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 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.

The assumed values and published VSWR curve indicate an antenna system half power bandwidth of 5.6kHz 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.

Duffy

For example, Duffy (2014b) analyses the Chameleon M loop. Based on the larger diameter of the conductor (25.4mm) and smaller loop perimeter (2.873m), we might expect it to have a loop inductance of around 61% that of the LNR Precision loop. Lower inductance is one of the factors of the equation for Q, and for half power bandwidth. The smaller loop of larger diameter conductor will tend to have lower Q, wider bandwidth, all else equal. You cannot simply infer relative efficiencies of these antennas based on half power bandwidth alone… any competent antenna designer know that.

Milazzo

Taking (Milazzo 2014) measurements for 7.1MHz, MinVSWR=1.345, Z at minVSWR=57-j14, BW at VSWR=3 35kHz (from the graph), we can explore the likely efficiency if we assume that radiation resistance was close to that of free space,

Above is the result of a more complete estimate of efficiency that includes not only half power bandwidth, but loop perimeter and conductor diameter. The estimated efficiency at 0.88% is significantly lower than a similar estimate for the LNR Precision loop, some 5dB lower but measurement of field strength from both loops in a relevant setting is the best way to compare performance.

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.

One could be forgiven for thinking that the market for these type of antennas are hams who aren’t much concerned with efficiency, but there remains a small number of hams interested in the technology of radiocommunications.

Conclusions

Based on measurements published by both suppliers, it is likely that the LNR Precision loop is significantly better than one of its competitors, the Chameleon M-Loop, but the difference between them cannot simply be inferred from the bandwidth measurements of each.

The calculator Calculate small transmitting loop gain from bandwidth measurement which captures most of the relevant factors, properly applied may provide the basis of a useful comparison between different implementations.

References / links

  • Chameleon. Oct 2014a. CHA M-LOOP Analysis. http://chameleonantenna.com/resources/CHA_M-LOOP_SWR_ANALYSIS.pdf(accessed 30/11/2014).
  • Chameleon. 2014b. CHA M-LOOP. http://chameleonantenna.com/PORTABLE%20ANTENNA/CHA%20M-LOOP/Chameleon%20M-LOOP.html (accessed 30/11/2014).
  • Duffy, O. May 2014. Small transmitting loop calculators – a comparison. http://owenduffy.net/blog/?p=1693.
  • Duffy, O. Nov 2014b. Chameleon CHA M-LOOP. http://owenduffy.net/blog/?p=2999.
  • Efficiency and gain of Small Transmitting Loops (STL)
  • LNR Precision. 2016, W4OP Small Transmit/Receive Loop. http://www.lnrprecision.com/loop-antennas/
  • Milazzo, C. May 2015. Chameleon CHA F-Loop Antenna Parameters: 5-30 MHz. http://www.qsl.net/kp4md/chafloop.htm.
  • Small transmitting loops
  • Yates, S. April 2009. Small Magnetic Loop Antenna Calculator ver. 1.22a.