73! Can I get away with no capacitor in my design? Yes, indeed EFHW antennas are very useful for SOTA or other field operation. The VSWR plots below show results from the same transformer tested on the bench with minimal output capacitance, with a primary compensation capacitor, either 100pF or 27pF included, with the transformer installed in its aluminum box including antenna connector feedthrough, and with the transformer with an 18 clip-lead on the antenna output. P.S. Do I count the six loops around the pvc in the measurement? Check your inbox or spam folder to confirm your subscription. Above 7MHz, there is very little temperature dependence at all. From our measurements of the box cooling curve, this would correspond to the box dissipating about 14W in still air. document.getElementById( "ak_js_1" ).setAttribute( "value", ( new Date() ).getTime() ); Enter your email address to subscribe to this blog and receive notifications of new posts by email. I am testing it with the tuned tank circuit and a coax to the tuner with and without a common mode rejection choke. The overall lenght is only 15m. My design objectives were: 1. 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. That will be the missing resistor in the resistor bridge of the below circuit. 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. To compensate for the antenna connector capacitance, the solution might be to add the appropriate inductance on the wire right near the box. He was afraid that being, Inverted V Antenna - A popular Dipole variant, The Center-fed Half-Wave Dipole Inverted V Antenna An Inverted V antenna is a very popular variant of the standard horizontal dipole. Basu Bhattacharya (VU2NSB). This will yield the maximum capacity of the variable capacitor. Basically, the loading coil is operating as a trap. This totally un-rigorous calculation just gives a quick idea of how much you have to be careful. The slim coil is torpedo shaped to prevent it being snagged in trees when deployed in 'the great outdoors'. The combination of the transformer with the end-fed dipole results in a positive reactance. This is indeed a classic antenna that is not only, Introduction to Omnidirectional Antennas In the literal sense, an Omnidirectional antenna is an antenna that radiates uniformly in all directions. I then used a piece of heat shrink on the outside to hold everything in place. The results are shown below. (The antenna wire could also be tuned for the lower end of 80 meters without the coil but the . Can a diode be used to rectify signals smaller than its threshold voltage? We also know, that both ends of the half wave antenna will have the highest voltage, thus impedance, which may reach up to several kilo-ohms. Understanding Antennas The Good, the Bad, and the Ugly. It performed well on both the 2m and 70cm bands. This illustration provides a graphical view of approximate comparative difference in expected Common-Mode-Current (CMC) that could flow on the transmission line to undermine the overall performance of the antenna system. An EFHW antenna has a very high impedance at the end point and you need a matching transformer to feed power to it. After cleaning it up and testing it, I installed it on the side of my house on top of a straight pole. The toroid with its many taps may be soldered directly on a multi-position switch. I first mounted the SO239 connector to the box. The purpose of the transformer is to step down the high impedance of a resonant EFHW, not as a resonant circuit or tank itself. 5) It is very hard to avoid stray capacitance on the output. The radiation pattern of this antenna on most bands produces multiple lobes. In his article, I will try to bust some of the myths associated with Inverted. If you need to change it, is necessary to recalculate the secondary turns, and the resonance frequency needs to be adjusted also. point to the loading coil is 20.2m and this sets the 40m resonance at 7.1MHz, which in turn dictates the responses of the harmonically related bands 14MHz, 21MHz and 28MHz. I've always had issues with 10m loading up properly so I'll also be adding a compensation coil 78 inches from the feed point on the wire. Be sure to take a look for additional analysis and conclusions. used with a load such that the input impedance Zin is approximately 50+j0, Gin=0.02S; Duffy, O. It is better to build one for 160 and 80m, perhaps add 17m to the design described in this article, and for higher bands it is easier to use a linked dipole. 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. A ktelez mezket * karakterrel jelltk. Further if your antenna requires a biggeer counterpoise on some bands, you can increase the size of the radiating element or improve your Tuner by using fine tuning capacitor or altering the coils slightly by varying the coil windings (compressed/spread out) slightly. The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes. Jos van den Helm's (PA1ZP) article published in Radcom: Fig.1 - Withdraw the radiating element carefully to avoid damaging the matching components A few years ago I bought a secondhand Diamond X50 dual band (144/430) antenna at a radio car boot sale. These dimensions were derived from modelling the antenna in EZNEC. Lets do a quick calculation to see how much an issue is this output capacitance. They will be adjusted for resonance at or close to the CW portion of the bands. Disclaimer | Privacy Policy | Cookie Policy | Terms of Use. See the second completed tuner depicted below. The antennas are set up as a sloper as in my SOTA outings. 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. Many people use an inductor placed at the near end to make things level out on the higher bands. . Looking at several builds and discussions, such as commercial antennas (e.g. Do your own experiments with your own set up, 2019 - 2022 by Dan Koellen AI6XG. As you look to use this transformer at higher power levels, I can imagine that the bottleneck might be voltage breakdown or corona in the windings. The final turns ratio turned out to be 3:29. The transformer is wound as an autotransformer, 3+21 turns, ie 1:8 turns ratio. IIRC they call it a "compensator". In this case, it looks important. Copyright 2014-2023. When you are discussing a distributed capacitance, it is difficult to know exactly what you are measuring. The EFHW's SWR will be affected by height above ground, type of ground, feed line interaction, and any nearby conductive surfaces. The latest news about Efhw Compensation Coil Experiment. Above is a sweep of the uncompensated prototype with a 3220+50 load. This is based on random lengths of the antenna, which is transformed with the un-un into a medium-ohmic impedance range with reactive components. The biggest uncertainty in these measurements was the actual power level used. 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. More designs can be found by Googling "80m loaded dipole.". Although it is simple to build and is a multiband antenna, it has several attributes that . 100W on a Wire Antenna - Is it good enough for DX? So the differences I see in the data sheets hysteresis curves are not the correct explanation. Hence, even operating at 1000W on the 160m band, the cores would not saturate with the 5-turn primary. This was a little bit of a compromise as the last 20ft of the antenna had to run down a fence to fit it into the garden. The following is the most up-to-date information related to EFHW Compensation Coil Experiment. Being end fed, it needs a transformer to transform the higher impedance of the wire to the 50ish ohms the radio is expecting. It is likely that next time the tuning may be set according to the already found positions, and it will not be far off, requiring only slight adjustment. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. The capacitor makes very little difference to the low frequency behavior, but it reduces the input VSWR significantly at the high end. = .00 00 00 00 0 258 Nevertheless, this is a rather curious result that might have derating implications for the 160 and 80 meter bands. I tossed the line as far up as I could in a tree. I used a section of 40mm external diameter PVC waste pipe and 1mm diameter enameled magnet wire to make the coil. Because we are considering this tuner for SOTA activations or casual portable operation, we do not need to design it for more then 100W. . I checked the inductance again to make sure this hadnt been overly affected, and it remained unchanged at 109.8H, good enough! I used the coil inductance calculator at 66pacific.com and after a few tries came up with 83 turns over a length of about 83mm as shown below. Best regards and thanks again for your comment. Consequently, this is how the capacitance values are determined. I strung up a nominal 80m half-wave antenna wire into the trees, snaking it through the open screen door to the work bench where I was doing the tests. Thanks for your comments. At equilibrium, the heat leaving the box due to cooling will exactly equal the heating power input. When the transformer was mounted in its box and the output connected to an antenna connector feedthrough, the observed capacitance increased dramatically to about 6.0 pF. I needed to add an additional coil (approx 12.5 uH) about . I was curious why this capacitor was added to the transformer. I decided to push for best efficiency with a 5-turn primary. It was difficult to work out where it was coming from but I found that it was more noticeable when it was windy outside. I ran WSPR at 10mW for about 2 hours and got the results below. It tests out well on the antenna analyzer for 3.5 to 29.7 MHz. Hence, there is no optimum single-fit solution. I was pleasantly surprised that the dips were so perfect across all of the bands and the SWR was low enough to not really need a tuner. This antenna is working for 80,40,20,10 and 6m bands. One end is about 5 meters high on an extended fishing pole. I recommend a good reading on this subject by Steve Yates, AA5TB at: www.aa5tb.com/efha.html. A week or two later I started to notice an occasional knocking sound inside the house. Hence after about 3pF of output capacitance, the source will have a hard time charging up that capacitance before it is time to start discharging it. Note the material 43 hysteresis curves shown below for 25 C and 100 C has a smaller open area (and hence lower losses) at the higher temperature. I was a little long on 80m not surprisingly, so folded back about 40cm of wire leaving about 2.1m beyond the coil. Throw one end of the antenna wire, extended with a rope, as high as you can on a three, and walk away with the other end and attach it to the tuner. We use cookies to optimize our website and our service. As I reexamined my formula, you pointed it out correctly, there is an error in it. A 100pF silvered mica was connected in shunt with the transformer . Elnzst krnk, de elsikkadt az zenete. The previous table already contains these measurements. I am building an EFHW antenna for my new. The orientation of these lobes varies with the frequency band of operation thus resulting in directional inconsistencies between bands. I also built one that is heavy-duty handling easily up to 100W for 80/60/40m bands. To be safe, better not get much above 120 C. 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. Cmzett: Weince joseph How can we help? One of the many confusing terms used in Workers' Compensation, "IME" stands for Independent Medical Evaluation or Independent Medical Examination. However, in reality, the term Omnidirectional needs to be taken in the context of the communication. It is recommended to use at least 2W non-inductive resistors for this purpose. 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. 73 Nevertheless, I will also describe, how it is designed, if one decides to embark on building it for different bands. Even at 15W the voltage at the ends may reach 100-200V. 3) Total losses are approximately independent of core volume. It is worth experimenting with its length also. Why had I not noticed this before? Look at the compensation coil slide. After some internet research, I settled on making the former from a paint roller. One possible explanation is that the core losses drop with temperature. The usual problems of having the multiple band resonances all line up appear here with the end-fed design, similar to the same issue with off-center-fed designs. End Fed Half Wave (EFHW) Antenna Upgrade Part 1 - The Matching Transformer. On the ends of each winding I slip the end under the last turn and pull tight to lock the winding in place. Note that building loss into antenna system components is a legitimate and common method of taming VSWR excursions, eg TTFD, CHA250, many EFHW transformers, but in some applications, users may prioritise radiated power over VSWR. Graphed with a log scale, it makes a nice straight line. A lot of radio amateurs have made similar antennas to this with larger transformers for higher power handling. PART - 2 : Most EFHW antenna configurations resort to the use of broadband impedance transformers at the feed-point to attain acceptable SWR on all operational bands. From the condition, it looked like it had been installed outdoors for a few years, so I got it at a good price. Hello, a wonderful approach for the problems.. The 2 coils are 35uh and 90uh. Note that the resonances do not necessarily line up harmonically, there is commonly some enharmonic effect.