doublet tuning

[p.larner]

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?.

[iMo]

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.

[p.larner]

so f i make the windowline longer does that bring the resonance lower in freq?.

[p.larner]

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?.

[A.Z.]

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 idea

https://www.m0pzt.com/80m-cobra-doublet/

and put the antenna in "open air"

[Andy Chee]

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.

[vk4ffab]

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?.

The 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.
 

[A.Z.]

just in case

https://ftp.unpad.ac.id/orari/library/library-sw-hw/amateur-radio/ant/docs/Introducing%20the%20All-Band%20Doublet.htm

http://on5au.be/content/a10/wire/abd.html


notice that you WANT a 1:1 BalUn

https://www.m0pzt.com/g3txq-tuner-baluns.png

and 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

[A.Z.]

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 one

https://pa0fri.home.xs4all.nl/ATU/FRImatch/FRImatcheng.htm

it 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 page

http://dl4cs.de/funktechnik/ant/fri-match/en.htm

and 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

[A.Z.]

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

[p.larner]

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.

[A.Z.]

how is the windowline routed to the shack ? hope not through walls/floors it isn't a coax and if it isn't in free air or, worse, passes nearby electrical cables, metal pipes and so on ...

[p.larner]

the 450windowline feeder hangs down from the loft hatch strait to the balanced terminals of my sem tranzmatch atu.

[A.Z.]

the 450windowline feeder hangs down from the loft hatch strait to the balanced terminals of my sem tranzmatch atu.

distance 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

[p.larner]

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?

[A.Z.]

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?

see my loop suggestion.

[vk4ffab]

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?

The 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.

[p.larner]

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

[vk4ffab]

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

You 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.

[A.Z.]

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

[p.larner]

so i will ask once more what is a better way to know whitch leg to change to try and get them equal?.

[A.Z.]

so i will ask once more what is a better way to know whitch leg to change to try and get them equal?.

pick a pair of RF AMmeters and measure the currents, you...

[p.larner]

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

[A.Z.]

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

[vk4ffab]

so i will ask once more what is a better way to know whitch leg to change to try and get them equal?.

I 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 quote

 Notice 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.htm

I can also recommend this book as well. https://home.arrl.org/action/Store/Product-Details/productId/2012451048

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

The 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.