-wavelength antenna up with 30 radials for starters and tuned up the base so that the feed point was a near perfect match in the middle of the 20M band. Immediately the antenna started out “on the run”. I was working contacts in the Far East, Australia, New Zealand, and Europe with respectable reports. Why it we "hams" are never satisfied with success. There always has to be something better, and we constantly violate the old adage "if it ain't broke". Several years ago from my then QTH in Kansas City I erected a phased array system consisting of a pair of 40 meter ¼ wavelength mono-band Zero-Five antennas. Setting them 90 degrees apart (1/4 wavelength), adding about 30 radials for each antenna, and firing them with a Comtek phasing system proved out to be a great system. The results were excellent and the antennas were providing the 3 to 4 db directional gain promised in the computer models. When we moved to California I set up a similar system with equally good results despite the rather poor ground conductivity compared to my former QTH in Kansas City.
40M Verticals on the property line ¼ wavelength apart.
Phased arrays (PA) have been around for years but until recently have had few takers in the amateur community. It helps to understand the dynamic that occurs with PA. If done correctly a pair of antennas set up as PA will come close to doubling the transmitted power directional It is not a magic trick. The PA cannot add power but only refocuses the existing power directionally.
When two verticals are lined up in a certain direction they are referred to as "end fired" arrays. A good rule of thumb to remember is the array always fires in the direction of the element with the lagging feed current. By altering the phasing you can basically concentrate the power lobe directionally. Wow, how does this happen? It helps to think about a full wavelength of RF energy a sine wave that travels 360 degrees at warp speed or your frequency. Visualize the waveform as it goes from zero to its most positive point and then back to negative territory as it heads south, and finally back up to zero. At one wavelength it has traveled 360 degrees at over seven million times per second on 40 meters! At the 90-degree point it has advanced one-quarter of a wavelength. OK…so if we put two identical verticals spaced 90 degrees (one quarter wavelength) from each other a receiving signal coming in line toward the direction of antenna "1" will arrive at antenna "2" 90 degrees later due to their physical separation (this assumes that the receiving signal is traveling horizontally, which is never the case as signals arrive at different angles a fact we can ignore for the moment). If we want to null the signal coming toward antenna "1" we can do this by making the feed line to antenna "2" electrically one-quarter wavelength (90 degrees) longer at the feed point. Now the two received signals will be 180 degrees out of phase, 90 degrees due to the physical separation, and an additional 90 degrees stretched out with the quarter wavelength phasing line. In this condition the two more or less cancel each other out. Bingo we can now eliminate possible unwanted signals coming toward antenna one.
Just the reverse affect happens when the receiving signal arrives from the opposite direction, or in line with antenna "2". This time the 90 degree phasing line on antenna two delays the signal to match its arrival on antenna one at the feed points. Now the two received signals are in phase with each other and combine to enhance the result. The same principal works on your transmitted output with a 3 to 4db directional gain resulting from the in phase combining of the output signal! Lee Barrett (K7NM) outlines a few basic requirements for phased arrays (PA) to work effectively. They are as noted:
Since my 40-meter quarter wavelength PA have proven to achieve the considerable gain in the phased mode why not end fire two 5/8 20M wavelength antennas? At this point it seemed a second opinion would be a good idea. In the past I have corresponded with Alan Chrisman who many considered to be one of the great antenna gurus for ham radio. I fired off an email to Allan and he put the concept into motion by running a computer model. His response follows:
Alan's analysis clearly confirmed a marked performance improvement with 5/8-wave length antennas, and significantly better results with two phased versions. This was all of the encouragement that I needed to proceed with my plan for the second 5/8 wavelength 20M vertical. I like both Alan Chrisman and Lee Barrett’s ideas for phasing systems and matching networks. ON4UN's "Low Band Dxing" also has many excellent examples of phased arrays and matching networks for those wanting to "home brew" their own systems. I don't have a lot of time for building antennas or their matching networks so I bought the Comtek 20M phasing box to go with the two Zero-Five 20M verticals. Following the advise of the experts I made sure the feed lines from the Comtek system were "electrically" (not physically) ¼ wavelength using an MFJ antenna analyzer. (If you don’t have an antenna analyzer you can come close to the desired length by multiplying the 90-degree physical length by the velocity factor (available from the coax manufacturer) to get the "electrical length".)
Most of us know that when you position two antennas close to each other they interact in what is termed "mutual conductance". This phenomenon results in a change in resonance for both entities, and has to be tuned out. This is very important in order to reduce standing waves on either feed line that might otherwise alter the phase of the signal at the feed point. With the adjustable capacitors on the Zero-Five antennas this turned out to be rather easy. By going back and forth I was able to reduce the SWR very close on both antennas to the same level, and nearly flat or to a 1:1 ratio at 14.2 Mhz. Adding another set of radials to the new 20M vertical, and installing the Comtek 20M phasing system and feed lines, I was almost ready to go. Lee Barrett recommends that you tie the radial ends together in a big loop to balance out the currents so I followed his advice which I have discovered to be very effective.
Once everything tested out I was ready to go. I set the phasing toward the West and called CQ. Immediately a Hawaiian station called back and reported an S9 plus 30 over signal. I asked him to record the change when I reversed direction. He reported that my signal went down to the noise level and was barely audible. Wow! This is going to be a lot of fun when cycle 24 finally comes out of the bottoms.
20M 5/8 Wave Length Verticals Spaced 1/4 Wavelength Apart
Who says verticals do not work well? Have them give me a call on 20M.