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[construction with fishing poles] [construction with aluminium tubing]
©2007-2010 F. Dörenberg N4SPP
This antenna project is a compact center-loaded wire-dipole. Center-loading is the most efficient way of loading a dipole or mono-pole. (ref. "Loading of short antennas" by Doug/WB6BCN). Read here how loading coils work. My short dipole is based on a design from Frank/G3YCC (silent key) and adaptations by Gary/KJ5VW.
A description of my KGD-20-FD micro-dipole (2x 50cm / 2x 20") is here. It works surprisingly well!
The G3YCC 'Shorty' Dipole for 14 MHz by Frank, G3YYC (SK)
The KJ5VW 20 Meter Mini Yagi by Gary, K5VVW
How Does an Inductor or Loading Coil Work? by Tom, W8JI
Basic dimensions :
Nominal antenna dimensions in imperial and in metric units
I have used a loaded-dipole calculator program by VE3KSK to double-check dimensions and loading coil parameters. for a 2nd opinion. For the given dimensions of the antenna, it claims I need center-loading-coils of 10.3 μH, compared to the the actual 8.9 μH of the coils of my fishing pole antenna. Still in the ballpark, though: over 25 years of professional engineering experience has taught me that anything within ±20% is within engineering accuracy!
As a sanity check, I used a loaded-dipole calculator program by VE3SQB, to reverse-calculate the measured coil values back to required coil dimensions for the given coil wire gauge. Came up with consitent values. Both programs probably use the same formula! However, the VE3SQB program indicates a coil value around 10 mH, about 1000x bigger than what I am using. The GUI should say μH instead of mH... Here is a nice on-line calculator for Center Fed dipoles by Martin, K7MEM (more java calculators on his website). And of course you can use HamCalc freeware from George, VE3ERP (which includes all sorts of calculators).
2007-2008: construction with fishing poles - very lightweight
Rather than using wooden dowels for the construction, I used light-weight telescoping fiberglass fishing poles. I found 3 meter (9 feet) poles for 7 euros a piece at the local supermarket ($9 at that time, $11 mid-2008) . They weigh only 130 grams (4½ ounce) each, and measure 1m15 (less than 4 ft) when the 3 sections are collapsed. Don't use poles with graphite/carbon! I used the dimensions of the driven element of the KJ5VW yagi as a baseline. Note that using these fishing poles rather than 4 ft wooden dowels, results in a much lower weight than that of the KJ5VW approach: two of these dipoles + PVC boom + three PVC T-pieces only weighs 2.4 kg (5.3 lbs), compared to almost three times as much for the KJ5VW yagi!
This is how I made my loading coils:
each coil core consists of a 7 cm (2¾") long section of ¾" Schedule-40 PVC pipe from the building supply store (21.4 mm outer diameter; 15.6 mm inner). This is not the lightweight light gray pipe, but heavy duty, thick-walled "rigid non-metallic conduit - above ground underground" pipe. However, that is not critical for this application!
I used #22 AWG (0.64 mm Ø, 0.33 mm2) insulated hook-up wire (Radio Shack item 278-1218, 90 ft for $5.99 (mid-2008))
5m7 wire per coil (12+ ft): 2m2 (7+ ft) for the coil itself, and 1m5 (5 feet) sticking out beyond each coil-end.
with this type of wire, the required 32 turns produce a coil that is just over 5 cm long (2"). Drill feed-through holes with a distance of 5.1 cm (2") between the holes (not center-to-center) to nicely fit the coil. See photo below. I used a 5/64" drill bit.
I have drilled out a small groove from the edge of the coil core to each of the holes.
the wire is fixed in place with small dabs of waterproof glue on the outside of the first and the last winding. I have used the universal 2-component epoxy glue UHU Endfest 300; Gorilla Glue also works great. Used the latter for a second dipole that I needed to construct a 2-element yagi beam antenna.
Note that, as always, "Harry's Law of
Coils" applies. As Harry (SMØVPO) says:
1. You cannot wind coils like I, and I cannot wind coils like you.
2. Coil-winding data is a constant that varies from person to person.
Blank coil cores (notch of groove visible) Finished 32-turn coil
The diameter of the bottom section of the (tapered) fishing poles is such that a 45 cm section of 18 mm diameter dowel rod can be inserted snugly. Hence, the two poles fit on a 1 meter rod, which leaves about 10 cm of exposed dowel between the poles, for installation.
All components - ready for installation (1 ft ruler added for size reference)
Initial installation is the same as the one I use with my Cobra folded dipole: 9 meters of 300 ohm flat twin-lead, plus a 4:1 balun and a ferrite-bead choke balun at the tuner end of the feed line. Quick testing (29 August 2006) shows that the antenna works as an antenna! Appears to be noisier than my other antennas (Slinky Coil and Cobra). Have not trimmed the antenna for resonance at any particular frequency (not critical, as I'm not using it with a coax cable) - just made sure it is symmetrical. Tuning shows an SWR dip that is much more narrow-band than with my other antennas - probably a good sign (hi). Does tune on 40 and 80.
Installation detail with junction box
Hanging from the ceiling
Update March-07: gave myself a very nice Dipmeter kit as a 2006 Christmas present. Have used it to "prune and tune" the wire stubs beyond the coils down to 1m18 (3 ft 10½"), to obtain a resonance frequency of 14,100 MHz if measuring directly at the antenna (no feedline connected to the antenna). With the flat cable attached, the resonance frequency at the cable end is about 500 kHz lower. In addition, there is a sharp/narrow resonance dip (high Q) just above 18 MHz. Probably related to the coils (they are loading coils, but of course they have stray capacity between the windings, so they are an LC-circuit with a resonance frequency, like trap coils). Still trying to figure out whether to measure at the antenna or at the cable end, when pruning. Apparently, an antenna is resonant at the frequency where the SWR is lowest...
Update March-08: have moved to a new apartment in August of 2007, again in Toulouse, and acquired a miniVNA antenna analyzer in February of 2008. Resumed playing with this antenna. This time I used 50 Ω coax as feedline, as per the original design (see references at the top of this page). In this case: 12 meters (40 ft) of RG58 C/U (MIL-C-17), which should account for ≈ 0.7 dB of attenuation per the spec sheet. Indeed, the miniVNA finds 12.26 m (40.22 ft) and 0.68 dB loss.
The plot below shows a minimum SWR of 1.34 at the resonance frequency. The latter now is 14,656 MHz (compared to 14,100 and 14,500 before). Oops: did I prune the antenna too much already? With my digital dip meter I find exactly the same resonance frequency as the miniVNA.
Update 26-April-08: with the help of the miniVNA antenna analyzer, I have tuned the antenna to 14,142 MHz. Had to extend the wires beyond the coils by 12 cm - yes, my previous prune-and-tune effort had gone a tad too far! First I added 25 cm (10"), and trimmed it down from there, 1-2 cm at a time. Resonance frequency went up by about 50 kHz per centimeter trimmed (≈125 kHz per inch). Final wire length beyond the coils is now 130 cm (4'3"); wire length on the inside of the coils is (still) 112 cm (3'8"), with an additional 12 cm (4¾") to the coax. Overall, close to the nominal dimensions of 122 cm (4 ft) on either side of each coil...
The final analyzer plot is shown below. Resistance at the resonance frequency appears to be close to 50 Ω. With and without the antenna tuner, I get about 55 watts output power (compared to 50 W with my Cobra-dipole when tuned); with the tuner, I can get the SWR down from about 1.5 to 1.1. I came across a stable carrier signal on 14,050 MHz; turning the antenna +/- 45º and +/-90º, I found about 4 S-points difference between the null and main direction of the dipole. Unfortunately, with this test station at the null direction, the mini-dipole and the Cobra-dipole are parallel, which made a comparison difficult.
Test set-up on my terrace
Had a brief QSO that same day with Federico (ISØWAV on Sardinia, about 800 km / 500 mi), who gave the mini-dipole a 2-S point advantage over the Cobra-dipole. The mini-dipole was parallel to the Cobra-dipole, with the wires approximately pointing toward the station (which would be the null-direction). Had solid transatlantic QSOs that evening with N1ABA (New Hampshire/USA) and N2TA (New York). Not bad for the size of the antenna! Tunes fine to SWR=1:1.
Update 26-July-08: according to on-line calculators for airwound coils (e.g., here), the loading coils should have an inductance of about 8.1 μH (assuming the PVC core and the wire insulation have negligible effect). Not that it matters what the inductance is (it seems to work), but out of curiosity I decided to check this. I made an extra coil, without the 1½ m wire at each end. I tried three different methods: series- and parallel-resonance with my miniVNA antenna analyzer, and parallel-resonance with my dipmeter. Used a 100 pF capacitor for the LC-circuits. Note that you can not use a cheap RLC-multimeter for this, as most of these instruments do not determine capacitance and inductance values that are valid at RF frequencies (they often use well below 100 kHz). My report with details of the measurement set-ups and results is here.
Test-coil with parallel capacitor and series-resistor
The various methods found resonance frequencies of 5.31 - 5. MHz. Using the formula below (or plugging in the numbers into an on-line calculator, e.g., here), I determined that the actual inductance of the coil is about 8.9 μH. Within 10% of the predicted value, not bad at all! Note that capacitors typically have poor tolerances with respect to their nominal value. When new, ceramic disk caps typically have +80/-20% tolerance, milar polyester typ. ±5 or ±20%, tantalum typ. ±10 or ±20%, metalized polypropylene typ. ±5, ±10, or ±20%, electrolytic typ. ±20%, etc. A handy overview of capacitor markings is here.
Also used the loaded-dipole calculator program by VE3KSK, for a 2nd opinion. For the given dimensions of the antenna, it claims I need center-loading-coils of 10.3 μH rather than the actual 8.9 μH of my coils. Close enough!
Also used a loaded-dipole
program by VE3SQB, to reverse-calculate the coil values. Using the 8.14 μH
predicted above, using 22 AWG, 2" coil length and 21.4 mm coil inner diameter,
the calculator comes up with 32 turns!
2010: construction with ALUMINIUM TUBING
Why? Why not!
Tubing instead of wires: using standard 250 cm tubing from the local Do-It-Yourself store. It has 7.5 mm OD, 5.5 mm ID. Tap M6 thread into all ends. M6 x ?? screws (hex head, so you can use a (socket) wrench to really tighten.
Adjustable tuning rods (inserted into the tip of the outer Alu sections) instead of pruning the antenna wire (idea taken from the original KGD design, modified from threaded to unthreaded rods, per my KGD-20-FD design); 25 cm sections of 4 mm diam. brass rod (unthreaded) and simple plastic strain-relief fitting for cables (UK: "cable gland", D: "Kabelverschraubung"). The Alu tubing fits tightly into the fitting. Pieces of shrink tube along the brass rod: insulate it from the Alu tube: capacitive tuning.
Coil cores: lenght?? diameter?? Screw-on caps. From gardening section of the Do-It-Yourself store.
Another Do-It-Yourself store antenna.
Adapter to 40 mm OD PVC "mast" in umbrella stand on terrace.
The components of the aluminium version of my mini dipole
(aluminium tubing not shown)
The assembled components of the aluminium version of the my dipole
The center insulator
with 32-to-40 mm adapter
(I am using a 40 mm diam. PVC "mast")
Simple hand tap for M6 thread
Without additional support, the dipole "legs" really droop a lot, unlike my original fishing pole construction. So I have glued a 30 cm (1 ft) section of 32 mm OD pipe on top of the center insulator T-piece, and attached mason's string (1 mm diam. non-stretch/pre-stretched multi-strand dacron or similar; nylon stretches way too much) from the top of this post to the coil cores. See photo below. Much better!
(Initial) coils: 0.8 mm CuL, 34 wdgs.
Initial resonance frequency - without tuning rods: kHz.
Too low? Too high? --> tuning rods or Alu length, or coil windings (remove wdgs to increase resonance freq)
Insert tuning parameters table
Insert miniVNA plot
The dipole deployed
on my terrace