Franklin antenna – how does it work?

(Franklin 1924) described a technique to cophase sections of a long antenna by “concentrating alternating half wave length portions of the wire within a small space, by winding such portions as inductance coils or by doubling such portions back on themselves so that there is practically no radiation from these portions”.

Let’s explore his second option, as unlike the first, it does work reliably.

Above is an NEC-4.2 model with current shown (magnitude and phase). The stubs conductors are all defined from top to bottom. Continue reading Franklin antenna – how does it work?

End Fed Half Wave matching transformer – 80-20m – LO1238 variant

A reader of End Fed Half Wave matching transformer – 80-20m asked if a good transformer could be made with with a FT114-43 core.

The original transformer above comprised a 32t of 0.65mm enamelled copper winding on a FT240-43 ferrite core, tapped at 4t to be used as an autotransformer to step down a load impedance of around 3300Ω to around 50Ω. Continue reading End Fed Half Wave matching transformer – 80-20m – LO1238 variant

End Fed Half Wave matching transformer – 80-20m – 2xFT240-43 variant

A reader of End Fed Half Wave matching transformer – 80-20m asked if a better transformer could be made with a stack of 2 x FT240-43 cores and using half the turns.

The original transformer above comprised a 32t of 0.65mm enamelled copper winding on a FT240-43 ferrite core, tapped at 4t to be used as an autotransformer to step down a load impedance of around 3300Ω to around 50Ω. Continue reading End Fed Half Wave matching transformer – 80-20m – 2xFT240-43 variant

Diagnosing a possible antenna problem by comparison with a baseline

Recently I have had difficult reaching the local DMR repeater on 70cm, and needed to check that the antenna system had not deteriorated.

I took a baseline measurement with an AA-600 after some refurbishment work in Jan 2018, and was able to compare a current sweep to that baseline.

Above, a wide Return Loss sweep of the Diamond X-50N with feed line compared to the baseline (the thin blue line). Continue reading Diagnosing a possible antenna problem by comparison with a baseline

Matching a 5/8λ ground plane – a single stub tuner example

A chap seeking details for a matching inductor for his 5/8λ vertical on 20m reported “my AA54 RigExpert analyser gives the following reading (SWR 8,2). (R 81,5). (X -158) ” measured looking into a “length of rg58 about 15-20 cm” and asked “is the inductor coil going to be enough or will I need an L match to bring the real resistance to 50 ohms”. Continue reading Matching a 5/8λ ground plane – a single stub tuner example

Trapped dipole

Some time ago I wrote some articles on so-called Coax Traps, and an example design of an Inverted V dipole for 80 and 40m.

A coax trap (before cross connection).

The whole subject of trapped antennas elicits a lot of online discussion that is often more about semantics than understanding. Continue reading Trapped dipole

RF transformer design with ferrite cores – saturation calcs

Ferrite cored inductors and transformers saturate at relatively low magnetising force.

#61 material example

Lets work through an example of a FT50-61 core with 10t primary at 3.5MHz.

Magnetic saturation is one limit on power handling capacity of such a transformer, and likely the most significant one for very low loss cores (#61 material losses are very low at 3.5MHz).

Let’s calculate the expected magnetising impedance @ 3.5MHz.

Zm=0.966+j144Ω, |Zm|=144Ω. Continue reading RF transformer design with ferrite cores – saturation calcs

RF transformer design with ferrite cores – initial steps

A review of transformer design

In a process of designing a transformer, we often start with an approximate low frequency equivalent circuit. “Low frequency” is a relative term, it means at frequencies where each winding current phase is uniform, and the effects of distributed capacitance are insignificant.

Above is a commonly used low frequency equivalent of a transformer. Z1 and Z2 represent leakage impedances (ie the effect of magnetic flux leakage) and winding conductor loss. Zm is the magnetising impedance and represents the self inductance of the primary winding and core losses (hysteresis and eddy current losses). Continue reading RF transformer design with ferrite cores – initial steps

An online expert on the unsuitability of #43 for HF UNUNs

An online expert recently advised:

…The spec for type 43 makes it clear that it should never be used for HF unun construction. It is specifically engineered with a complex permeability that makes the core lossy on most HF frequencies. Since an unun is not a TLT (transmission line transformer) but rather an autotransformer, a low loss core is essential for efficient operation….

Now it contains the very common FUD (fear, uncertainty and doubt) that masquerades as science in ham radio, but without being specific enough to prove it categorically wrong. To a certain extent, the discussion goes to the meaning of efficient operation. Continue reading An online expert on the unsuitability of #43 for HF UNUNs

High end VSWR compensation in a ferrite cored RF transformer

The article Estimating the Insertion VSWR in a ferrite cored RF transformer discussed the importance of sufficient magnetising impedance to InsertionVSWR at low frequencies.

Above is a low frequency equivalent circuit of a transformer. Although most accurate at low frequencies, it is still useful for RF transformers but realise that it does not include the effects of distributed capacitance which have greater effect with increasing frequency.

The elements r1,x1 and r2,x2 model winding resistance and flux leakage as an equivalent impedance. Whilst for low loss cores at power frequencies, flux leakage is thought of as an equivalent inductance, purely reactive and proportional to frequency, the case of lossy ferrite cores at RF is more complicated. Winding resistance with well developed skin effect increases proportional to the square of frequency, but with lossy ferrite cores will often be dwarfed by the loss element of leakage impedance. Continue reading High end VSWR compensation in a ferrite cored RF transformer