Common mode choke | 1:1 current balun | Unun
160 - 10m / 2KW
A Common Mode choke / 1:1 "balun" aims at preventing
- your coaxial cable from sending back some RF (causing RFI's)
- your coaxial cable from being part of the TX antenna (coaxial braid must NOT be radial of your vertical antenna)
- your coaxial cable from catching local noise that could ruin your reception.
- It also helps your symmetric antenna (dipole's, yagi's, ...) to be really symmetric, by avoiding distortion of its radiating diagram
A good choke / 1:1 balun must have the following mandatory performances over its entire bandwidth
- > 30dB rejection
- Impedance > 3KOhms
- SWR < 1,1:1 and insertion losses < 0,1dB
- Rs > |Xs| (your device MUST be resistive and NOT REACTIVE, else it can be either counter-productive, and even harmful to your station
Do you know a vendor who specifies all those requirements at the same time ?
- Be sure of what you get -
Our device has typically over the entire 160 up to 10m bandwidth
- From 31 up to 35dB of rejection
- impedance > 3KOhms (up to ~ 5KOhms)
- SWR < 1,05:1 and insertion losses < 0,08dB
- Rs > |Xs|
It is easy to have a good rejection or a good SWR or a good impedance. When lucky, this is the only thing that is specified by other vendors. But it is a lot more difficult to gather all of them, including R > |Xs|, over the entire 160 up to 10m.
==> This requires a special toroid mix, a specific number of windings and a specific way of winding the coaxial around the toroid.
Why Rs > |Xs| ?
G3TXQ : Reactive chokes have the disadvantage that they can "resonate" with a CM impedance path that is also reactive but of opposite sign - in some cases actually increasing the CM current flow rather than choking it
I may be conservative but do not trust people saying that a single FT240-sized toroid can handle more than 1KW PEP SSB : This is the best way to burn it, especially if you have an intense activity (pile up, contest etc).
You only need 1KW ==> check out my other items
Each device will be tested before shipping, like it would be mine : I will not ship a device which is not compliant with all those requirements.
Disclaimer
- This device is intended to be used on a 50-Ohm load (antenna). Any mismatch will degrade its performances. For a SWR = 2:1, decrease the power by 30/40%.
- Power rating 2KW SSB down to 500W in FT8 (may be conservative but...)
What you get :
- 2x genuine FT240 Fair-rite toroid, with windings made of RG142 coaxial cable, already and securely tied around the core.
- 5cm extra coax length on each side to enable you to fit it in your own set-up/enclosure.
- Tested before shipping
Any questions? Any requests? Drop me a message