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 Re-hanging my 40m delta loop - Which balun? 
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Post Re-hanging my 40m delta loop - Which balun?
When I had my full wavelength 40m horizontal delta loop up previously, I fed it with a 4:1 current balun and it worked incredibly well.

Martin, G8JNJ plotted it for me and you can see the full details by clicking here.

He suggested that I might get a bit less loss using a 1:1 current balun rather than a 4:1 but I'd expect the impedance of the loop to be a couple of hundred ohms so surely the 4:1 is a better choice?

I appreciate that another way to do it is with a stub of 75 ohm feeder but a) I don't have any and b) the feed point will be right by where the feeder enters my loft so I don't want to run it in close proximity to all the crap in my there.

I've got the 4:1 400w balun I used previously but I also have a 1:1 1.5kw balun.

I can't find the original wire so I'll be building a new one if the weather holds off tomorrow. I'll start with the wire at something like 42m long and trip as required.

Which would you suggest and why?

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Sun May 06, 2012 6:16 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
You won't need a balun just use a quarter wavelength of 75 ohm satellite coax if you're struggling for 75 ohm coax... obviously that's only if you're using the loop for 40 metres only Keith.

Full wave loop is supposed to be about 100 ohms so a 4:1 balun won't help, my last horizontal 40 metre loop was set out in a square and fed using cheap satellite coax spliced with two PL259s and a barrel connector... no problems powering it with my trustee FL 2100B and the feed point was on the section above the ridge on my house surprisingly no TVI just set fire to the loft insulation... joking, it worked ok but as it literally described the perimeter of the back garden which was tiny it had to go... replaced it with a groundplane and that did a great job of receiving QRN, much better than the loop. :blackeye:

I assumed the velocity factor for the satellite coax was .66 and pruned to fine tune.

Cheers, Dave.


Sun May 06, 2012 7:56 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
I've got a lump of 75ohm coax perfect for the job somewhere in one of my sheds. I bought two lengths incase I cocked up when I made mine.

I use it on mine and it works fine and dandy.

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Sun May 06, 2012 7:58 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
Hmm, well I do want it to be multiband.

Surely if I use 75 ohm stuff for matching, that will be part of the aerial and will radiate?

Plus, 400 watts up some TV coax?

When the aerial was up before with the 4:1 balun, it worked really well. Inter G and EU was superb and I also worked DX on it as well.

I made a note of the SWR measurements previously before I took the loop down, I didn't have a VNA then so I couldn't check anything else but here they are.

40m
7.000 – 1.5:1
7.100 – 1.4:1
7.200 – 1.5:1

20m
14.000 – 1.6:1
14.100 – 1.7:1
14.200 – 1.8:1
14.300 – 1.9:1
14.350 – 1.9:1

15m
21.000 – 1.6:1
21.100 – 1.6:1
21.200 – 1.6:1
21.300 – 1.6:1
21.400 – 1.6:1
21.450 – 1.6:1

10m
28.000 – 1.5:1
28.500 – 1.3:1
29.000 – 1.5:1
29.500 – 1.9:1
29.700 – 2.1:1

It's not matching the coax, the SWR is only at those levels in the bands so it's on those which are harmonically related to the primary wavelength.

It's such a quandary!

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Sun May 06, 2012 11:17 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
Unless you're running digimodes flat out I'd expect that the "TV coax" would be fine.

If you don't want a monobander don't use 75ohm coax.

I wanted a very efficient antenna for 40m. I didn't want to loose power in the balun and wanted to make the most out of every last drop of power I put into it.

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Mon May 07, 2012 8:33 am
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
Hmm, I may have been talked into this. It's always been a multiband aerial but I suppose I do have the beams for higher up.

I also do run data modes at full tilt during contests so perhaps something like Webro WF125 would be more suitable than boggo TV or satellite cable. It's a bit thinner than RG213 but not much so it'd go into a standard PL259 easily enough.

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Mon May 07, 2012 9:48 am
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
I bought rg216 or whatever it is called that is 75ohm stuff from wh westlake. The spare stuff I have is thinner but would handle the power okay I'd guess. I think G3TXQ said it'd be fine for the run.

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Mon May 07, 2012 9:51 am
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
I only used mine for SSB on 40 Keith and it genuinely was cheap crap coax... copper weld from memory with a mylar foil screen as well as the barest of copper braid.

The 1/4 wavelength of 75 ohm coax is a matching stub... Steve will explain, neatly side stepping the fact that I can't. :blush:

This is weird though as I was looking at sky loops the other day... Twilight Zone music playing in the background.

Cheers, Dave.


Mon May 07, 2012 8:14 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
OK, I've done the calculations and I think I've got the correct measurements.

(299.8 / 7.1) x 1.05 = 43.27m of wire for the loop

(75 / 7.1) x 0.66 - 6.97m matching stub.

That's assuming a centre frequency of 7,100kHz and a velocity factor of 0.66 for the 75 ohm coax to be used as the stub.

The stub looks a little short but the figures shouldn't be telling porkies. Obviously I'll cut the loop a little long to allow for adjustments.

With a stub that length I'll need to patch some RG213 on the end for it to reach the shack.

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Wed May 09, 2012 10:21 am
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
Feed the loop at the nearest point and your existing feed coax is probably long enough, assuming that you have an existing feed coax that is... the 75 ohm coax section goes on the end of your 50 ohm feed and then split apart and on to the loop.

I was reading about the stub this morning in my old ARRL antenna handbook while laying in the pit and they described it as an impedance transformer with a range typically between 50 and 600 ohms, from memory my 75 coax was about 22.5 feet long Keith.

Cheers, Dave.


Wed May 09, 2012 8:21 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
6,970mm looks right for the quarter-wave transformer; the loop dimension will vary depending on height, and particularly on whether the antenna wire is insulated or not.

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Steve G3TXQ


Sat May 12, 2012 3:47 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
Interesting, I didn't realise insulated or non insulated would make much difference to wire length - I'll just make sure I build it long so I can trim it.

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Sat May 12, 2012 6:13 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
g6nhu wrote:
Interesting, I didn't realise insulated or non insulated would make much difference to wire length

I've seen anything up to 5% difference; 3% is a typical figure.

5% would move a 14.1MHz antenna down to 13.4MHz !!

Steve G3TXQ


Sat May 12, 2012 6:42 pm
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
I'm going to have extra coax in the loft when I do this because I'll just connect the 75 ohm coax to my exisiting run, is it worth winding the spare cable into an ugly balun?

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Thu May 17, 2012 8:45 am
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 Re-hanging my 40m delta loop - Which balun? 
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Post Re: Re-hanging my 40m delta loop - Which balun?
Suck it and see. I used an ugly balun when my antenna was multi banded but the second I turned it into a single band antenna the RFI pretty much stopped.

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Thu May 17, 2012 8:57 am
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