A week or two later I started to notice an occasional knocking sound inside the house. It should be no surprise that 80pF is close to optimal. It is basically an end fed half wave dipole on 20m/40m and an end fed full wave dipole on 10m. An end fed half wave antenna, or efhw, is a variation of a half wave dipole, but it is fed from one end. To be safe, better not get much above 120 C. The final turns ratio turned out to be 3:29. More designs can be found by Googling "80m loaded dipole.". If you want to minimize leakage inductance, the way to do that is to keep the two windings very close to one another. As it turns out K1RF had similar questions about the capacitor and published data showing that the capacitor does improve SWR performance at frequencies higher than 20 meters. 1. Thank you for an interesting and informative read. The SWR results are as follows: (LogOut/ . The EFHW's SWR will be affected by height above ground, type of ground, feed line interaction, and any nearby conductive surfaces. The cooling rate is more difficult to calculate because it involves how heat moves through the structure, but it is easy to measure. To calculate the impedance ratio, use the following equation: A completed tuner is shown here, which depicts the inductance assembly. However, let me write about it from a slightly different perspective that was not very obvious from the articles I read. I am currently working on measurements of an EFHW antenna. 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. Below about 7 MHz, transmission losses increase as the core temperature increases. Make sure the first wire is wrapped evenly around the whole toroid. MyAntennas), G0KYA, K1RF, K1TA, K6ARK, N4LQ, W8JI AND WA7ARK; they all have a primary capacitor of 100 pfd to 220 pfd. So, I embarked on testing it for myself. Above my old grey-haired head to be certain BUT well written enough to guide the neophyte through it all. 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. The basic question before us is how does an antenna work? = .00 00 00 00 0 258 Below is the schematic of the final build. On the ends of each winding I slip the end under the last turn and pull tight to lock the winding in place. I did struggle with the C in pF calculation..I seem to be getting hung up on the correct decimal placement..for example, for the 40-meter Capacitor requirement, I get .000000000258: C pF = .000001 / [ ( 2 * 3.14 * 7 MHz) ^2 ] * 2 uH The aluminum box is the logical heatsink, so I cut a few scraps of aluminum to make a thermal connection to the bottom of the box and to the lid, adjusting with aluminum shims so that when the box lid is screw down, the entire thermal assembly, with the cores in the middle, is under a little compression. In this case =1058 seconds, about 18 minutes, is the characteristic thermal equilibration time in still air. It may be one or even five kilo-ohms. The slim coil is torpedo shaped to prevent it being snagged in trees when deployed in 'the great outdoors'. From experience, I found that with this tuner my SWR was better by simply disconnecting the counterpoise at lower bands. The 2 coils are 35uh and 90uh. Finally the tank circuits specification is complete, which is sown below. A graphical presentation of the mathematical analysis of EFHW antenna SWR variations on account of variations in counterpoise length as well as the Unun impedance transformer at the feed-point. Nevertheless, this is a rather curious result that might have derating implications for the 160 and 80 meter bands. I used my IC-751As power meter and an in-line Siliconix power meter as well as a voltage probe on the feed cable. (The antenna wire could also be tuned for the lower end of 80 meters without the coil but the . Fill in your details below or click an icon to log in: You are commenting using your WordPress.com account. Once completed and tape-wrapped, I applied a layer of liquid electrical tape over the surface and the interior wire connections, ready to go outdoors. Wrap the first wire around the toroid 27 times (I used a black wire for this). Compensation. 2. That does not seem inconsistent with the expectation calculated above of about 10% core loss at 3.6MHz. However, a key restriction is its 25W power rating, due to the small matching transformer. It will present its parameters, so anybody with basic home-brew skills will be able to build it. Each geodesic great circle path displayed on the map originates from your location that is derived Read More, The Great Circle Map GCM We present an automatically rendered Great Circle Map GCM based on your location derived from your Internet IP address. One of the many confusing terms used in Workers' Compensation, "IME" stands for Independent Medical Evaluation or Independent Medical Examination. From the condition, it looked like it had been installed outdoors for a few years, so I got it at a good price. The SWR for the 20 meter EFHW antenna was < 1.2:1 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. Most informative and appreciated. I wanted to test the hypothesis that an inductance near the box would compensate for the output capacitance and further improve match on those bands. You can start with the links below. Could it be something to do with the new vertical aerial I had recently installed? 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. It is hard to wait an infinitely long time for the box to equilibrate to a given power level. EFHW Antenna Frequency Compensation Lead 5,144 views Jan 20, 2019 188 Dislike Share Save Steve Ellington 9.92K subscribers 8 inches of wire attached to the antenna post can lower the frequency. The change in heat content in the box as it cools can be written: where and are the ambient temperature and heat content respectively. I am building an EFHW antenna for my new MTR3b, but do I need to include the capacitor for 20, 30 and 40 meters? The photos above show a method that works fairly well. This will allow good matching of the half-wave radiating antenna wire to the 50 coaxial cable at virtually any location. Toll Free 800-532-2645 636-532-1523 Fax: 636-532-8259. Thirty turns #28 are wound on the two FT114-43 cores. At FT82-43 matching transformer for an EFHW I wrote about the likely losses at 3.6MHz of a common design using a FT82-43 ferrite core with a 3t primary. Thank you Gary. The EFHW's SWR will be affected by height above ground, type of ground, feed line interaction, and any nearby conductive surfaces. The finished tran, End Fed Half Wave (EFHW) Antenna Upgrade Part 1 - The Matching Transformer. A transformer was wound on a FT50-43 toroid with 3 bifilar turns followed by another 31 turns of #26 enameled magnetic wire. After some internet research, I settled on making the former from a paint roller. This is perhaps one of the most hotly debated rookie topics in. I started with a small piece of PVC pipe and drilled a few holes in it to run the wire through. Wire antennas with an Un-Un-transformer 1: 9 have become more and more popular. (LogOut/ A design by VK3JEG is only 66-ft. long, about the size of a 40m dipole. From the condition, it looked like it had been installed outdoors for a few years, so I got it at a good price. The blue trace above shows the results of adding a 6-turn 2 coil about 6.5 ft. from the box. Elnzst krnk, de elsikkadt az zenete. Select components with good insolating materials and properly chosen, relatively high voltage rated parts. The latest news about Efhw Compensation Coil Experiment. And not all hf antenna installation are same, with most requiring bit of tinkering. Change), You are commenting using your Facebook account. I use shock cord to connect the insulator at the end of the wire to my fence. 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. Many people use an inductor placed at the near end to make things level out on the higher bands. No wire counterpoise is used to reproduce the SOTA setup. used with a load such that the input impedance Zin is approximately 50+j0, Gin=0.02S; Duffy, O. A few comments have been made about why the capacitor is needed: 'Improves higher frequency UNUN performance' and 'Compensates for UNUN primary leakage' - K1RF In an urban area we are much more restricted, especially if we need to be stealth about it. The cooling curve I measured is shown below. The impedance matching circuit is essentially a simple parallel resonant LC circuit. And of course, a counterpoise with various lengths. Lets call this element the LoadAttenuator. But I would caution its usage for higher power than 10-15W. The coil uses 26 AWG (27 SWG) enamelled copper wire close wound on a machined plastic former. The SWR was low, but not unity, so the peak voltage was lower than the 50 ohm match value when measured with the high-Z scope probe. The impedance per unit length of a lumped transmission line is where L and C are values per unit length. Your calculation is correct, the result I get is also .000000000258, which is clearly wrong (it is Farads instead of pF). Your article was well written and very informative, thanks. The procedure was to run for around 30 minutes starting from the box cool at ambient temperature. My design objectives were: 1. Above, 3t on the primary delivers Gcore<0.002S. 100W on a Wire Antenna - Is it good enough for DX? 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). 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. Measurements were made on the unmounted transformer with just a ~2 inch length of lead from the secondary to the 2450 load and sampling resistor (jig connections as shown in the photo above). I place a 2 inch coil of the antenna wire with 3 turns (green) or 6 turns (brown) close to the box. 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. The overlaid VSWR plots are shown below. The permeability rapidly goes away above the Curie temperature, about 160 C on the data sheet. Just the bare 80m wire is shown in red. A 100pF silvered mica was connected in shunt with the transformer primary. Lets do a quick calculation to see how much an issue is this output capacitance. If you stare at the above frequency sweeps you will discover a few things. For the antenna wire itself, I used 119 feet of 22g stranded hookup wire. Gut feeling says that should be possible with just a couple of FT114-43 cores, similar to the design by John Oppenheimer, KN5L. It is worth experimenting with its length also. In this case, it looks important. http://www.hamuniverse.com/randomwireantennalengths.html, https://km1ndy.com/diy-491-unun-impedence-transformer-for-end-fed-half-wave-efhw-antenna/. Un-Uns for Endfed-Antennas (EFHW) by DK7ZB. We do not know what the actual impedance will be at the end of the antenna radiator. I am planning to use this latest one at the Field Day next year. This gave the RF some place else to go, trapped by the coil and capacitively shunted to ground, rather than on to the transceiver. (30 votes, Rating: 5.00) - Please vote the article with your valuable star rating. Another design by KB0ZZ is a little longer, but resonates on both 40m and 80m. I can give a link if you need. i liked it a lot, you give good ideas, many thanks. I wish I had a simple answer, but really this is where some experimentation can pay off. I was curious why this capacitor was added to the transformer. 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. A/l for the chosen core is 3.5 times that of a FT82-43 yet it is only 1.6 times the mass. The transformer was attached to the 30 meter EFHW antenna and turns were removed for best match to 50 ohms. Transmitting on 15 meters was impossible because of RF into the radio. A week or two later I started to notice an occasional knocking sound inside the house. With very little counterpoise, the antenna is being fed at a high impedance point so a rather large impedance step-up is employed. blue: 250pF, to much compensation. The SWR for the 20 meter EFHW antenna was < 1.2:1, The SWR for the 40 meter EFHW was ~1.4:1 for 40 meters and 1.2:1 for 20 meters (full wavelength), For my 20, 30 and 40 meter EFHW antennas I will not use a capacitor in the primary. I tried to operate from a hotel, and learned the hard way that this type of antenna is not suitable for such location. A 100pF silvered mica was connected in shunt with the transformer . Disclaimer | Privacy Policy | Cookie Policy | Terms of Use. It is quite possible that designing more than three or four bands into a tuner will lead to a failure. The SWR results are as follows: Considering the compromised installation (ends about 2.5-3m above ground, inverted V with peak at 8m) I am pretty happy with this so far. It performed well on both the 2m and 70cm bands. In comparison to the EFHW antenna, a simple dipole offers a far better prospect. The computation method is also touched on. We also need to know what are the frequencies or bands for which we plant to use this antenna. Why had I not noticed this before? For fun, I attached an 18 clip lead to the secondary output and observed 7.2 pF due to the dangling clip lead. Isn't it? Loss (to mean PowerIn/PowerOut) can be calculated in dB as -|S21|-LoadAttenuator-MismatchLoss=18.64-18.16-0.03=0.450dB, or an efficiency of 10^(-0.45/10)=90.2%. It is not necessary to calculate the above; the results are sown in the following table. The negative effects of local QRM on an, How does antenna radiation occur? 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 needed to add an additional coil (approx 12.5 uH) about . I am building an EFHW antenna for my new. See the second completed tuner depicted below. I ran WSPR at 10mW for about 2 hours and got the results below. The in-band SWR improved significantly with this localized inductance to compensate for the boxs capacitance. 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.