All thumb nails in this document expand to photos with full descriptions.
Quad Antenna Design
Having successfully operated for many years with various types of three element beams, mainly for convenience, I decided to try and build a two element three band cubical quad. Many months were spent searching for the best model dimensions shown in various antenna handbooks, and information found on the Internet. I eventually wrote a simple program using Windows Excel spread sheet to translate formula's into actual wires in a 3D dimensional space coordinated system. Suitable for exporting out of "Windows Excel" and into "Eznec" the antenna-modeling program, by Roy Lewallen. See Quad Plots
From the spread sheet I was able to obtain the boom length, director and reflector loop sizes, and many other details such as wavelengths, side dimensions, and for convenience displayed the results in feet, inches, and metric measurements. For those interested in playing with this program or just looking, follow Quad Maker.
Many hours were spent trying to obtain the best 50 ohms impedance match that fell within the amateur band of interest. It was interesting to note that an acceptable match was always obtained some 50Khz to 100Khz out of band and always lower in frequency when swept for best 50 Ohms impedance match in Eznec.
The further away from the desired frequency the better the match became. It appeared impossible to find any combination that best matched impedance and front to back ratios. A trade off was always at the expense of the poor front to back ratios. -25Db or more was a basic target in all program trials and analysis. It was noted that closer the loop dimensions (not spacing) became to one another, the better the front to back ratio's became. Unfortunately, the feed impedance always rose sharply, again always on the lower side and outside the band edges.
A similar effect could be obtained with larger or smaller loop sizes, and changing the boom length. There was considerable latitude in all dimensions as to the forward gain within a few percentage of the desired operating frequency. The optimum result for best front to back ratio for any given loop size was to make the reflector between 3% and 5% larger. Whilst some manufactured and home constructed quads, claim a good match to 50 Ohm feed line, this is not born out using Eznec.
The criteria, and release, from many ponderables is to ignore the feed impedance that result from various dimensions, which being what it will be, will always be different when the antenna is mounted on the tower. This problem of antenna to coaxial impedance matching can be address easily with matching methods, discussed later, and should not be of concern, or cloud the construction issue.
I have in the past constructed cubical quads with the feed line mounted on the lower vertical arm, as opposed to the more convention way. I could find no conclusive proof of any difference between the two, again from a construction point of view; it was easiest to fit the matching box, near the ground with the bottom wire horizontal, and wire side's vertical. This then places the feed point at a convenient height for tuning and adjustments.
I have always been suspicious of the composition of this type of construction aluminium as opposed to that sold by antenna manufactures. The material composition may have additives and may not be what it seems, especially the electrical properties. These kinds of materials used, as spreader arms do not present any problems, if broken up with insulators to prevent odd harmonic resonance. Anyone that has seen or been involve in raising a quad antenna on top of a fixed tower, will testify that this a very nerve racking experience.
There is simply nothing to get hold of to stabilize the rotation of the antenna, and is liable to get tangled up in all sorts of odd place. Aluminium spreaders are not forgiving when under tension, in awkward situations. Aluminium spreader arms were rejected as not the most suitable material, for a fixed guyed tower installation. In search of fiber glass arms, I contacted several antenna manufactures, most without any response. The exception being Cubex in USA.
The proprietor Norman W4QN replied very efficiently with details of price and shipping cost. My choice, eventually settled for a fiber glass kit, comprising eight spreader arms, two spider clamps, boom, and boom to mast clamp. Saving me the trouble of local fabrication. For very little extra cost the arms are supplied pre drilled and supplied with very flexible hard drawn copper wire, pre cut for both director and reflector loops. Each attachment point is marked along the wire, and additional pieces of solid wire supplied for corner wrapping to prevent flexing, and the arms rotating or slipping side ways out of alignment.
In Thailand I have not found a source of hard drawn copper wire, so was pleased with my purchase of this kit, which still allowed me plenty of room for experimenting, if desired. One point to note is that the wire supplied is best cut with a sharp masonry chisel and hammer. Wire cutters don't even mark the wire, and will trash a good pair of electrical cutters.
Laying the four spreaders on the ground and affixing them to the spider with stainless jubilee clips provided, uncoiling the wire, treading, corner rapping the wire where indicated took only a short time. This was repeated with the second loops. After all loops are fixed, one can get a feel if all the dimensions are equal or not. I was pleased with the results as the fiber glass arms appeared to line up without twisting or being pulled out of shape. At first, I was concerned about the arms being pre drilled in case of causing weakness. The size of the hole to diameter of the internal wall thickness of the fiber glass dispelled this concern.
The distance from the center boom to bottom loops, and a few feet extra for clearance call's for a very tall "A" frame ladder to support the antenna. The first problem is to hoist the antenna from a horizontal position to a vertical position. To accomplish this I constructed a kind of derrick, out of a single 4 meter 2 inch steel pipe, guyed with steel rope and three four foot iron stakes driven into the ground. I placed a winch on the lower half of the pipe, and a pulley arrangement on top so that I could slowly wind up the antenna into a vertical position, so that the boom rested on the "A" frame ladder. The ladder method has the added advantage that the coaxial feeder is easy to install, also fixing the mast clamp to boom with correct alignment. See Jin Pole / Derrick Photo's
The antenna already looking very impressive, in its upright position, now meant that I could commence the next step of impedance matching the feeder point to the coaxial cable. First how to connect the coaxial cable to the middle of the driven loop between the arms. Fortunately, I had purchased from Cubex some 3/8-inch nylon halyard rope. I designed a hanging system thereby hanging and bolting my matching boxes onto the nylon vertical nylon rope. With final alignment, all three gamma boxes hung vertical down from the driven spider end of the boom, and the antenna loop wire stretched horizontally out from each box. Loop wires are not under stress as supported at mid point. The impedance matching boxes can be easily be adjusted up or down the nylon rope for a perfect symmetrical mechanical alignment.
Matching antenna to 50 ohms coaxial cable
I opted for the slightly more complicated Omega Match, where two variable capacitors do the matching without the need for moving the tap once set to mid point. I am fortunate to own an MFJ259 analyzer, which when connected to the two open feed wires at mid loop point, give the resonant frequency and impedance details. I was able to repeat the same measurement over several days to establish stable measurement condition for all three bands.
As a test, I opened circuited the reflector loop and measured the resonance point of each driven loop of all three bands. These turned out to be exactly in the canter of each band, as it was supposed to be, so abandoned all further thoughts of using other measurements. I finally decided to cut the reflector mid center lower loop point, for further tests with tuning the reflector. I used paxalin strips with a heavy-duty wire straps, crimp on cable lugs, and bolted them together with 0 BA nuts and bolts, tightening with a pair of pipe grips. These straps could be removed if required for inserting tuning tails, if required.
Using VE3ERP Gamma matching program found in section "C" item 02. It is easy to obtain capacitor values, and gamma rod spacing, and length of rod required. I can highly recommend the omega matching for its simplicity of tuning, and no more difficult than dipping the plate and load knobs of a linear amplifier. I found the length of the gamma tap to be virtually none sensitive, and was initially adjusted for a match when the capacitors were mechanically at mid point.
Initially I intended to join all loops all together. Looping from one matching box to the next with the feed line. However, I experienced, as expected some interaction when tuning from one band to the next. I explicitly wished to avoid any interaction, and decided to install a separate relay in each matching box. This effectively disconnected the coaxial cable when not in use. Whilst the single pole relay removed the coaxial, the loops remained active, so as not to disturbed the design parameters, the loops themselves were not broken. This modification completely removed any interaction between other loops, when playing around with the matching network.
One other luxury was that I purchased an Ameritron 8 way coaxial remote switch. This was mounted directly under the rotator. Separate coaxial tails were installed between each Omega matching box, and the remote switch. The only down side is that two extra control cables are required. A four core for the remote control, and a separate four core for the gamma relays, using one live core for each band and a common.
During temporary construction, though the antenna was only a few feet off the ground, I entered the ARRL worldwide phone contest, and very pleased with the results at such a low height. One important factor I overlook was waterproofing of the gamma matching boxes. These measured 6X4X3 inches, made of PVC plastic with a removable lid.
Although I only drilled three holes, one on the top and one each side, for the tuning spindles, water entered the first day of our rainy season, mid October. After refurbishment I sealed both sides of each hole with the glue gun and rapped the lid with insulation tape after closing. This is the beginning of the third year without any problems.
Concerning the construction of the omega impedance box, this is none critical, but components should be placed so that connecting wires are as short as possible with safety margins for any arching. I used TXing type variable capacitors with large air gaps, but whilst the voltage developed across each capacitor is fairly low, I could not calculate it.
The current is certainly high being several amps. Coaxial connectors (PL259 sockets) were fitted to each matching box, to facilitate inserting the MFJ259 analyzer. Using back to back connector there is little chance of the VSWR being any different at the shack end after tuning. I once climbed the tower but forgot to change the relay over to the correct loop. I don't think that I will make that mistake again. See System Layout Drawing
With the receiver on dummy load, nothing is heard but the very strongest BC stations. The limitations are the cross talk within the Amertron box. Ameritron's specification sheet lists worst case as - 60Db. Switching to the 20-meter quad lifts, the S meter on a quiet band to around S5, mostly made up of atmospheric or local impulse noises. It is easy to detect the change in background noise when rotating the antenna towards the sun. Still tuned to 20 meters and changing the antenna switch to another loop, reduces the background by several S points.
I plotted the antenna azimuth response on a rock steady wide band radar pulsing noise. The radar site is believed to be several hundred miles away, and is assumed to be an over the horizon radar detection system. The resultant plot, could consist of very high angle signal reception, and not really a true picture of the quad's performance. However a signal is a signal, so with the AGC turned off, and lots of attenuation inserted, a plot was undertaken. See 10, 15 and 20 meters antenna plots
The very odd skew shape on the left side in the plot is believed to be reflection from a nearby mountain, which is two thousand feet high, at a distance of five Kilometers You will also notice the very deep side nulls almost -30 Bb in the plot, which is good, for direction finding, tuning for a side null, as opposed to maximum front signal. The front to back ratio is not as good as my target figure of -25 Db but without trimming the reflector, is fairy decent at around -18Db.
@ Copyright 2005 Constructed by Tower Co - Last Updated May. 2007