Stacking two ferrite cores of different permeability for an RF inductor

One of the magic ham recipes often proposed is to stack two ferrite cores of different permeability for an RF inductor, but an explanation is rarely offered, I have not seen one.

An explanation

Starting with some basic magnetism…

The inductance of an inductor is given by \(L=N\frac{\phi}{I}\).

For a closed magnetic circuit of high permeability such as a ferrite cored toroid, the flux is almost entirely contained in the core and the relationship is \(\mathcal{F}=\phi \mathcal{R}\) where \(\mathcal{F}\) is the magnetomotive force, \(\phi\) is the flux, and \(\mathcal{R}\) is the magnetic reluctance. (Note the similarity to Ohm’s law.) Continue reading Stacking two ferrite cores of different permeability for an RF inductor

Shorting winding sections of a ferrite cored EFHW transformer

A chap recently posted some advice on construction of a dual ratio transformer for EFFHW antennas, advice with an informative pic, but without measurement evidence that it works well.

Pictured is a dual UnUn. I made this for experimenting. It’s both a 49 and 64 to 1 UnUn.

The 49 to 1 tap uses the SS eye bolt for the feed through electrical connection and the SS machine screw on the top is the 64 to 1 connection. If I want to use the 49 to 1 ratio, there’s a jumper on the eye bolt that connects to the top machine screw where the antenna wire is attached. The jumper shorts out the last two turns of the UnUn. Disconnect the jumper from the top connection and now you have a 64 to 1 ratio.

Continue reading Shorting winding sections of a ferrite cored EFHW transformer

A desk review of the MiniPa100 kit – #1: characterise the output transformer

This article is one in a series of a desk review, a pre-purchase study if you like, of the MiniPa100 kit widely sold on eBay and elsewhere online.

One of the first questions to mind is whether it is likely to deliver the rated power, so let’s review the MOSFET output circuit design from that perspective.

Sellers mostly seem to need to obscure the MOSFET type in their pics, so essentially you buy this with no assurance as to what is supplied, no comeback if the supplied MOSFET is not up to the task. Online experts suggest the MOSFET is probably a MRF9120 (or 2x IRF640 in a 70W build). The amplifier claims 100W from 12-16V DC supply.

Note that this module does not include the necessary output filter which will lose 5-10% of the power from this module.

In this case Carlos, VK1EA, connected a sample output transformer (T2) core from a recently purchased MiniPa100 kit to a EU1KY antenna analyser. The fixture is critically important, it is at my specification. Continue reading A desk review of the MiniPa100 kit – #1: characterise the output transformer

(How) does this balun work? A variation on the theme …

My article (How) does this balun work analysed a balun configuration sent to me by a correspondent, apparently published on Youtube channel TrxBench.

Essentially, my analysis was that it comprises two 12t winds of two wire transmission line in parallel on the ferrite ring. The potential benefit was that the characteristic impedance Zo of each transmission line is probably close to 100Ω, and the parallel combination is probably close to 50Ω.

Online experts following fashion are opining that a low Insertion VSWR balun is better made with two wire line(s) than winding a single 50Ω coax line. They make these claims without evidence, I am not convinced.

In that vein, here is a variation on the TrxBench balun above.

The designer describes it: Continue reading (How) does this balun work? A variation on the theme …

N6THN’s novel balun – flux leakage

N6THN’s novel balun presented measurement of the Insertion VSWR of the subject balun, and N6THN’s novel balun – an explanation gave explanation that included mention of flux leakage as a contributor to the quite high inductance per unit length of the transmission line formed by the two windings.

A correspondent suggested that with a ferrite core, flux leakage is insignificant. This article calculates the coupled coils scenario.

The balun as described

Above is the ‘schematic’ of the balun. Note the entire path from rig to dipole. Continue reading N6THN’s novel balun – flux leakage

N6THN’s novel balun

One sees lots of articles and videos on how to make a current balun suited to a low VSWR antenna. This one was recommended in an online discussion on QRZ.com. N6THN might not have invented this balun, but he made a video of it.

In this case, it is described in the referenced video as part of a half wave dipole antenna where you might expect the minimum feed point VSWR to be less than 2.

Apologies for the images, some are taken from the video and they are not good… but bear with me.

The balun as described

Above is the ‘schematic’ of the balun.Note the entire path from rig to dipole. Continue reading N6THN’s novel balun

Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2×2631540002 – measurement of Zcm

Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Fair-rite 2631540002 suppression sleeves – design workup presented a desk design of a low power balun. This article presents measurement of common mode impedance Zcm of a prototype using a nanoVNA.

Above is the prototype 2631540002×2 wound with 3.5t of RG316. Continue reading Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2×2631540002 – measurement of Zcm

Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – measurement of Zcm

Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – design workup presented a desk design of a low power balun. This article presents measurement of common mode impedance Zcm of a prototype using a nanoVNA.

Above is the prototype 2843009902 binocular wound with 3.5t of RG316. Continue reading Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – measurement of Zcm

Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – design workup

The article Low power Guanella 1:1 balun with low Insertion VSWR using a pair of Jaycar LF1260 suppression sleeves describes a current balun with low Insertion VSWR for operation at modest power levels. The design was based on Jaycar LF1260 cores which are readily available in Australia.

This article presents the workup of a balun with similar design objectives using a low cost Fair-rite 2843009902 binocular core (BN43-7051).

Above, a pic of the core. Continue reading Low power Guanella 1:1 balun with low Insertion VSWR using a Fair-rite 2843009902 binocular – design workup