Small untuned loop for receiving – optimal loop load resistance

Small untuned loop for receiving set out a model for calculating the S/N degradation of an active untuned small loop antenna system.

The calculations in Small untuned loop for receiving – Trask noise and gain analysis might prompt the question of what is the optimal resistive load for an untuned small loop.

This article explores the topic for a simple model where the equivalent noise temperature of the amplifier is independent of source impedance.

A simple model for a small loop

We can construct a simple model where the loop behaves as a fixed  pure inductance, and its load is a fixed pure resistance.

This is a reasonably good model for a small loop, perimeter < wl/10, not too bad for perimeter up to wl/3.

The source impedance becomes the loop’s inductive reactance Xl which is proportional to frequency, and the load is Rl.

Above is a plot of the relative power developed in the load vs the ratio of Rl/Xl.

There is a maximum where Rl=Xl, and the power captured falls away either side. Continue reading Small untuned loop for receiving – optimal loop load resistance

Small untuned loop for receiving – Trask noise and gain analysis

The article Small untuned loop for receiving mentioned Trask’s active loop amplifier.

(Trask 2010) published a two stage design using passive augmentation, arguing certain benefits of the approach.

  • Zin=2.25Ω
  • NF 2.42dB
  • Voltage gain 36dB
  • OIP2 80dBm
  • OIP3 40dBm

This article presents a noise gain analysis for the 8m perimeter loop used in the article Small untuned loop for receiving to achieve a S/N degradation of no worse than 1dB at 7MHz.

The analysis assumes linear components, that there is no significant intermodulation distortion in the preamplifier. That is a significant challenge on which success of the system depends.

External noise

From the above chart (ITU-R P.372-12 (7/2015)), we can take the external or ambient noise figure Fa to be about 45dB at 7MHz, Ta=290*10^(45/10)=9.17e6K. Continue reading Small untuned loop for receiving – Trask noise and gain analysis

Small untuned loop for receiving

This article walks through a case study for a small single turn untuned loop with attached 50Ω balanced preamplifier and 50Ω coaxial output to a high grade communications receiver. The objective is to achieve system S/N ration not poorer than 1dB below the external S/N (ie ExternalS/ ExternalN).

Such an antenna has utility in that it can be rotated to null out a strong noise source from a direction other than the desired signal.

The analysis assumes linear components, that there is no significant intermodulation distortion in the preamplifier. That is a significant challenge on which success of the system depends.

This is a rework of an earlier article which presented a ‘back of the envelope’ noise and gain analysis now presented as a more accurate model embodied in a spreadsheet to allow convenient exploration of variations to the scenario.

External noise

From the above chart (ITU-R P.372-12 (7/2015)), we can take the external or ambient noise figure Fa to be about 45dB at 7MHz, Ta=290*10^(45/10)=9.17e6K. Continue reading Small untuned loop for receiving

The Mobius strip loop – ham benefits

(Baum 1964) describes his “Moibus strip loop” (sic).

In fact it is not made from a strip conductor but rather a circle of round tube  with a gap at the top, and containing a transmission line which is cross connected to the outer tube at the gap.

Two main features are claimed for this antenna:

  1. cancellation of induced Compton currents in the centre conductor due to incident gamma radiation; and
  2. transformation of the feed point voltage V to 2V at the transmission line at the loop feed T joint.

Feature 1 is claimed to improve S/N when irradiated by gamma radiation, the effect would be of most benefit in the event of a nearby nuclear bomb. Given that most ham stations are not EMP hardened, this is unlikely to be of material benefit to those ham stations. Continue reading The Mobius strip loop – ham benefits

Quiet HF antennas and E and H fields in the near field zone

Hams often postulate that certain HF antennas are “low noise’ antennas.

There are many possible explanations for why an antenna captures less noise power than another, this article discusses the distribution of electric and magnetic fields (E and H) very near to a radiator, and the power captured by antennas that respond more to E or H fields.

Electromagnetic radiation consists of both and E field and a H field, and they are in the ratio of η0=µ0*c0Ω, the so-called impedance of free space, often approximated to 120πΩ or 377Ω. Close to a radiator there are components of E and H additional to the radiation components, the ratio of E/H is not simply 377Ω.

Fig 1.

Fig 1 shows the magnitude of the ratio E/H near a quarter wave vertical over average ground at 3.6MHz. |E/H| depends on location near the antenna, and with increasing distance it converges on 377Ω.
Continue reading Quiet HF antennas and E and H fields in the near field zone

LNR Precision small transmitting loop

LNR Precision have announced a small transmitting loop for amateur radio.

This article is a revision to take account of recently updated information published by LNR filling in some of the gaps in their original page. It is encouraging to see better product descriptions and measurement data.

Description

The antenna is described at (LNR Precision 2016).

The loop itself appears to be 3/8 Heliax or similar (nominally 9.5mm outer conductor diameter) in a rough circle of 45″ (1.143m) diameter.

Little information is given of the internals, but the promotional material gives a VSWR curve for a matched antenna at 7.065MHz. To their credit, they give the height above ground and ground type for their tests.

The VSWR=3 bandwidth scaled from the graph is 18kHz.

If we assume for a moment that the VSWR measurement was captured at a substantial height above ground, its behavior approaches that of the antenna in free space. Taking the assumption that the published curve is similar to the antenna in free space, we can estimate efficiency based on earlier assumptions. Such antennas very close to ground have a directivity of about 6dB (dependent on ground parameters), and that can be used with efficiency to estimate gain in proximity to ground.

The assumed values and published VSWR curve indicate an antenna system half power bandwidth of 15.6kHz and Q of 453 which implies efficiency of 2.8%.

The actual value for radiation resistance is likely to be with -50-+100% of the free space value used, and that rolls up as an uncertainty of +/-3dB in the calculated efficiency and gain. Continue reading LNR Precision small transmitting loop

Current and voltage implications of a small transmitting loop power ratings

This article gives a simple method for calculating the key voltage and current in a small transmitting loop using observed or expected behaviour and Calculate small transmitting loop gain from bandwidth measurement.

Method

Above is a model hypothetical 1m diameter loop of 10mm conductor on 40m with 1% radiation efficiency.

Lets say it is rated for input power being the lesser of 10W continuous, or 30W PEP SSB. Continue reading Current and voltage implications of a small transmitting loop power ratings

CHA P-Loop 2.0 small transmitting loop

Chameleon have released their CHA-P-Loop 2.0 small transmitting loop. This article considers the likely efficiency on 40m based on their published measurements and Efficiency and gain of Small Transmitting Loops (STL).

Description

The antenna is described at http://chameleonantenna.com/CHA%20P-LOOP%202.0/CHA%20P-LOOP%202.0.html.

This analysis does not consider the proprietary Power Compensator option for lack of sufficient information.

The loop itself appears to be LMR400 coax or similar (nominally 8.0mm outer conductor diameter) in a rough circle of 34″ (0.863m) diameter.

Little information is given of the internals, but the promotional material gives a VSWR curve for a matched antenna at 7.15MHz. To their credit, they give the height above ground and ground type for their tests, though elevation above ground was between 1/2 diameter to a full diameter of the P-LOOP 2.0 is a little vague.

Basic loop (34″)

The VSWR=3 bandwidth scaled from the graph is 27.0kHz. The shape of the curve near minimum suggests that were the scan points sufficiently close, the minimum VSWR would be very close to 1.0 and it is taken as 1.0.

If we assume for a moment that the VSWR measurement was captured at a substantial height above ground, its behaviour approaches that of the antenna in free space. Taking the assumption that the published curve is similar to the antenna in free space, we can estimate the gain and efficiency based on earlier assumptions.
Continue reading CHA P-Loop 2.0 small transmitting loop

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