Winding an Un-Un

[Squarewave]

Hello.

I attempted to make my own 9:1 unun but something isn't quite right.

I have one which I bought years ago, so basically made a copy of it. I used my VNA to measure it with a 450ohm resistive load across the ground and antenna connection to give me an idea what to look for. It's a good match and the return loss is an acceptable level.

So the one I made is wound on FT140-43. Frequency sweep is 3mhz t0 30mhz. At the lower frequency end, it starts off ok, but then the SWR goes crazy high around midway across the sweep all the way to the end of the sweep.

I checked and checked and checked again, windings are ok, connections are ok, but for some reason, as the frequency goes up, so does the SWR and at the moment I'm lost as to why that is? Any ideas welcome!

[T3sl4co1l]

How many turns, how arranged?

Tim

[A.Z.]

as said, it depends from the turns count which, in turn depends from the core material, in your case, start from this design

https://vk6ysf.com/unun_9-1_v3.htm

which uses your same core, then, willing to further flatten the response, you may experiment by inserting a fixed capacitor across the coax connection, a value between 100pF and 150pF may be a good starting point, just ensure that the capacitor can stand a voltage of at least 500V or (better) more

Also, ensure to use non inductive resistors for the 450 Ohm load, when running your VNA tests !

[Squarewave]

How many turns, how arranged?

Tim

Trifilar, started off with 5 like my bought unun, then tried 9 which other build threads have done, both no good.

[Squarewave]

as said, it depends from the turns count which, in turn depends from the core material, in your case, start from this design

https://vk6ysf.com/unun_9-1_v3.htm

which uses your same core, then, willing to further flatten the response, you may experiment by inserting a fixed capacitor across the coax connection, a value between 100pF and 150pF may be a good starting point, just ensure that the capacitor can stand a voltage of at least 500V or (better) more

I'll try that one and see what happens.

The bought unun, I think is 43 material, but measures out to be a FT114-43 and uses two of them stacked. With my 140-43, I've tried two and one.

Also, ensure to use non inductive resistors for the 450 Ohm load, when running your VNA tests !

Yes, as I said above, resistive load.

[A.Z.]

as said, it depends from the turns count which, in turn depends from the core material, in your case, start from this design

https://vk6ysf.com/unun_9-1_v3.htm

which uses your same core, then, willing to further flatten the response, you may experiment by inserting a fixed capacitor across the coax connection, a value between 100pF and 150pF may be a good starting point, just ensure that the capacitor can stand a voltage of at least 500V or (better) more

I'll try that one and see what happens.

The bought unun, I think is 43 material, but measures out to be a FT114-43 and uses two of them stacked. With my 140-43, I've tried two and one.

Also, ensure to use non inductive resistors for the 450 Ohm load, when running your VNA tests !

Yes, as I said above, resistive load.

you "think" is a #43 or IS IT a #43 ? Because you can't tell the material from the toroid size ! And the material characteristics will determine the required windings, the rule of thumb, whenever choosing a material for a given RF transformer, is to pick the material type which, at the given frequency range, requires less windings; a good starting point for infos is

http://toroids.info/

then, if you are not familiar with RF transformers calculations, there are several online calculators which may help you; as for my capacitor suggestion, it will have negligible/no effect on a purely resistive load, to see what it does, you'll need to add some reactance

[Squarewave]

The core I'm winding on is a 43, bought from a store selling Fairite branded cores.

The one I think is a 43, is the bought unun which measures spot on with the VNA.

[Squarewave]

I'm winding now according to the website you linked to, then will pop in on the VNA for test.

[Squarewave]

Exactly the same terrible results.

Go to the bought unun and it's fine 

[MarkF]

Not my bag.  But this might help.

https://youtu.be/VAV9Wws-Bs0?t=75

[Squarewave]

No, doesn't help, but thanks anyway.

So, no idea why, but if I do exactly the same as any build thread/video, I can't get it to work. It should be obvious that I'm doing something wrong, so I check, then check again, then again, it's exactly the same wind but the same terrible results.

[T3sl4co1l]

Should be this one, for anyone curious.
https://www.fair-rite.com/product/toroids-5943002701/

So that's what, about 4cm turn length?  And it's three windings stacked to make an autoformer?  So the overall electrical length is 4cm * 5t * 3 = 60cm, or 1/4 wave about 125MHz, less velocity factor (twisted pair probably 80% so 100MHz).

Shouldn't be too bad.

LF limit is, let's see, AL = 0.8 so 5t is 20uH, somewhat less at ~MHz; into 50 ohms that's 0.4MHz cutoff.  So say 4x above that should be okay, which seems to be the case here.  Maybe more if you're putting a lot of power into it (watts?) because the core material is a bit lossy by some MHz.

Photo of the build?

Tim

[Bud]

The bought unun, I think is 43 material, but measures out to be a FT114-43 and uses two of them stacked. With my 140-43, I've tried two and

The bought unun may actually have two different material cores to better cover the frequency range. Did you measure both cores or just one?

[A.Z.]

No, doesn't help, but thanks anyway.

So, no idea why, but if I do exactly the same as any build thread/video, I can't get it to work. It should be obvious that I'm doing something wrong, so I check, then check again, then again, it's exactly the same wind but the same terrible results.

could you post some pics of the unun you wound ?

[Squarewave]

The bought unun, I think is 43 material, but measures out to be a FT114-43 and uses two of them stacked. With my 140-43, I've tried two and

The bought unun may actually have two different material cores to better cover the frequency range. Did you measure both cores or just one?

They look both the same in size, as the top one is easy to get to, that's what I measured.

[Squarewave]

No, doesn't help, but thanks anyway.

So, no idea why, but if I do exactly the same as any build thread/video, I can't get it to work. It should be obvious that I'm doing something wrong, so I check, then check again, then again, it's exactly the same wind but the same terrible results.

could you post some pics of the unun you wound ?

Should be this one, for anyone curious.
https://www.fair-rite.com/product/toroids-5943002701/

So that's what, about 4cm turn length?  And it's three windings stacked to make an autoformer?  So the overall electrical length is 4cm * 5t * 3 = 60cm, or 1/4 wave about 125MHz, less velocity factor (twisted pair probably 80% so 100MHz).

Shouldn't be too bad.

LF limit is, let's see, AL = 0.8 so 5t is 20uH, somewhat less at ~MHz; into 50 ohms that's 0.4MHz cutoff.  So say 4x above that should be okay, which seems to be the case here.  Maybe more if you're putting a lot of power into it (watts?) because the core material is a bit lossy by some MHz.

Photo of the build?

Tim

I'll post photos of what I've done so far. Two attempts have been taken apart for another attempt. Will also try again in a moment.

[A.Z.]

if possible, post also some pics showing the setup you're using to measure your unun

[Squarewave]

Here's a photo of one I recently wound on a FT240-43 using 5 turns.

Results also attached.

I'll shortly put up the results for the bought unun and post a photo.

[Squarewave]

The bought Un Un

[Squarewave]

Oh, bare in mind, the small wire I'm using is for bench test purposes, I'm not wanting to waste any more enamelled copper wire.......

[Squarewave]

The enamelled wire on FT140-43

[A.Z.]

Here's a photo of one I recently wound on a FT240-43 using 5 turns.

Results also attached.

I'll shortly put up the results for the bought unun and post a photo.

shorten those wires, lay them out properly to avoid parasite coupling and use a coax connector, also, maybe I'm wrong, but those resistors don't seem to be non inductive to me; your VNA setup to test your unun is badly screwed, imVHo

[Squarewave]

There is a coax connector.

Did you see the results for the bought unun? Do those results with the same resistors look unacceptable? I thought they were ok as at worse, SWR is just above 1.33 which is fine for matching.

[Squarewave]

Going to test this one next

Tested

[T3sl4co1l]

The resistors are fine.  Carbon or metal film type axial resistors have a spiral stripe that becomes inductive up in the GHz (and even then, for fairly low R values, up to say 200-300 ohms; above which, lead capacitance dominates instead).

The stray wiring is not however!  Trim that tight and short between core and connector(s).

Tim