Review of Dunlavy’s STL patent gain claims

(Dunlavy 1967) sets out his description of a wide range tunable transmitting loop antenna and makes a broad efficiency claim of better than 30% (-5.3dB) for his system.

Minimum efficiencies of 30 percent are attainable with practical designs having a diameter of only 5 feet for 3-15 Megahertz coverage.

In a context where extravagant claims are often made for such antennas, his claims warrant review.

Dunlavey gives an example embodiment in approximate terms.

Practical loop designs for use in the range of 2-30 megahertz will utilize copper or aluminum tubular conductors having a diameter of 3 inches to 5 inches. A typical design for 3 to 15 Megahertz operation would be constructed as shown in FIG. 2 with a primary loop 4 having a diameter of about 5 feet and tuned by a high voltage vacuum capacitor 5 having a capacitance range of approximately 25 to l,000 picofarads. The tuned primary loop should be made of aluminum or
copper tubing having a diameter of approximately 4 inches-5 inches. The diameter of the feed loop, which is designated by the reference number 6, for 50 ohms impedance should be approximately l0 inches.

Lets take a perimeter of 4.8m (dia=5′) and copper conductor diameter of 100mm (4″) as the dimensions for further exploration.

Screenshot - 01_06_16 , 07_46_16

Above, Dunlavy’s Figure 5 gives gain relative to a monopole above perfectly conducting ground. Continue reading Review of Dunlavy’s STL patent gain claims

Some thoughts on a two turn small transmitting loop

Small transmitting loops (STL) are very popular with hams, and a fashion is developing for N turn loops. This article lays out some thoughts on a 2 turn STL.

Firstly, to the meaning of “small transmitting loop’. There are a range of definitions used, and they mostly centre around the concept of a size sufficiently small that current is approximately uniform. The issue is about the meaning of sufficiently. Accuracy of estimation of radiation resistance of small transmitting loops sets out a rationale for a single turn loop for criteria that perimeter<λ/10.

This article will compare NEC-4.2 models of loops with the following key parameters:

  • 1m diameter (the loop perimeter is 0.07λ);
  • 20mm copper conductor;
  • frequency is nominally 7.1MHz;
  • 16 segments per turn
  • when not specified as in free space, the loop centre is 1m above ‘average’ ground (0.005,13);
  • the loop is directly fed in the middle, opposite to the tuning capacitor position, cap down;
  • pitch is 0.15m.

The model is sensitive to all these parameters. Continue reading Some thoughts on a two turn small transmitting loop

Review of N6PAA’s 40m STL

(N6PAA nd) describes several small transmitting loops (STL) and gives some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

This review focusses on his 40m STL.

The loop is a circle of perimeter 3.83m which at 7.1MHz is 0.091λ which is at the top end of the strictest criteria for an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops). It appears from his pics that the bottom of the loop is about 1.5m above real ground, so we expect a significant ground loss resistance component in Rtotal.

N6PAA gives a measured VSWR curve for the matched antenna, and the VSWR=3 bandwidth as scaled from the graph as 20kHz, from which we can calculate the half power bandwidth and eventually, efficiency. There is some suggestion that some measurements were taken indoors, this analysis assumes that the relevant measurements were taken outdoors as pictured. Continue reading Review of N6PAA’s 40m STL

Review of KK5JY’s 40m STL

(Roberts 2010) describes several small transmitting loop (STL) and gives some meaningful performance measurements. It is rare to see such measurements and he is to be congratulated.

This review focusses on his 40m STL.

The loop is a circle of perimeter 4.3m which at 7.1MHz is 0.102λ which is at the top end of the strictest criteria for an STL, the common formula for radiation resistance Rr of a STL fail for perimeter above about 0.1λ (see Accuracy of estimation of radiation resistance of small transmitting loops). It appears from his pics that the bottom of the loop is about 2m above real ground, so we expect a significant ground loss resistance component in Rtotal.

Roberts gives the VSWR=2 bandwidth as 5.4kHz, which if we assume that it was adjusted for a perfect match mid band, we can calculate the half power bandwidth and eventually, efficiency. Continue reading Review of KK5JY’s 40m STL

Trial of prototype stand alone GPS logger

An upcoming project calls for a stand alone GPS logger.

The requirement is for a GPS stream that allows correction using RTKLIB, but this trial is of a lesser GPS as proof of concept.

SAGPS001

Above, the equipment consists here of a Ublox NEO-6M based GPS module (~A$15 incl on eBay) at left, an Openlogger (~A$15 incl post on eBay) at right, and a 12V-5V converter (~A$7 from Hobbyking) at bottom. The latter is a 5A converter, way overkill, but it was on hand. The GPS module has a 3V regulator on board for the NEO-6M chip.
Continue reading Trial of prototype stand alone GPS logger

Reconciling W5QJR’s loop formulas

Ted Hart inspired interest in loops for transmitting applications with his article “Small, high efficiency loop antennas” (Hart 1986).

He included a table of recommended designs, the following is an extract of the table rows relating to an octagonal loop with perimeter=20′ (6.1m). The tube specified was 3/4 copper pipe which has an OD of 22mm. Continue reading Reconciling W5QJR’s loop formulas

Reconciliation of field strength to efficiency calculator with Boswell’s loop measurements

(Boswell et al 2005) discussed a small transmitting loop (STL) and offered predictions and measurements of performance.

Boswell’s loop is 1m diameter of 22mm diameter copper tube.

This article is a reconciliation of Calculate efficiency of vertically polarised antenna from far field strength with Boswell’s predictions, measurements and efficiency calculations.

Boswell-Fig06Above, Fig 6 from Boswell shows his prediction of the field strength of a 100% efficient loop at several distances, and measured field strength. He calculated efficiency from the difference between predicted lossless and measured. Continue reading Reconciliation of field strength to efficiency calculator with Boswell’s loop measurements

On ignoring capacitor losses in Small Transmitting Loops

There are a host of design tools for Small Transmitting Loops, spreadsheets, online calculators and conventional applications you download and run on your PC.

Almost all ignore capacitor loss… and I say almost so that I am not wrong, I have never seen one of these tools that does include capacitor loss.

NEC study of Small Transmitting Loop Q vs frequency contained a graph of the elements of feed point resistance from and NEC-4.2 model for a small loop. Key parameters are:

  • Octagonal loop of 20mm copper with area equal to that of a 1m diameter circle, loop perimeter=0.104λ at 10MHz;
  • centre height=2m;
  • Qcap=2000;
  • ground=0.007/17; and
  • freq=1-10MHz.

This analysis only extends up to 10MHz, because for perimeter>λ/10, the formulas used by most of these simple calculators are in error for other reasons.

Clip 070

Above, the four elements on log scale. Continue reading On ignoring capacitor losses in Small Transmitting Loops

NEC study of Small Transmitting Loop Q vs frequency

Recent comments elsewhere on the shape of the plot of measured Q from (Austin et al 2014) gave reason to explore the behaviour of Q for a ‘good’ Small Transmitting Loop (STL) using an NEC-4.2 model.

The term Small Transmitting Loop means a loop sufficiently small that there is not a significant departure from smaller loop behaviour. Essentially this is true for perimeter less than about λ/10.

NEC-4.2 model

Key parameters are:

  • Octagonal loop of 20mm copper with area equal to that of a 1m diameter circle, loop perimeter=0.104λ at 10MHz;
  • centre height=2m;
  • Qcap=2000;
  • ground=0.007/17; and
  • freq=1-10MHz.

This is a quite practical small transmitting loop with current that is approximately uniform around the loop. Continue reading NEC study of Small Transmitting Loop Q vs frequency

K4PP’s 1m Small Transmitting Loop

K4PP described his Small Transmitting Loop (STL), including details of its construction and measured VSWR response.

The loop is a 1m diameter circle of 12.7mm dia copper tube with a high Q vacuum cap for tuning.

K4PP01

Using a quality capacitor and copper tube, this loop should be as efficient as they come for its size and location. Continue reading K4PP’s 1m Small Transmitting Loop