Help with high swr on wire diapole 11 mtrs

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original45
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Help with high swr on wire diapole 11 mtrs

Post by original45 » 05 May 2016, 17:07

Followed a guide to make a wire antenna today. I have cut my wire for each side to be finished at 8' 6" .
Both sides are equal . I have insulated both ends with rubber o rings larder type.
Both ends on both wires are folded back and twisted around itself so length is exactly 8' 6".

The centre as I intend using it as inverted v I have from the wire end where it is folded back I have soldered 9" of cable to provide my feeds to the coal pl .

From what I can see and tell I have followed instructions correctly I intend to use the higher channels 30/40 on the uk 27/81 fm frequencey.

I connected swr mtr and its way out . I'm getting swr of 5 right in the red.
I untwisted my trim end and increased length by 1" made no differance.
Before I attempt to swr it again can someone confirm the wire lengths I need . And that the extra 9" I attatch end from the folded back wire in centre is or isn't relevant to the wire length in total each side.

Connections are clean and solid .

Help please .
Cheers

VintageTin
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Re: Help with high swr on wire diapole 11 mtrs

Post by VintageTin » 05 May 2016, 17:54

Worth adding a 1:1 balun at the feed point.
Malcolm

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Buick Mackane
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Re: Help with high swr on wire diapole 11 mtrs

Post by Buick Mackane » 05 May 2016, 18:00

Is the wire you used for the elements insulated ? Here is a dipole calculator, Just punch in the desired frequency and hit calculate

http://www.csgnetwork.com/antennaedcalc.html
Some days will stay a thousand years, Some pass like the flash of a spark, Who knows where all our days go ~ Stuart Adamson.

original45
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Re: Help with high swr on wire diapole 11 mtrs

Post by original45 » 05 May 2016, 18:23

I will look at a Balun .

I just calculated wire length should be 16" 9 and 7/32 for 27 .91125 . I did reduce length after by folding wire back at my trim end. Didn't lower swr.
But will try again tomorrow.

Yes all the wire I have used is still insulated accent the fed end which I bared half inch to solder.
It's white thick wire with two layers of insulation black inner grey outer I think it is switch wire .
Should I strip all insulation off then ?.

Never made one before and can't have a antenna fitted upside now .

Everything I read said it was simple . Tut.

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Buick Mackane
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Re: Help with high swr on wire diapole 11 mtrs

Post by Buick Mackane » 05 May 2016, 18:27

original45 wrote:
Yes all the wire I have used is still insulated
well where it doubles back at the ends will add to the overall electrical length. You said something about adding an extra 9* Where too ?
Some days will stay a thousand years, Some pass like the flash of a spark, Who knows where all our days go ~ Stuart Adamson.

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prettypolly
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Re: Help with high swr on wire diapole 11 mtrs

Post by prettypolly » 05 May 2016, 18:44

Has the wire been outside a long time is it new old it could have water in gression or if you want you can have my old wire dipole Inc free post .

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Re: Help with high swr on wire diapole 11 mtrs

Post by original45 » 05 May 2016, 18:47

Ah ok .what I have done is I've used insulated wire.
I measured out 9 ,3" of wire. I market my length that I thought was correct 8' 6 " and folded both wire ends back over and twisted the extra back down the wire element.

The end tail on my feed end once folded back I then added a 9" piece to connect to the s.o plug that my pl will connect into.

So if I understand you correctly even if I have folded excess back down the element wire this effects antenna length.

If I'm right then even if I fold back wire against itself to form the ends around the insulators this wire has to be part of the full length and I guess the centre feed too then .
If this is correct I'm well too long or have I miss understood .

Thanks for the help bit above me this but I will get their . With help.
Cheers

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Re: Help with high swr on wire diapole 11 mtrs

Post by Buick Mackane » 05 May 2016, 19:04

original45 wrote:
So if I understand you correctly even if I have folded excess back down the element wire this effects antenna length.
yeah, had you used uninsulated wire it would have terminated to the correct electrical length when you folded it back, So looks like your a few inches long either side, By the way iv'e never made a dipole, But thats my understanding of it,
Some days will stay a thousand years, Some pass like the flash of a spark, Who knows where all our days go ~ Stuart Adamson.

original45
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Re: Help with high swr on wire diapole 11 mtrs

Post by original45 » 05 May 2016, 19:10

I just had a search on net ,looks like you are correct if wire is insulated and folded back down itself it acts in its full length as the antenna.
So I need to re measure and trim trim trim.

Not made one ,well that makes two of us ,however you have the knowledge
. And thank you for sharing it.

Thanks for the offer of buying old wire but that would be too easy prettypolly .

Will try again tomorrow and see how she goes.
Thank for the guidance will report back results on swr once re cut .
Cheers

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prettypolly
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Re: Help with high swr on wire diapole 11 mtrs

Post by prettypolly » 05 May 2016, 20:38

It's a full working dipole swr 1.1.1

2E1IIP

Re: Help with high swr on wire diapole 11 mtrs

Post by 2E1IIP » 05 May 2016, 21:15

A Dipole fed with 50 Ohm coaxial cable will read correctly about 1.5:1 due to the miss-match, you will see a lower VSWR due to feeder losses.

PVC coated wire will appear electrically longer for a given length per frequency due to the velocity factor (about 0.95 for PVC coated cable), so if you cut the wire for the exact dimensions at the center band you will find that it is too long and will need trimming.

So using the formula 300/f therefore 300/27.500 MHz = 10.9m * 0.95 = 10.35m giving you a half wavelength of 5.17 m, 2.58m each leg which will get you close. (8.48 feet in old money)

Using a BalUn will not match the 75 Ohm dipole to your 50 ohm feed line, it will however limit the chance of your coax radiating and distorting the radiation pattern, a simple common mode choke consisting of 5 turns of coax about 5" to 6" diameter will be enough.

Good luck.

original45
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Re: Help with high swr on wire diapole 11 mtrs

Post by original45 » 05 May 2016, 21:38

Wow thats very impressive , I will make a Balun and follow your 8.48feet measurement.

Thank you very much for the full explanation .

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Re: Help with high swr on wire diapole 11 mtrs

Post by kr0ne » 05 May 2016, 22:46

I notice you mentioned the wire has double insulation, I hope there is nothing in the outer insulation to strengthen/armour it that might have a negative impact! Just a thought...

You can still fold back insulated wire to shorten it, RF behaves very differently from DC or low frequency AC and will largely ignore the insulation. After all, it gets out and into the Ether without too much bother! It's not entirely straightforward though and the insulation will have an effect if you do this.

There is a short but intersting article at the below link where an American amateur describes his experiences doing this. It's not a guide how to do it yourself but it might help paint a clearer picture for you. ;)

http://n6mw.ehpes.com/FoldWriteUp.pdf

original45
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Re: Help with high swr on wire diapole 11 mtrs

Post by original45 » 06 May 2016, 10:12

Had a go this morning.
I rimmed equal length each side whilst checking swr taking a few mm each time. I believe I'm at 102 inches now .
Initially swr dropped from 5/6 down to 2.9.
Then as I chipped a little more off swr went back up to 3.2 ish.

Not sure if this is relevant but when I touched swr mtr case the needle dropped a minute but more like needle thickness.

I have been testing it in my garden and neighbor one side has a 16' trampoline of steel construction so I'm going to cut a new wire and go to open field to re test.
Although I'm not sure it's close enough o radiate back so much?.

Bit disappointing but if I'm honest too whilst my knowledge is little in his I've enjoyed making it.

This is a hand held CB radio I'm testing it with,this wouldn't be an issue would it ?.

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Re: Help with high swr on wire diapole 11 mtrs

Post by original45 » 06 May 2016, 10:22

Thanks for the link I have just read it but will read it again and refer to it.
As for double insulation the inner cover is more like rubber black in colour. The outer looks just like mains cable grey in colour.
IDE guess it's not armoured
I'm ground to leave the wire straight tie clip tightly ends of bottoms of legs to nylon cord so no bends back on legs base.
And try simular up top at feed ends still insuring their insulated.

I had even tried adjusting leg angle in and out no differance their.

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