Electronics > RF, Microwave, Ham Radio

Winding an Un-Un

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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:

--- Quote from: T3sl4co1l on May 14, 2022, 03:40:55 am ---How many turns, how arranged?

Tim

--- End quote ---

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

Squarewave:

--- Quote from: A.Z. on May 14, 2022, 07:04:06 am ---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

--- End quote ---

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.


--- Quote from: A.Z. on May 14, 2022, 07:04:06 am ---Also, ensure to use non inductive resistors for the 450 Ohm load, when running your VNA tests !

--- End quote ---

Yes, as I said above, resistive load.

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