Common-mode chokes (by G3TXQ)
The following chart presents the results of impedance measurements made on a variety of common-mode choke implementations across the frequency range 1MHz to 30MHz. Amateur frequency allocations are indicated approximately by the vertical grey bands.
The colours of the bars indicate the magnitude of the CM (common-mode) impedance; however, depending on the style of choke and the type of ferrite material used for the core, that impedance might be mostly Resistive, mostly Reactive, or somewhere in between. The black bars at the bottom of the coloured bars indicate the range of frequencies over which the choke impedance is predominantly Resistive - that is Rs>Xs. No black bars are shown for the air-cored chokes because their impedance is almost entirely Reactive apart from a very small band of frequencies around resonance.
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; see the section at the bottom of this page for a detailed explanation. Resistive chokes have the disadvantage that if they have insufficient impedance to reduce the CM current to a very low value, there may be significant core heating.
Aim to choose a choke which has a high impedance and is Resistive over the frequency range of interest.
I hope to add more data as I make further measurements.
Let's take the example of a 20m half-wave dipole erected 30ft above average ground. It is fed by RG213 coax which drops vertically away from the dipole and whose braid is earthed at the “shack end” to a moderately effective 20O ground. The centre conductor of the coax is connected to the left-side dipole leg, and the braid to the right-side.
In this example an inductive reactance of +j200O is obviously a “worst case” and you would be unlucky if the choke was exactly that impedance; but note that any choke reactance between 0 and +j400 will reduce the CM path impedance - and therefore increase the braid current - to some degree.
As we vary the length of the coax, the braid path impedance changes. When the coax is close to a quarter-wave long the CM path is high-impedance and relatively little current flows along the braid whether we include a choke or not; when it is close to a half-wavelength long substantial current flows if we don't include a choke. But there is no length of coax where an “unlucky” reactive choke impedance could not make things worse!
The situation gets more complex with a multiband antenna - in fact the potential for a Reactive choke exacerbating the situation on at least one of the bands increases.
table shows for a range of coax lengths from 20ft to 70ft on this model
the braid current without a choke and with a worst-case inductive choke;
it also shows the impedance required in a Resistive choke to keep the
braid current 30dB below the level of the dipole current.
coax lengths a Reactive choke has the potential to increase the CM current,
sometimes by a very significant factor
Thank you G3TXQ, for updates look on: www.karinya.net/g3txq/chokes/
klopt niet geheel maar zo snap je hem wel...
Om het even uit te leggen nemen we een dipool antenne...dat was immers zo'n beetje de basis van alle antennes.
Een dipool (zoals
het woord zegt bestaat uit twee polen die beiden gelijk zijn.
Nu is het ook de
bedoeling dat de antenne gevoed word met een signaal dat in balans is.
Zover zo goed...
Nu hebben we twee
Met de eerste oplossing
"vernachelen" we de boel...we komen immers niet met een oplossing
maar we "gaan om" met het probleem. (de RF-choke/mantelstroom
Nou, wat gebeurt
er dus bij je antenne...
Dan kijken we even
weer naar die coax en het signaal op de heen weg.
er zijn trouwens
ook een boel dingen die die mantelstromen beinvloeden...
De oplossingen hebben
we al aangedragen...
Nu moet je er ook
niet "als de dood" voor zijn: Om bv op vakantie even een dipooltje
op te hangen zonder een 1op1 balun of mantelstroom filter is geen probleem.
Nou, eh zoiets?