Tuesday, November 21, 2017

Matching the 2 and 6 meter cycloid dipoles to 50 ohms

The "Cycloid" dipole - a circularly-polarized antenna capable of (more or less) omnidirectional (toward the horizon) radiation was discussed in previous posts:
Figure 1:
The 6 and 2 meter Cycloid dipoles at the WA7X beacon.
The 2 meter cycloid dipole has been in service since 2001 and while the 6 meter
cycloid was (mostly) built at about the same time, it has been in service
only since 2015.
Click on the image for a larger version.
The 2 and 6 meter antennas were installed at the site of the WA7X beacon to impart a circular polarization on the transmitted signals, making them (generally) agnostic to the antenna used by the listener - which is to say that it wouldn't matter whether the receive antenna was vertically or horizontally polarized.  The use of circular polarization also reduces the problem where the ionospheric reflection may rotate the signal to the "other" plane and cause fading at the receive location owing to cross-polarization.

While this antenna is described as being "omnidirectional", that is not true in the proper sense of the word.  Its circularity and most of its radiated power is directed toward-ish the horizon (at some elevation angle) in all directions while relatively little energy is radiated at a high angle upwards or downwards - and what is being radiated in those directions not likely to be very circular.  As with any antenna, the proximity of the coaxial cable, metallic support and feedline will, no doubt, skew the pattern in some way - and this antenna is no exception - but this is unavoidable.

While the dimensions of the "antenna" part of the Cycloid dipole are spelled out in the linked article(s) above, details related to matching of these antennas to 50 ohms is not - with only the suggestion that a "1/2 wave matching network" be used.  While this matching network is very simple, it may be unfamiliar to some, so what follows is a paraphrased response to an email on this very question.

Matching the cycloid dipole to 50 ohms:

While I carefully noted the dimensions of the dipole when I designed them, we never precisely measured the various dimensions of the matching networks of the 2 and 6 meter Cycloid Dipoles as the need for precise replication would render them as mere "starting points" and they are simply stub-tuned 1/2 wave sections - only the dimensions of the actually "antenna" portion are on the web page.

In retrospect, a full 1/2 wave section was probably an overkill as a 1/4 wave may have sufficed - but that "extra" bit of open-wire balanced transmission line (e.g. the portion of the pipe between the coax tap and the "cycloid" part of the antenna) worked out well to provide physical support, rigidity, and counterbalance, would not cause any significant loss, and it all but guaranteed that we would be able to find a good match. in almost any conceivable situation.

The details of the matching network and its tuning are thus:
  • We used about 1/2 wave length of copper tubing protruding from the "back" of the antenna, used also to support the main antenna body.  As can be seen from the pictures, it was folded upon itself, zig-zagging to reduce its overall size.  This extra weight can help to counter-balance the antenna itself.
  • There is a 1/2 wavelength coaxial balun to go from 50 ohms unbalanced to 200 ohms balanced using small 50 ohm coaxial cable.  This type of balun well-described in literature and one of several online calculators may be found here:  http://n-lemma.com/calcs/dipole/balun.htm .  For the 2 meter antenna we actually used some small, 50 ohm hardline (the RG-58-sized equivalent of "UT-141" PTFE coax) that was obtained on the surplus market, but RG-8x or even RG-58 would have been fine.
  • When the antenna was tuned, it was mounted on a nonmetallic support (a fiberglass ladder) placing it several feet/meters above the ground and an MFJ analyzer was connected to the far end of the coax (10-15 feet away) to minimize the effect of having a person too close to the antenna and affecting tuning.  For initial tuning, it should be mounted to the same type of mast as that which will be used for permanent mounting.  For the antennas at the WA7X beacon, plastic pipe has proven to be durable with the 6 meter antenna being mounted using black ABS sewer pipe.  In the case of the 2 meter antenna, it was mounted using some PVC piping that seems to be holding up despite being out in the weather for well over a decade.
  • We prepared two nonmetallic sticks - 5-6 feet long (1x1, wood dowels, bamboo, small plastic pipe, etc.) and one of these had a piece of heavy wire to use as a shorting stub and the other had the balanced (200 ohm) side of the coaxial balun, also connected to 2 wires. The wires/balun were simply taped to the end of the stick to allow contact to be made.
  • Make sure that the copper pipe from which the matching section is made is clean and free of oxide using steel wool or sanding with fine-grit paper to allow a reliable connection while sliding the connections back and forth - both for finding a match and for ease of soldering.
  • At the position farthest from the "antenna" portion, the sliding shorting bar was placed while sliding connection to the balun was placed near it, on the "antenna" side of the shorting bar.
  • With the sticks, the two pieces (coax attachment and short) were slid around to achieve 50 ohm match.  While the two sliding portions are held in place, another person marks their position with a permanent marking pen on the antenna when a match is found.  It is easier to move the connection on the balun back and forth while watching the VSWR while slowly moving the shorting bar back and forth, looking carefully for a match.  Typically, the two connections will be fairly close to each other as seen in Figure 2, below.
  • Once a preliminary match is found, the sliding shorting bar is replaced with a piece of heavy, solid wire (#10-#14 AWG) that is wrapped around the pipes at the marked position. The other sliding bar (on the balun) is then re-checked for a good match, the shorting bar's position tweaked as necessary.
  • Once the position of the shorting bar has been established, the wire on the balun section is wrapped around the pipe at the location of the best match, allowing the wooden stick to be removed.  The positions of the two connections are then tweaked by sliding the wires back and forth for best match.
  • The two connections are soldered in place, and the match re-checked.  If it is OK, the connections are sealed and the match re-checked and adjusted as necessary.
  • In both Figure 1 and Figure 2 one can see small pieces of acetal (e.g. Delrin tm) plastic on the matching network - this material being chosen for its low RF loss characteristics and its durability to UV exposure.  Note that PTFE (a.k.a. Teflon tm) would have also worked well.  Some of the pieces (those at the far left edge of the matching section) are used for mechanical support, but the others are used for fine tuning:  The position of these pieces of dielectric slightly alter the tuning.  After the antenna was fully assembled, these were moved back and forth for the best match and secured in place with blobs of RTV (e.g. Silicone tm) sealant on both sides.  (RTV does not stick to this plastic, but the blobs keep it from moving about.)
As can be seen in Figure 2 on both the 2 and 6 meter antennas, the attachment point of the balun is fairly close to the shorting bar.  The proximity of the coaxial cable balun to the match may affect tuning a bit, so it must be fixed into place before the final tuning is done.

Figure 2:
Annotated image showing the locations of the shorting bars, coax baluns, balun connections and the rain shields on
the 6 and 2 meter cycloid dipoles.  The matching network on the 6 meter antenna is longer than
necessary to allow its far end - which is electrically neutral - to be clamped to the mounting pipe and attached to
a ground wire for static discharge/lightning protection.  The connecting cables were secured with good-quality
electrical tape and black "zip" ties, which were also covered with electrical tape to protect them from UV.
Click on the image for a larger version
Note that for each antenna a "snow/rain shield" was placed over the top of the matching section to minimize the effects of moisture, but the addition of this shield did change the tuning, as did the addition of RTV sealant on some of the connections, so final tuning must be done with such hardware and sealant in place.

The entire procedure is a lot easier if there are 2 or 3 people participating as it is pretty tricky for a single to hold two wires on sticks in place and mark them. If there is only one person available, the shorting bar wire would be wrapped around the (clean!) pipes at a position correlating to about 0.4 wavelength on the pipe and the balun portion slid back and forth to see a "dip" in the VSWR, iteratively adjusting the shorting bar back and forth experimentally while sliding the connection from the balun to get the best match.

As I noted, it is possible that a 1/4 wave section would have been fine, but we just used the 1/2 section as there would be no doubt that it could be matched - and we wanted to minimize the hassle related soldering/unsoldering things as much as possible.  Importantly, this type of match - using the large pipes and "open wire" line - is very low loss compared to many other matching networks (e.g. those using small wound coils and discrete capacitors) and it contributes to the mechanical strength of the antenna itself.


This page stolen from ka7oei.blogspot.com


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