Matching a 5/8λ ground plane

The 5/8λ ground plane is regarded by hams widely as a superb antenna for DX, and since the main reason for modern ham radio is DX, it is an antenna of interest.

The idea behind the 5/8λ ground plane popularity is that claim that it has higher gain at low angles than a simple 1/4λ ground plane.

The 5/8λ ground plane is not resonant, and the feed point impedance is hardly suited to direct coax feed.

The chart above is for a 5/8λ ground plane elevated to 5m height above average ground (0.005,13). The feed point impedance in this case at 5/8λ radiator height (14.2MHz) is about 110-j485Ω. Continue reading Matching a 5/8λ ground plane

A check load for antenna analysers with UHF series socket

Hams embrace the UHF series connectors like no one else, including for its use on test equipment where its performance is lacking.

This is the likely reason why it is so hard to find low VSWR 50Ω terminations with UHF series plug. It is rare to find something with VSWR quoted in specifications, and nigh on impossible to find one at a reasonably low price.

On the other hand, SMA terminations start at about $2 each (posted), and it is not too hard to find ones specified with VSWR<1.2 to several GHz.

Above is a low cost, low quality solution. It is a SMA termination selected from a bunch using a high accuracy DMM (selected, R is 49.86Ω) and a SMA(F)-UHF(M) adapter, total cost $7 (posted) (but you might be advised to buy 5 loads to select the best one). Despite the specification, they are probably only good to 100MHz, and can be unreliable. Continue reading A check load for antenna analysers with UHF series socket

Extrapolating VSWR of a simple series resonant antenna

An online expert helped recently helped his Small Transmitting Loop (STL) disciples with:

Also remember that the bandwidth given by the calculators is the half power point. That’s equivalent to an SWR of about 4.3 at the ends.

Whats that?

Most STL, and lots of other resonant antenna systems exhibit a classic VSWR curve being that of a approximatly constant resistance in series with an ideal capacitor and inductor.

Clip 222

Above is that classic VSWR curve.  Continue reading Extrapolating VSWR of a simple series resonant antenna

Finding the inductance of the outside of LDF4-50A

There are applications for estimating the inductance of the outside of LDF4-50A at radio frequencies.

For the purpose of calculating the inductance, the geometric mean radius is appropriate. This article offers two methods for estimating the geometric mean diameter (GMD) of the conductor.

Above a section of LDF4-50A.

 

Above is a magnified view of the profile, it is corrugated copper outer conductor with a shallow but not quite symmetric profile.  Continue reading Finding the inductance of the outside of LDF4-50A

The sign of reactance – SM6WHY’s take

As the popularity of low cost, low end antenna analysers increases, client software appears to enhance the capability of the analyser.

The SARC-100 is one of these low end analysers, it and its many close derivatives are marketed under various model names.

The sign of reactance discusses a major weakness of these and many other low end instruments in that they do not ‘measure’ the sign of reactance, displaying the magnitude of reactance and leaving it to the user to solve the sign problem.

SM6WHY is one of the many who have produced software for the SARC-100 that purports to solve the sign of reactance problem. He gives this graphic on his website to demonstrate the capability of his software used with a SARC-100 (which does not sense the sign of reactance).

Above is part of the graphic he offers. Though the image is poor quality, the VSWR plot appears smooth and quite typical of that which might be obtained by measuring an antenna system near its VSWR minimum.

However the accompanying Smith chart plot which has points plotted with both negative and positive reactance is inconsistent with the VSWR plot and appears flawed.  Continue reading The sign of reactance – SM6WHY’s take

Power in a mismatched transmission line

This is a republication of an article posted on VK1OD.net Jun 2012.

This article presents a derivation of the power at a point in a transmission line in terms of ρ (the magnitude of the complex reflection coefficient Γ) and Forward Power and Reflected Power as might be indicated by a Directional Wattmeter. Mismatch Loss is also explained. Continue reading Power in a mismatched transmission line

80m half wave dipole made from 0.91mm steel MIG wire

Hams being innovative come up with a myriad of cheap alternatives for wire for antennas. One of those alternatives is common 0.91mm steel MIG wire.

Steel MIG wire is often coated with copper and is claimed by some online experts to “work real good”, particularly as a stealth antenna.

But is it the makings of a reasonably efficient antenna?

This article applies the model developed at A model of current distribution in copper clad steel conductors at RF to estimate the effective RF resistance of the wire at 3.5MHz.

Copper coated round steel conductor (MIG wire) – 0.91mm single core

In fact copper is an undesirable and restricted contaminant of steel welding wire, high grade MIG wire is not copper coated.

Copper content is held to less than 0.05% in the core, and less than 0.05% in the coating… which on my calcs says the coating of common 0.91mm MIG wire is less than 0.125µm…. basically it is a small diameter wire with low conductivity and high permeability. Continue reading 80m half wave dipole made from 0.91mm steel MIG wire

80m half wave dipole made from galvanised fence wire

Hams being innovative come up with a myriad of cheap alternatives for wire for antennas. One of those alternatives is galvanised steel fence wire.

A small roll of galvanised tie wire can be purchased from Bunnings hardware for about $10 for 95m… so at $0.10/m it looks like an economical solution.

But is it the makings of a reasonably efficient antenna?

This article applies the model developed at A model of current distribution in copper clad steel conductors at RF to estimate the effective RF resistance of the wire at 3.5MHz.

Galvanised round steel conductor – 1.5mm single core

A sample of new unweathered wire was measured to determine the approximate zinc coating depth, it was 15µm. Note that zinc is a sacrificial coating and it will erode through life, so this study is an optimistic one of wire when new. Continue reading 80m half wave dipole made from galvanised fence wire

Skin depth in copper at 1.8MHz according to QRZ

Having just written again on skin effect and copper clad steel (CCS) conductors on HF, and the potential for less than copper performance, it was interesting to note a thread on QRZ where the OP asked for advice on the issue with budget CCS RG-11.

Two late posts as I write this were:

There really is no real issue with skin effect on HF bands with copper clad materials.

and…

At 1.8 MHz, the skin depth in copper is 0.654 micro-meters (.0000654 mm), so the copper cladding on the center conductor of most RG-11 type coaxial cables is more than sufficient for any of our current bands.

The specific advice above looks interesting, convincing even… but thankfully, the skin depth in copper is nowhere near either of the figures he gave. Continue reading Skin depth in copper at 1.8MHz according to QRZ