Wideband HF RF choke design for QRO operation. V2.

I have now continued the investigation into a possible design for a wideband HF RF choke for QRO applications. The criteria is that the R part of the R+jX load the choke present to the common mode current on a coax should be so high that there is low risk of overheating the core even for QRO operation. It turns out that two main loops that gives sufficient resistive (and reactive load) that covers the lower frequency range combined with another loop with smaller diameter and fewer cores that takes care of the upper frequency range will give quite good results. This is similar to what GM3SEK has observed and described in his publication. Vector network analyzer measurements confirms this.

Above the three choke sections can be observed
DSC_5174 DSC_5173 DSC_5167 DSC_5186 DSC_5175 DSC_5169
Above, the measuring setup can be seen. On the right there is a multicore choke that was  tried. It gives a big resistive and inductive peak low in the frequency range. Far more than required. The cost will be high due to the many cores and is not justified. Therefore a 3 + 3 + 2 design was found more optimal.

Above the resistive (blue) load to the common mode current, the inductive load (red) to the common mode current and the Q (green) for the choke can be studied. The choke presents around 1000 ohms resistive to HF current in the frequency range of approx 2,5-25 Mhz. Another material could probably have been added to prevent the drop above 25 Mhz. 10 Meter would be a bit marginal for QRO operation with a lot of common mode current.

Testing of a HF current transformer with a vector network analyzer

I wanted to check the load that a ferrite core with a secondary winding presents to the common mode current carrying conductor (the outside of a coax) when set up as a current transformer. The coax runs thru center, the secondary winding could be one or several turns loaded by a R+jø load.  (+jø load but the windings of the secondary will give some +jx component).

I did the following three S11 measurements with the VNA (M1, M2, M3):

M1) No secondary winding is present but the single turn is running thru the core

M2) A shorted two turn secondary and an open two turn secondary (winding is pulsed in and out off to be able to better detect a difference). The time domain is captured by the slow scan and sampling rate of the VNA.

M3) Only the primary winding is attached to the calibrated S11 measuring plane. (to measure the self inductance of the single turn in itself. This is an air-core measurements.)

Above is a similar test setup. The core is an “unknown” Amidon toroid core. The red conductor is the simulated coax common mode current path (a one turn loop). The green conductor is the secondary winding. The load could made by paralleling several resistors in series with the secondary (the blob on the left side, right picture). Note that the measurements below were done with a one turn loop and a short – no resistor. The measurement device is a VNA from DG8SAQ calibrated by O S L  references in the S11 measuring plane (the SMA in the end of the coax from the TX port).  
The left picture shows measurement 1) The mid picture shows measurement 2) the right picture shows measurement 3)

Edited: What is interesting to see is that the image to the left shows that this core has some resistive loss as can be seen on the blue trace. The Q is quite low. When there is a secondary winding present, the loss is shorted out but the inductancechanges since the inductance of the secondary is reflected into the S11 measurement plane. On the right image it can be seen that there is no resistive loss and a linear inductive reactance caused by the air core inductor (no appreciable drop off or frequency dependent effects, in the measuring frequency range)

What this tells me is that this core setup probably is not too well suited as a high current measuring setup for frequencies above 160m because it will affect the measuring circuit too much. Not in terms of the R element but because of the +jX element. The R is low to the RF current passing thru the core (in the common mode), but the +jX element will present a reactance to the RF current and thereby giving you lower current than should be expected without the core.  My analysis says that another core type should be selected or that a frequency compensation technique should be used. Alternatively that a lower turns ratio should be tried. However looking at the plot to the left, it can be seen that below 30 Mc*s^-1 the +jX component is too high. Perhaps this core has a too large permeability and that a lower permeability core should be used. The primary turn with secondary loading should give a low reactance on the primary. The voltage given over Rt would then be lower but the gain of the detector could be adjusted. (I may be wrong). Please comment if you have comments or suggestions.