JANUARY 9, 2019

Fine-Tuning the Christman Phasing System

Now the Hard Part

Tuning the array is the last task. 

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From flipping back and forth between NE and SW for a day or so, it has been determined that the F/B is maximized around 7.280 Mhz. Now that the NE vertical has radials and has been re-resonated, I will now walk the F/B curve down in frequency and park it where I want it, which is 7080 Mhz. This will afford excellent F/B in the CW and DX phone sub-bands between 7.2 Mhz and 7.0 Mhz. To do this I have to add a tiny bit of coax into the 71˚ phasing line out at the remote relay box. I am able to do this now because both verticals are exhibiting nearly-identical impedance and reactance sweeps. 

Methodology: Moving the F/B Ratio Curve Up and Down in Frequency by Adjusting the Christman Phasing Line Length

• First we have to calculate how long 71-degrees of RG-8X is at two frequencies: where it is presently maximized and where we want it to be maximized.
• Once we do that, we subtract the length of the shorter coax (71˚ @ 7.280 Mhz) from the longer coax (71˚ @ 7.080 Mhz) to see how much coax to add to the shorter piece to make it as long as the longer piece.
• Then we cut a piece of coax that length and solder PL-259s on each end.
• We then put on the waders, go out into the salt marsh and insert the little patch cable into the phasing line using a barrel connector. 

Calculating Christman 71˚ Phasing Lines

• [300/frequency] x feet/meter x velocity factor of coax  = 1 wavelength of coax at frequency specified.
• Since that length represents 360˚ of the frequency's sinewave, we multiuply this length by how much 71˚ is of 360˚. This is, of course, the fraction: 71/360. 
• So we multiply the 1 WL coax by 71/360 to arrive at how long 71˚ of it is. 

The 7.280 Mhz and 7.080 Mhz calculations are provided below:

[(300 / 7.280) x 3.28 x 0.82 x (71/360)] = 21.859 Feet @ 7.280 Mhz

[(300 / 7.080) x 3.28 x 0.82 x (71/360)] = 22.476 Feet @ 7.080 Mhz

We now subtract the longer coax from the shorter coax to see how much coax to add to the shorter coax to make it as long as the longer coax. 

This will lower the frequency of the F/B ratio curve from 7.28 Mhz to 7.08 Mhz. Or so I think. Thus,

22.476 - 21.859 = 0.619 feet or 7.4"

So the next step is to prepare the little piece of RG-8X coax, and solder a couple PL-259s on the ends. 

Fine-Tuning the Christman Phasing Harness

7" of RG-8X

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Believe it or not, this is all that is called for according to our observations switching the array back and forth for a day or so, as derived by our mathematical calculations. Seven (7) puny inches of coax inserted into the Christman phasing line out in the marsh. At times like this we think we might be crazy to think such a simple thing will walk the perceived F/B down 200 Khz to the design frwquency of 7.1 Mhz. But then we recall the original spark gap operators of a century ago, and the spirit they had for experimentation and for giving things a try based on their own observations. We put on the waders and head out to what several on the air inadvertantly refer to as "The Swamp". Let's see what happens after this simple adjustment. If anything, this constitutes one of the finer points of tuning phased verticals which, hopefully, others will perform. Remember, at this point we have received some fairly good F/B reports.

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Hank, K3YDX and Mike, VE3MEU

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