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
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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.

VNA 0,5-600MHz measurements of Q, R and X. Fair-Rite 2643167851 high loss material


Today I did some further measurements on the same Fair-Rite material as the other post (Fair-Rite 2643167851). I did the measurements with a calibrated open, short, 50 ohm load S11 measurement. The green trace is the resistive part of the impedance, the red trace is the reactive part of the impedance and the blue trace is the unloaded Q (ratio stored energy in the magnetic field in the core and windings to lost energy in the effective series resitance). As expected, the Q is one when the green and red traces cross each other. This material is not useable as a regular coil for energy storage (high Q, filters and such) over approx 15 Mhz. Below 15 MHz the Q can be quite high, however. This core is probably very good as a dampening material for RFI applications as GM3SEK has indicated. For High power operation when coax common mode current causes problems I think this core may be good. The reason is that the resistive component (real component) of the impedance is dominant. This means that even if a capacitive reactance cancels the inductive reactance, the resistive part of the impedance is still always present. This can be seen from the green trace above. Center of the plot is approx 300Mc/sek. It also rises with QRG up around 450 Mc/sec. At 2 meters and 70 centimeters wavelength it looks like even one turn on a coax (in a low impedance point of the coax!) this choke will do some good. At HF, with more turns it is possible to achieve 1000 ohms over a fairly large range.

Collinear airband antenna design

I wanted to test a simple airband antenna design since I had a roll of balanced feedline laying around in my shack doing nothing useful. First I had to measure the velocity constant of the feedline with my MFJ-259 to be able to come to an estimate of the required length of the matching section. I could also have used my vector network analyzer (VNA) to do that by the way.  To measure without interference from coupling to adjacent objects I did the measurement with the cable hanging out from my balcony as you can see in the left picture. The MFJ-259 was connected in the end and held by hand (that is a benefit of the battery operated MFJ 259 even if the instrument is not of the most accurate on the market). I wanted to make two antenna segments folded over each other. Therefore the top of the feedline is shorted and the currents will be in phase if the antenna is of a proper length. The matching section is a shorted line section that is tapped by the transmission line. The coil on the coax is a choke (I haven’t done any measurements on that choke yet by the way).

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The procedure I used to find the velocity factor of the balanced transmission line was to first measure a length of the feedline with a tape measure. Then I connected the MFJ-259 and found the frequency where the lowest reactance could be measured. (See pic two from left on the upper row above. You can see that the X is very low). This was done in the mode of the MFJ-259 where it is possible to measure both R and X. This is the quarter wave frequency of the line when the wave propagates in the line – not in the free air (“ether”). Then I calculated that frequency back to the wavelength with  y=300/f. I then divided the tape measure length by the calculated length and came to a velocity factor of 0,89. This is the ratio of the wavelength in free air and the wavelength in the transmission line. This is directly related to the propagation speed of the line when it operates in transmission line mode. From that I calculated the required length of the matching transformer and the approximate tapping point on that transformer to reach 50 ohms. Please note that you cannot use the velocity factor of the transmission line to calculate the required length of the antenna (only the matching section), since the RF currents on the two folded legs on the antenna are in phase and therefore the one lead is coupled to the ether and not to the other lead.


The picture above shows the SWR as measured with my vector network analyzer from DG8SAQ. The markers on the right side shows a 1:2 SWR bandwidth of 118,5 to 128,6 Mc/s which is OK. The reference level is 1:1 SWR. This level is lifted one division for clarity. (I think the Mc/s  is a cool way to express frequency by the way.)


Here my DG8SAQ1 kc/s to 1,3 Gc/s VNA is shown. It is connected to my PC via USB.

Conclusion: a combination of the MFJ-259, the DG8SAQ vector network analyzer, some balanced line and some coax can be used to make a good collinear airband antenna in less than one our at a cost of a few dollars. The antenna was screwed to a wooden section of my roof by a small screw by the way and can be removed in approx 2 minutes.