CM Choke on 49:1 transformer !?


I’m trying to put a 1:1 choke on my 49:1 EFHW transformer since I had problem with CMC’s when using digrig device on the phone.

So the idea was to put a second unun on top of the transformer, as a choke. Like I did for my 4:1 transformer at home. I couldn’t find any similar build on the internet and was a bit sceptical. However, I tried anyway, and here is the build:

Plugged to my 20m wire Inv V, I measured some SWR plots and it worked like a charm, apparently:

And someone told me: your curve is impossible unless you have a super lossy transformer

And then I decided to measure insertion loss… It seems like this guy is right, I measure -20dB of loss !!!

So what’s up with this ? Why can I not put this choke like I did ?



I believe you can do this but you need to have 0.05wl counterpoise connected to “ground” of transformer. If your CMC is perfect antenna would not have return path for RF.

I use following method to check losses in the transformer

Basically, antenna end of the transformer is connected to S21.

You measure all losses in the system from which you need to take away losses introduced by resistor (~2450 ohm). Measure resistor as precisely as you can and its losses are 10LOG(R+50/50) where R is measured value of resistor. Make all connections as short as you possibly can.

This method is described by Owen Duffy on his blog.

GL 73
Marek SP9TKW


Unfortunately it cannot be assessed based on the photo and text shown (without wiring and measuring arrangements).

Maybe the arrangement and wiring of my experimental 1:1 Guanella balun with connection socket for the short antenna leg can help you.


HB9BCB I believe mine is exactly the same as yours, as I can see from the pictures.

The 49:1 : 3 primary over 21 turns in total
1:1 CMC : 12 bifilar turns on each side (where you have 9)

What I don’t have is the counterpoise plug though. The two right ends of your 1:1 are directly soldered to the two entries of the 49:1 in my case.

I used a similar method but without any resistors. I did an open-short-load-isln-thru calibration and then measured logmag on channel 2. I will try with resistors even though I don’t really understand why they would be any necessary. I will read his blog as you suggest, he is usually very informative. Thanks

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This is the load to bring the output of the 1:49 impedance transformer (nominally 2450 ohms) to the input impedance of the VNA (nominally 50 ohms). In order to obtain a correct measurement result, the value of the load resistance would therefore have to be chosen to be 50 ohms lower… however, the effective transformation ratio of a real impedance transformer will usually be slightly smaller than the nominal one and thus cause a larger measurement error, hi.

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And here is the measuring fixture:

Yep ok, got it, thanks.

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I would suggest to make a two step process.

First measure just transformer + resistor and see losses in transformer only.

Then measure your CMC, which should be 50 ohm on input and output so you can connect it directly to nanoVNA.

Adding both results will give you total losses.

What’s more you can verify if both components are sufficiently efficient for your needs. At the end of the day one or the other might need some adjustment.

GL 73, Marek SP9TKW

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ok, i’ll take part in the guessing game :wink:

What Marek SP9TKW said seems to be a reasonable process. You shouldnt have so much loss. The 0.05lambda counterpoise is only needed for real operation, not for your measurement. Another remark: Your resistors are non reactive? You need to use resistors, without a complex part (inductive or capacitive).

Personally I had bad experiences with a metal housing for my Unun… so I would guess, that the toroids effect each other negatively. Try to seperate them, if possible, and repeat your measurement.

GL, Peter

Ok this is what I just did. With the same calibration I measured everything individually and together. I even added, in the comparative, another 49:1 that I made before (called “original” here). I used the calculator from Owen.

Exemple at 14 mHz:

S11 S21 Adjustment Insertion loss Efficiency %
49:1 + capacitor -43 -17,8 17,1 0,7 85
49:1 alone -13 -18,2 17,1 1,1 81
CMC alone -31 -0,16 0 0,16 97
49:1 + CMC -9,7 -23 17,1 5,9 28
49:1 + CMC + CP -32 -19,9 17,1 2,8 52
49:1 original + cap -30 -17,8 17,1 0,7 85
49:1 original -12,7 -17,9 17,1 0,8 88

I think it’s clear, insertion loss is acceptable when they are measured a part. When plugged together, catastrophy, efficiency drops massively (28%). It is a bit better with counterpoise (CP) (52%)

Saldy it seems like not. If I add 1.1 + 0.16 = 1.26 , far from the 5.9 measured !!!.

Yep, I separated everything for this test.

Interesting results. Maybe this is completely wrong, but I have the impression that the magnetic fields influence each other. How far apart were the two toroids during the measurement?

Both “flat” spaced with 1 - 2 cm. Same result when “on top” of each other.

Hi All,
The following was posted years ago, and is related; This also requires a counterpoise in actual use.

Nov '19

One thing to be considered, applicable to some of the simpler designs, is to use an N-turn Faraday shielded link instead of just N-turns. As many may know, this allows better isolation of the rig from the tuner / antenna, makes the antenna use the counterpoise instead of the rig, etc. see PIX: this is a 2-turn example, coax from the rig not connected to anything except by inductive coupling.

Best, Ken


I think what I’m trying to do is just impossible. I found this:

Since the coaxial shield is acting also as a counterpoise CMC should only be placed on the feed line if needed, & if you are using a EFHW antenna it needs to placed 0.05 of a wave length of the lowest band.


It means the CMC should be 2.15 m from the transformer when your lowest band is 40 m. I guess I will put it right at the radio then.

Using the coaxial cable shield with 0.05 lambda as a counterweightpoise after a 1:49 Unun… and then using a current balun is common practice… and also easy to make

Connection cable for various EFHW 40 m and higher bands:

EFHW with traps for 60m, 30m, 17m, 1:49, current balun, connection cable… all in one piece

73 Armin


I put the CMC in a separate box and performed new tests, together with my usual 2.7m coax RG58. The final diagram is like this:

Channel 1 —coax 0.5 m—CMC-- coax 2.7 m—49:1— Channel 2

S11 S21 Adjustment Insertion loss Efficiency %
49:1 v2 -14,2 -17,6 17,1 0,5 92
CMC -32,4 -0,07 0 0,07 98,5
Coax -29 -0,31 0 0,3 93
CMC + coax -26 -0,39 0 0,39 91,6
coax + 49:1 v2 -19,7 -18 17,1 0,9 82
CMC + coax + 49:1 v2 -17,28 -19 17,1 1,9 66

It’s a lot better but still quite lossy to my taste. I’m not sure I will continue my investigations. Since the CMC can now be taken apart, I will avoid using it when possible since the combination of the 2 things leads to almost 2 dB of loss, almost half the power …

Toroids are not a shielded magnetic circuit. If you think about the progression of the winding pattern, you realise that they always have an open loop which is the core outer diameter i.e they are also 1 turn wound around a very short tubular ferrite rod.

i.e. two toroids like this can couple quite well, and possibly at some frequencies where the leakage inductance is brought into resonance by antenna/cable/stray C, they would couple very well becoming a link tuned coupler.
So as you say, space them or put them in a T arrangement if you really must have them close.


There will always be losses…

Coax plugs and sockets, coax cables… straight or coiled, solder connections, wires… as connections or windings,… and also the antenna wire itself.

And of course these losses increase with frequency.

… it’s all ok! - as long as it doesn’t mean a loss of fun! :wink:

73 Armin


You are right there will always be losses but I have fun experimenting, playing with my VNA, toroids and excel files. I like my gear to be optimised.

But so far so good, I think we solved my problem. I also learned a lot on this little research and that’s also very satisfying.



Some more side conclusions of my experiments:

Effect of the wire diameter on the insertion loss, on a 140-43 toroid, 3/21 turns:

S11 S21 Adjustment Insertion loss Efficiency %
49:1 - 0,5 mm wire -14,2 -17,6 17,1 0,5 92
49:1 - 0,8 mm wire -43 -17,8 17,1 0,7 85

I’m also pleased to find out that my measurement technique seems on point now, thanks to this video. He measures a loss of 0.7 dB for the 3/21 on ft240-43 ferrite with what I believe is quite thick wire. Exactly what I measured for mine.

He made a very nice chart out of his experiments on cores and windings:

Edit : It seems like a better results for QRP 40 - 10 m is obtained with ferrite 2661102002. It’s type 61 material, but since we usually don’t need lower bands on qrp, it doesn’t matter (source). I would give it a try, but those are not available in Europe and I’m not willing to pay a 20€ shipping cost for a 4€ part. If someone has one of these here in the EU and would sell it …

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If you receive a 10W transmitter with S5, you have a total propagation loss of -137 db.

2db doesn’t play a significant role. But there is certainly one qso out of hundreds where 2db can be crucial.

Have fun exploring
73 Chris