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 UnUns - what is really happening! 
Silent Key

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Post UnUns - what is really happening!
"Regular readers" will know that I've been pretty scathing in the past about Baluns and UnUns wound on Iron Powder cores like the T200-2. I've even done the maths to show that at low frequencies they will depart massively from the impedance transformations they are designed for. Nevertheless, folk continue to use them and seem to be happy with the results. That led me to make some detailed measurements over the past few days on a 4:1 UnUn wound on a T200-2 core; the results are very interesting:

Firstly, I used EZNEC to predict the feedpoint impedance of a 43ft vertical over an excellent radial system on 80m; the answer was 11-j273, which is an SWR of 107:1. If we then insert a perfect 4:1 UnUn we would expect the impedance to drop by a quarter to 2.75-j68, which is an improved SWR of 52:1.

However, when I measured the real UnUn with an 11-j273 load, instead of the expected input impedance of 2.75-j68 I measured 367+j480 ...... crazy!!

What is going on? It turns out that at low frequencies where the UnUn makes a very poor transformer because of its low winding reactance, it begins to look more like a simple inductor. The reactance of that simple inductor cancels to some extent the capacitive reactance of the short vertical, and the combined tuned circuit formed from the non-ideal UnUn and the vertical actually raises the feedpoint impedance. The result is an even lower SWR than if the UnUn had been ideal!!

To see just what it means across the whole HF range, I built a 40ft vertical this morning and swept its SWR with and without the 4:1 UnUn. Here are the results - the red trace is with the UnUn and the orange trace without:

Image

As you can see, nearly everywhere the UnUn improves the SWR. The only exception is around the 1/4 wave resonance at 5.6MHz and the 3/4 wave resonance at 16.8MHz.

To illustrate how "non-ideal" the UnUn is, I also plotted the feedpoint impedance with and without the UnUn:

Image

With an ideal UnUn the bottom trace would always be 1/4 the value of the upper trace; as you can see it only gets close to that above 17MHz or so. Look down at the 80m region and you can see how the UnUn is transforming the impedance higher!

In summary: a UnUn wound on Type-2 material will massively depart from being an ideal transformer at low frequencies; but it turns out that the way it departs from the ideal can actually improve things if the antenna is electrically short at those frequencies!

What I'm left wondering is if the commercial suppliers of these UnUns and Verticals actually know all this and designed it to be so ..... or was it just "serendipity"?

73,
Steve G3TXQ


Wed Jun 22, 2011 2:18 pm
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 UnUns - what is really happening! 
Silent Key

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Post Re: UnUns - what is really happening!
Out of interest, this morning I swapped the 4:1 UnUn for a 9:1 and re-scanned. Here's a comparison - red trace is 4:1 and orange trace is 9:1

Image

The 9:1 does better on 80m, 30m, 20m, 15m and 12m; the 4:1 is the better on the other bands. Taken across all bands, the 9:1 is probably the better option for this particular vertical.

73,
Steve G3TXQ


Thu Jun 23, 2011 1:56 pm
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 UnUns - what is really happening! 
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Post Re: UnUns - what is really happening!
Interesting results Steve, serendipity or by design indeed :shock:

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Fri Jun 24, 2011 8:42 pm
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 UnUns - what is really happening! 
Silent Key

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To complete the story, here are the calculated losses for four different UnUns used with that vertical: a 4:1 wound on a T200-2; a 9:1 wound on a T200-2; a 4:1 wound on a FT240-61; and a 4:1 wound on a FT240-31:

Image

At an average power level of 100W, 0.6dB is the maximum loss you could tolerate for the FT240 size core, and 0.45dB would be the maximum you could tolerate for the T200 size core. So, used with that vertical, a 4:1 or 9:1 wound on a T200-2, or a 4:1 wound on a FT240-61 would be OK at 100W; none of them could tolerate average powers much above 100W - you'd need to go to a larger core or stack cores.

Steve G3TXQ


Fri Jun 24, 2011 9:52 pm
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 UnUns - what is really happening! 
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Post Re: UnUns - what is really happening!
I have to say, looking at the build quality of MOST of the commercial baluns (except a couple of "proper jobs") then I'd say serendipity rules the roost !!
We measured a 9:1 with a mini VNA at a club the other month, just "cos we could", and it went up and down like the preverbials drawers, without really being effective anywhere.
I shall spare the makers blushes by not saying the name.
Although it Seemed Rather Cheap to me !! :lol: :lol: :lol:

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Sun Jun 26, 2011 8:50 am
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 UnUns - what is really happening! 
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Post Re: UnUns - what is really happening!
Does the length of wire you use for the radiator have much bearing on the results Steve?


Sun Jun 26, 2011 11:45 am
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 UnUns - what is really happening! 
Silent Key

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G6TJS wrote:
Does the length of wire you use for the radiator have much bearing on the results Steve?


Across a range of different radiator lengths it seems that there's not much to choose between the 9:1 UnUn and the 4:1 as far as SWR is concerned except at the lowest frequency band; there the 9:1 is marginally better. What does change of course is the loss: the shorter you make the radiator the higher the UnUn losses, whatever the ratio.

Also, we shouldn't forget that short radiators result in some pretty high voltages. One UK manufacturer advertises a 19ft vertical for use on 80m at 150W pep. For that antenna to actually radiate 150W there would need to be 8kV peak across the UnUn! Of course that doesn't happen in practice because losses in the ground system and the UnUn ensure that the antenna radiates only a small fraction of the power applied.

73,
Steve G3TXQ


Sun Jun 26, 2011 3:32 pm
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 UnUns - what is really happening! 
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This thread has been interesting reading. A friend bought a 9:1 unun from a company who sells 10m poles with a wire up the centre, then a pole off eBay and taped a wire up it. After adding a single 10m radial as the commercial device he wasn't very happy with it. We recently opened up the unun black box after breaking the superglue. Inside was a ferrite ring with 9 turns of flat twin mains cable wrapped around it and around 2" of co-ax. It didn't seem to conform to diagrams of other 9:1 unun's I'd seen on the web. It's wired as follows. The red wire antenna terminal is connected straight through to the co-ax SO239 socket via 2" of co-ax. The co-ax braid is connected to the SO239 socket. The twin mains cable windings are in effect 2 trifilar windings. These are in series. The top end goes to the wire antenna red terrninal and the centre of the SO239 via the co-ax, the middle to the co-ax braid and SO239 outer, and the bottom to the black earth terminal.

All 9:1 unun drawings I've seen have used 3 trifilar wound windings and are completely different from this device. Is this a normal 9:1 unun?

Additionally the soldering is poor and there were 2 solder splashes stuck to the plastic case inner, and more solder stock to heatshrink covering the center of the 2 windings to the co-ax inner join. Not very neat for a pro built device.

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Sun Jun 26, 2011 5:04 pm
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 UnUns - what is really happening! 
Silent Key

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Steve,

Interesting - I've just drawn that out and if I have it right it's certainly NOT a 9:1 UnUn. It's actually a sort of 4:1 voltage balun with some unfortunate characteristics:

1) If you were to connect the black earth terminal to a good radial system you would force the SO239 GND to float above earth to a potential equal to that of the transmitted signal. That will drive masses of current along the coax braid.

2) If you happened to connect the black earth terminal to the SO239 GND at the box - perhaps because you connected some radials to the black terminal and the SO239 - you will have shorted out the lower bifilar winding; that in turn forces a short circuit across the SO239 !!!!!!

The 2" of coax does nothing - it might just as well have been a single piece of wire.

73,
Steve G3TXQ


Sun Jun 26, 2011 5:30 pm
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 UnUns - what is really happening! 
Silent Key

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Steve,

This is how I interpreted your description:

Image

If that's correct, imagine we apply an input voltage Vin across the SO239; that means there must be Vin across winding W1; and because it's acting as a transformer there must also be Vin across winding W2. That means the mid-point of the transformer and the metalwork of the SO239 MUST be forced to Vin volts above the black ground terminal.

If we then shorted the metalwork of the SO239 to the black terminal - my dotted line - you've shorted out the W2 winding, and than translates to a short across W1, and a short across the SO239.

Horrible device!

73,
Steve G3TXQ


Sun Jun 26, 2011 5:43 pm
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 UnUns - what is really happening! 
Silent Key

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I just remembered that one of the big US antenna companies made the same error a few months ago and supplied a device wired as a 4:1 voltage balun instead of a 4:1 UnUn with their untuned vertical. It became a hot topic on the eHam forum, and the company eventually conceded their mistake.

Steve G3TXQ

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Sun Jun 26, 2011 5:47 pm
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 UnUns - what is really happening! 
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This is a picture of the horrible thing. Excuse the grubby bench. Yes, your drawing matches mine.


Attachments:
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IMG_0011.jpg [ 77.83 KiB | Viewed 1139 times ]

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Steve M0SVB
Sun Jun 26, 2011 5:55 pm
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 UnUns - what is really happening! 
Silent Key

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Yep - it's a conventional 4:1 voltage balun!

Your friend can easily turn it into a 4:1 UnUn by moving the coax braid to the black terminal, and the coax centre to the bifilar centre tap.

73,
Steve G3TXQ


Sun Jun 26, 2011 6:04 pm
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 UnUns - what is really happening! 
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Post Re: UnUns - what is really happening!
G3TXQ wrote:
Steve,

Interesting - I've just drawn that out and if I have it right it's certainly NOT a 9:1 UnUn. It's actually a sort of 4:1 voltage balun with some unfortunate characteristics:

1) If you were to connect the black earth terminal to a good radial system you would force the SO239 GND to float above earth to a potential equal to that of the transmitted signal. That will drive masses of current along the coax braid.

2) If you happened to connect the black earth terminal to the SO239 GND at the box - perhaps because you connected some radials to the black terminal and the SO239 - you will have shorted out the lower bifilar winding; that in turn forces a short circuit across the SO239 !!!!!!

The 2" of coax does nothing - it might just as well have been a single piece of wire.

73,
Steve G3TXQ


That's an interesting observation you make Steve, I had a discussion with a chap re his rally bought 10m, 9:1 UnUn fed, antenna and told him he would need a good radial betwork when installing it, he told me that the manufacturer advised him that it does not need any radials and should not be connected to any, what I take from this is he knows exactly what would happen if the end user did install a ground network.

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Sun Jun 26, 2011 7:22 pm
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 UnUns - what is really happening! 
Silent Key

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G7DIE wrote:
That's an interesting observation you make Steve, I had a discussion with a chap re his rally bought 10m, 9:1 UnUn fed, antenna and told him he would need a good radial betwork when installing it, he told me that the manufacturer advised him that it does not need any radials and should not be connected to any, what I take from this is he knows exactly what would happen if the end user did install a ground network.


You might be interested to see the plots here:

Image

These were SWR scans made on my 40ft vertical "testbed". The blue trace is the SWR with no UnUn, no radials and no choke; it's doing roughly what you might expect a 40ft vertical to do, with 3 "dips" around 5.6MHz, 16.8MHz and 28MHz.

Now look what happens if you put a choke on the coax and stop current flowing on the braid - the red trace; the SWR is nearly always greater than 50, and where it does dip the dips are at the "wrong" frequencies. That's what happens when you prevent the coax acting as a counterpoise.

In summary:

* Use a UnUn if you want to reduce coax losses cause by high SWR
* Use radials and a choke if you want to keep current off the braid
* Use no radials and no choke if you want current to flow along the outside of the coax
* Whatever you do don't try a choke with no radials - you've left nowhere for the ground current to flow.

Steve G3TXQ


Sun Jun 26, 2011 8:20 pm
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