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 What Balun/Unun should I use?

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Templar
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Templar

Call Sign : 26-CT-3550 Ham:- M7DEW
Posts : 66
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QTH or Location : Huddersfield West Yorkshire. UK
Equipment Used : Yaesu FT-450D. CRT SS 6900n. CRT Micron
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PostSubject: What Balun/Unun should I use?    What Balun/Unun should I use?  Icon_minitimeMon Jun 15, 2020 9:53 pm

Hi all.
A mate of mine has given me a homemade wire antenna (with a counterpoise) to put in my loft and try. It resembles a long wire endfed with as I said a counterpoise. His works fine, he doesn't use a Balun or a Unun, but I'm having difficulty with mine.
I'm trying to get it resonant on T5.
My Analyser is telling me the lowest my SWR will go is 5.5 this is after much trimming ect from an original reading of 13:1
However, my impedance and resistance (r & z?) (sorry don't know which way round it should be) ls reading 9 and 16 respectively. Now I do know these figures need to be at or around 50 ohms.
So my question is as in the title, what do I need to fit at the feed point to bring R&Z to 50?

Hope I've explained it well enough for you to understand what I'm trying to achieve,
because it baffles me. The wire as is receives quite well compared to my Dipole. I'm not however going to try transmitting until I get better numbers.
Cheers and thanks in advance for any advice.
Dale
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Northern Crusader
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Call Sign : M0GVZ
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PostSubject: Re: What Balun/Unun should I use?    What Balun/Unun should I use?  Icon_minitimeWed Jun 17, 2020 4:57 am

Impedance is "resistance" for AC which is what RF is.

You need to look at two figures. You need to find where X is which is the reactance which will be at or very near 0 when you get to the resonant frequency of the antenna. You then need to look at the value of R and see what that is. It won't be 50 Ohms however it should get to 50 Ohms a little bit higher in frequency than where X=0 is.

The problem with putting one in the loft is that it'll "couple" to any wiring or any ducting or copper plumbing piping also in the loft area which will make it appear electrically longer. You have an antenna analyser so I would widen the frequency range you're sweeping to well below the 11m band. In fact I'd probably start as low as 24 MHz and do a sweep up to 28MHz and see if there's a dip.

As for the BALUN/UNUN yes you need one. For 11m band I'd go buy a FT240-61 mix toroid and you want 12 turns of RG58 around it. You need to use a toroid rather than just making an air wound one because you'll need a lot of choking with that antenna and air wound ones are both very narrow in bandwidth and reactive so it's easy to get it wrong. 12 turns of RG58 on a FT240-61 mix toroid will give you >8k Ohms choking over about 5MHz well and truly covering both 10 and 11m bands and it'll be entirely resistive.



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43CT016
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PostSubject: Re: What Balun/Unun should I use?    What Balun/Unun should I use?  Icon_minitimeWed Jun 17, 2020 7:26 am

I’m not sure what analyser you have, but  impedance (z) is made up of resistive (R) and reactive (x) components, all helpfully measured in ohms, so Z could be your complex impedance and R the resistive component.  What analyser is it?  My old analyser will show resistive and reactive components, but not the breakdown of the reactive component.

Ideally you want as close to 50ohm resistive impedance as possible with no reactive components, or the reactive components cancelled out. My analyser would show this as R=50 and X=0, or thereabouts.

For your SWR to be that high though, something is obviously wrong, and if you do just have a piece of wire with no matching that’s a good place to start....but if it is wire and counterpoise, how is it wired exactly?  How long is the wire? Is it running near pipes or other wiring?

If you sweep it, does the SWR get noticeably better at other frequencies, and coincidently the impedance get closer to 50 ohms? If so, it’s a tuning issue. If not, it’s likely something else, like a major mismatch.

Typically an end-fed long wire of any type has an impedance far higher than 50 ohms, so not sure how your mate is getting a match with no transformer or unun, unless it’s not really a long wire at all.  Convention usually sees a 9:1 used to match a random wire (and in fact, it’s not random, but certain specific lengths known to work and not be multiples or fractions of the wavelength you’re working on) and a 49:1 or 64:1 transformer to match the far higher impedance of an end-fed half wave (EFHW) which is what I use as a multi-band antenna.

The fact you have figures way lower than this makes me wonder if something else is going on?

Bit more info on the antenna would hopefully help troubleshoot further.
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Northern Crusader
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PostSubject: Re: What Balun/Unun should I use?    What Balun/Unun should I use?  Icon_minitimeWed Jun 17, 2020 6:19 pm

As above, knowing the length of the radiating element would help. If it was over 1/4 wave/9ft long it'll be going over 50 Ohms no matter what you do, if it got to 1/2 wavelength long without a matching circuit you'd be looking at a feedpoint impedance in the thousands of ohms.
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