Dipole Antenna Length - Page 1
Forum section: electronics / rf-microwave

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 11, 2024, 06:18:32 pm
content: I am getting mixed reports on the optimum length of a dipole antenna.My understanding is the two dipoles consist of 1. the ground/shielding, 2. the other/positive.I've also heard of a rule, 468/frequency in MHz to give an approximation of the length of the wavelength in feet. I've heard you further divide that by two to get each element length: each element is then 1/2 the wavelength.Now I found a resource suggesting the length of the total dipole should be half the wavelength, each element a quarter of the wavelength:For my reference frequency let's do 117MHz, so 468/117MHz gives a wavelength of 4 feet.If I follow the Antenna Dipole calculator in SDRAngel I get the following:So each element is 2 feet, or half the wavelength. If I follow the youtube video's recommendation, each element of the dipole should be 1 foot, or 1/4 the wavelength.I'm at a loss and am on the verge of breaking out the university physics textbook. What works best here?

author: A.Z.
author_rank: Super Contributor
timestamp: April 11, 2024, 06:40:10 pm
content: in meters it's easierLambda=300/MHzand then apply a correction factor to deal with the conductor speed and the end effect, assuming copper you will then haveLambda=Lambda*0.965at this point, for a dipole your total length will beTotLen=Lambda/2and each arm length will then beArmLen=TotLen/2all the above in ideal conditions, you will usually cut the arms longer and then, trim them equally to resonance

author: RFDx
author_rank: Regular Contributor
timestamp: April 11, 2024, 06:49:32 pm
content: Stick to the calculator in SDRangel.Length (not wavelength) of a lambda/2 dipole antenna in feet is ~468/frequency, with frequency in MHz. For 117MHz both arms of the dipole would have 2 feet, 4 feet in total for the whole antenna.

author: A.Z.
author_rank: Super Contributor
timestamp: April 11, 2024, 07:06:45 pm
content: is it better learning the calcs or using a calc w/o knowing how it works ?Oh and then Lambda=Wavelength in case you weren''t aware; what's next the dipole gain ? Well, at 0.6 Lambda height it's more than 7 dBi, now maybe you'll need to search for dBi

author: radiolistener
author_rank: Super Contributor
timestamp: April 11, 2024, 08:20:58 pm
content: Quote from: Fried Chicken on April 11, 2024, 06:18:32 pmHalf wavelength dipole resonant frequency is very close to a half wavelength. It should be trivial from antenna name.It is not exactly half wavelength but very close.Actual frequency may vary depends on the wire thickness, presence of isolation material around wire (include air humidity, etc) and height above ground.You can precisely tune your dipole with antenna analyzer by cutting wires step-by-step.Also note that antenna feeder may be a part of your antenna, especially if you're don't use balun to match half wavelength dipole with coax cable and don't use RF-chokes to cut-off common mode currents. In that case coax cable length and geometry may significantly affect your antenna resonant frequency.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 11, 2024, 11:26:17 pm
content: I should add what I'm doing. I boughtthis kitand am exploring the frequency spectrum. I've finally managed to reliably pick up the airport ATIS which was faint until I messed around with the antenna. I found full wavelength total length worked better than half wavelength total length.What I'm wondering is what the logic/physics here is. That video that I posted is throwing of my conception completely

author: DimitriP
author_rank: Super Contributor
timestamp: April 12, 2024, 12:20:56 am
content: QuoteOh...."rabbit ears".You should have opened with thatForget calculating and "adjust antenna for best reception".

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 12, 2024, 12:52:24 am
content: Quote from: DimitriP on April 12, 2024, 12:20:56 amThat's what I did, but i'm curious about the physics and how come I'm getting different recommendations

author: radiolistener
author_rank: Super Contributor
timestamp: April 12, 2024, 12:53:27 am
content: Quote from: Fried Chicken on April 11, 2024, 11:26:17 pmThe logic/physics here is that full wavelength dipole has bad VSWR, so its output has high mismatch with feeder input and as result it gives much more weak signal to the receiver.Much more weak signal "worked better" because your receiver performance is bad and it is overloaded with signal from a good antenna.The same issue for any cheap Chinese radios like Baofeng/Quansheng/etc. If you connect it to a good external antenna it can't work in the modern city environment, because it is flooded with high signal levels. This is because such cheap radios have low dynamic range and unable to see weak signals in presence of a strong signals on different frequency.In other words, this is not dipole works better when it has length equal to a full wavelength. This is your low quality radio works better when antenna gives more weak signals due to bad antenna performance. It happens because your radio cannot work with a more strong signals from half wavelength dipole.

author: radiolistener
author_rank: Super Contributor
timestamp: April 12, 2024, 01:25:48 am
content: But on the other side, the more large antenna has better efficiency, so if you use proper antenna matching with feeder, you can get better antenna performance. But it also needs to include to analysis of losses in the more long wire and losses in matching circuit.Here is a chart for dipole radiation resistance depends on it's length:As you can see, dipole with length 1 lambda has radiation resistance about 200 Ω. So, if you attach it to the 50 Ω coax cable through 1:4 impedance ratio transformer it may give you better performance due to higher efficiency of more large antenna.More longer wire resistance can be compensated with using more thick wire. So, the efficiency of such antenna will depends on your matching transformer efficiency.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 12, 2024, 05:40:54 am
content: Wow. This is not in any way what I was expecting. I thought it would have something to do with the ground of the AC signal being negative and the other positive causing a higher voltage across the antenna --> ground and therefore the full wavelength would be stronger. This was my gut assumption, but would mean the youtube video so highly praised is incorrect.I've looked up the input impedance of my radio:Quote from: RTL-SDR CompanyAt this point I feel I need to either break out my university physics textbook, or accept things are the way they are. I'm not sure what dipole radiation resistance is... I'm also not sure what's meant by feeder input.

author: radiolistener
author_rank: Super Contributor
timestamp: April 12, 2024, 08:32:01 am
content: Quote from: Fried Chicken on April 12, 2024, 05:40:54 amhttps://en.wikipedia.org/wiki/Radiation_resistanceRadiation resistanceis that part of an antenna's feedpoint electrical resistance caused by the emission of radio waves from the antenna.In other words, this is a part of antenna input resistance which leads to energy losses for EM radiation.The antenna also has a part of resistance due to unwanted energy loss due to heating.For receiver antenna you can view it as a signal source resistance.Quote from: Fried Chicken on April 12, 2024, 05:40:54 amFeeder input is a transmission line input end point which is used to put RF signal from source.For receiver antenna this is transmission line endpoint which is used to connect to antenna.If you're using coax cable as transmission line, it will be coax cable end which is connected to antenna.

author: radiolistener
author_rank: Super Contributor
timestamp: April 12, 2024, 08:48:25 am
content: Quote from: Fried Chicken on April 12, 2024, 05:40:54 amFirst and the main goal - you're needs to make sure that the antenna impedance is good matched with your feeder (coax cable) impedance.For receiver, input port impedance has some effect, but if there is some minor impedance mismatch, this is not so critical like for transmitter.Regarding to RTLSDR, its impedance depends on the frequency.For example my RTLSDRv3 input port has Z = 63.1+j14.6 Ω at 117 MHz, so there is no needs to worry if you're use it with 50 Ω or 75 Ω coax cable.You're needs to take main care about good matching between the cable and antenna impedance.If you want to match your cable with full wavelength dipole, it's better to use 50 Ω coax cable, because it will be more easy to match with 200 Ω antenna impedance by using 1:4 impedance ratio transformer (1:2 turns ratio).For comparison, with half-wavelength dipole it's better to use 75 Ω coax cable, because half wavelength dipole impedance is about 73 Ω, so you can use 1:1 impedance match transformer as balun.PS: adding several RF chokes on coax cable near antenna and near receiver can help to reduce noise interference from home equipment.

author: A.Z.
author_rank: Super Contributor
timestamp: April 12, 2024, 09:48:43 am
content: Quote from: Fried Chicken on April 11, 2024, 06:18:32 pmexactly, for a resonant dipole, offering an impedance around 75 Ohm that's the sizeQuoteLet me do that in meters (I use metric units so it's easier for me), let's see...Lambda isC/F, we assume C as 300 and F as expressed in MHz, so we have300/117which gives us2.564(meters), this is thefull wavelenght (Lambda), now we calculate the WHOLE dipole spread (both arms) as Lambda/2 that is 2.564/2 and we get1.282which is our 1/2 wave dipole overall spread, at this point each arm of the dipole will have a length of a quarter wave, so 1282/2 will give us0.64which tells us that each arm of our dipole, for a resonance at 177MHz should have a length of64cmor2.09ftQuotesee above... it isn't "half" but one quarter.Note: in the above calculations I didn't consider the "end effect" shortening to keep things simple, but willing to consider it you may read thishttps://www.kb6nu.com/dipoles-shorter-half-wavelength/notice that I usually stick with a reduction factor of about 0.965 for 2.5mm diameter copper wires, in this case this would mean that the arms length would become 0.64*0.965 that is 0.618 (61.8cm or 2.02ft)

author: A.Z.
author_rank: Super Contributor
timestamp: April 12, 2024, 10:30:50 am
content: Quote from: Fried Chicken on April 11, 2024, 11:26:17 pmThat kit may be ok for V/UHF and then, only if you go listening outside in some place far away from the urban "electrosmog cloud", willing to get serious you'd better consider putting up a better antenna and place it outside, be it on a balcony or a window if not over the roof, connect it with a run of RG6 (TV/Sat) coax cable and start listening

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 12, 2024, 02:35:20 pm
content: Quote from: A.Z. on April 12, 2024, 10:30:50 amIn fact I'm about to buy some pizza pans and build a pizza pan antenna and stick it in the attic via the TV coax (I already bought the adapters).Lots of examples:https://turbofuture.com/industrial/How-to-Build-Your-Own-Planar-Disk-Antennahttps://thenack.com/2016/11/planar-disk-antenna/http://www.wa5vjb.com/references/PlanarDiskAntennas.pdfhttps://wa5vjb.com/references/Pizza_pan_vertical_2016%20WA5VJB.pdf

author: A.Z.
author_rank: Super Contributor
timestamp: April 12, 2024, 02:53:04 pm
content: Well, if you are only interested in SHF signals up to the GHz, that one may be a start, but then don't expect it to give you some decent reception down at UHF/VHF or HF, they just won't perform; if you're going to install an antenna up the roof, my suggestion is to start with a discone, most of them will offer acceptable performance from 25MHz up to about 2GHzWilling to get down to HF/VHF with decent performance, here's my suggestionPick this small 9:1 BalUnhttps://www.amazon.com/Balun-One-Nine-v2-Barebones/dp/B07XJRTF94and put it inside a plastic waterproof junction box, then use some 16AWG insulated wire and lay it out over the roof (or inside attic if you have a wooden roof) forming an horizontal loop and connect the ends of the wire to the balun, cut the loop wire wire at the opposite side from the Balun and insert a resistor with a value around 500...600 Ohm, the latter will flatten the impedance curve allowing to obtain a good match over a wide bandwidth, such a terminated loop will also offer the advantage of being less "noisy" than other types of antennas, yet it will be able toset the SNRand allow you to pick up signals from MW up to around 50MHz at least

author: ahbushnell
author_rank: Frequent Contributor
timestamp: April 12, 2024, 09:23:00 pm
content: Quote from: A.Z. on April 11, 2024, 07:06:45 pmA dipole is not 7 dBi gain. It's about 2.15 dBi.https://en.wikipedia.org/wiki/Dipole_antenna

author: fourfathom
author_rank: Super Contributor
timestamp: April 12, 2024, 10:59:11 pm
content: Quote from: ahbushnell on April 12, 2024, 09:23:00 pmThat's gain in free space.The height above ground of a half-wave dipole will cause the gain to increase at some elevation angles, and decease at others. I don't have the numbers handy, but I believe that the 7dBi value is possible. Obviously the quality of the ground also is a factor.

author: uer166
author_rank: Super Contributor
timestamp: April 12, 2024, 11:16:33 pm
content: Quote from: fourfathom on April 12, 2024, 10:59:11 pmWould this be freznel zone interaction? I thought antennas were always specced in free space, since the distance to GND is not really a controlled parameter. If you have obstructions in Freznel zone (like a ground), then the pattern gets pretty arbitrary.Edit: pulled out Kraus book, 2-19 claims that directivity of a Sine power pattern (I assume a 1/2 wave dipole is pretty close to that) has directivity of 1.27, which is 2.08dB.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 12, 2024, 11:24:53 pm
content: Quote from: radiolistener on April 12, 2024, 08:32:01 amSo kind of like eddy currents?QuoteI'm confused. "Transmission line input end point" - so the end of the antenna? Where the antenna connects to the receiver? That's where the resistance in a coax cable is put, at the endpoint?

author: uer166
author_rank: Super Contributor
timestamp: April 12, 2024, 11:27:34 pm
content: Quote from: Fried Chicken on April 12, 2024, 11:24:53 pmIt's the location where the transmission line ends, and the antenna element(s) begin. In your case it's where Coax is connected to the 2 dipole elements.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 12, 2024, 11:28:15 pm
content: Quote from: A.Z. on April 12, 2024, 02:53:04 pmI'm not wanting to spend more money beyond what I have. I've now purchased these pizza discs, and plan on putting two of them together as shown. This is a limitation I have placed on myself. I currently use the SDR to listen to ATC and pick up ADSB signals.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 12, 2024, 11:30:06 pm
content: Quote from: uer166 on April 12, 2024, 11:27:34 pmOn the dinky included dipole antenna, they pulled the shielding and put it on one dipole as the negative, and then took the inside and merely connected it to the other dipole antenna.There's a resistance here? I mean it must certainly be negligible?

author: uer166
author_rank: Super Contributor
timestamp: April 12, 2024, 11:39:16 pm
content: Quote from: Fried Chicken on April 12, 2024, 11:30:06 pmIf you're asking about the radiation impedance, then that "resistance" as you called, is a physical manifestation of the wave interaction. You want to have it either match to your coax transmission line impedance (50Ohms), as well as your receiver input (also 50Ohms), or convert it to 50Ohms if it isn't already. You place that matching conversion thingy at the antenna feed point as discussed.There is another thing called loss resistance, which you want as low as possible, since it just converts current to heat. You can't do much about this other than minimize in antenna design. You can expect 50-80% antenna efficiency (50-20% of energy lost to heat) in general. It's caused mostly by skin effect where current in antenna elements crowds to the metal surface.Comment about pizza pan antenna: since you're not using a balun (and an image in one of your links showed coax soldered to one of the pans), it is not a full dipole. It's a simple monopole antenna with a GND plane of a specific shape. Since the element is round, you get lots of bandwidth out of it, though efficiency probably not the best.If you make it out of carbon steel, which has a resistance many times that of copper, the efficiency is yet more shit.

author: A.Z.
author_rank: Super Contributor
timestamp: April 13, 2024, 06:56:14 am
content: Quote from: ahbushnell on April 12, 2024, 09:23:00 pmSIGHanother "internet expert"

author: A.Z.
author_rank: Super Contributor
timestamp: April 13, 2024, 07:14:47 am
content: Quote from: Fried Chicken on April 12, 2024, 11:28:15 pmthat "pan" antenna will possibly work ok for ADS-B which is in the GHz range, but NOT for ATC which is down to the 100s MHz range, again, consider a discone if you want to get some acceptable performance over the frequency range you are interested in

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 13, 2024, 01:48:56 pm
content: Quote from: A.Z. on April 13, 2024, 07:14:47 amWait really? Why are all these RTLSDR lunatics recommending this if the rabbit ears work better? Well what else am I going to do with the pizza pans I have now?Does a discone antenna need to be oriented vertically?

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 13, 2024, 01:55:49 pm
content: No no no no no no no.This is all a distraction. I still want to know what's going on with the dipole antenna length. If I have a dipole antenna at full wavelength, wouldn't it pick up exactly nothing in a theoretical perfect world? Would it cancel out? If that guy's video is to be believed this is exactly the case....But is a radio measuring across the two sides of the dipole, or is it measuring across the two sides of the dipole and then the ground? Maybe I should ask this in a physics forum. I really want an ideal theoretical answer, not a practical one. I believe it's a simple question and there should thus be a simple answer no?

author: A.Z.
author_rank: Super Contributor
timestamp: April 13, 2024, 02:13:20 pm
content: a 1/2 Lambda dipole will offer good matching at 50/75 Ohm and a bidirectional pattern with the lobes perpendicular to the dipole axis, making the dipole longer you'll loose the match (but it can be recovered) and the lobes will split into multiple smaller ones, and when you approach or go over a full Lambda the lobes will point more and more toward the ends of the armsnote: a 1/2 wave dipole is one whose arms are 1/4 Lambda each, so the total lenght from end to end is 1/2 Lambda (1/2 wave)

author: radiolistener
author_rank: Super Contributor
timestamp: April 13, 2024, 08:56:06 pm
content: Quote from: Fried Chicken on April 12, 2024, 11:30:06 pmThis is a bad cheap antenna, because such connection leads to use outer braid of coax cable as a part of antenna. So, technically this is not dipole, because it uses coax cable and your computer shell as a part of antenna. And such antenna will receive a lot of noise from your computer and from all electronic equipment placed across coax cable path.In your case one arm of antenna is a half length of such dipole, but the issue here is that second arm is a half length of such dipole + the length of your coax cable + length of your computer shell + length of mains wire + etc. And the second arm is not a part of dipole, but complicated geometry wire.Proper connection of coax with dipole antenna needs to use balun and RF choke in order to match symmetric antenna with non-symmetric coax cable and block unwanted currents between antenna and outer braid of coax cable.It prevents to sucking of noise from equipment around coax cable path and eliminates exposure of nearby equipment with high strength RF field from coax cable during transmission.Half wavelength dipole in a free space has about 73 Ω impedance. So it well match with 75 Ω coax cable, but there is needs balun in order to match symmetric dipole antenna with non symmetric coax cable.Since both have the same impedance, you can simply use 1:1 transformer for that. Since primary and secondary coils are isolated it prevents current flow from antenna to outer braid surface of coax cable, in that way coax cable braid is excluded from antenna.

author: A.Z.
author_rank: Super Contributor
timestamp: April 13, 2024, 09:12:54 pm
content: a simple W2DU choke could solve that issue, yet, that antenna sucks for a number of reasons

author: radiolistener
author_rank: Super Contributor
timestamp: April 13, 2024, 09:25:03 pm
content: Quote from: A.Z. on April 13, 2024, 09:12:54 pmSimple RF choke cannot eliminate parasite currents to enough level, because they are very high. It can reduce these currents, but can't eliminate it at all. Using balun to match symmetric antenna with non-symmetric transmission line is more easy way to eliminate these currents.So, the best way is to use balun. And then, for better results, you can add some additional RF chokes to block currents due to capacitive/inductive coupling of coax cable braid with antenna within near field zone.

author: A.Z.
author_rank: Super Contributor
timestamp: April 13, 2024, 09:27:09 pm
content: you know what a W2DU choke (and BalUn) is, don't you ?

author: radiolistener
author_rank: Super Contributor
timestamp: April 13, 2024, 10:00:19 pm
content: When I said "RF choke", I mean specific case - ferrite bead on coax cable braid or winding the coax cable in the form coiled coax choke or both in combination.And when I said "balun", I mean specific case - the component which is especially designed to match balanced and unbalanced lines, it may include impedance match and it needs to break coax cable and solder/connect balun component in the middle.What I wanted to say exactly in this case is that using 1:1 transformer balun is better way to match coax cable with dipole than using ferrite beads on coax cable or coiled coax choke.The reason is that it needs to use a lot of ferrite beads or use a lot of coax windings to get good enough performance and it will be very inconvenient to use. While adding 1-3 ferrite beads or coax windings will be not enough to get good performance in comparison with transformer balun.But ferrite beads or coiled coax choke can be added in addition to transformer balun in order to reduce effect of capacitive/inductive coupling of coax cable outer braid with antenna within near field region. For better result it should be placed at 1-2 meters away from antenna. While transformer balun needs to be placed at feeding point of antenna. Such combination allows to achieve the best results.

author: A.Z.
author_rank: Super Contributor
timestamp: April 13, 2024, 10:19:10 pm
content: ok, you don't know what a W2DU (aka Maxwell) choke/balun is, and how, when and why it was born; fine, no problem

author: radiolistener
author_rank: Super Contributor
timestamp: April 13, 2024, 11:11:06 pm
content: Quote from: A.Z. on April 13, 2024, 10:19:10 pmI know about W2DU balun which he talking on his book "Reflections III - Transmission Lines and Antennas":As you can see and as I said above, this is specific case which I meant exactly when I said "RF Choke".Do you notice that Walter Maxwell wrote in his book "many baluns embody some form of coupling transformer"?This is what I meant when I said "balun".From my point of view it is pretty clean. But I don't understand what you're not agreed with, could you please explain?

author: MartinL
author_rank: Regular Contributor
timestamp: April 14, 2024, 02:09:32 am
content: Fried Chicken: unfortunately, this thread has diverged from your quite reasonable beginner's questions about antenna theory, into an argument between experts seeking to demonstrate who among them is the bigger expert. Perhaps said experts could take their dispute elsewhere?Let's come back to your questions.Quote from: Fried Chicken on April 13, 2024, 01:55:49 pmThat video is, quite simply, wrong.The argument it presents is that in a full wavelength dipole, you would need to have currents travelling in opposite directions within each element, and that those opposing currents would have to cancel each other out, resulting in nothing being radiated from or received in the antenna.But the whole idea of those opposite currents cancelling out is false in the first place. The narrator seems to think that either you can't have two currents flowing in opposite directions within the same piece of wire, or that if you do, their effect will somehow cancel out such that there is no radiation from the antenna.But when talking about AC, it is perfectly normal to have different currents flowing at different points along a wire. In fact it's impossible not to. Whenever you have an AC current flowing in a wire, then the current in that wire will vary, all the way from one direction to the other and back again, at intervals of one wavelength all the way along the length of the wire.Those "opposite" currents at different points do not cancel out, because they are not in the same places. In fact, they are all part of the same AC current. An AC current does not just vary over time: it also varies over space.And their conclusion, that a full-wave dipole could not radiate, is also false. In reality a full wave dipole will radiate or receive just fine.The reasons that half-wave dipoles are preferred are practical, rather than fundamental:- A resonant antenna acts as a filter that preferentially receives or transmits a particular frequency.- A half-wave dipole is the shortest resonant antenna for a given frequency, and therefore the smallest and cheapest to build.- The impedance at the centre of a half-wave dipole is one that is easy to work with.These properties are of great concern when e.g. building a transmitting station at long wavelengths, but in your case, none of them are critical:- Your receiver is very capable of selecting what frequency to listen to and filtering out others; a resonant antenna can help, but isn't essential.- In the case of your rabbit-ears antenna, you're free to vary its length to whatever works best within its range; there's no cost to extend it fully.- Your receiver has more than enough variable gain to compensate for loss due to the antenna impedance not matching that of the radio.As such, you're quite likely to find that a different configuration - e.g. extending the rabbit ears fully to get a larger antenna which can collect more of the available signal - will get you better results than trying to achieve a half-wave dipole configuration.Quote from: Fried Chicken on April 13, 2024, 01:55:49 pmSimple answer: it is measuring across the two sides of the dipole. There doesn't need to be any "ground" involved. A dipole antenna works just fine floating in space. Having a planet nearby is a complication, rather than a requirement, for this type of antenna.More correct answer: it is not really measuring across the two "sides", but rather measuringat the feed point. By the feed point, I mean the point in the middle where the antenna has been split. Usually for a half-wave dipole this is in the middle, but you can place the feed point anywhere. The position of the feed point will determine the impedance seen by a radio connected at that point.

author: iMo
author_rank: Super Contributor
timestamp: April 14, 2024, 12:47:45 pm
content: As the OP is interesting in reception only (that is my current understanding) the special/exact tuning of his "dipole" antenna is not necessary..Moreover, you would need a broadband balun in the feedpoint of that dipole (Edit: on that picture there is a "balun/common mode choke" on the coax but it should be closer to the feedpoint, imho), otherwise with unbalanced feeder (the coax cable) one leg of the dipole gets grounded.But again - with that antenna on your picture above and your SDR receiver it does not matter whether the antenna will be cut exactly for a resonance (and a dipole or "doublet" is a single band antenna anyhow).So simply calculate the lamda/4 for a frequency of choice and set the length of each leg to that number. That it is.In case you want focus your reception on special bands (especially vhf/uhf) I would recommend an yagi antenna as that would help you with the gain and directivity much more.PS: the "dipole antenna" when made properly is lambda/2 of length (left leg + right leg), where a single leg's length is lambda/4.In reality it will be a little bit shorter as it has been said above.The coax length plays no role in the calculation (it is not a part of the dipole) when with proper impedance matching, it means when the impedance of coax used is the same as the impedance of the dipole's feedpoint and the input of your SDR receiver.The impedance of the dipole itself is the impedance in its feedpoint (the point between the legs, in your case at that plastic part), and it depends on the angle between right and left leg.Thus with changing the angle between the legs in your kit you may do some SWR improvement (with legs in the same plane the impedance is around 73ohm, with some angle like 30-40degree off the plane it will be around 50ohm).So basically you may tune your telescopic antenna (as depicted in your kit) with the leg's length (frequency) and leg's angle (impedance).You will get the best signal when the legs are set to the lambda/4 each and the angle to aprox 30-40degree off the plane, provided your coax is 50ohm and the SDR's input is 50ohm as well (with max signal strength when the transmitter is in the max of the front/back lobe of your dipole's radiation pattern and, of course, the transmitter's antenna with the same polarization as your antenna is - it could be "vertical" or "horizontal").

author: ahbushnell
author_rank: Frequent Contributor
timestamp: April 14, 2024, 01:47:13 pm
content: Quote from: fourfathom on April 12, 2024, 10:59:11 pmFind a link.

author: A.Z.
author_rank: Super Contributor
timestamp: April 14, 2024, 02:26:55 pm
content: Quote from: ahbushnell on April 14, 2024, 01:47:13 pma link ? check the pdf I attached (reply #25), then if you want a linkI may post it tooOh and then, if you desperately want a link, you may try this onehttps://www.w8ji.com/antennas.htm

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 16, 2024, 05:21:49 am
content: Quote from: MartinL on April 14, 2024, 02:09:32 amThis offers a lot of clarity, and explains how I can pick up frequencies my antenna theoretically shouldn't pick up. The radio picks up FM stations without any antenna attached at all! I tried finding Walter Lewin's lecture where he explains and builds a working radio antenna (and trolls Harvard by making an illegal broadcast in their name).

author: radiolistener
author_rank: Super Contributor
timestamp: April 16, 2024, 11:24:15 am
content: Quote from: Fried Chicken on April 16, 2024, 05:21:49 amThe example of antenna in visible optic band is a telescope. You can see Mars on the sky without telescope, and you can see it through piece of water pipe. But if you want to see detailed picture, you're needs to use properly designed telescope.Exactly the same thing for radio antenna. You can see some signal with using rusty nail, you can even use it for transmission. But if you want to get good sensitivity and listen good quality radio with no noise interference and with no distortions, you're needs to use properly designed antenna.The main issue of a random wire antenna is that it is not selective for the frequency of interest and it sucks the noise from nearby equipment, so the useful signal is flooded in a high noise interferences.Using random wire instead of proper antenna is like using random piece of glass instead of proper telescope to see Mars on the sky.

author: Fried Chicken
author_rank: Regular Contributor
timestamp: April 12, 2025, 04:47:14 pm
content: Quote from: uer166 on April 12, 2024, 11:39:16 pmI made it out of aluminum using 16" Pans.I think it gets slightly better reception than the "dipole" I was using? I connected the ground plane at multiple points, and now shoved it in the attic using the existing house coax wiring.I think it's slightly better than the antenna I was using before

author: A.Z.
author_rank: Super Contributor
timestamp: April 13, 2025, 09:52:39 am
content: Quote from: ahbushnell on April 14, 2024, 01:47:13 pmooooohh... a link changes the game for you, I seetry this onehttps://www.voacap.com/2023/antennas/squeezing-decibels-out-of-dipole/no video, sorry, you'll have to read

author: Andy Chee
author_rank: Super Contributor
timestamp: April 14, 2025, 12:54:57 am
content: Quote from: A.Z. on April 13, 2025, 09:52:39 amI think you're a year late to reply to that one

author: A.Z.
author_rank: Super Contributor
timestamp: April 14, 2025, 06:48:38 am
content: agreed, I missed that post, and it needed a reply

author: A.Z.
author_rank: Super Contributor
timestamp: April 18, 2025, 01:48:14 pm
content: Quote from: Fried Chicken on April 11, 2024, 06:18:32 pmthere' s no "two dipoles" just the two ARMS of a SINGLE dipole antenna, the TOTAL lenght is usually 0.5 Lambda so each arm is 0.25 Lambda

author: A.Z.
author_rank: Super Contributor
timestamp: April 18, 2025, 05:19:59 pm
content: oh and since I'm at ithttps://www.aa5tb.com/efha.htmlHTH