3/8 wave antennas - Page 1
Forum section: electronics / rf-microwave

author: A.Z.
author_rank: Super Contributor
timestamp: February 15, 2023, 02:56:12 pm
content: Hi there, I've been fiddling a bit with the 3/8 wave antenna idea, it's rather simple, in short you setup a 0.375 lambda radiator, add a capacitor at the feed end to "cancel" the reactance and then feed the resulting antenna using a 4:1 or a 6:1 balun transformer, nothing too complex, but running some NEC simulations the results seem promising, for example the 4NEC2 model belowCode:CM -------------------------------------------------------CM File: dip38.necCMCM 3/8 lambda, resonant dipole; fed using a 6:1 BalUn, twoCM capacitors are placed at the inner end of each arm toCM compensate the reactance, set the impedance to 300 OhmCM in 4NEC2 before running the model simulationCM -------------------------------------------------------CE' --------------------------------------------------------' symbols and calculated values' --------------------------------------------------------SY freq=29.000 ' calc frequencySY test=freq ' test frequencySY wire=0.00125 ' wire radiusSY lfac=0.965 ' shortening factorSY wave=(300/freq) ' wavelengthSY wfac=0.75 ' wave fractionSY leng=(wave*wfac)*lfac ' total dipole lengthSY cent=0.02 ' center (feed) segmentSY arms=(leng/2)-(2*cent) ' dipole arm lengthSY hfac=0.60 ' height factorSY feed=(wave*hfac) ' feedpoint heightSY hvar=0.0 ' height variation +/-SY term=feed+hvar ' arms end height (for V or inv-V)SY fara=1000000000000.00 ' FaradSY capa=14.2 ' pFSY cond=(capa/fara) ' capacitors valueSY segl=51 ' longer wires segmentationSY segm=21 ' midsize wires segmentationSY segs=1 ' short wires segmentationSY fedw=1 ' feedpoint wireSY feds=1 ' feedpoint segment #' --------------------------------------------------------' calculate the endpoint (for inverted V dipoles)' --------------------------------------------------------SY vert=abs(feed-term) ' vert side of triangleSY endp=(sqr(abs((arms^2)-(vert^2)))) ' pythagorean formula' --------------------------------------------------------' arms structure (the center stub is a trick)' --------------------------------------------------------GW 1 segs 0 -cent feed 0 cent feed wire ' centerGW 2 segm 0 -cent feed 0 -endp term wire ' armGW 3 segm 0 cent feed 0 endp term wire ' arm' --------------------------------------------------------' ground parameters' --------------------------------------------------------GE 1GN 2 0 0 0 13 0.005' --------------------------------------------------------' wires loading (insulated copper wire) and capacitors' --------------------------------------------------------LD 7 0 0 0 2.1 wire ' insulationLD 5 0 0 0 58000000 ' copperLD 0 2 1 1 0 0 cond ' capacitorLD 0 3 1 1 0 0 cond ' capacitor' --------------------------------------------------------' ext kernel calc' --------------------------------------------------------EK' --------------------------------------------------------' feeding' --------------------------------------------------------EX 0 fedw feds 0 1.0 0 0' --------------------------------------------------------' frequency' --------------------------------------------------------FR 0 0 0 0 test 0' --------------------------------------------------------' end of model' --------------------------------------------------------ENwhich defines a 3/8 lambda (ok, 0.75 lambda) dipole shows a pretty good gain and bandwidth, so... before going on and building something, I was wondering if someone here already went down the same path and built some 3/8 wave antenna (be it a GP or a dipole) and with which resultsTIA

author: antenna
author_rank: Frequent Contributor
timestamp: February 15, 2023, 11:43:41 pm
content: Only trouble matching inductive elements is that the series capacitor you need isolates the antenna element from DC ground. That allows static to build up. I think that is why most people build antennas in the capacitive regions ~ the inductance can be used to DC ground the antenna.

author: coppercone2
author_rank: Super Contributor
timestamp: February 16, 2023, 04:05:47 am
content: how is this resistor to solve the problem? 7W 1megaohm ceramic resistor, it should be good to 2300Vhttps://www.digikey.com/en/products/detail/ohmite/104BA105KDS/10126381

author: antenna
author_rank: Frequent Contributor
timestamp: February 16, 2023, 06:11:14 pm
content: That resistor is $23 before shipping, not sure that is a solution. It might be overkill too. 1000w (guessing) into 300Ω is 550v RMS. Also, that makes about 0.3w across 1MΩ, a bit mroe than I would want to throw away. Why not just take 5 quarter-watt 1M resistors and string them in series? Now that 550v RMS is across 5M for a total dissipation of 0.06w. The resistors can take the voltage and current, and you get to keep the $40 that was about to go to digikey. Is 5MΩ too high for static dissipation?

author: A.Z.
author_rank: Super Contributor
timestamp: February 16, 2023, 06:18:23 pm
content: Quote from: coppercone2 on February 16, 2023, 04:05:47 amwhich resistor, and what problem ?if it's about my post then there's no resistor in that antenna, so either open another thread or explain your postor just, stop waving and move on

author: antenna
author_rank: Frequent Contributor
timestamp: February 16, 2023, 07:59:49 pm
content: He was attempting to solve the static buildup issue I pointed out by bridging that capacitor with the resistor so the element is no longer isolated from ground. If your element has static buildup, the radiation pattern will get messed up.

author: antenna
author_rank: Frequent Contributor
timestamp: February 16, 2023, 08:08:56 pm
content: And I followed that up with a conversation about resistor ratings and expense. You can skip the bleeder resistor, but good luck predicting the radiation pattern. 4NEC2 doesn't account for the wind blowing across your antenna.

author: A.Z.
author_rank: Super Contributor
timestamp: February 16, 2023, 08:11:28 pm
content: Quote from: antenna on February 16, 2023, 07:59:49 pmok I see, but... imagine a vertical, 3/8 stylus, 4 radials 1/4 wave, a capacitor between stylus and radials (place it very low on ground, lay radials almost on ground), would you put a resistor there ?

author: antenna
author_rank: Frequent Contributor
timestamp: February 16, 2023, 08:24:58 pm
content: I see, you want to use the shunt capacitor to bring the impedance to something purely resistive (although obviously higher than 50Ω) and then use a transformer from there to match that resistive impedance to 50Ω. That will probably work, but is the gain improvement worth the transformer loss and parts cost?

author: A.Z.
author_rank: Super Contributor
timestamp: February 16, 2023, 08:45:22 pm
content: Quote from: antenna on February 16, 2023, 08:24:58 pmgain in simulations is around 2 dBi but with pretty low takeoff angle, which is good for DX signals, considering that the (vertical, but the horz dipole has some merit too) antenna is a few inches from ground...

author: antenna
author_rank: Frequent Contributor
timestamp: February 16, 2023, 09:16:43 pm
content: Sounds like a fun project, I'd like to hear how it performs if you end up building it!

author: Co6aka
author_rank: Unknown
timestamp: February 20, 2023, 02:30:20 am
content: I used a axial lead choke to ground, and from what I've seen that's what commercial antennas use. Feedpoint impedance is around 200-Ohms so a very high resistance isn't needed; 20x is 4k-Ohm. Reactance and voltage rating are the important parameters.Quarter-wavelength radials aren't necessary, though they'll work OK. I used chicken wire, plus about a dozen random length wires, plus all of the chain link fences bonded together.And two 6-foot ground rods at the base in case the Sparks of Thor came my way. Feedpoint was elevated at about 3-feet above ground, and the base of the radiator sat on a ceramic insulator from a utility pole.For the higher HF bands it's better to elevate the feedpoint (rooftop, towertop, etc) and construct a ground-isolated counterpoise. Feedline should pass through a common mode choke before the 1:4 transformer (or whatever ratio works best) and the secondary of the transformer should connect to the radiator and the counterpoise, not to the coax shield.The best part of the 3/8-wave antenna is that you can easily multi-band it with 1/4-wave "stubs" connected at the corresponding 1/8-wave-up point on the radiator. There isn't a lot of interaction unless the bands are close.For VHF, one could make a nice 50, 146, 222 and 440 multiband antenna for very low cost. Or 10m/6m.

author: A.Z.
author_rank: Super Contributor
timestamp: February 20, 2023, 09:06:37 am
content: Quote from: Co6aka on February 20, 2023, 02:30:20 amInteresting ideas, thanks, just to add some more infos, the whole 3/8 idea came to me after reading thesehttps://ei7gl.blogspot.com/2016/11/three-eight-wave-ground-plane-antenna.htmlhttps://rrc.org.au/2019/10/21/a-3-8-wave-vertical-for-20m/which describe a 3/8 GP, I became curious and tried the same idea with a dipole, and it seemed to work

author: Co6aka
author_rank: Unknown
timestamp: February 20, 2023, 04:21:35 pm
content: Another thing you can do is add a coil and capacitance hat at the top of the radiator, which will physically shorten and electrically lengthen it. Handy for the lower HF bands. You can do the same with multiband "stubs" connected to the main radiator. I've used fiberglass tube at the top of the radiators for this purpose.The slickest 3/8-wave HF antenna I've seen was built for 160m with aluminum tower sections, a fiberglass pole at the top, and six or eight aluminum rods making up the capacitance hat. Off the tower sides at the appropriate 1/8-wave-up points were the "stubs" for 80m, 60m, 40m and 30m. The base of the stubs was made with aluminum plate and brackets, and the stubs were anchored to the tower at a few points with fiberglass rod. Spacing of the stubs from the tower was about 4-6 inches.