Can I get away with no capacitor in my design? uBITX HF Transceiver Build - My Adventures in Engineering I checked the inductance again to make sure this hadnt been overly affected, and it remained unchanged at 109.8H, good enough! application of Kirchhoff's current Law on a typical EFHW antenna configuration. Note that the resonances do not necessarily line up harmonically, there is commonly some enharmonic effect. EFHW 80m extension - MI0KXX Yes, indeed EFHW antennas are very useful for SOTA or other field operation. It is common to use "compensation" coils of a few turns of the antenna wire to improve the alignment of the bands. An end fed half wave antenna, or efhw, is a variation of a half wave dipole, but it is fed from one end. It performed well on both the 2m and 70cm bands. The tests here were using a dummy load on the transformer, and that did allow confirmation of the design and expected loss at 3.6MHz. There are many good descriptions on the net about this type of antenna. Above is a sweep of the uncompensated prototype with a 3220+50 load. Nothing drastic but likely the SWR on the summit will vary as conditions and the setup change. The parasitic reactances near the feed-point also vary between installations. The transformer is wound as an autotransformer, 3+21 turns, ie 1:8 turns ratio. 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The overlaid VSWR plots are shown below. In an ideal setting, the center of the antenna will have the lowest voltage, consequently impedance, which is why it is possible to feed it by 50 or 75 coaxial cable. We would like to try to fly a kite with a halfway antenna wire and see what miracle we achieve. I will go back and have the article corrected. Being a more efficient design that some, it might result is a wider VSWR excursion that those others as transformer loss can serve to mask the variations in the radiator itself. The technical storage or access that is used exclusively for statistical purposes. 1) More primary turns and primary inductance improve the transformer efficiency and low end performance. Hence, there is no optimum single-fit solution. Gut feeling says that should be possible with just a couple of FT114-43 cores, similar to the design by John Oppenheimer, KN5L. Therefore the Great Circle Map generated below should be Read More, Multiband End-Fed Half-Wave EFHW Antenna The End Fed Half Wave antenna or the popularly known EFHW antenna has been around almost ever since the inception of HF radio. Why does antenna radiation occur? In my research on End Fed Half Wave (EFHW) antennas I found that nearly all have a capacitor in the primary (rig) side of the transformer. Typically the IC-751A would read about 60W on its power bridge, the Siliconix would show >50W on the 50W scale and 45W on the 500W scale, so there is ~20% uncertainty. A short 80m antenna for my lot - KB6NU's Ham Radio Blog The antennas are set up as a sloper as in my SOTA outings. I decided to play with this to see what I could accomplish. The previous table already contains these measurements. For our transformer at 100W (==> 71 Vrms into 50 ) , the core cross section for the two FT114s is 0.74 cm2 , consider the lowest frequency as 3.5 MHz and our 5-turn primary, then plugging in the numbers we find Bmax = 0.012 T. Lets consider a much more challenging extreme: 1000W and the 160m band. Newtons Law of Cooling tells us that heat flow is proportional to the temperature difference. I then added two 220pf capacitors in series between the center conductor and ground. However, the insertion loss of the packaged transformer would be intolerable on the 10 meter band if it appeared the same way with an antenna wire connected. From our measurements of the box cooling curve, this would correspond to the box dissipating about 14W in still air. I would mention first its ease of installation. I cut 2.5m of 16AWG wire, added a connector, and hooked this up to one end of the inductor. Apply for the Job in Software Engineer - EDI at St. Louis, MO. In case you need to perform calculations, use the formula: Where the frequency f is in MHz and the inductance L is in H. We do not know what the actual impedance will be at the end of the antenna radiator. The transformer was attached to the 30 meter EFHW antenna and turns were removed for best match to 50 ohms. This transformer has more surface area than a FT82-43 based one, so it has higher capacity to dissipate heat, and it is more efficient, so it will have higher power capacity than the FT82-43 based one. Don't sweat the digits to the right of the decimal point too much, the SWR readings changed as the feed height and droop of the antenna changed. Being end fed, it needs a transformer to transform the higher impedance of the wire to the 50ish ohms the radio is expecting. As is often the case, packaging was determined by what boxes I had laying around. (LogOut/ The final turns ratio turned out to be 3:29. Hence, we need to move along and start to include the antenna with our testing. The transformer fits sideways in the box with a few millimeters to spare. My goal was to describe the parameters of an antenna that can be easily build, yet show enough theory for someone to design his or her own antenna and antenna tuner. Calculate the capacity for the resonance at the lowest frequency to be covered with the lowest inductivity taken in consideration. Also find news related to Efhw Compensation Coil Experiment which is trending today. The cooling rate is more difficult to calculate because it involves how heat moves through the structure, but it is easy to measure. See the table above. The coil came out at 89mm in length rather than the theoretical 83mm not surprisingly, but the inductance was bang on at just over 109H. A Brief Synopsis of the Antenna Features & characteristics, EFHW Antenna Geometry versus Center-Fed Dipole. With a very short counterpoise it is difficult to resonate the fundamental with any drive impedance. Ive spent some time on digital modes over the last week and and my signal reports seem very similar to the pre-80m-extension period, which is good, and I am getting out well past 1000 miles on 20, 40m and 80m digital. For my build, I started with a waterproof enclosure from the hardware store, a panel mount SO239 connector, an FT240-43 toroid, two 220pf 15KV capacitors, and 14g enameled copper wire, and some hardware to connect it all up. It turns out that we can easily measure the effective parallel secondary capacitance by looking at the primary self resonance when the secondary is open. Compensation. We also need to know what are the frequencies or bands for which we plant to use this antenna. The SWR results are as follows: I needed to add an additional coil (approx 12.5 uH) about . Will there be the same performance for single band EFHWs? Nevertheless, the EFHW antenna had in the Read More, SSN, SFI, Solar Data for HF Radio Propagation Here are some of the important Solar activity parametric data that are responsible for influencing the behavior of the Ionosphere on earth. This happens to place the coil near the peak of a current maximum for the 20m band, which coincidentally appears to have significantly improved the match on that band. It not a loading coil. Thermal gasketing between the cores and the aluminum heat sinks could help a lot, as would a single piece of aluminum rather than the stack up of thin sections I used. Start by wrapping the wire around the toroid. I particularly want to know how counterpoise affects the common mode currents on the coax. With the entire winding length short, the flux from the primary need not travel all the way around the core to link all of the secondary turns. For a quarter wave that is about 8 nS. I went with a 49:1 unun to match the unbalanced antenna to the unbalanced feed line. The slim coil is torpedo shaped to prevent it being snagged in trees when deployed in 'the great outdoors'. Moreover, this antenna produces several inter-lobe nulls creating multiple band dependant shadow regions in its coverage. The transformer was attached to the 30 meter EFHW antenna and turns were removed for best match to 50 ohms. End-Fed antennas are useful in many ways, includes easier mounting of cable at one end (freeing the antenna from unnecessary weight of cable) & antenna maintenance in future. The latest news about Efhw Compensation Coil Experiment. The Truth, ,and Nothing But The Truth (as I've experienced it) Transmitting on 15 meters was impossible because of RF into the radio. Feeling brave, I decided to tape my coil up and solder connectors on and hope I would be one of the lucky ones who wouldnt need much coil adjustment (will I be right??). The SWR for 30 meters, 1.05:1, was the lowest of all three antennas since the transformer was tuned for impedance match at 30 meters. End Fed Half Wave Antennas: Is a Primary Capacitor Really Needed? Consequently, lets pick the lowest frequency 7,000MHz that we need to be able to tune the inductance to, for each of the taps on the inductance. I decided to push for best efficiency with a 5-turn primary. Im a hundred watt guy, so do not have a lot of experience with high voltage RF, but I know that ~5kV peak-to-peak RF voltage that you would expect at the 2450 output at kilowatt power levels could begin to cause you some trouble. Selecting the variable capacitor that can be varied from this minimum to maximum capacity, will allow tuning to resonance for 40, 30, and 20m bands at each of the switch position of the tuner. If the antenna is not tuned properly, and it represents other than 50, the diode will rectify the current flowing across the bridge and the LED will light up. I will publish my findings, but I cannot not promise to complete it until early fall. Note that there is some uncertainty in the measurements, but we can be confident that the loss is no where near the figure estimated for the FT82-43 design. blue: 250pF, to much compensation. I wish you lots of productive experimentations and successful portable operations. The coil uses 26 AWG (27 SWG) enamelled copper wire close wound on a machined plastic former. Some of my research showed that a small compensation coil can be added about 2 meters from the transformer to pull down the SWR on the higher bands. We need to measure the inductances of the toroid with the tapped coil. The highest frequency could be14.350MHz. The maximum B field in the core is proportional to the imposed voltage, as: where A is the cross sectional area of the core, f is the frequency and N is the number of turns. http://www.hamuniverse.com/randomwireantennalengths.html, https://km1ndy.com/diy-491-unun-impedence-transformer-for-end-fed-half-wave-efhw-antenna/. 5) It is very hard to avoid stray capacitance on the output. I installed an eyebolt on one end as a pull-point, and a BNC connector on the other. How to tune a multi-band EFHW antenna using a loading coil Thirty turns #28 are wound on the two FT114-43 cores. Nevertheless, I will also describe, how it is designed, if one decides to embark on building it for different bands. EFHW Antenna Frequency Compensation Lead - YouTube High self-resonant frequency. But I would caution its usage for higher power than 10-15W. I tossed the line as far up as I could in a tree. EFHW Compensation Coil Experiment - YouTube Wire antennas with an Un-Un-transformer 1: 9 have become more and more popular. And of course, a counterpoise with various lengths. RC = 2500 x 1.0e-12 = 2.5 nS Now consider the period of a 30 MHz wave. 1. Wrap the first wire around the toroid 27 times (I used a black wire for this). A tutorial on estimating the impedance of a toroidal ferrite cored inductor for radio frequencies, Pulse test of a Hantek 6204BD 4 channel 200MHz USB DSO and supplied probe. Other observations along the way: 4) Tight windings or backwinding over the primary can significantly reduce leakage inductance. Thanks for your comments. Isn't it? Impressive article and so well done that I may give it a try. The wire as I strung it up seemed to resist having a good SWR for the 20m band. This totally un-rigorous calculation just gives a quick idea of how much you have to be careful. It is not necessary to calculate the above; the results are sown in the following table. In that case, expected efficiency (meaning PowerOut/PowerIn) of the transformer was less than 65% at 3.6MHz. A typical schematic representation of an EFHW antenna with a short counterpoise and driven with a impedance transformer Unun. Inverted V Antenna A popular Dipole variant, EMI-RFI and Noise related HF Radio Station performance loss. Is it any good? Assuming, most of us use the 40/30/20m band for HF SOTA activation, so the following table shows the SOTA frequencies, and the calculated half wave radiator wire of the antenna. The next important parameter is the parallel resonant circuits resonant frequency. Select components with good insolating materials and properly chosen, relatively high voltage rated parts. It is important that resonance may be achieved at each tap of the inductance. The End-Fed Half Wave Antenna a bit Differently I can give a link if you need.
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