[TowerTalk] Christman Phasing help!

[Gary Johnson]

Rick brings up some important topics related to phasing networks and the design of this particular 2-element array. BTW Rick, your 7-Hex design is a thing of beauty :-)

It was designed for 40 CW during CQP, was manned for years by the skilled hands of Mike, WA6O (SK), and he was very satisfied with its performance. The primary goal was to have a deep null off the back to reject in-band SSB coming from the nearby 4/4 stack. So the array spacing etc. were optimized for F/B more than gain. 

I’ve purchased and deployed thousands of feet of coax over the years and every bit is checked for Vf and loss. I’ve never had a piece of Bury Flex outside of .80-.84 Vf. But I did have a 1000’ roll of similar DXE-MAX that was at 0.77… Anyway, phasing lines must ALWAYS be individually measured and trimmed for exact phase shift. 

As for adjustment of the relative phase, indeed LC networks are the way to go. Direct observation of current at the antenna feedpoints is the last word, with no inferences required, so that’s why I went with current transformers and a VNA for this measurement. It is very important to isolate any instrumentation from the antenna currents. A CM choke is placed behind the current transformer for decoupling. Then the VNA and measuring cable are not a part of the antenna system to any substantial degree. 

Capacitors are always an interesting topic for QRO operation. The choice of affordable and very robust Russian K15Y doorknobs was based on advice form Pavel, VA6AM, who pointed out that all his QRO bandpass filters (and those from 403A) use those caps extensively. Those networks, being high-order, have much higher sensitivity factors (partial derivative of H(s) w.r.t. C) than this simple phase shift network, making their fairly high tempco even less of a concern here. Just for fun, I simulated the change in phase shift for a 10% change in C, which would be 66 degC for an N1500 cap. It was about 3 degrees. My wobbly verticals and galloping radials are surely worse than that.

This being a learning exercise as well, I took great care to study and measure every aspect. Determining initial L and C values is not trivial and the measure-simulate-adjust loop is enlightening and successful. Anyone with the time and interest in an endeavor like this will find it very rewarding.

73,
Gary NA6O

> 
> From: "Richard (Rick) Karlquist" <richard at karlquist.com <mailto:richard at karlquist.com>>
> 
> Great article Gary.  It wasn't clear whether the purpose of this was 
> just a receive antenna or if it was also used for transmit.  In any 
> event, my SOP when I do want a transmit array, is to first build a 
> receive array in order to make it easy to tune up using small receive 
> type components.  Then the receive components can replaced by QRO 
> components of the same value.
> 
> Figure 7 tells the essential story.  This is exactly what I do.  Quarter 
> electrical wave drive lines.  I see that the bury flex lines appear to 
> be based on a velocity factor of 82%.  I know that is what the vendor 
> claims, but I bought a batch of bury flex and it had a VF in the low 
> 70's.  I don't know if you measured it or just assumed 82%.  If it 
> measured 82% then ... you got lucky.  There is an LC delay network 
> inserted in front of just one of the verticals.  The beauty of this 
> paradigm is that you can dispense with EZNEC, and DG8SAQ, etc. and 
> simply experimentally adjust L and C until the VOLTAGES at the drive 
> ends of the coax are 90 deg out of phase and of the same magnitude. 
> Then the CURRENTS at the antennas will be likewise.  The phase and 
> magnitude can be determined by the "3 voltmeter" method described in the 
> article I previously cited in this thread.  Then there is an L C 
> MATCHING network to transform whatever the drive impedance is to 50 
> ohms.  Again, this can be tuned up experimentally with just an SWR 
> meter.  The impedance matching adjustment doesn't affect the 
> phasing/pattern.
> 
> A couple of additional comments.  You say that is it OK to use N1500 
> capacitors because the tempco doesn't matter.  True, but what does 
> matter is the dissipation factor for the high drift dielectric, which is 
> something like 2%.  Better to use NP0/C0G ceramics or else mica.
> 
> You also say that the verticals have "plenty" of high angle radiation to 
> work close in stations.  Every vertical I have built was completely deaf 
> to stations within a few hundred miles.  If yours are not as such, then 
> I would check for feedline radiation, etc to explain what you are seeing.
> 
> BTW, I normally use 1/8 wave spacing between a pair of verticals.  You 
> pick up a dB or so additional gain that way.  It is easy to retune the 
> LC phasing network to work with the reduced spacing.
> 
> Keep up the great work at N6RO.
> 
> 73
> Rick N6RK