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Two layers of 1mil Kapton tape are placed over the winding. Could it be something to do with the new vertical aerial I had recently installed? Thanks! The combination of the transformer with the end-fed dipole results in a positive reactance. I am building an EFHW antenna for my new. I then used a piece of heat shrink on the outside to hold everything in place. Subscribe to our newsletter and receive regular updates on new posts and articles. The last few years have seen a resurgence of interest in the end fed half-wave wire EFHW antenna. However, in reality, the term Omnidirectional needs to be taken in the context of the communication. I found that sometimes the multimeter that was measuring the temperature would give unreliable information when RF was present, so the off periods provided regular intervals for measuring the temperature. Thanks for your comments. You can start with the links below. 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. Home Antenna Wire Antennas Multiband End-fed Half-wave EFHW Antenna. 'Capacitor flattens SWR at higher frequencies' - WA7ARK. The parasitic reactances near the feed-point also vary between installations. And of course, a counterpoise with various lengths. (Tvir s tvbeszl gpek s kszlkek) Ksznm. With no counterpoise wire it will be the coaxs shield providing it. To explore parameter space a little, I modeled a couple of lengths for the counterpoise and see how the wire is resonant across the band when fed with various impedances at the driven point. The model results show the need for a not entirely insignificant counterpoise. Note: If we change the primary number of turns, the impedance ratios will considerable change also. It is very hard to know what will be the best solution for a given antenna installation if you want to hit all of the bands exactly. A typical schematic representation of an EFHW antenna with a short counterpoise and driven with a impedance transformer Unun. I was a little long on 80m not surprisingly, so folded back about 40cm of wire leaving about 2.1m beyond the coil. A design by VK3JEG is only 66-ft. long, about the size of a 40m dipole. From our measurements of the box cooling curve, this would correspond to the box dissipating about 14W in still air. For . I ran WSPR at 10mW for about 2 hours and got the results below. The difference here is that the target bands are low frequency. This will yield the maximum capacity of the variable capacitor. inductance as one FT240-43 and is able to withstand 1kW 50/50% duty cycle with help of a cooling fan. = 258 * 10^-12. Another design by KB0ZZ is a little longer, but resonates on both 40m and 80m. An end fed half wave antenna, or efhw, is a variation of a half wave dipole, but it is fed from one end. I've been using a 1:8 ratio transformer successfully in other EFHW antennas, so I planned the same for this one. I placed the box in a cool oven and let it get up to about 200 F, soak for a while, and then I took it out, hung it in still air and measured the temperature as time passed and the box cooled. 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 think the idea is to begin by tuning 40m operation, adjusting the wire length as needed. Yes, indeed EFHW antennas are very useful for SOTA or other field operation. A 100pF silvered mica was connected in shunt with the transformer primary. Best results are with 2450 drive impedance and the 3.3m counterpoise, so it is not surprising that 49:1 transformers are the common choice. Thirty turns #28 are wound on the two FT114-43 cores. I am already seeing interesting things with it, but not enough measurements to make any conclusions yet. Ismerje meg a hozzszls adatainak feldolgozst . 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. Nagyon j ez a cikk. An End Fed Half Wave (EFHW) antenna for operation on 20, 30 and 40 meters was built for SOTA operation. This reveals we only need to compensate for the reactive component. I came up with a very simple way to build these coils on the antenna wire that proved quick and easy to change and test, involving small piece of PVC pipe and a piece of Velcro shown below. The point is to get the characteristic time constant for the cooling, which is the reciprocal of the exponent coefficient that Excel shows for the fitting equation. PART - 1 : The EFHW antenna as a radiating and receiving element is a decent multi-band antenna with good efficiency on all bands, provided it is driven at its feed-point using a well configured and well-matched driving source. Since the tuners location is best if it is handy for the operator, one end of the end-fed antenna is also close to the operator. Effects of Local Ambient QRM on HF Radio Local ambient QRM (Noise) from various electrical appliances and other sources in the neighborhood often adversely affect HF radio communication. I installed an eyebolt on one end as a pull-point, and a BNC connector on the other. EFHW coil and 80m extension in place (coil visible at tip of red arrow) With the coil and its wire attached I fired up my AA-35 Zoom analyzer from RigExpert and had a look. blue: 250pF, to much compensation. For the antenna wire itself, I used 119 feet of 22g stranded hookup wire. The equation in the article is corrected now. I used four parallel #28 wires (not twisted) for the primary. Scaling the small core to higher power levels would mean that the flux level in the ferrite core would increase. My favorite SOTA antenna is the End-Fed Half Wave Antenna. They will be adjusted for resonance at or close to the CW portion of the bands. The above illustration displays a typical set of Azimuth and Elevation sections of the radiation patterns that one might expect from a horizontally configured EFHW Wire Antenna deployed at an average height of 40 ft. above ground level (AGL). 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. It is fun to build them and measure the lower and upper frequency limits and to see how close the calculation was. Disclaimer | Privacy Policy | Cookie Policy | Terms of Use. How to get started on ISS Cross band Repeater? With the cooling curve and a good estimate for the heat capacity of the box, now it is just a matter of running some power through the transformer and measuring the the temperature rise from ambient to be able to determine the power that turns to heat in the transformer. The equation should have a 10^6 and not 10^-6 in the nominator. It performed well on both the 2m and 70cm bands. The solution was to make a small coil of the coax feed line, thrown on the ground, before connecting it to the radio. I am currently working on measurements of an EFHW antenna. 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. I tossed the line as far up as I could in a tree. 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. Transmitting on 15 meters was impossible because of RF into the radio. Wire antennas with an Un-Un-transformer 1: 9 have become more and more popular. However, to minimize capacitance, you want to spread turns and keep windings apart, so what is the right approach? is the initial disequilibrium temperature, is the equilibration time constant, and is the heat capacity. We have been in the middle of Storm Arwen and Strom Barra here, so I had the pole down for a few days. (LogOut/ Impressive article and so well done that I may give it a try. Yes, there is a coil a few feet from the box but it is NOT TRAP, it is no secret as well, actually, everything in this antenna was published and patented by few people a long ago. clear. If you stare at the above frequency sweeps you will discover a few things. The orientation of these lobes varies with the frequency band of operation thus resulting in directional inconsistencies between bands. %PDF-1.4 Above is a sweep of the uncompensated prototype with a 3220+50 load. This is perhaps one of the most hotly debated rookie topics in. Note: Crosswalk CAA administers the LIHEAP in Franklin, Jefferson, and Williamson Counties. I was curious why this capacitor was added to the transformer. Now I just need to build or acquire a decent radio to utilize this new antenna. Let us touch on some necessary theory to begin with. From experience, I found that with this tuner my SWR was better by simply disconnecting the counterpoise at lower bands. This animated illustration shows the harmonically related current conditions that exist on the EFHW antenna wire on various HF bands. I started with a small piece of PVC pipe and drilled a few holes in it to run the wire through. The high frequency performance was extraordinarily sensitive to anything that could add capacitance to the output lead. The End-Fed Half Wave Antenna a bit Differently, The End-Fed Half Wave Antenna a bit Differently Coventry Amateur Radio Society, Ismerje meg a hozzszls adatainak feldolgozst, 40 M R/7E azaz a kirlyi ejternysk rdillomsa, jra megy az SSTV tjtsz 144,500 MHz-en. Do your own experiments with your own set up, 2019 - 2022 by Dan Koellen AI6XG. The capacitor makes very little difference to the low frequency behavior, but it reduces the input VSWR significantly at the high end. You can see the coil in the picture below (red arrow). End Fed Half Wave Antenna Coupler Schematic - 7-30MHz. Tuning the antenna to its resonance will result in 50 seen at the output of the SWR circuit. = .00 00 00 00 0 258 An end fed half wave antenna, or efhw, is a variation of a half wave dipole, but it is fed from one end. Tap and primary high-side are soldered together near the middle of the secondary winding; the ground side of the autotransformer is located over the low-end of the secondary turns. This is not an optimal value, benefit may be obtained by exploring small changes to that value. But systems that work better increase the prospects of contacts. I have been offered input VSWR curves for such a configuration, and they are impressive but VSWR curves do not address the question of loss / efficiency. The worse SWR was 1:1.7 at the top end of 20 meters. Wideband RF components ultimately succumb to the non-ideal behavior of materials and assembly methods. Most of the time 5 to 20W will be adequate for activation anyway. As I reexamined my formula, you pointed it out correctly, there is an error in it. VSWR<1.8 over all of HF. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. The impedance per unit length of a lumped transmission line is where L and C are values per unit length. This is why we need the impedance transformer from the 50 coaxial feed-line to the several kilo-ohm antenna connections. We also need to know what are the frequencies or bands for which we plant to use this antenna. Be aware counterpoise or ground radials are required for all hf antennas for better performance. I tried to operate from a hotel, and learned the hard way that this type of antenna is not suitable for such location. Because of the division of power between the 3220 resistor and VNA input, there is effectively an attenuator of -10*log(50/(50+3220))=18.16dB, so |S21| has a component due to this division. The SWR figures were as follows: FT240-43 132ft/100pF capacitor 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. Ive been thinking about getting an antenna up for 80m, and Ive come across a number of options that interest me and would suit the space I have available realistically no matter what I go for it will be a very compromised antenna, but still much better than none. What are its disadvantages? IIRC they call it a "compensator". It may be one or even five kilo-ohms. Cmzett: Weince joseph How can we help? The coil uses 26 AWG (27 SWG) enamelled copper wire close wound on a machined plastic former. A few comments have been made about why the capacitor is needed: 'Improves higher frequency UNUN performance' and 'Compensates for UNUN primary leakage' - K1RF If we wish to limit the ferrite temperature to 130 C, then assuming ambient temperature of 30 C, we can tolerate a 100 C temperature rise. It is hard to wait an infinitely long time for the box to equilibrate to a given power level. 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??). Lets call this element the LoadAttenuator. What kind of balun is this? The blue trace above shows the results of adding a 6-turn 2 coil about 6.5 ft. from the box. This gave me an effective coil diameter of 40.5mm. nanoVNA-H can firmware be updated if it has a broken USB socket?