doublet tuning - Page 1
Forum section: electronics / rf-microwave

author: p.larner
author_rank: Frequent Contributor
timestamp: November 24, 2023, 06:08:28 pm
content: i have a doublet,its a lashup but its the best i can do with the space i have,it consits of a feedpoint indors at the apex of my roof,one leg goes under the roof tiles and lays on the roof towards the gutter,the other leg zigzags on my rafters,its fed with about 12ft of 450 ohm windowline,it has its lowest swr at about 3.2 Mhz,it is needed to be multiband from 80 and up,it seems too long,do i shorten the antenna elements ot the windowline feeder?.

author: iMo
author_rank: Super Contributor
timestamp: November 24, 2023, 06:22:35 pm
content: I would shorten the dipole's legs (elements). Do it carefully, not to cut off too much of the wire.The windowline feeder does not radiate when the legs are perfectly symmetrical, in your case it will radiate most probably.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 24, 2023, 09:20:21 pm
content: so f i make the windowline longer does that bring the resonance lower in freq?.

author: p.larner
author_rank: Frequent Contributor
timestamp: November 25, 2023, 04:37:38 am
content: it may seem a daft question but getting to the element legs is a pita,can i test each leg as an endfed with my nanovna get each one where i want then connect the balun and measure from its output?,at the moment its lowest swr freq is about 3.2 ish Mhz,so will shortening the feeder raise the lowest swr point,i assume the wndowline is adding to the length of the antenna elements?.

author: A.Z.
author_rank: Super Contributor
timestamp: November 25, 2023, 07:59:33 am
content: a multiband doublet isn't usually resonant, just use an atu at the bottom of your ladder line, and if you are space constrained you may consider this ideahttps://www.m0pzt.com/80m-cobra-doublet/and put the antenna in "open air"

author: Andy Chee
author_rank: Super Contributor
timestamp: November 25, 2023, 08:07:28 am
content: Despite descriptions of doublet antennas as “all-band”, there are definitely combinations of feeder length and dipole length that may be untunable by your ATU.https://kv5r.com/ham-radio/ladder-line/ladder-line-page-3/An example of a good ATU might be one that can match antennas between 10-250 ohms. A crappy ATU might only be able to tune 16-150 ohms. Check your ATU manual.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: November 25, 2023, 08:25:02 am
content: Quote from: p.larner on November 25, 2023, 04:37:38 amThe long answer:SWR and resonance are 2 different things. The feedpoint impedance of a dipole cut to resonance is about 75 ohms. That equals and SWR of 1:1.5, when its fed with a 50 ohm feedline. Now consider a doublet, which is a dipole being fed with 300, 450 or higher ohm feedline. The feedpoint impedance has not changed, its still just a dipole and if its cut to resonance its feed point impedance is about 75 ohms but its being fed now with a 300 ohm feedline. The SWR is now 300 divided by 75, or 1:4.No idea if you care about the CW or SSB portions of the band, but for argument sake, if 3.6Mhz has an swr of about 4 to 1, connect that feedline to a balanced tuner, fiddle the knobs a bit so the radio sees 50 ohms, key up and have some fun.The point of using an open line feeder in a multiband antenna situation like this is so you do not have the SWR losses associated with using a lower impedance feedline. Without boring you with a whole load math, feedline losses are inversely proportional to the characteristic impedance and a doubling of the feedline impedance halves the losses.The Short Answer: Do as A.Z. said and connect it up and get on the air.

author: A.Z.
author_rank: Super Contributor
timestamp: November 25, 2023, 02:58:05 pm
content: just in casehttps://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htmhttp://on5au.be/content/a10/wire/abd.htmlnotice that you WANT a 1:1 BalUnhttps://www.m0pzt.com/g3txq-tuner-baluns.pngand that the CHARACTERISTIC impedance of the window/ladder line has NOTHING (ok almost) to do with the impedance seen at the end of the line

author: A.Z.
author_rank: Super Contributor
timestamp: November 26, 2023, 09:12:10 am
content: also, to match the doublet to your RTX you'll need a capable ATU, and most ATUs built into rigs, won't suffice, so... if you are ok with homemade stuff, have a look at this onehttps://pa0fri.home.xs4all.nl/ATU/FRImatch/FRImatcheng.htmit will serve you well from 80 up to 10 meters (and maybe 6 meters - didn't try it up there), pick the schematic from DL4CS further down the pagehttp://dl4cs.de/funktechnik/ant/fri-match/en.htmand it will be fine, to connect it to your window line, just use a 1:1 Guanella and, if needed, a short run of coax; notice that adding relays (in place of the rotary switch) and a couple of stepper motors (or R/C servos to turn the variable caps) it will be easy to obtain a remote tuner

author: A.Z.
author_rank: Super Contributor
timestamp: November 26, 2023, 12:49:54 pm
content: a suggestion, pick a spool of insulated wire and use it to form a loop around your roof, connect the ends of the loop to a run of window line dropping down, outside the building and kept, using spacers, at (at least) 20 inches from the walls, place a 1:1 at the end of the line and from there, enter the shack using a run of coax, connect the coax to a tuner (e.g. the FriMatch) and try the antenna

author: p.larner
author_rank: Frequent Contributor
timestamp: November 29, 2023, 06:56:03 am
content: theres a bit of a saga with this,the swr went high on many bands,i started pruning the legs to get a match with my tuner,after a lot of buggering about i found one leg of the 450 windowline has a break,getting to measure each leg length is not possible,can i test each leg at the end of the windowline(tuner end) against ground with my nanovna and try and match the legs that way as i didnt take note of how much i removed from each leg.

author: A.Z.
author_rank: Super Contributor
timestamp: November 29, 2023, 03:22:36 pm
content: how is the windowline routed to the shack ? hope not through walls/floorsit isn't a coax and if it isn't in free air or, worse, passes nearby electrical cables, metal pipes and so on ...

author: p.larner
author_rank: Frequent Contributor
timestamp: November 29, 2023, 05:17:23 pm
content: the 450windowline feeder hangs down from the loft hatch strait to the balanced terminals of my sem tranzmatch atu.

author: A.Z.
author_rank: Super Contributor
timestamp: November 29, 2023, 07:34:09 pm
content: Quote from: p.larner on November 29, 2023, 05:17:23 pmdistance it from the hatch and the , try the loop setup i suggested, use a 1:1 (a guanella will do. fine) to pass from twin to coax, enter using coax to the ATU

author: p.larner
author_rank: Frequent Contributor
timestamp: November 30, 2023, 01:51:06 pm
content: the doublet legs are unequal/unbalanced,how can i tell what one needs to be longer to match them,i cant get to both ends of each leg,only the atu end,can i measure each leg as an endfed against ground using my nanovna and look for the swr dip on the lowest band,ie80m?

author: A.Z.
author_rank: Super Contributor
timestamp: November 30, 2023, 08:22:11 pm
content: Quote from: p.larner on November 30, 2023, 01:51:06 pmsee my loop suggestion.

author: vk4ffab
author_rank: Frequent Contributor
timestamp: November 30, 2023, 09:53:05 pm
content: Quote from: p.larner on November 30, 2023, 01:51:06 pmThe antenna will always be unbalanced because the install does not have symmetry and is not away from obstructions, one leg is installed on the roof, the other zig zags. So in reality, it does not matter all that much that the legs are unequal in length. So pretty much do not worry about it. The antenna is never going to be a performer because you have to make so many compromises just to install it.Tune it up, get a match and call CQ. Sometimes, forgetting 80m has to become part of the compromise. Unless you have a great need to join in the 80m geritol net to tell the bloke down the street you are not dead yet, sticking to 40m and up does make life a whole lot easier. In my old place in the city, I had 2 bands only 40m and 15m and between the 2 of them I worked dxcc with SSB and CW only with QRP power levels in a high noise environment. And then that antenna was a coil shortened dipole because I only had space for an antenna 15m long.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 01, 2023, 01:59:04 am
content: the issue is at the moment the missmatc kills caps on my chineese kit atu-100,i have 500v silver mica replacements but getting the antenna legs the same length would be a start,please tell me how i can measure there length as i originaly asked instead of going off on a tangent.73

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 01, 2023, 05:33:26 am
content: Quote from: p.larner on December 01, 2023, 01:59:04 amYou are at qrp power levels yeah? And you are blowing 1Kw rated caps in an L match tuner? Last I read you were using a manual Z match. If you go and read your other thread about ganging air caps I started to explain some fundamentals. You are going to have to do some work here to understand what is going on and then do a much better job of explaining your question, which I answered,the length of the legs in your antenna do not matter and will never be electrically equal even if they are physically equal to the billionth of a millimeter(bold for emphasis) because all the obstructions they face being near metal adds capacitance, zig zagging adds inductance. Its a dogs breakfast and you might not like the answer, but that does not mean the answer is a tangent, it means you do not understand things. Electrical and physical length are two different things.If you want to measure their length, pull them down and use a tape measure. You cant do a TDR because a single wire is not a transmission line. I get that you want to bring down the swr viewed at the tuner to about 50 ohms, so just cut 2cm off the end of each leg until you achieve that. Yes the ends, not the middle, not the feedpoint, the ends because if you cut at the feedpoint and then pull the wires to shorten them it will change the stray capacitance and inductance from the obstructions and throw everything off again. If you cant get to the ends then it sucks to be you, we cannot rewrite physics to suit your compromises.I am going to assume you measure off 2 20m lengths of wire to make the elements and not just hung random lengths of wire in your roof. If you did that, then they are near enough to being physically equal and if you want to change the impedance seen by the antenna port on the tuner, you need to trim the ends, or lengthen them until you get 50 ohms at the tuner. Oh and one last thing, do not use the ATU-100 with a balanced feeder, its not designed to work that way, you need to use a balanced tuner.

author: A.Z.
author_rank: Super Contributor
timestamp: December 01, 2023, 12:00:24 pm
content: then, there is to say, that a multiband doublet is NOT resonant, by design, and for some good reasons, but the OP seems to have his own ideas about it "things must work as he thinks" and will probably discard any suggestion which is against his "how I think it MUST work" paradigm, it would make more sense to explain a blind man a color than trying to convince him to understand how things really work... oh well

author: p.larner
author_rank: Frequent Contributor
timestamp: December 02, 2023, 10:54:12 am
content: so i will ask once more what is a better way to know whitch leg to change to try and get them equal?.

author: A.Z.
author_rank: Super Contributor
timestamp: December 02, 2023, 12:41:10 pm
content: Quote from: p.larner on December 02, 2023, 10:54:12 ampick a pair of RF AMmeters and measure the currents, you...

author: p.larner
author_rank: Frequent Contributor
timestamp: December 02, 2023, 05:30:31 pm
content: i dont have rf ammeters only a nanovna,besides that doesent answer the question i asked!,if you cant ansew my specific question then please dont waste everyones time with replies that do not answer what asked ok.73

author: A.Z.
author_rank: Super Contributor
timestamp: December 02, 2023, 05:39:14 pm
content: well build it, just a handful of pieces from the junkbox, nothing special, then if you want to sheet out some $$$ you may go for an MFJ-835 but considering the simplicity of the circuit (just a diode, some resistors and a meter plus a switch or two meters) I think it may be worth building it, and then, maybe LEARNING something, also since the ham/hobbyist is usually someone who builds stuff and learns, and not one which pays $$$ and is just a button pusher and a knobs fiddler... ok that comes later on

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 02, 2023, 09:24:44 pm
content: Quote from: p.larner on December 02, 2023, 10:54:12 amI know you think we are all idiots talking out out butts, so here is something from the authority on the subject of antenna L. B. Cebik, W4RNL. And I quoteNotice that the precise length is not critical.https://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htmI can also recommend this book as well.https://home.arrl.org/action/Store/Product-Details/productId/2012451048Quote from: p.larner on December 02, 2023, 05:30:31 pmThe fact that you dont understand the answer does not mean he did not answer the question, with a pair of rf ammeters you can measure the currents in each leg of the antenna at the same time and prune the ends until you see the same current in each leg. Meaning they are electrically the same length. Its still a pointless thing to do because it does not matter with a doublet that the legs are the same length to any degree of precision.As for me, I wont be wasting anymore time on your threads, if you cant grok basic facts then its beyond any of us to help you learn. Perhaps elmer google will give you answers you wanna hear. 73 OM i wish you the best of luck.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 04, 2023, 09:47:24 am
content: just looked at the smith plot at 3.775 megs on my vna on each leg against ground,one reads 92 ohms the other 250 ohms,both points are in the upper half of the chart and close to the rh side,any ideas how to get it nearer 50 ohm?.73

author: A.Z.
author_rank: Super Contributor
timestamp: December 04, 2023, 11:49:15 am
content: Quote from: p.larner on December 04, 2023, 09:47:24 amw/o knowing how you installed the whole thing and the window line it's impossible to answer, I don't think people here has some kind of "crystal ball" or can "read your mind", if you want to be helped you'll need to offer as much details (and possibly images) of your setup, otherwise you'll be on your own, and then you'll also need to learn to listen to suggestions instead of pretending that things work "as you want" instead of "as they work for everyone else"

author: p.larner
author_rank: Frequent Contributor
timestamp: December 04, 2023, 02:29:18 pm
content: the antenna is a lashup but its the best i can do as i live in a top floor flat in a concervation area,it is approx 2 legs of 20m long so 40m total,its centerfed with 450 ohm ladderline thats about 4m long,the feedpoint is indoor at the apex of my roof,one leg goes under the tiles down one side of the roof,then from the edge of the roof comes back indoors and runs up the side of my stairs,the other leg zig zags over the roof joists,the legs are unbalanced do to not taking note when pruning,it goes thru a 1-1 balun into a homebrew tmatch amu,is an electricaly long antenna less reactive than a short one,ie will it be less stress on my atu matching on 80 if its longer than a half wave on 80?.

author: A.Z.
author_rank: Super Contributor
timestamp: December 04, 2023, 05:47:21 pm
content: there's nothing to prune in a doublet, reread the infos at the links which were already posted WITH CARE, make a checklist and check your antenna setupnotice that I suggested an horizontal wire loop "all around your roof" for some reasons, in particular. if you aren't able to run ladder line or window line all the way to (or near) the shack it won't be a bad idea considering an antenna fed using a transformer and a run of coaxand then, assuming you are in an HOA "environment" and want to put out your nose on as many bands as you can arrange and don't care about contacting "whatever", have a look herehttps://hflink.com/antenna/but please, do NOT try to ... no ok, that's up to you

author: Andy Chee
author_rank: Super Contributor
timestamp: December 05, 2023, 02:50:09 am
content: Quote from: p.larner on December 04, 2023, 09:47:24 amIf you can't squeeze any more antenna into your roof space, consider adding more feedline to get your SWR down.http://oz1db.dk/no_tuner/no_tuner_eng.htmlhttps://web.archive.org/web/20191219204049/http://www.w5dxp.com/notuner/notuner.htmEither that or build a better ATU which can match wider range.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 05, 2023, 04:07:07 am
content: but the smith plots are in the upper part of the screen so that says its inductive so too long for the freq?.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 05, 2023, 04:24:21 am
content: If you can post a picture of your smith chart, we can actually use it to calculate the correct feedline length to reach 50 ohms (or close enough to match).

author: p.larner
author_rank: Frequent Contributor
timestamp: December 05, 2023, 05:00:39 am
content: i doubt i can get a pic filesize low enough to post,at the moment i use my homebrew tmatch with a 4-1 balun,i would rather use my atu-100 tho,without it blowing its caps to bits that is.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 05, 2023, 05:16:02 am
content: would i be better of getting the antenna legs as long as possible and sweep that?.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 05, 2023, 10:31:22 am
content: Feel free to experiment! That’s what this hobby is supposed to be about!The thing to learn from this is that both dipole legs AND feeder length, affect SWR.(and of course objects in the vicinity of the antenna and/or feeder affect SWR. This includes the ground/earth/dirt as an “object” in the vicinity i.e. antenna height above ground).

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 05, 2023, 08:49:31 pm
content: Quote from: p.larner on December 04, 2023, 02:29:18 pmTHERE IS YA PROBLEM. Of course when you prune this antenna you will not see much of a change if any when you take off a cm or 2 because the antenna is more stray capacitance and inductance than it is actual antenna. Its kind of impossible. You can tune it up and hope for the best, but trying to do anything to make it more than that is futile.How much physical space do you have in the roof cavity? Can you draw a map with some dimensions on it so we can see what you are actually working with. But at a guess, I am going to say 80m is not going to be a band you have. 40m and up will be a distinct possibility depending on the space you are working with, if you use traps to electrically shorten the antenna. If you were to install an antenna in the following shape in the roof, how many lineal meters would that cover per leg? Would that equal 7 meters or there abouts? If you can get 7m per side this way, you can get a reasonable antenna going for 40m and up happening.Or if 80m is a must, install the antenna as I have drawn below, if possible, and wind the remainder of the wire onto a 100mm pvc pipe keeping about 1m between the coil ends. It will be kind of a coil shortened folded dipole then, it wont be a performer on 80m, but it will work. You can then prune the end coils down until you are happy with it on 80m and it will work reasonably well on 40 and up. Having coils on the ends will electrically shorten the antenna, but reduce bandwidth you will have to pick what end of the band you want, cw or ssb, it wont cover both. This way you can control the strays in the system somewhat and make your life easier. It will probably need a 1:1 at the antenna end and a 4:1 at the tuner end. Then you can use the ATU100 with it.

author: A.Z.
author_rank: Super Contributor
timestamp: December 07, 2023, 04:50:00 pm
content: The point is... an "all band" doublet is different from a RESONANT dipole, the second one can be pruned to bring it to resonance and obtain an impedance near 60 Ohm, the second one, that is the doublet is "NOT resonant" by design, you WILL need an AMU (Antenna Matching Unit) to match the impedance (R+X) seen at the end of the feedline and make the TRX "happy", also, the usual way to minimize feedline losses when using a doublet (or a similar "non resonant" antenna) is to use a "balanced line", that is ladder or window line, but the latter must be installed carefully, that is the line MUST stay away from other conductive objects and cannot be passed through walls the same way one does with coax, this means that if you can't run the balanced line "in the clear" (for most of the distance between the antenna feedpoint and the TRX) you'd better consider a different solution; some possible ones are an horizontal wire loop, which may be run all around the roof perimeter (and will be invisible) and which could then be fed at a corner using a 4:1 and a coax feeder, alternatively, if you can't (or don't want to) use an AMU, you may consider a terminated antenna like a T2FD or a Butterfly, those will offer an almost flat impedance curve over (almost) the whole HF bands and, again, can be fed using coax; then the whole idea of putting the antenna under the roof is acceptable ONLY and ONLY if you have a wooden roof/structure, if instead we're talking about bricks and/or reinforced concrete, then it's game over and you'd better try finding a way to put the antenna OVER the roofThen, assuming you've a wooden roof another possible antenna which can be fed using coax, is the fan dipole, in such a case you may consider something like thishttps://www.qsl.net/af4ps/iad.htmlthat being said, please connect the BRAIN before blindly trying stuff PRETENDING that it must work just because you "don't have time to read/learn"

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 08, 2023, 01:37:29 am
content: Quote from: A.Z. on December 07, 2023, 04:50:00 pmI am not sure that reading and comprehension are the OP's strong suits. As the saying goes, you can lead a ham to knowledge, but you cant make him think. That said, he lives in a council flat in the UK, so he wont have a lot of roof space to do anything, maybe 7m by 5m. So it would require a bit of creative thinking to make something usable, but its quite obvious he is not really after input. Sometimes its better just to leave them with it rather than waste your time.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 08, 2023, 06:02:41 am
content: heres the smith plot of one leg against ground,ive rejoined the wire i cut off of one leg,can only post one pic so will post the other leg pic on a new reply,due to file size limits

author: p.larner
author_rank: Frequent Contributor
timestamp: December 08, 2023, 06:03:55 am
content: other leg smith vna plot.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 08, 2023, 08:24:19 am
content: What does the smith chart look like when you connect both legs to VNA simultaneously?

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 08, 2023, 09:44:39 am
content: Quote from: Andy Chee on December 08, 2023, 08:24:19 amI wanna know what is ground when you only have a single wire? I am not sure he has actually measured anything that is real or at least useful. What he has done is replace one half of the antenna system with some part of another system, that does not give you anything of meaning other than this new antenna system has a feedpoint impedance of x, x of what in relation to what though? if you are not measuring the actual antenna system, or one half left floating, have you actually measured anything real?I could be wrong here, but his ground (not that it is a ground its the other half of the antenna system) should probably be a resistor with a value equal to the half the expected feedpoint impedance of the antenna system, at least then you are comparing apples to apples. Half a dipole is a monopole after all and to measure just one half of a dipole and have the measurement mean something, it needs to be referenced against a known standard, not the water pipes in the house or the earth of an electrical socket, but the other half of the antenna system, that way when the monopole becomes a dipole again after pruning, its feedpoint impedance should be close to what it should be because its referenced to a dipoles feed point impedance.I probably should perform the experiment and make sure my theory is fact, but it sounds legit in my head at least HAHAHA. Antenna are a system, if he wants a feed impedance of 50 ohms just cut bits off each end until that happens, they dont need to be the same length, off center fed is a thing even if its 30cm the feed point impedance has changed.

author: A.Z.
author_rank: Super Contributor
timestamp: December 08, 2023, 10:09:45 am
content: first, take your 1:1 BalUn, terminate it with 50R and sweep it with the VNAnext, connect the 1:1 to the end of the balanced line and repeat the sweep with the VNAPost the Smith charts resulting from both the above

author: p.larner
author_rank: Frequent Contributor
timestamp: December 08, 2023, 04:35:57 pm
content: the ground i refer too is all my copper pipework maybe in total 50m +,i live in an old house with 16 flats,all the pipework is linked.

author: A.Z.
author_rank: Super Contributor
timestamp: December 08, 2023, 05:07:17 pm
content: Quote from: p.larner on December 08, 2023, 04:35:57 pmyou have no idea about what you're saying nor know the difference between an electrical ground and an RF ground, you seem to be just guessing around and supposing, a nice way to waste YOUR time, not the same when it comes to OTHERS' time, but then it sounds like you don't care about other people's time value

author: Andy Chee
author_rank: Super Contributor
timestamp: December 08, 2023, 05:21:36 pm
content: will you be connecting the copper pipework to your ATU?If not, then VNA measurements connected to the copper pipework are irrelevant.

author: A.Z.
author_rank: Super Contributor
timestamp: December 08, 2023, 07:06:39 pm
content: Quote from: Andy Chee on December 08, 2023, 05:21:36 pmAndy... judging from the 4th reply herehttps://www.eevblog.com/forum/rf-microwave/low-voltage-jamming-for-practical-jokes/msg5210407/#newI think he's just a poor "wannabe" trollAndrew.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 08, 2023, 07:48:20 pm
content: I don't know how wise it is to mention the the use of "resonant breakers" on water pipes, lest add more confusion!https://www.arrl.org/files/file/Technology/tis/info/pdf/7910018.pdfIt's something I've always wanted to try out, but I've never had an opportunity or scenario to do so.

author: A.Z.
author_rank: Super Contributor
timestamp: December 08, 2023, 08:19:59 pm
content: Quote from: Andy Chee on December 08, 2023, 07:48:20 pmwell... he may just use inductive coupling to the pipes and then use an L/C to match the whole thing and use it as an antenna <grin>

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 08, 2023, 09:00:00 pm
content: Quote from: p.larner on December 08, 2023, 04:35:57 pmSo you could not tune the old antenna with 2 legs, how do you think you are going to prune this new antenna which is 50 meters of copper pipe on one side and 20m of copper wire on the other side and have joining the 2 wire legs back together actual have a meaningful result? You cant.Its time to start reading some books OM to have a better understanding of how antenna systems work. Failing that you need to start listening to, understanding and replying to questions from people in this thread so that you might learn something.Now, if you actually want some help, hit the quote button and say yes. Then go pull down the antenna and we will design something from scratch that will fit in the space you have and do as much of what you require as practical. Your situation is a lot of compromises and needs proper thought to get the best results and it starts by telling us just how big this roof cavity is? do you have a balcony? Is 80m a must?Other than that, it looks like no one is going to waste their time when the end result is futile.

author: p.larner
author_rank: Frequent Contributor
timestamp: December 09, 2023, 09:38:22 am
content: the pipes are just to give the vna something to work against when measuring the doublet legs,athough they are also connected to the case of my atu,my loft area is about 3x5m,i cant have anything visible as its a conservation area,one arm goes under the roof tles and runs down the roof then doubles back indoors on the first floor,80m is needed for an inter "G" net i go on daily,it seems to work ok but my auto atu dont like it,it keeps frying caps,just would be nice to tame it a bit to use the atu without caps poping.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 09, 2023, 10:27:35 am
content: Quote from: p.larner on December 09, 2023, 09:38:22 amThis is definitely NOT the way to measure the length of doublet legs!As already mentioned, equality of length is not important, your 1:1 guanella balun should be sufficient to handle any current imbalances.If your ATU is being burnt out due to impedance mismatches, you can insert more feeder line (or indeed prune feeder line). You can use the smith chart to work out how much feeder line you need, but you need to measure the antenna properly in the first place, NOT against the water pipe!!!

author: A.Z.
author_rank: Super Contributor
timestamp: December 09, 2023, 10:32:22 am
content: Quote from: Andy Chee on December 09, 2023, 10:27:35 amI already wrote him how to take the measurements. either he's an idoit <grin> or he is trying to troll.

author: Andy Chee
author_rank: Super Contributor
timestamp: December 09, 2023, 05:15:14 pm
content: Quote from: p.larner on December 02, 2023, 05:30:31 pmRF ammeter is an easy build:https://owenduffy.net/module/icm/Definitely worth having one to measure wire currents under radio key down conditions (i.e. your radio connected to the antenna instead of your VNA).Measuring the current in each leg of the windowline will allow you to adjust the doublet legs for balance (the currents should be equal).But again, if you're burning up caps in your ATU, this may or may not be a current balance problem. Using a 1:1 guanella balun should rule out balancing as a problem. So if you continue to blow up your ATU with your 1:1 balun connected, you know the problem has nothing to do with unequal doublet legs.

author: A.Z.
author_rank: Super Contributor
timestamp: December 09, 2023, 05:37:03 pm
content: Andy... wasting your time imVVVho

author: Andy Chee
author_rank: Super Contributor
timestamp: December 09, 2023, 06:17:25 pm
content: I've got time to waste.... otherwise I'd be trolling on another internet forum myself

author: A.Z.
author_rank: Super Contributor
timestamp: December 09, 2023, 06:20:26 pm
content: Quote from: Andy Chee on December 09, 2023, 06:17:25 pmdittoalthough... if this is trolling. then we're in a really poor situation

author: vk4ffab
author_rank: Frequent Contributor
timestamp: December 09, 2023, 09:09:20 pm
content: Quote from: p.larner on December 09, 2023, 09:38:22 amYeah this does not work like that. Well it does, but not in a way that is useful to a dipole. Essentially what you are trying to do is get each leg to about 35 ohms so when you join them back together as an antenna the feedpoint is 70 ohms IE resonance for a dipole. But if you do that with one half of the system with an unknown impedance, pruning down one leg to 35 ohms is in reference to the unknown other half of the antenna so the value is meaningless, when put back together as a dipole you wont know what you will get as the entire system has changed.Quote from: p.larner on December 09, 2023, 09:38:22 amOk, not a lot of room to play with.Quote from: p.larner on December 09, 2023, 09:38:22 amAnd 80m is a must for a net.I am no confined space antenna guru, but the options as I see them are a magnetic loop or hellically winding. Think CB antenna here, its a short fiberglass rod with wire coiled around it to make a long antenna much shorter. But instead of a fiberglass rod, use 40mm pvc tubing installed in such a way you can get to the ends to prune it. It will be quite narrow banded, but that should not be a problem. And plenty of people make ham stick dipoles for confined spaces, this will just be a bit bigger.The less bends and obstructions the more predictable the antenna would be. If you had a balcony and did not mind taking the antenna in and out whenever you wanted to use it there might be some better options. Other than a former, you need a 1:1 at the feedpoint and a 4:1 at your auto tuner.https://www.strictlyham.com.au/browse-by-type/antennas/mobile-antennas/hf80clA pair of these would make a usable 80m dipole that is only 4.4m wide. A single one could be put on a tripod on a balcony using your pipework as the ground plane, coult also wind a 80m monopole into a 7m squid pole bigger is better, none of these might not be very efficient and only be one band, but those are some other options. Short antenna might be less than 10% efficient, but on 80m for the short distances you have nationally it probably wont matter.Notice the theme, you need to electrically shorten the antenna.

author: A.Z.
author_rank: Super Contributor
timestamp: December 10, 2023, 08:24:32 am
content: here's another short antenna for the 80m bandhttps://www.nonstopsystems.com/radio/frank_radio_antenna_KGD.htmand here's an alternativehttps://www.loopantennai3vhf.com/_files/ugd/1aa222_185d0c04919743469c50caf82488bc1f.pdfand another onehttps://hflink.com/antenna/Broadband_Butterfly_Terminated_Dipole_Antenna_BBTD_house_roof_version_1a.jpgwhich may fit the available space, then to adjust the match, one will just need to adjust the termination resistor value

author: A.Z.
author_rank: Super Contributor
timestamp: December 10, 2023, 11:05:14 am
content: oh, and here's a quick-to-try antenna; pick a lenght of coax, say 3+5+3 given your available space, at one end of the coax solder a 50 Ohm resistor (or, e.g. four 200 Ohm resistors in parallel) ensuring that the resistors can dissipate around 60% of your max TX power, at the other end of the coax solder an SO-239 connector but REVERSING it, that is solder the coax braid to the center pin and the coax center conductor to the outer of the connector, lay the antenna (coax) to form an approx horizontal wide U shape in your available space, connect the coax to it through a 1:1 choke and give the antenna a try

author: p.larner
author_rank: Frequent Contributor
timestamp: December 12, 2023, 03:14:46 pm
content: well time for an update,i found all the pieces of flexweve i had prunned from the legs,i now get this when connected to my vna without a balun,this am i went on the barrometric net on 3.775,i got some 5-9 reports ranging from schotland to liverpool,average seemed about s5,most said its the best they have heard me,on the outside leg that runs down the roof,where it re enters the building it now runs at 90 degrees from where t enters the building,before it went strait up the stairs so was basicaly folded back on itself with just a few feed in height separating them,this is the plot from my vna with both legs connected to port 1,anyway it seems to be working the best it has done so far.,i was usng a 4-1 balun+my homebrew tmatch atu,73.

author: A.Z.
author_rank: Super Contributor
timestamp: December 13, 2023, 04:31:03 pm
content: Quote from: p.larner on December 12, 2023, 03:14:46 pmYou reallydon't get it and don't know how to use a VNA, do you?To test the antenna, connect a1:1 Guanella BalUnat the end of the 450 Ohm line, then connect the other end of the BalUn at the VNA input port and check the antenna.

author: A.Z.
author_rank: Super Contributor
timestamp: February 17, 2024, 03:57:16 pm
content: and ... instead, he's going on with his crap

author: p.larner
author_rank: Frequent Contributor
timestamp: February 18, 2024, 11:19:26 am
content: time for an update,i built a new L match atu,its based on a nye viking design,the main variable wide spaced cap was a 1000pf unit with a roller inductor,dont know the inductance as my component tester sees it as a resistor!,it matched the antenna fine on 40+80m but failed higher 20-15-10m etc,this needed about 4 feet adding to the windowline to match 80+40m,i stripped the cap and removed 2/3 of the plates,this worked for the higher bands but then did not have enough C for 80+40m,i then had 1400pf of high voltage silver mica caps switched in for the lower bands,it now matches ok from 80 thru 10,its been a lot of messing about,took a good 4hrs stripping the roller coaster due to bolts snapping due to rust,i have doorknob caps that i will replace the silver mica caps with at a later date,the only issue now is sorting the swr sensor out for the twin needle swr meter,its took some doing as i had a stroke a few years ago and a foot operation,i have issues walking and have lost most of the use of my left hand,been a battle but at least i am on air 80 thru 10,thank everyone for helping,cheers +73.+ some pics of the crusty roller before rebuilding.

author: tggzzz
author_rank: Super Contributor
timestamp: February 18, 2024, 11:26:30 am
content: Quote from: A.Z. on February 17, 2024, 03:57:16 pm... all over the forum in far too many threads.He's on my ignore list, but I still see the threads he startsEvery time one rises up into my sight, it goes on my "ignore thread" list.This forum has been going that way recentlyEDIT: my personal/individual irritation is, on its own, completely irrelevant of course.

author: p.larner
author_rank: Frequent Contributor
timestamp: February 18, 2024, 11:38:16 am
content: for tggzzz,sorry if i offended you but it was to say thanks for folks help and to update people on this saga,i tried to pm you to find you blocked my pm,seems you are a troll,its certainly the actions of one you display.

author: factory
author_rank: Super Contributor
timestamp: February 19, 2024, 01:12:35 pm
content: Quote from: tggzzz on February 18, 2024, 11:26:30 amThankfully Mr Alias has been banned for the second time, over on the vintage radio forum.David

author: A.Z.
author_rank: Super Contributor
timestamp: February 19, 2024, 08:24:18 pm
content: thanks...

author: vk4ffab
author_rank: Frequent Contributor
timestamp: February 19, 2024, 08:59:28 pm
content: Quote from: p.larner on February 18, 2024, 11:38:16 amDon't worry about them, just keep on doing what you are doing and try learning something new everyday.