Ve6fi Amateur Radio
80 Meter Antennas
I built a phased array of four inline verticals for 80 meters. This is sometimes referred to a directional end fire array.
The verticals are almost 0.25 wavelengths high and are 0.25 wavelengths apart. Each self-supporting tower (vertical) has 48 radials, each 0.25 wavelengths long in a circular pattern around the base.
I modeled the array on the antenna modeling software called ‘Eznec’. Modeling allows one to see the antenna patterns one can obtain with different heights, spacing and radials. It is a lot easier to move elements in a model than it is to try moving them in the field.
This vertical phased array has a gain of 9.2 db dipole, a theoretical f/b of 39 (actual to date is 27db) and a take off angle of 18 degrees. The beamwidth in the direction aimed is 100 degrees. This vertical array is located approximately 600 feet from the radio room and is fed with 0.75 foam cable.
Matching - Matching these antenna means that you have to match the towers so that you have the correct amount of power going to the respective tower, and have the correct magnitude and phase of current into the base of each vertical. Instead of using lengths of coaxial cable for phasing and matching, I used L C components. I utilize the articles in Ham Radio magazine 1983 by K2BT and on line help from Greg W8WWV.
Some of my Learning’s:
One does not make the verticals exactly 0.25 electrical wavelengths long. You make them slightly shorter so that you can make the reactive portion equal to zero at the operating frequency. A quarter wave vertical has a theoretical impedance of 36 + j20 ohms and at resonance you want something theoretical like 36 + j0. In effect you make the vertical about 5 % shorter. This percentage depends on your average diameter of your vertical. W2PV discusses this in his book ‘Yagi Antenna Design’
The magnitude of currents into the base of the verticals follows the binomial theorem, which would be the ratio of 1 amp each if you have two verticals, 1,2,1 ratio if you have three verticals and 1,3,3,1 ratio if you have 4 verticals as in my case.
When verticals are 0.25 wavelengths apart one would think that you would phase them 90 degrees apart such as 0, -90, -180 and –270. I have found that the theoretical optimum for maximum gain and best front to back ratio is 0, -116, -232 and –348 degrees.
You cannot measure the driving impedance of a vertical – you have to calculate it. You can measure the self-impedance of a tower but make sure all the other towers are above ground when you do the measurement. Having another tower connected will throw off you measurements because of the mutual coupling. YOU HAVE to calculate the driving point impedance, which is the impedance your transmitter will see. The driving point impedance takes into account the currents in all other verticals.
Verticals are a low maintenance antenna and can be arrange to give you gain at low take off angles in different directions. The networks are in place and the remote switching is completed and now this 80 meter array is in use at VE6fi.
For more details on the construction and calculations on this array go to the 80 Meter Vertical Array Details