Tuesday, October 27, 2015

6 meter cycloid dipole for circular polarization

A 2-meter version of this type of antenna (the Cycloid Dipole) has been discussed here before - see the August 5, 2013 entry, "A Circularly-polarized 'Omnidirectional' antenna" - link.

Way back in 2000 or thereabouts I slaved over a hot keyboard and bruised my branium with the voluminous numerical output from the NEC2 program - a decidedly user-unfriendly antenna analysis and simulation tool - and derived the dimensions of a 6 meter "cycloid dipole".  I wasn't shooting for 6 meters, specifically, but the initial "stab" at dimensions seemed to indicate via simulation that, in this general frequency range, the structure that I'd inputted exhibited a vague semblance of the desired characteristics - namely, omnidirectional properties near the horizon and circular polarity with a reasonable axial ratio, so I ended up with an antenna at that frequency.
Figure 1: 
The "Ring-and-Stub" form of the Cycloid dipole.

This is a strange-looking antenna in either its original round ("ring-and-stub") form (Figure 1) or the easier-to-build "square" shape seen in Figure 2.  As noted in the earlier article, the round version had been used for FM broadcast use but the bending of round elements (not to mention inputting the model into NEC2 manually!) was deemed to be too difficult for "amateur" construction so it was worth the extra effort to crunch some numbers and run a lot of simulations to "empirically" derive the optimal dimensions for a "square" antenna that seemed, on computer, to function identically to the round one.

Figure 2:
 The "square" version of the antenna along with the matching network.
The ultimate result is the form of the antenna seen in Figure 2.

As can be seen, the form is basically the same, but it may be built with with things that you can find at any hardware store - namely copper pipe, couplers, caps and elbows.

Once I had been able to derive the 6 meter dimensions I did a linear rescaling to 2 meters - the frequency range of interest.  According to NEC2 the desired properties (omnidirectionality, axial ratio) were not well preserved so a bit more tweaking of the various dimensions was required to "dial" it in.

This 2 meter antenna was then implemented in hardware in the form of copper water pipe using standard pieces of hardware soldered together.  Because the antenna's feedpoint is a complex match (e.g. not 50 ohms and highly reactive) a 1/2-wave matching line was used, fed with a 200 ohm balanced source constructed using a 1/2 wave section of coaxial cable:  This sort of arrangement is not only very low loss using a "balanced line" consisting of copper pipe as the tuning section, but being fed with a balanced feed it is also quite symmetrical.  Finally, noting that it was very susceptible to detuning, an acrylic plastic shield was formed over the top of the matching network to keep it free of snow and rain.

This antenna was installed in about 2001 on a "temporary" mount consisting of ABS pipe at the mountain cabin belonging to Glen, WA7X, a site at an elevation of approximately 8500 feet (2600 meters) in central Utah, about 75 "air" miles from Salt Lake City.

The antenna seemed to work very well.  Those who had heard the 2 meter beacon when it was using a vertical J-pole and were using horizontally-polarized antennas for reception reported an increase in signal strength.  As of the time of this writing (October, 2015) this "temporary" installation is still in place and no maintenance has been done on the antenna and in the years since, the 2 meter beacon has been heard all over Utah and various parts of the western U.S. via Meteor and, possibly, Auroral and tropospheric propagation.

Shortly after the 2 meter antenna was constructed a 6 meter version was also built, but it was too large and heavy to support itself so it (literally!) sat around for well over a decade.

Earlier this year the 6 meter J-pole to which that beacon was connected seemed to have failed, exhibiting a high VSWR (around 5:1) and signals were down by 1-2 "S" units.  Rather than repair the J-pole it was decided that the 6 meter Cycloid should be (finally!) put into service - but first, the wobbly 1/2-inch copper pipe structure had to be stabilized.

That was the job of WA7X, the beacon owner.  Since it had held up well on the 2 meter antenna, ABS pipe was used again to support the antenna structure - with more pieces than before.  As with the 2-meter Cycloid, a 1/2 wave matching network consisting of parallel sections of copper pipe was used, fed with a 200 ohm coax balun and to keep the various parts of this assembly mechanically stable, Delrin (tm) plastic sheets were obtained at a local distributor, cut, holes drilled into them and used to maintain the spacing.

The end result can be seen in Figure 3, below.  A diagonal piece of ABS is used to support the "vertical" elements.  The bottom section of the matching network is attached to the ABS pipe without worry of losses as it is "beyond" the active section and is inert at RF and it is to that section that a ground wire is attached.
Figure 3:  
 The installed 6 meter Cycloid dipole along with its smaller 2-meter cousin.
Click on the image for a larger version.
As with the 2 meter version, the matching network is very sensitive to changes in velocity factor or reactance and it was observed that as a piece of the Delrin (tm) that was used to maintain the spacing was moved around, the tuning was changed, so three extra pieces were cut - one on the section above where the feed was attached and two more on the section above that.  When the antenna was finally completed, these pieces were slid back-and-forth to obtain a 1:1 VSWR at the beacon frequency (50.070 MHz) and then secured in place with blobs of RTV (Silicone (tm)) adhesive.

Finally, a "rain shield" was installed over the top of the matching network, attached to a piece of ABS pipe via a right-angle connector attached to the top of the pipe supporting the antenna.  Getting the antenna "up there" was a challenge as it weighs quite a bit, but with a bit of rope and the grunts of three people it was hoisted to its final destination, the cables connected and...

The VSWR was terrible - around 5:1.

As it turned, the J-Pole was fine all along, but the connection of the outer shield of the 1/4" Heliax (tm) to its RF connector had work-hardened due to vibration from wind and broken loose.  Replacing that connector with a carefully-constructed splice on the end of the Heliax using a short length of RG-8X (it's only 50 MHz!) and some PTFE "pipe tape" as a heat-resistant insulator, this (now) flexible jumper showed a 1:1 VSWR and a quick call to an amateur located near Salt Lake City revealed that when received on a horizontally-mounted Yagi the signal was at least an S-unit higher than before.

Since then, more people have had the opportunity to check out the signal from the beacon.  As expected, those that have horizontally-polarized antennas have reported noticeably stronger signals while those with vertically-polarized antennas reported slightly weaker signals as there is an apparent 3 dB loss (around 1/2 "ideal" S-unit) due to polarization losses between the vertical antenna and the circular wavefront.

It will be interesting to gauge by the reports during the next 6 meter season how well this antenna works, particularly since the signal that it radiates is now agnostic to the polarization of the antenna being used for reception and the vagaries of propagation's effect on polarization  - and also to see how this antenna holds up compared to its smaller, lower wind-load 2-meter relative.

For dimensions of the 6 and 2 meter versions refer to the August, 2013 article linked above and again here - link.


This page stolen from "ka7oei.blogspot.com".