A common theme among online experts is to measure, or ask for measurement of a common mode choke connected between the centre conductor of a VNA’s tx and rx ports. That raises the question of whether |s21| is meaningful, whether it in any useful way characterises the choke as a component of an antenna system.
In another long running discussion on QRZ about End Fed Antennas, WA7ARK offered a contribution:
(1) Back in post #30 I showed that with a halfwave wire fed close to its end works just like the same wire fed in the center; the only difference being the feed point impedance. I let EzNec figure this out; I didn’t have to explain it with any mysterious “displacement” currents. Shown as (1) in the attached.
Since, in the model, the source is a constant current source, that forces the current on either side of the source to be equal, and the radiation pattern predicted by EzNec reflects that, because the patterns for the end-fed and center-fed match… (go back and look at post #30)
His post #30 is of a 67′ dipole at 66′ above poor ground @ 7.18MHz, fed at one end.
An online expert somewhat exasperated that the audience hasn’t absorbed his wisdom elaborated in apparently many previous posts said:
We’ve been round and round on this discussion but in a current mode balun aka a common mode choke the losses due to the windings and core are common mode not differential mode losses. You DO NOT dissipate your transmitted and received signals, which are carried as differential mode signals, as choke losses.
I know you’ve been reminded of this many times and don’t expect you to accept it now but that’s how common mode chokes work.
An online expert recently expounded on detailed design of a balun, this is an excerpt about wire sizing.
The wire gauge used limits the current handling capacity of the wire, run too thin a wire and it will heat up. Run much too thin of a wire for the power in use and it will fuse open. Current carrying capacity of wire is typically rated for either power transmission applications or chassis wiring applications. The latter, and higher, current capacity for a wire is relevant to designing a balun. How much current your 50 watt signal generates depends on the impedance its looking into. If you’re talking about a 50 ohm system, with a perfect match you’ll deliver one amp through your balun wires when driving 50 watts into it. Allowing for say a 4:1 SWR the worst case current(@12.5 ohms) is 2 amps. If you’re using this as a tuner balun, perhaps to drive a multi-band doublet then the impedance can vary widely so over sizing the wires is easy insurance. Here’s a table of wire current carrying capability: https://www.powerstream.com/Wire_Size.htm
For convenience, the relevant part of the table linked above is quoted for discussion.
The Amidon AB_200_10 2-30MHz, 1KW balun and knock-offs have been around for a very long time, I recall Dick Smith selling them in the early 1970s in Australia.
They were regarded as the epitome of the art… but it was not a very well understood art.
Lets analyse the common implementation as a Ruthroff 4:1 voltage balun in a 50:200Ω scenario.
Ruthroff 4:1 voltage balun
In this implementation, Amidon’s instructions show 16 bifilar turns on a T200-2 core.
A very simple model is to consider the device as an ideal transformer with a shunt magnetising impedance equal to the impedance of the 16t winding that appears across the 50Ω terminals. This has its greatest effect at low frequencies and although it is specified from 2-30MHz, lets analyse it at 3.5MHz.
The powdered iron core has very low loss at 3.5MHz, sufficiently so that we can ignore the imaginary component of µr for this analysis and take µr to be 10+j0.
Above is a calculation of the magnetising impedance and admittance under those assumptions. The magnetising admittance (0.00-j0.0134S) appears in shunt with the transformed load admittance (0.02S) so we can simply add them to find the admittance seen by the transmitter (0.02-j0.0134S). Continue reading Review of the Amidon AB_200_10 balun
Online experts all have a preferred core material, but there is a dearth of measurement data to show the difference in actual use. If someone recommends a particular core material and cannot provide measured Zcm data to support the recommendation, regard it as a weak recommendation.
Beware the magic of unobtainium… just because something is hard to get is not an indication that it is desirable.
This article describes a Guanella 1:1 current balun which has high common mode impedance (Zcm) and low Insertion VSWR. It is for application on antennas that have low VSWR(50) on at least some bands, especially if they would be used without an ATU on some bands.
The purpose of the balun is to minimise common mode feed line current which may contribute to EMC problems when transmitting, and contribute to increased ambient noise when receiving. Reduction of feed line common mode current also helps in achievement of expected load impedance characteristic, radiation pattern and gain. This article gives measured Zcm, but the definitive test of the effectiveness of such a balun is direct measurement of common mode current Icm… and it is so easy.
Example applications are half wave centre fed dipoles, fan dipoles, trapped dipoles, G5RV with hybrid feed, ZS6BKW, trapped verticals, monopoles, ground planes.
To obtain low Insertion VSWR, the choke will be wound with 50Ω coax, to demonstrate the practicality of the design budget (but good quality) regular (ie solid PE dielectric) RG58C/U will be used. Foam dielectric is NOT recommended. Solid PTFE coax could be used, but avoid coax with steel cored inner conductor, it may be lossier than you think at low frequencies with the silver cladding is relatively thin.
The candidate core is the readily available FT240-43 (Fair-rite 2643803802, 5943003801), it is a low cost NiZn ferrite with medium µr, and its µr and loss characteristic contributes to a broad high impedance choke well suited to this application.
My article G3LNP balun explored the operation of the G3LNP 4:1 balun on a 200Ω asymmetric load and found it exhibited extreme Insertion VSWR on what should have been an ideal impedance transformation but for the asymmetric element.
The balun is in fact a Voltage Balun and cannot be expected to work properly on asymmetric loads.
A correspondent proposes that the balun probably works very well on a nearly symmetric load such as a half wave dipole.
There are two aspectes to this proposition:
the assumption that a common half wave dipole implementation is nearly symmetric; and
G3LNP described a 4:1 balun for HF antennas in Radcom Nov 2017.
Above is the schematic supplied by G3LNP. He describes the dashed link at the bottom as optional, but uses it in his prototype so this analysis is with that link installed. The prototype used equal lengths of coax (1m PF100, an RG-6 like coax), and the toroidal choke appears to be 8t on a T130-2 powdered iron core.
Exploration of behaviour of baluns on extreme asymmetric load often reveals whether they work properly for asymmetric loads. Continue reading G3LNP balun
Walt Maxwell (W2DU) described a simple common mode choke or 1:1 current balun using ferrite sleeves slipped over a coaxial cable.
Maxwell gives the choking impedance of two of his recommended chokes in Fig 21-3 from (Maxwell 2001). He does not give any detail of how he arrived at the curves, and in correspondence declined to give any detail.
This article focusses on a linear design for HF using 50 x FB-73-2401 (2673002402) ferrite sleeves.