from Bob Raynor, N4JTE on April 6, 2012
Once your antenna experiments move beyond the basic dipole and the plethora of its variations you’re probably thinking about how to get some real, not theoretical, gain in your direction of choice and maybe knock out some of those stations and noise behind you while chasing a weak DX or stateside contact. What follows is my attempt to convince you to try an easy guaranteed method of phasing with 2 driven wire elements and achieve double your ERP with 24 db of front to back.
Commercial aluminum and various wire beams are the mainstay of most beam systems and do an admirable job. However it has been my experience when dealing with two or three element wire beams that there can, and will be, a lot of hit and miss with the element spacing and lengths if you want optimum results. For example, the old nugget about making the director 5% shorter and the reflector 5% longer than the driven element can be a misleading starting point if you want more than a mediocre beam.
Having built too many to count and spending many hours/years tuning, pruning etc. I can tell you at this point that the mutual impedance, height, ground conditions etc. are going to change that 5% and in some cases the best parasitic beam on the air, as opposed to modeling, will end up with equal physical lengths on some of the wires of a three element array! As can be seen in the plot, the front to back max on the commercial beam design is at a very narrow bandwidth to allow the max gain of 8.61 dbi; which is at a different frequency then the max front to back, a compromise not found when using a phased driven design.
The diagram above is the best case scenario for a commercial 3 element aluminum beam at a 1/2 wl high. Food for thought.
Of course you can build a dipole wire beam on 40 using the tried and true 62,65,68 length and it will work relatively well, but you might be missing out on some valuable gain and front to back without realizing the shortcomings.
Sure it’s going to outperform a dipole and the model says it’s should work like gangbusters in theory, but that always changes in my backyard. And you will be extremely lucky if you get close to the predicted results in the illustration even with the narrow band limitations!
Phased Driven Arrays:
I stayed away from phasing lines for a long time because I figured it was beyond my abilities and the commercial hybrid systems are very high priced so I never really explored the concept to fairly recently with verticals and dipoles. John, ON4UN opened my eyes to the simplicity of doing it correctly and saving $500!
There are some driven single wire phased designs which are limited to bidirectional and obviously no front to back, gigantic disadvantage at least at my qth on the East Coast.
We’ve all seen that nifty page in the ARRL antenna book illustrating the gain and patterns for two element verticals based on phase angles and spacing, btw, it all applies to horizontal wires also!
Seems like a simple matter to just cut a couple of 135 or 90 degree lines put a tee connector in and get all that gain and direction, well getting the phase lines accurate is easy. You don't need an analyzer, just a radio, dummy load and an external accurate swr meter and you’re good to go. BTW, using velocity factor to cut degree lengths is pointless.
Unfortunately you can’t just hookup the two phased accurately cut feedlines to your verticals or dipoles and expect any kind of the gain or pattern you read in the antenna books. Why not? you might ask, and the reason is simple in explanation not so easy in implementation.
To fully benefit from phasing driven elements the current/ voltage must be the same at both feedpoints to the elements, easier said than done.
By using the impedance transforming characteristics of 90 degree phaselines and element spacing, you can essentially force feed the equal correct current/ voltage to each feedpoint, no need for expensive hybrid circuits etc. ON4UN’s lowband dx book specifies for a 90 degree phase shift the equal voltage points end up being at 84 degrees. http://www.va7st.ca/home.html/2009/11/christman-phasing-calculator/ VA7ST has a great calculator for calculating phase line length for any frequency or band, saves a lot of work and a great antenna page also. This approach is perfect for a 90 degree carderoid pattern and the resulting gain when you have a 90 degree lagging feedline. The actual use of 84 degrees was determined by ON4UN’s feedline calculator. A simple relay can be incorporated to switch directions instantly by adding a 71 degree delay line. And that’s when 20 to 30 db of front to back shows up like a light switch, always astounding to me!
The amazing part was how well all of this vertical stuff transfers to phasing horizontal dipoles, the unknowns of ground radial systems and inherent losses are diminished by getting things up in the air. By sticking to the Christman and quadrature 90 degree feeding and spacing all the uncertainties are gone in this type of phased driven array. Now you know for sure that the pattern and gain are actually what the ARRL book and modeling programs illustrate!
Driven phased 2 element wire dipoles, correctly phased, will give you at least 25 db of front to back over a wide bandwidth, in my case on 40 meters from 7135 to 7210, while maintaining a true 3db of gain over single dipole. Going back to the plot above you can see that you get better bandwidth and the same gain as a three element beam using a phased 2 element design.
And of course you can make it instantly reversible from the shack. I have been using this design on 40 and 80 meters for awhile and because it works so well it has been the longest experiment in the air I have ever built and I am REAL picky.
I hope you take the time to learn and implement driven inexpensive phaselines into your next experiment, you will not be disappointed.
Tnx for reading,