Wednesday, December 27, 2017

XM240 Update

Although I purchased my Cushcraft XM240 in 2015 it was only in use for the first half of 2016 when it was at a height of 21 meters. The antenna had been strengthened per (most of) the recommendations by Dave Leeson W6NL. It did well on 40 meters but with several negatives:
  • The cheap muffler clamps used on the aluminum channel element-to-boom clamp in lieu of the stock Cushcraft clamps did not work out. They could not prevent rotation of the elements on the boom in high winds. The thin steel saddles had little grip and if tightened further would only damage the boom.
  • The same was true of the muffler clamps used for the boom-to-mast clamp. At least boom rotation could be corrected by climbing the tower, which cannot be done for the element clamps.
  • Although tuned per the instructions (interpolated between the two lower settings) for the lower part of the band the antenna resonated ~7.125 MHz rather than the estimated 7.075 MHz. At 7 MHz the SWR rose to 3 requiring the use of a tuner, the risk of loss over long coax runs and undesirable extra steps for band changes and frequency excursions in the heat of a contest.
I set myself the goal of correcting these problems before raising it onto the new 150' tower. I was partly successful. With more time I could have done better, but in the larger plan this antenna's quirks were not highest priority. It was enough to get it working reasonably well even if not to the utmost possible for a small and compromised yagi such as this.

Boom-to-mast clamp

Please refer to the adjacent picture for the this and the next sections. This is a larger version of one published in an earlier article.

The previous owner's muffler clamps -- 2 for the boom and 4 for the mast -- were replaced. For the boom I substituted the two Cushcraft stock saddle clamps, originally intended to attach the stock boom clamp to a mast of up to 2.5" diameter.

They are snug on the 2.5" O.D. boom, but required reaming the holes on the custom ⅜" plate to make them fit. Ordinary muffler clamps do not follow any dimension standard, varying between suppliers despite being stamped with the same size, in this case 2.5". The Cushcraft clamps are truly 2.5" I.D.

Rather than 4 muffler clamps for attaching to the mast I used two DX Engineering saddle clamps. Twice the number of muffler clamps does not compensate for their poor strength under axial load. I carefully selected spots on the plate to drill the 4 holes for the mast clamps to minimize the risk of weakening the plate due to the multiplicity of holes already there.

Integrated truss support

In coordination with the new mast clamp holes mentioned above I utilized two existing holes on the large plate to bolt the aluminum angle stock for the integrated truss. This innovation is required since the truss extends above the tower mast.

The truss is set up and adjusted on the ground. Once up the tower I eyeballed the alignment and declared it good. I was wrong. Only later did I notice that the boom wasn't exactly horizontal. The error of ~2° downward is small enough that the performance impact is not significant. But it does look a bit odd from afar since the booms of the XM240 and TH6 are not quite parallel.

I suspect I made the error because at first the mast clamps were not fully tightened so that I could manoeuver the yagi. I snugged the clamps tight at the very end when my ground helpers had to leave, delaying lifting it up the mast. Apart from looking less than perfect I am not concerned about it. Very likely the antenna will come down next year and I can fix it then if necessary; when it goes back up it may not need the integrated truss support.

Element-to-boom clamps

To avoid weakening the aluminum channel used to support the elements with more holes I decided to revert to the stock Cushcraft boom clamps rather than use better quality saddle clamps. Installing them according to the manual was interesting in that it required a specific tightening sequence for the bottom, middle and top fasteners. Nevertheless I did it as prescribed.

The only omission was the boom truss which is supposed to attach to these clamps. I don't like the idea of a ¼" stainless steel bolt threads radially loaded so I continued with the custom boom clamps for the truss.

The Cushcraft clamps were a bit of an enigma. Despite following the manual and no matter how I fiddled with the alignment of bolt holes in the clamp and the element channel it was impossible to make the elements perfectly square with the boom. Although I did the best I could the elements are not as parallel to each other as they ought to be. The error is small enough to be of little importance to performance, yet it is aggravating. Whether the error is mine -- I don't see how this can be true -- or that of Cushcraft/MFJ quality control is difficult to say.

Another quirk, though not one attributable to Cushcraft, is that with the stock boom clamps for the elements the elements are not in the same plane. When one element is horizontal the other is not. This is due to the machining of the custom boom insert. This is not an issue for muffler clamps securing the elements to the boom since the elements can be rotated to make the elements co-planar. Again, the error is small and not a hindrance to performance.

Tuning adjustment

The tuning challenge is this: decrease the SWR in the CW segment of 40 meters without changing the gain and F/B curves. From experience using the XM240 the positioning of the F/B and gain curves are where I want to keep them, providing acceptable performance between 7.0 MHz and 7.2 MHz. Two element yagis, especially those with shortened elements have narrow optimum bandwidths for gain and F/B so you don't want to mess with those metrics if they are already correct.

Why the feed point impedance differs from what the manual promises is not clear to me. One other contester I've spoken to noticed the same behaviour. He deals with it in the shack by adjusting the match on his tube amplifiers. Modelling isn't an option to fully investigate the antenna's performance since NEC2 is inadequate and NEC4 is not an investment I am willing to make.

All is not doom and gloom since there is another way to get what I want without extreme effort. The method relies on understanding how yagis do what they do. In particular that the gain and pattern, and their associated electrical parameters are unaffected by modest changes to the driven element. By modest I mean with respect to the element's physical dimensions and matching network, if any, at the feed point. Changes to parasitic elements are another matter entirely, and must not be touched for the following procedure to work.

Since this antenna has no matching network -- only a common mode choke -- and I want to keep this simple I am constrained to making changes to the length of the driven element, in particular the tubes serving as the elements tips.

Although NEC2 cannot be accurate for this antenna I created a proxy model a couple of years ago to investigate aspects of the antenna that interested me. The model uses constant diameter elements and therefore makes no allowance for reactance errors due to the tubing taper schedule -- Leeson's stepped diameter correction in EZNEC does not support loads (coils in this case) or capacity hats. The resulting model is frequency shifted downward and likely alters the pattern and impedance a small amount. Even so it is good enough for what I had in mind.

In the second of the two proxy model SWR curves each half of the driven element has been extended 5 cm (2"). This moved resonance down 50 kHz without changing the frequency range of the pattern. Ignoring the inaccurate frequencies calculated by NEC2 it is the amount of downward shift that is of interest since the actual antenna will do the same. Just imagine, if you will, that the lowest SWR shifted from 7.125 MHz to 7.075 MHz.

You might guess that we could push the matter further if CW is of utmost importance. That would be wrong. While a subtle difference notice the reduction of 2:1 SWR bandwidth below resonance from ~105 kHz to ~85 kHz. The reason is that radiation resistance drops sharply near maximum gain, which for a 2-element yagi with a reflector is below resonance. Further lengthening of the driven element reduces the SWR bandwidth and increases the minimum SWR.

Out of some concern with the reliability of my proxy model I extended the halves of the driven element 1.5" (3.7 cm) a little less than the modelled 2" (5 cm). Once up the tower I measured the SWR with my analyzer and saw that the minimum SWR was 1.1 at 7.085 MHz and the SWR at 7.0 MHz was 2.0. With some relief I declared the mission accomplished.

Back in the shack the frequency of minimum SWR shifted upward to 7.1 MHz while the SWR at 7.0 MHz stayed at 2.0. This is not surprising since used 50 Ω coax is rarely 50 Ω, and such is the case for the 330' (100 m) of LDF5-50A tested and used to feed the XM240. The effect of impedance variation is more noticable at low SWR than at higher SWR. If you think about it for a few moments I think you'll see why.

Tram experience

The XM240 is the heaviest antenna I've trammed up a tower, coming in at ~75 lb, or 10 lb heavier than the stock antenna due to the W6NL designed mechanical improvements. It looks pretty nice in the picture I posted earlier. The reality was not quite so wonderful.

My lawn tractor was not entirely up to the task of tensioning the tram line with the heavier antenna. Some sag is normal on the line even with high tension since the weight of the antenna is concentrated at a single point by the pulley. The lack of adequate tension became noticable as the antenna approached the tower.

Sag at the upper end of the tram line meant that the angle of the antenna's motion gradually shifted towards the vertical. My friends on the haul rope experienced this as an increased effort to move the antenna due to loss of mechanical advantage.

That was a minor problem compared to the changed orientation of the yagi. With the haul rope angle becoming increasingly vertical the element tips rotated upward. That made my job on the tower more difficult as I struggled to rotate the boom so that the mast clamps could be installed.

Next time I'll consider using a fixed anchor for the tram line and a winch so that tension will not be limited by the small tractor's traction and weight. I nixed the offer from one of my friends to use his car since the valuable vehicle is at risk of body damage should the antenna swing just a small amount as it lifts off the ground. The elements of a 40 meter yagi are long!

LCA performance update

Soon after acquiring the antenna I began looking into the performance of the element loading coils. There was a story going around that the coil Q was ridiculously low, coming in at under 100. If true the loss would be excessive. I wondered if it would be worthwhile to replace the Cushcraft LCA (4 of them) with a high Q home brew replacement.

Based on my own inspection of the coils I determined that the low Q assessment could not be true. This is because the fibreglass form is hollow not solid as assumed in the one ham's calculation. I estimated the Q to be closer to 200, and therefore with a loss less than -1 db. Earlier this year I received an email from Bernard F6BKD describing his measurement of the LCA with an HP Q meter and got a value of ~200. Now I have the data to believe the coil loss is no worse than in my model.

This is still not ideal. I can live with it until I replace the XM240 with a higher performance yagi (or two) for 40 meters or upgrade it to a W6NL Moxon, thus eliminating the LCA entirely. For now it continues with its stock electrical design. However I did was wrap the coil anchor screws with cable ties since the self-tapping screws are reported to loosen over the years.

Isolated reflector element is still isolated

Both elements of the XM240 are electrically isolated from the boom. Many owners of the XM240 short the reflector element to the boom claiming that it reduces noise -- by draining static buildup on the element -- and adds to the top loading when shunt feeding the tower on 160 meters.

I didn't do this because, quite frankly, I forgot. I have since had instances of static QRN on 40 meters with the XM240 and not on the inverted vee during rain and snow showers. This may indicate that I should have shorted the reflector. The matter is more complicated than that and there are other possible explanations for the observed static. Next time the antenna comes down I'll rectify the problem, just in case.

The future

I haven't yet said how the antenna performs at its new height of 47 meters above ground. That will come in a later article on how all the antennas play. For now I'll only say that it does very well.

Over the winter I will rethink my long term plans for 40 meter high performance antennas. Due to the mechanical challenge I am not yet ready to tackle a full size 3-element yagi on 40 meters. The tower and rotator are up to challenge so it is not out of the question that I'll follow that route. What I can say is that I remain unsatisfied with the performance of 2-element yagis since the gain, F/B and SWR bandwidths are narrow.

The W6NL Moxon conversion of the XM240, or the alternative built from scratch solves the F/B and SWR problems but not the gain. That will require another element. Despite this I remain impressed at what Dave Leeson has accomplished with the XM240 conversion and the performance objectives he met in the design. Many hams have done the conversion and are very pleased with the result

There will be more to report on this subject in the coming months. In the meantime there are many other antenna projects of higher priority. I am well set on 40 meters for the next months with the XM240 as it is and complemented with the inverted vee lower down the tower.

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