JANUARY 4, 2019

Tuning Phased Verticals

Waited for low tide at noon before venturing out to tune the two verticals. The process is to disconnect the verticals from their feedlines and attach the MFJ analyzer to the base of one of the verticals. Walk the resonance (lowest reactance) to the design frequency by adjusting the length of the vertical. When done, move over to the other vertical and do the same thing. Today, here in the salt marsh, we hope there is enough ground conductivity to allow each vertical to exhibit a feedpoint impedance (at resonance) close to 36 Ohms.

When we measured the experimental vertical (hereafter called the "Southwest Vertical"), it was resonating at 7.3 Mhz -- 200 Khz above our design frequency. This means we have to take it down and lengthen it by about 6". After completing this task, and reinstalling the vertical, the MFJ analyzer showed the following readings.

Southwest Vertical: 37 Ohms

Resonating on 7.1 Mhz with a 37 Ohm feedpoint impedance and 7 Ohms reactance.

Purfect!

***

The Southwest vertical ends up resonating at the 7.1 Mhz design frequency with a feedpoint impedance of 37 Ohms and 7 Ohms reactive. This is outstanding since the theoretically-ideal impedance is 36 Ohms. I can bring out the reactance by dismantling the vertical, cleaning its junctions and put them back together with dielectric grease (Editor: This is exactly what happens when the vertical junctions are cleaned a few weeks later. See the February 18 update for details). So we now off in the muck over to the Northeast vertical to see what's up with it. Remember, the Southwest vertical remains unattached to its feedline to render it invisible to the Northeast vertical.

Northeast Vertical: 41 Ohms

Resonating on 7.2 MHz with a 41 Ohm feedpoint and 6 Ohms reactance.

***

With only 3 ground rods, the Northeast vertical resonates at 7.195 Mhz with a feedpoint impedance of 41 Ohms and 6 Ohms reactive. Not bad! The resonant frequency needs to be brought down 100 Khz to the 7.1 Mhz design frequency. This would entail shortening the vertical radiator if it was not for the fact that the addition of its ground raidals will likely change the resonant frequency. So we hold off and leave the vertical as is. Obtaining 41 Ohm feedpoint impedance with only three 2.5' ground rods testifies to the conductivity and permitivity of the salt marsh, and is close to the 37 Ohms exhibited by the other (Southwest) vertical. We're psyched as we slosh through the muck back to the shack to check out what the feedline reads when the array is run through its three phasing states.

Wet Boots in the Shack

Checking the Feedline Numbers

Removing our waders at the sliding glass door, we attach the MFJ analyzer to the coax feedline and measure the three phasing states: Northeast, Southwest and Broadside (Omni). We should take these readings at the remote phasing relay box in the salt marsh but the tide is rising too fast. We also note that losses incurred in lengthy coaxial runs mitigate high SWR readings, making things look a lot better in the shack than they are at the feedpoint. We hope for as little change as possible in the MFJ analyzer readings as we toggle between NE and SW positions because this indicates the system is balanced, e.g. the two verticals exhibit identical impedance and reactance sweeps across the 40 Meter band.

Northeast: 37 Ohms

7.1 Mhz design frequency resonance • 37 Ohms feedpoint impedance • 5 Ohms reactance.

***

In the shack, the system reasonates at the design frequencey exhibiting a 37 Ohm feedpoint impedance with 5 Ohms reactance. This will likely change after deployment of ground radials under the Northeast vertical as discussed. Obviously we are pleased. Now, the moment of truth. What are the readings after we flip the system to the Southwest? Drum roll, please...

Southwest: 41 Ohms

7.1 Mhz design frequency resonance • 41 Ohms feedpoint impedance • 3 Ohms reactance.

***

We are pleased to see that the numbers show 41 Ohms with 3 Ohms reactance. They, too, will change after we install radials under the NE vertical and tune it to the 7.1 Mhz design frequency. We then check the third, "Broadsude" position achieved when the phasing line is shortd causing the verticals to in-phase. This condition always exhibits a problematic impedance in the Christman phasing system. 

Broadside (Omni): 41 Ohms

7.1 Mhz design frequency resonance • 41 Ohms feedpoint impedance • 20 Ohms reactance.

***

NOTE: These readings, however encouraging, are taken from the shack. True system readings are obtained at the remote relay switching box in the field. Once we install the ground radials under the Northeast vertical, we will remeasure all readings.