Precise RF have announced two small transmitting loops for amateur radio, this article looks at the Precise High Gain Loop.
The antenna is described at (Precise RF 2017).
Above is an extract from a table in the brochure comparing the subject antenna to some others.
On a quick scan, the standout figure is gain of 2.8dBd presumably at a loop height of 4.57m (15′), and without qualification of frequency. Elsewhere in the brochure there is a note that 80m requires an optional ‘resonator'… presumably a larger loop.
Lets review the meaning of dBd
The ITU Radio Regulations (ITU 2012) gives us a definition for antenna gain that captures the meaning of dBd that is accepted by most regulators and industry world wide.
ITU's Gd is gain relative to a half wave dipole (dBd)
b) gain relative to a half-wave dipole (Gd), when the reference antenna is a halfwave dipole isolated in space whose equatorial plane contains the given direction;
The gain of a half wave dipole in free space is 2.14dBi, so gain give in dBd can be converted to dBi by adding 2.14.
In this case the claim of 2.8dBd gain is equivalent to 4.94dBi.
An NEC-4.2 model was constructed at 7.1MHz of a helix of 16 segments of the same diameter as the Precise RF loop and 6.35mm conductor radius. The model places the bottom of the loop 4.5m over average ground so includes ground losses and includes copper loss for a round copper conductor. It does not include tuning capacitor loss or the additional loss expected of a braided main loop conductor (and that is likely to be significant). No feed system loss is included, but that should be insignificant.
The model is optimistic because of omission of braid related loss and capacitor loss.
Above is the gain plot from the optimistic model. The model gain is -6.4dBi (-8.54dBd), 11.34dB short of the claimed 4.94dBi (2.8dBd).
Lets do some calculations based on the reported efficiencies above. Feed R at resonance was 0.0642Ω, and the radiation efficiency figure tells use that 6.9% of that is radiation resistance Rr, Rr=0.069*0.0642=0.0044Ω. Loop loss (conductor only in this case) Rloop is (1-0.1533)*0.0642=0.0544Ω, and therefore equivalent ground loss resistance Rg is found by deduction from the total Rg=0.0642-0.0544-0.0044=0.0054Ω.
Note that Rr and Rg are sensitive to height above ground and the ground type.
Above is a plot of the VSWR and -ReturnLoss curves. The half power bandwidth is the difference between frequencies where VSWR=2.618 or ReturnLoss=6.9dB. The half power bandwidth is 3.7kHz.
We can use some values from the NEC model to improve the accuracy of Calculate small transmitting loop gain from bandwidth measurement for this loop as configured.
The adjustment to Rr/Rrfs and Directivity capture the variation from free space conditions for the loop in this exact scenario. Within the precision of the data entry, the models reconcile.
The VSWR curve of a matched antenna allows measurement of its half power bandwidth which can be used along with key physical properties to estimate its efficiency.
If the bandwidth of the loop at this frequency, height and ground context was measured to be other than 3.7kHz, plugging the measured value would estimate the efficiency and gain.
Do try this at home
You can try this at home… but we aware that if using NEC-2 of its short segment limitation and that compliance may meaning using less than the 16 segments used in this model, resulting in some small modelling error.
We should expect that Rr and Rg are sensitive to height above ground and the ground parameters.
The models above of an optimistic implementation of a loop of similar size but plain round copper conductor and no capacitor loss do not support the claimed gain of 4.94dBi (2.8dBd).
The radiation efficiency of the real antenna is likely to be less than the optimistic model's 7%.
The real test is to measure field strength in-situ and calculate the efficiency in the same way the industry does for MW BC antennas.
Second to that is to measure the half power bandwidth in-situ and infer the efficiency using the technique employed by Calculate small transmitting loop gain from bandwidth measurement using appropriate ground proximity adjustments as done above.
The seller does not publish a set of VSWR curves, and you might wonder why.
Beware of antenna gain quoted in dBd, the author may use the measure incorrectly and it is an obfuscation of the actual gain and for some reason appeals to the innumerate.
- Duffy, Owen. 2012. dB, dBi, and dBd. https://owenduffy.net/antenna/concepts/dBd/index.htm (accessed 14/04/12).
- ITU. 2012. Radio Regulations. ITU: Geneva 2012.
- Calculate small transmitting loop gain from bandwidth measurement