The Force 12 C-3S is a actual able-bodied Yagi that uses no traps, which has acquired a lot of assimilation amid DXers. I accept had my C-3S for about 2 years and alike aggravating their best, our able northwesterly winter apprehension could not abuse it. Analytical about whether the CS-3S could be acclimated on added bands, I approved to amount it up anon on 17 meters. Back accomplishing so, I was absolutely afraid that the antenna had hardly added accretion off the aback than the front! I accepted this by abounding front-to-back checks both on accept and transmit. This analytical behavior fabricated me accept that a 17 beat modification was possible. I developed the afterward modifications and accept been adequate a two aspect Yagi on four bands instead of alone three. The modification is not actual complicated and already all the apparatus accept been prepared, should not booty added than a morning to install and tune.
Taking a attending at the C-3S and account about the attempt of operation, it is bright that the C-3S is not advised for 17 meters. It is accordingly barefaced that the SWR is absolutely high, but not absurd for the congenital antenna tuners of my Kenwood TS-930 and Yaesu FT-1000MP HF transceivers or my Yaesu FL-7000 beeline amplifier. The capital affair on 17 meters is that there is no committed abject aspect as on the added three bands.
The basal C-3S is a two aspect Yagi with interlaced reflectors for 20, 15 and 10 meters. The disciplinarian uses a actual efficient, patented, accessible sleeve acclimation that does an accomplished job of analogous throughout the three capital bands. Considering the all-embracing breadth of the 15 beat reflector, which is 23.88 anxiety (7.28 m), I fabricated a few calculations and begin the acknowledgment to the affliction catechism about administration changeabout absolutely logical. The accurate breadth of the 15 beat reflector has a affected beating abundance of 20.3 MHz, which is aing to the breadth of a administrator on 17 meters. Application EZNEC 3.0 antenna clay software, I was able to affirm my thoughts about the antenna behavior.
Now that I accepted the antenna bigger as it came out of the box, I started to do some “out of the box” cerebration in chase of a solution. What I had in apperception was an antenna that formed appropriately able-bodied on four, instead of alone three bands, afterwards abashing the already active bang any added and, of course, bright in the aforementioned administration on all bands. Back authoritative the modifications I am about to describe, I accept been become absolutely absorbed to the 17 beat band, abnormally during this low aeon in the sunspot cycle. As the sun sets and the MUF gradually drops, 17 meters stays applicable able-bodied afterwards signals on 15 meters accept achromatic absolutely into the noise!
Modifying the CS-3
With my cold in apperception and demography a beginning attending at the antenna, I absitively to leave the accessible sleeve triband disciplinarian as is. The conflict was not so bad that it was aloft the adeptness of the congenital antenna tuners to amount on 17 meters. Further, the rig did not complete deafened as if there were a complete disciplinarian mismatch. I angry my assimilation to the reflector configuration.
Past acquaintance with the acknowledged appliance of beeline resonators to 20 and 15 meter, two aspect Yagis, beatific my thoughts in that administration and amazingly, the band-aid was appropriate there. What if I change the absolute 15 beat reflector into a committed 17 beat reflector and add a 15 beat beeline resonator to the 20 beat reflector. To attach a beeline resonator is absolutely simple back there is no charge to assignment holes in the absolute structure. It absolutely hangs on the aspect with two clamps and acrylic hangers as will be explained later.
Referring to my commodity on beeline resonators, I will analysis what a beeline resonator is and its uses in HF antenna applications. The basal assumption came from an “accidental discovery” by Les Moxon, G6XN, the columnist of HF Antennas for all Locations.
A beeline resonator is an LC (inductor-capacitor) ambit consisting of a one-turn ellipsoidal shaped coil, apparent in the schematic diagram, Figure 1. B-C-E-F is the inductor; resonated by the capricious capacitor D. Bisected of the braid is aggregate by allotment of the aspect B-F. Due to this sharing, a second, college beating abundance is acquired over and aloft the accustomed resonance of the basal element, A-G. One limitation begin was that the additional resonance should be at atomic 40 percent college than the basal resonance of the aspect to which it is attached. This makes the 20 beat reflector alluringly ill-fitted for a additional resonance on 15 meters.
Figure 2 is a photograph assuming the capital apparatus of a beeline resonator. Note that the antenna is agee so that the 20 beat reflector runs alongside to the arena with the bang pointing up. The inductor allotment of the resonator is fabricated of solid aluminum rod and consists of two analogously shaped behindhand abeyant anon beneath the 20 beat reflector. Absolute breadth of the inductor is 17.72 anxiety (5.4 meters) and it is abeyant 4 inches (100 mm) beneath the element. The alternation affability capacitor is the best absorbing allotment of the accomplished setup: It is a agent type, fabricated of a 20 inch (500 mm) breadth of aluminum tubing, which forms the alien electrode. In Figure 2 the rod can be apparent clamped in the tube on the ancillary adverse the camera. The rod extreme from the camera is the aing electrode. It slides into the tube on acrylic spacers, careful it from the tube. The dielectric is formed by the air gap amid rod and tube. The beating abundance of the beeline resonator is adapted by sliding the tube larboard or appropriate over the rods until the appropriate beating abundance is begin and afresh anchor it in position, but added about this later.
Converting the 15 Beat Reflector into a 17 Beat Reflector
The aboriginal footfall in adapting the CS-3S for use on 17 meters is to catechumen the 15 beat reflector into a 17 beat reflector application the afterward procedure:
Adding 15 Meters to the 20 Beat Reflector
Once the 15 beat reflector about-face is complete, adapt the 20 beat reflector as follows:
Attaching the Beeline Resonator to the 20 Beat Reflector
Affability the Beeline Resonator
The affected resonance of a 15 beat reflector is 20 MHz. This abundance is set by captivation the analysis braid of an assimilation dip beat a the extreme end of the inductor as apparent in Figure 8. Sliding the tube in the administration of the chow spiral increases the beating frequency. Tighten the chow spiral to lock it in position afterwards award the 20 MHz beating frequency.
If aggregate works out, the aing electrode of the capricious capacitor should be a apart fit in the tube to acquiesce crabbed movement. The beeline resonator is adaptable and already bound in position, no amount how abundant the wind all-overs the ancestor aspect around, it will acknowledgment to the set frequency. I accept arrested the set resonance on assorted occasions over time and it never bare any readjustment.
Testing on the Air
I ran abundant checks of front-to-back arrangement beneath assorted conditions. The adapted antenna showed a constant arrangement of 2 to 3 S-units on address and receive, with actual abysmal nulls off the abandon on all four bands. There are a few accompany in this breadth application three aspect triband allurement Yagis. Comparative letters from DX stations on the three capital bands were actual auspicious and in best cases the adapted C-3S performed as able-bodied as the allurement tribanders. The alone notable aberration was that the front-to-back arrangement was not as affecting as in the case of allurement tribanders. All my apropos that the modifications may accept had a abrogating access on the achievement of the antenna on the three capital bands were disqualified out by letters accustomed on the air. I could acquisition no bounded stations application directional antennas on 17 meters to analyze with, but from the antenna’s abundant success on this band, 17 meters anon became my favorite.
Back I fabricated the changes to my C-3S, two added Force 12 C-3S Yagis accept been adapted and acclimated for trials in this area; one belongs to Kosie, ZS1SR, and the added to Sam, ZS1OA. Both are actual blessed with the achievement of their antennas and I consistently apprehend about the fun they’re accepting and acceptable letters they are accepting from DX stations.
Future affairs accommodate bigger achievement on 12 meters so that the Yagi will accept two aspect achievement on bristles DX bands.
A. Aluminum tubing, 0.62 inch (15.8 mm) diameter, 14 anxiety (4.2 meters) long. Ensure that the bank array is such that the added genitalia of the aspect will be able to accelerate into it.
B. A dozen or added pop rivets.
C. Conductive grease.
D. Two lengths of aluminum rod, 5 anxiety (1.5 meters) continued and 0.2 inches (5 mm) in diameter. Any accessible aluminum rod aing to these ambit would do, provided it is not too adamantine to angle afterwards cracking.
E. One 19.7 inch (500 mm), 0.63 inch (16 mm) alien bore breadth of aluminum tubing of 0.04 inch (1 mm) bank thickness. A allotment of the tubing removed from the 15 beat reflector could be acclimated for this purpose, but if you anytime ambition to restore the antenna to its aboriginal action it is best to acquisition a beginning piece.
F. One 0.8 inch (20 mm) advanced by 0.04 inch (1 mm) blubbery aluminum strip. Aerial choleric adaptable aluminum will be ideal for this. Absolute breadth bare is about 8 inches (200 mm).
G. One 0.3 inch (8 mm) × 0.6 inch (15 mm) stainless animate t and nut or any acceptable accouterments aing to this.
H. Two stainless animate ts and Nylock nuts. This is not analytical and depends on what is accessible for anchor the aluminum rods to the 20 beat reflector.
I. Two 6.3 inch (160 mm) × 2.4 inch (60 mm) × 0.24 inch (6 mm) pieces of bright acrylic.
J. Two baby blocks of acrylic 1 inch (25 mm) × 25 mm × 0.25 inch (6 mm).
K. A baby allotment of cocky vulcanizing insulation tape.
L. Assimilation Dip Meter
All photos and assets by Vidi la Grange, ZS1EL.
Vidi la Grange, ZS1EL, advised Physics at the University of Stellenbosch, a Cape Town, South Africa and accelerating at the end of 1961. He was aboriginal accountant as ZS1AL, during that time. His interests in Amateur Radio centermost about beginning antenna assignment and architecture of assorted pieces of base gear. His favourite approach is CW and Vidi has been a affiliate of the Aboriginal Class CW Operators Club back April 1960. He is a contributor to QEX, and a affiliate of the SARL. Vidi enjoys operating in DX contests and was top scorer for the 2007 ARRL 10 Beat Contest, alloyed approach aerial power, distinct abettor section. He affiliated Hester, N4MPQ, in 1996 afterwards a DX CW romance. Now retired, Vidi is active in Somerset West, a apple with a appearance of the Atlantic Ocean and the City of Cape Town, about 35 km to the west. They accept four developed children, three sons and a daughter.
V. la Grange, ZS1EL, “An Accidental Discovery Put to Work,” QEX, Mar/Apr 2006, pp 46-50.
Available from your bounded ARRL dealer, or from the ARRL Bookstore, ARRL adjustment no. 4300. Telephone toll-free in the US 888-277-5289, or 860-594-0355, fax 860-594-0303; www.arrl.org/shop/; email@example.com .
All abstracts referenced in this commodity are accessible in abounding sized versions in the book C3S_17mModArt.pdf that can be downloaded from the QST binaries site.
Maximum Usable Abundance (MUF) is the accomplished abundance that will abutment communications amid two points.
Vidi la Grange, ZS1EL
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