In the case of the EFHW 49:1 transformer, the major shortcomings of the most popular current designs are excessive leakage inductance and poor heat transfer from the ferrite material to the environment. 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. The solution was to make a small coil of the coax feed line, thrown on the ground, before connecting it to the radio. This gave the RF some place else to go, trapped by the coil and capacitively shunted to ground, rather than on to the transceiver. 101: The Independent Medical Evaluation - "IME" By: Cynthia M. Hennessey Attorney. The coil uses 26 AWG (27 SWG) enamelled copper wire close wound on a machined plastic former. The blue trace above shows the results of adding a 6-turn 2 coil about 6.5 ft. from the box. 73 After doing the ferrite transformer scaling experiments last time, and learning a bit more about what matters in these transformers, it was time to make a stab at designing one. Im very pleased with that indeed. Then record the change in temperature recorded at the core from the probe, and the elapsed time so I could corrected for not reaching perfect equilibrium. I look at the end-fed as just an extreme example of an off-center-fed antenna. This will allow good matching of the half-wave radiating antenna wire to the 50 coaxial cable at virtually any location. Some say you do not need a counterpoise, but that is false. In comparison to the EFHW antenna, a simple dipole offers a far better prospect. The radiation pattern of this antenna on most bands produces multiple lobes. Nagyon j ez a cikk. 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. I found it very useful to label the tuner with switch and capacitance positions. Nothing drastic but likely the SWR on the summit will vary as conditions and the setup change. Just the bare 80m wire is shown in red. I then added two 220pf capacitors in series between the center conductor and ground. 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. Change). The Counterpoise is a short, typically one twentieth (0.05 *. ) Current Balun 1. Your article was well written and very informative, thanks. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. 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 combination of the transformer with the end-fed dipole results in a positive reactance. A week or two later I started to notice an occasional knocking sound inside the house. %PDF-1.4 We do not know what the actual impedance will be at the end of the antenna radiator. I suspect that a small coil near the transformer is generally a good idea, whether it is my transformer design or others, since it is very difficult to get the antenna wire out of the box without incurring some excess capacitance. I first mounted the SO239 connector to the box. Above, the magenta curve is measurement of a real transformer from 1-11MHz with nominal resistance load and three compensation options: cyan: 0pF, too little compensation; magenta: 80pF, optimal compensation; and. Another design by KB0ZZ is a little longer, but resonates on both 40m and 80m. I was a little long on 80m not surprisingly, so folded back about 40cm of wire leaving about 2.1m beyond the coil. P.S. http://www.hamuniverse.com/randomwireantennalengths.html, https://km1ndy.com/diy-491-unun-impedence-transformer-for-end-fed-half-wave-efhw-antenna/. And of course, a counterpoise with various lengths. The technical storage or access that is used exclusively for anonymous statistical purposes. A 100pF silvered mica was connected in shunt with the transformer primary. The goal of the temperature characterization is to determine how much temperature difference (core to ambient) there will be for a given level of power dissipation. 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. MyAntennas), G0KYA, K1RF, K1TA, K6ARK, N4LQ, W8JI AND WA7ARK; they all have a primary capacitor of 100 pfd to 220 pfd. 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. The comparison is done on 4 bands - 10m, 20m, 40, and 80m. 2006. In fact, this was the second one I had built, as I had also made one for a 30m/15m end fed antenna. Update: Full size 132ft EFHW For one month I replaced the 66ft wire and loading coil and replaced it with a full 132ft of wire, connected to the FT240-43 balun. I added some Christmas colored zip ties to hold everything in place. 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. Above is a thermograph of the transformer at 20W input at 3.6MHz. I decided to play with this to see what I could accomplish. After cleaning it up and testing it, I installed it on the side of my house on top of a straight pole. One cable is the coax feedline, the second is the supply to the cooling fan and control of relays. A Brief Synopsis of the Antenna Features & characteristics, EFHW Antenna Geometry versus Center-Fed Dipole. ROB@COMCOIL.COM During portable operations, I noticed that I forgot the toggle switch in the tuning position, just to discover that no one calls me back to my relentless CQ-ing. How to get started on ISS Cross band Repeater? RC = 2500 x 1.0e-12 = 2.5 nS Now consider the period of a 30 MHz wave. I went with a 49:1 unun to match the unbalanced antenna to the unbalanced feed line. The second smallest number I measured was 2.7pF, derived from the self resonance at 2.72 MHz with the magnetizing inductance (measured at 25.8 H) with the secondary open and the output wire folded to a small 1/2 length. 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 does not need any special components; in fact, most of us may have them in our shack drawers already. The bridge than will measure zero voltage across the bridge, and the LED will turn off. This could be achieved by increasing the spacing of the windings, hence reducing the interwinding capacitance. A method for estimating the impedance of a ferrite cored toroidal inductor at RF. stream The finished tran, End Fed Half Wave (EFHW) Antenna Upgrade Part 1 - The Matching Transformer. It is recommended to use at least 2W non-inductive resistors for this purpose. The orientation of these lobes varies with the frequency band of operation thus resulting in directional inconsistencies between bands. Essentially, the matching is done with a tank circuit, which represents the highest impedance when it is tuned to resonance. The latest news about Efhw Compensation Coil Experiment. Im not interested in trialing coil-loaded dipoles at the minute (from my limited reading bandwidth can be a real issue) so the options I like include: The G0KYA/PD7MAA/IK0IXI EFHW extension to cover 80m calls for a 110uH coil placed at the end of the EFHW, with a further 2-2.5m of wire beyond this. The 1:64 transformer is wound on 3 stacked FT240-52 toroids, this provides same. This antenna is working for 80,40,20,10 and 6m bands. 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. Ez az oldal az Akismet szolgltatst hasznlja a spam cskkentsre. Research salary, company info, career paths, and top skills for Software Engineer - EDI . This will yield the minimal capacitance of the variable capacitor. 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. 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. My limited experience with these antennas also suggest that one must be cognizant of the RF current pushed onto the feed-line coax. HA5CBM Mikls, Finally the tank circuits specification is complete, which is sown below. Select components with good insolating materials and properly chosen, relatively high voltage rated parts. Although it is simple to build and is a multiband antenna, it has several attributes that . Another completed tuner, which is more compact then the pervious one shown, also it is lighter, more suitable for portable operations. I swept the antenna with a nano vna from 2 to 30MHz and there were nice dips at each band. Once that's done, the instructions say "adjust the coil as necessary" to bring the other bands into resonance. The coil is just 12 turns of wire wrapped around a 0.7 inch diameter tube, located about 6 feet past the antenna's feedpoint. First, note that the transformer has low transmission loss up to nearly 50MHz. The overall lenght is only 15m. With most operations you only transmit for 50% of the time, and since a small transformer with four or more primary turns can achieve >90% efficiency, that implies that I really only need to dissipate ~5W. 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. Thanks again Jim. . But Ill find the error of my ways.and there are various on-line calculators to help me ensure I get the decimals and (H, mH, uH, pH, etc.) The antenna is effective and the materials and construction are of a very high quality. From our measurements of the box cooling curve, this would correspond to the box dissipating about 14W in still air. The previous table already contains these measurements. If you need to change it, is necessary to recalculate the secondary turns, and the resonance frequency needs to be adjusted also. 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.
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