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THE DOUBLE TWIN QUAD ANTENNA
The quad antenna in all its forms has become the preferred antenna for the members of our club. Over the years many designs were built for the different ham bands. When we built the first version of the " DOUBLE Twin Quad Antenna" for the two-meter band we were very surprised, if not deeply impressed, by its apparent performance. One day we were receiving strange S 9 signals which happened to come from a "cash register" at a distance of several kilometers. We tracked down the signal and got an S 5 signal with our mobile radio parked in front of the shop! In this project we will describe the building of a 70
cm version of this performing antenna. Our contest team members were
unanimous in rating this antenna as excellent. Its performance rivaled the
performance of a 19-element Yagi during several
contests. SOME THEORY The Double Twin Quad Antenna is based on the well known "end fire principle". This principle uses two radiators spaced 1/4th wavelength apart and both radiators are fed with a 3/8th wave open line which results in a theoretical gain of 9 dBi in one direction.By also using a reflector, theoretically one can achieve an extra gain of 4 dBi, for a theoretical 13 dBi gain. The larger active surface (capture area) of this antenna seems to result in a higher sensitivity so you can pull in the real weak ones! Different spacings and different delay lines are possible and there is ample room for further experimenting in optimizing this antenna.In order to avoid currents on the coax, the antenna is fed through a quarterwave sleeve forming a very effective choke balun. The characteristic feed impedance of this antenna is 35 ohms. The feed impedance is matched to the 50-ohm coax using a quarter-wave stub by moving the contact points up and down the stub.It might also be important to note that the wind load factor of this antenna is smaller than for a comparable Yagi and that it is very easy to mount on any antenna mast.YOUR SHOPPING LIST FOR THE LOCAL HARDWARE
STORE Jnote : In the US, some of the metric articles aren’t available commonly, except in the larger cities, so there are two lists shown side-by-side.The US equivalent parts work equally as well as the metric, but one must bear in mind to use US thread-gauge when making threads if you are using the US screws and all-thread rods... Personally, I prefer to use metric measurements for the length and width, but again, some materials were not available, so "American" width parts were used. The originals of this antenna were built in Europe. Parts List (a) One aluminum square tube (square cross cut 20 by 20 mm) ¾" square tube for the boom. Length will finish out at 60 cm. 23-5/8" long(b) Two stainless steel threaded rods (all-thread) with a diameter of: 5 mm and a length of 37 cm 3/16" x 14.5"Jnote: I used galvanized #10-24 all-thread. This is very close to 3/16. (they will serve as the spacers between the two individual quads and the reflector) (c) Two aluminum strips: 10 mm wide and 2 mm thick with a minimum length of 155 cm ½" wide by 1/16"(These may be cut from an aluminum plate if needed, but suitable strips should be found in most hardware stores. They will be bent into shape and form the twin radiators) (10mm = 25/64 inches, 2mm = 5/64 inches, 155cm = 61 inches) Jnote: I was unable to purchase ½" aluminum strips longer than 36 inches, so changed this specification to Five aluminum strips, ½" wide x 1/16" x 36" long, to include enough for the two strips needed in the next item (d).(d) Two aluminum strips of 10 mm width by 2mm 1/16" x ½" x 29 cm but with a length of 29 cm (29cm = 11 13/32 inches)(these will be used to make the open feed line/delay line) (e) Four square nylon (or any other suitable plastic) blocks with a thickness of: 15 mm and 25 by 25 mm long ¾" x 1" x 1"(These blocks can be cut easily from a breadboard. They will be used as the "corner" insulators.) (f) Two plastic or nylon rods with a thickness of at
least: 10 mm and a length of 15 mm 3/8" thick x 5/8" long
(g) Two plastic rods made of nylon or a suitable other
plastic with a length of: 40 mm and a diameter of 15 mm
1- 9/16" x 5/8" (These will serve as the spacers at the center of the loops.) (h) Two lengths of aluminum strip: 10 mm wide and a length of some 50 cm 25/64" x 19-11/16"Jnote: I used 1/8" x ½" x 50 cm strips here for strength considerations. (These will be used to mount the reflector to the boom.) (i) A length of heavy copper (or stainless steel, if you prefer) wire: 1.5 mm square, length of 40 cm #12 American Wire Gauge wire, 40 cm long(j) A length of copper tubing with a diameter of: 15 mm and a length of 17.5 cm ½" "hard drawn" copper pipe17.5cm = 6-7/8" (k) A length of coax (RG-213) of 20 cm length ( 7-7/8 inches)(l)
A chassis connector
SO-239 (m) Four or so stainless steel bolts: diameter 5 mm, length 20 mm #10-24 x ¾" preferably of the hex or ‘phillips’ type(n) Eight stainless steel nuts: to fit bolt threads in (m) and (b), along with accommodating washers as you see fit.(o) Two stainless steel bolts:Jnote: I used #6-32 brass for these. diameter 2mm, length 20 mm #6-32 x ¾" you will also want nuts and washers to match these. (p) Eight tiny sheet metal or wood screws, approximately: 1.5 mm x 10 mm #4 x 3/8"(q) One piece of heavy duty chicken wire or some kind of mesh: 60 cm by 50 cm 23-5/8" x 19-11/16"Jnote. I used ½" x ½" mesh "hardware cloth" screen, similar to Pascal’s pictures. (This will serve as the reflector and you can use your own imagination freely) NOTE: USE THESE HANDY CONVERSION CALCULATORS AND TABLES FOR LENGTH CONVERSIONS TO U.S. LENGTHS IF NEEDED:CONVERSION CALCULATOR FRACTION, DECIMAL TABLES Note: When cutting the chicken wire for the reflector, make sure that it has an odd number of openings so it will be easier to mount the boom in the center of the reflector. Also see to it that the threaded rods or bars will be in the center of an opening in the chicken wire (see two photos below by Pascal).
AND OFF WE GO Jnote: this puts one hole 6 cm in from each end of the boom. Carefully center your holes and bore clear through both sides of the boom. It may be helpful instead, to drill four holes carefully measured, two on each side of the boom if you are not using a vertical drill press. Bend the screen wire at some 10 cm from the edge along the longer side (50 cm) on both sides of the screen. The angle should be some 45 degrees. The two aluminum strips (50 cm) are now woven through the openings of the chicken wire. The spacing between these two strips should equal the spacing between the two holes we drilled before in the boom. On the back side of the reflector we can now rivet some aluminum strips to the ones we wove through the chicken wire (see pictures above). This will ensure a good and stable connection of the reflector to the boom.Now put the boom on the inside of the reflector just built and mark the two holes in the boom on the aluminum strips.
Now you know where to drill the holes (5mm) in the two strips. Put a nut on the threaded rod at some 5 cm and push the rod through the boom and through the aluminum strips. Then use a second nut to fix the threaded rod in position and adjust and tighten both nuts. Do the same with the other threaded rod (see drawing of boom with threaded bar and spacers). It is also a good idea to use washers with each nut. See drawing. Take the four 2.5 cm plastic or nylon blocks and drill a hole through the center of each. The holes need to be tapped for the size thread of the threaded rod (all-thread) that you purchased in (b) and can then be used to correctly space the two radiators at the correct distance from the reflector. 1/4th wavelength here means a distance of 17.5 cm between the reflector and the first radiator and another 17.5 cm for the second radiator. See drawing of boom and spacers above. The two rods or bushes (spacers) are made of nylon or
any other suitable plastic. They are cut to a length of 40 mm, then small
holes are drilled in the ends and are tapped with a 5-mm, thread about 1
cm deep (for details, see the drawing of the feeding of the antenna).
J: I drilled mine end-to-end and threaded to the depth of the die on
both ends. In one of the nylon or plastic rods we drill two each 2 mm
holes perpendicular to the threaded holes within one centimeter of each
end. This rod will be used for the first radiator and for the mounting of
the quarter-wave stub (see photo below). FORMING THE RADIATING ELEMENTS Jnote: I added this section because I was so lost by this point, I had to e-mail WIM, who was extremely helpful to explain the measurements and how they related to the quad shapes. I built four quad elements and riveted them together in pairs, this because I could not get long enough strips of aluminum to make it out of one continuous strip.Study this diagram below carefully, and you will see
the symmetry of the sides coming into play. These are fairly simple to
make, just pay attention to what you are doing. I must caution you at this
point, the diagram is NOT to scale, so don’t let your result or my
narration scare you.
I will start you out a point A in the drawing above and move
counter-clockwise until we arrive at I. Refer back to the drawing as
needed. From point C which you have just made, measure 14.5 cm and mark the location of point D. Study the diagram, then use your wrench or vise and place a sharp 90 degree bend at point D, moving toward E. Measuring from point D to C now should be 14.5 cm. If not, re-bend a little, and try to keep it nice and straight. Now that you have established D, measure from point D to point E, be careful now, this should measure 17.5 cm. Mark point E and place a sharp 90 degree bend toward point F. Point F should measure 17.5 cm from point E. Bend point F sharply at 90 degrees toward point G. Point G should measure 14.5 cm from point F. At Point G, you want to install one more 45 degree bend. This should make your "quad element" look similar to the above diagram, except from point G toward I will be very long. Measure from point G toward point I, just 5 cm and mark this place. It should be even with point A. If so, cut the strap at this point. Now, you have to make four of these and put them together in pairs. Here is how I worked that out. Points B and H are where you drill your main mounting holes, where the bolt goes that holds the delay line, the stub, and the quad all together. These holes are located 2 cm from A and I (the ends of the metal) respectively. For my 10-24 bolts, I used a 3/16" hole. Be sure to attempt to carefully center this hole in the strap. To hold the quads together, I measured in from A toward B 1 cm and put a 1/8-inch hole, and from I, I measured in 3 cm toward H, and put a 1/8-inch hole. You will find that when you fit two such contraptions together that you can use two 1/8-inch pop-rivets or #6-32 screws to hold two quads rigid. So be sure your holes line up before you do anything permanent like a rivet. Once you have made your two double
quads, you are ready to mount them to the 2.5 cm plastic squares.
From the picture above it will be clear how you can mount the two radiators to their plastic mounting blocks. If everything is done correctly then the two radiators are spaced correctly at the required 1/4 wavelength. Use small self-tapping screws (sheet metal screws) for the plastic-mounting blocks. You can round off the edges of the blocs for a better fit with the radiators.Jnote: my plastic was very stout, and required that I pilot the small holes for these screws. It actually helped them to mount squarely, also. THE
QUARTER WAVE STUB
The quarter-wave stub (picture above) is made from the heavy copper wire (or stainless steel) and shaped in U form. Each leg should be 1/4th wavelength at your operating center frequency. The spacing between the two legs should be 2.5 cm. We can now insert the quarter-wave stub in the nylon rod and mount the rod into position in the first radiator. See photo of the feed system below.
Note that the smaller nylon/plastic rod pieces have been drilled and tapped similarly to the large one, but with just one hole for the copper wire to pass through. By placing a nut under the head of the screw, you have two adjustments, one to tighten the screw into the ¼ wave stub, then you have the nut to tighten down against the lug loops on the ends of the choke balun coax. More information on this later on...
The idea now is to fix everything into position by tightening the two nuts and bolts. In its final position the bolts should go some 10 mm deep into the rods. The two strips forming the open feed line
(delay line - see Photo above) to the second radiator are each 28 cm long,
(starting length, before drilling and trimming) Jnote: look at the picture and notice that each of the ends have one radical 90 degree bend and a 90 degree twist so that it does not interfere with the radiating element. I bent my large curves over a 4" diameter jar so that they were uniform, after using a pair of adjustable wrenches to make the twist. Be sure you twist the ends in opposite directions from each other, otherwise you may have difficulty getting both ends of both feed line segments to be parallel to each other..While you are in the mood for bending, be sure the two feed lines, once fashioned, are approximately the same shape, parallel to each other, and should measure 17.5 cm from center of one mounting hole to the center of the opposite end’s mounting hole... The two ends of the delay line segments should line up 180 degrees with each other when finished.
BUILDING THE QUARTER-WAVE SLEEVE OR CHOKE BALUN
The choke balun is in the center of picture above. Jnote: Please pay careful attention, this is perhaps the most difficult part of your antenna to build. You will be using a high-heat, high wattage soldering device! Please be careful! In order to suppress the currents on the shield of the coax we need to make a choke balun. For the choke balun we make use of a quarter-wave length of copper tubing. The coax (RG-213) will be stripped of its insulation over a length of 2 cm. The copper shielding is now folded back and we strip the inner conductor of its insulation over a length of some 5 mm Solder the center of the
coax to the center
of the PL connector.
Solder the shield to the edge of the
SO-239
Solder the copper tube to the shield
Use a length of shrink sleeve tubing to waterproof this connection. At the other end of the coax at a distance of 1/4th wavelength we split the shield and the inner conductor and solder a cable shoe (ring/lug) to both conductors. We now insert the coax in the copper tubing and, using a high wattage soldering iron, solder one end of the copper tube to the coax shield and to the cold end or ground of the SO-239 connector. The cable shoes should also be protected with some shrink sleeve. It is also a good idea to use some silicone sealant to further waterproof these connections. The choke balun can now be mounted with a small clamp in the middle of the reflector and should be mounted perpendicular to the reflector. Again, see picture of the mounted choke balun. CONNECTING THE CHOKE BALUN TO THE QUARTER-WAVE STUB To connect the choke balun to the stub, use the
two small pieces of nylon or plastic rod. We drill two threadable holes in
the end and thread these so they can accept 5 mm (or #6-32) bolts. Holes
should be less than the diameter of the bolt. Perpendicular to these tapped holes we drill again small 2-mm (1/16") holes to allow shifting up and down of the coax connection along the quarter-wave stub. All this seems terribly complicated but with careful attention to the drawing and the close up pictures, things will get a lot clearer.
AND FINALLY, THE MATCHING OF THE ANTENNA You can now move the connection points up and down the stub and affix them into position when we have found the correct point for 50-ohm impedance. Matching of the antenna (not tuning !) can be done by trial and error. We do not have a standard procedure for this. The trick is simply to move the connection points up and down until you reach the lowest SWR. This will be easy for those who enjoy the use of an antenna analyzer. Electrically the stub has no resistance where
it is shortened. A quad has a Z from approx 30 ohm so the feedpoint is
near the shortened point. As the front quad is ½ wave ahead, it is fed by
the aluminum strips. The feedline is Z=50 ohms and feeding point is a
little higher (same as in the J-POLE antenna).
One last hint: It is always good to use a 1/2 wavelength or a multiple thereof for the coax feeder to your SWR meter. This is to ensure that you can read exactly what is going on at the feed point of the antenna. Once you have completed the matching, any length for the coax should do. If the antenna cannot be mounted in the clear when matching is done, you may wish to put the antenna with its back on the floor or ground pointing straight up.Have fun and lots of success building the
DTQA. Email Wim here for questions: ON4CJA@gmail.com Email Jerry KD7UGC using this email
address jzs(at)xmission.com Here’s Jerry, KD7UGC with his first DTQA...
Editors note: This antenna project should make a fine addition to your antenna farm.Although the project can be a bit complicated at times, these dedicated Hams have spent many hours tweaking the project. If you have problems or questions, please feel free to contact them via email....they will be happy to help you get it on the air!....N4UJW   Hamuniverse.com uses Green Geeks Web Hosting! |
