Surface mount Common Mode Choke Suggestions for HF

[Jeff Weinmann]

Hello All,

I'm going to try my hand at building a Microvert Antenna, and the design calls for a 1:1 Common Mode Choke like the one ON7IR built:

https://on7ir.blogspot.com/2020/10/microvert-antenna.html

I'm looking to build my version for a WSPR application which will be using power in the 10 to 100 milliwatt range, so the choke can be MUCH smaller.   I'm looking for a proper Surface mount Choke  or 1:1 Balun,  that will work for 10 to 80 meters.  My first guess is the one below.  I am looking for one with similar dimensions if this one is not well suited for my application:

https://www.digikey.com/en/products/filter/common-mode-chokes/839?s=N4IgTCBcDaIIYGMC2B2ADANjSAugXyA

I plan on making my own PCB for this design.

Thanks and 73!

[T3sl4co1l]

You need a data line type choke, one made with twisted or parallel pair, something like that.  The side-by-side coil kinds are almost as much leakage inductance, and will unmatch the tank quickly.  Shop for an impedance of at least several kohms at Fo.

There appears to be a lot of mysticism about this antenna in the cluster of links on the page; to be clear, it is an absolutely, positively standard electrically-short dipole with built-in antenna tuner (fixed for the band of interest, which fortunately is quite narrow; they're only working the HAM bands I take it).  The connection to that dipole seems a bit peculiar, making it at the end (where the choke is) rather than in the middle with a balun -- for which you will get exactly the same results, with a much smaller choke (or a balun proper).  The difference is, making the connection at the end, the choke must withstand the full resonant voltage of the element, a big ask and likely not achieved by the toroid shown (which is probably around 2kohms at driven frequency, and about equal parts reactance and resistance; the equivalent circuit is this in parallel with the small capacitance of the elements, plus whatever else is on the cable; I would recommend a ground connection below the choke, to keep whatever line current is left, away from the transceiver).

As an electrically-short antenna, operation is equivalent to a magnetic loop of the same dimension, if that's any help.

Tim

[Jeff Weinmann]

Thank you for the reply,

According to your suggestions this data line choke has 8Kohm impedance:

Does this look like It may work?

https://www.coilcraft.com/en-us/products/emi/data-line-common-mode-choke/general-speed/lpd6235/lpd6235-682/

Again I am only using 10 to 100 MILLIWATTS MAX.   

Jeff

[T3sl4co1l]

It's only 500 ohms at 10MHz..?

That part might do, or the family anyway doesn't look too bad.  The 223 or 104 might do better.  But it depends on which band you're targeting.

What frequency were you shooting for, anyway?  This isn't a wideband antenna, it's made for one narrow band at a time.  If you want to work multiple bands 80-10m you might as well make one for each and swap 'em out..?  I mean the elements could be common, you only need to change out the chokes and whatever fine tuning, but yeah...

Tim

[Jeff Weinmann]

Tim,

I plan on placing a PCB low pass filter before the choke and targeting the monopole's frequency

Bands I plan on targeting:

Radio ---> 10 meter LPF --> CM Choke --> coax as counterpoise, cut for 10 meters, --> MicroVert antenna.

The PCB will have the LPF then the Choke on one board. 

Jeff

[richard.cs]

There appears to be a lot of mysticism about this antenna in the cluster of links on the page; to be clear, it is an absolutely, positively standard electrically-short dipole with built-in antenna tuner (fixed for the band of interest, which fortunately is quite narrow; they're only working the HAM bands I take it).  The connection to that dipole seems a bit peculiar, making it at the end (where the choke is) rather than in the middle with a balun -- for which you will get exactly the same results, with a much smaller choke (or a balun proper).  The difference is, making the connection at the end, the choke must withstand the full resonant voltage of the element, a big ask and likely not achieved by the toroid shown (which is probably around 2kohms at driven frequency, and about equal parts reactance and resistance; the equivalent circuit is this in parallel with the small capacitance of the elements, plus whatever else is on the cable; I would recommend a ground connection below the choke, to keep whatever line current is left, away from the transceiver).

Yes looking through the details of the antenna it is all a little unconvincing. I suspect that the end connection comes mainly from not wanting to admit that the feedline is the main radiating component and the fact that the choke losses make the return loss look good. At the end of the day the whole structure is ~10m long (8.2+1.7) for the 40m band, it's only a modest reduction in length of 0.5 from a resonant 20m and probably performs less well than a conventional centre-loaded symmetrical design of that length.