This article is a tutorial on using an NEC to model a small transmitting loop in proximity of ground.
NEC-4.2 model parameters:
- single turn;
- 1m loop diameter;
- 20mm OD conductor;
- loop centre 1.5m above ground;
- ‘average’ ground (σ=0.005, εr=13);
- 20 segments in loop;
- conductor loss modelled as 0.0033Ω per segment;
- tuning capacitance 197pF with Q=1000 (ie 0.112Ω series R).
Note that NEC-2 is more restricted in the size of segments for good results, and this same problem will require fewer / longer segments in NEC-2, and give slightly different results.
The model tuning capacitance and frequency were adjusted to resonate at about 7.1MHz.
Above is a VSWR plot of the matched main loop, half power bandwidth (ie between VSWR=2.6 or ReturnLoss=6.99dB points) is 12.5kHz, and we can calculate Q=7106/12.5=568.5.
A model run at 7.106 gives us several interesting metrics.
Gain is 9.77dB, and as expected maximum gain is at the zenith. Continue reading Loss components in an NEC model of a Small Transmitting Loop
Last update: 29th December, 2022, 5:45 AM
Small transmitting loop enthusiasts search for explanations of why their antennas are so fantastic.
One of those fantastic explanation is from KK5JY:
I have spent some time thinking about this discrepancy, and how to account for it within the typical ham home-made loop. This is not to say that I am asserting this as correct, but I suspect there are straightforward reasons why the efficiency of a small loop of typical construction could be better than the classic formulae predict.
One simple possibility has to do with construction. Many loop designs, mine included, use open-ended copper tubing for the radiating element. Mechanically, this means that the loop itself actually has two conductors, wired in parallel. One is the outside of the loop conductor, and one is the inside of the loop conductor. The reason for this is skin effect. Anybody who has run high power RF into a coaxial cable that is poorly matched to a balanced antenna is familiar with the “feedline radiation” effect, where the shield of the coaxial cable forms two conductors, with current flowing on both. In the loop case, The outer and inner surfaces of the loop conductor are connected together at the ends, so the two conductor shells carry current in parallel. Depending on the difference in diameter of the two surfaces, the effective increase in surface area can be almost 100%, roughly doubling the surface area of the main element. “But the inner conductor is shielded from the environment by the outer conductor,” someone might object. This is true for the electrical field, but not the magnetic field, which just happens to be the largest component of the EM near-field created by this type of antenna. A small loop is driven almost completely by the magnetic field generated by the driven element, and the lines of magnetic flux cut both the inner and outer surfaces of the main (large) loop, inducing current flow into each one, independently, and the two are able to create a combined magnetic field around the antenna.
Continue reading Exploiting waveguide mode of the loop conductor in a small transmitting loop
Last update: 12th June, 2017, 9:01 AM
A correspondent has been tearing his hair out trying to replicate my VSWR plots of some STL.
Above is an example where the Z0 has been set to 0.0901847Ω which is the feedpoint impedance of the loop at resonance. Continue reading 4NEC2 plots of STL VSWR
Last update: 6th June, 2017, 1:00 PM
The ‘net abounds with calculators for design of small transmitting loops (STL), and most estimate the voltage impressed on the tuning capacitor. Most of these calculators give an incorrect estimate.
This article describes a measurement based approach to estimating the capacitor voltage for a STL.
Continue reading Estimating the voltage impressed on the tuning capacitor of a small transmitting loop
Last update: 6th June, 2017, 11:50 AM
The ‘net abounds with articles describing easy to build low cost small transmitting loops (STL).
This article describes measurement of a STL for 4MHz using RG213 coaxial cable for the main loop and its tuning capacitance, and a smaller plain wire loop for transformation to 50Ω. Continue reading A QRP small transmitting loop evaluation
Last update: 6th June, 2017, 10:27 AM
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. Continue reading Precise RF small transmitting loop
Last update: 5th June, 2017, 8:26 AM
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
Last update: 28th May, 2017, 9:49 AM
Shunt matching a loaded HF whip with just a VSWR meter gave a direct answer and supporting explanation to an online poster’s question about optimising an 80m loaded mobile vertical with shunt matching, specifically the inductor needed and an adjustment procedure.
The original poster clearly had the impression that this improvement of the original VSWR=1.3 would make a large difference.
The only other option for me is to remove the shunt and set my swr back to 1.3:1 and not be able to communicate.
Continue reading Shunt matching a loaded HF whip – discussion
Last update: 24th May, 2017, 7:57 PM
A question was asked on one of the popular online forums:
How to get the most out of an 80 mobile antenna?…I am using a hustler antenna and I had the swr down to 1.3:1. I started researching how to make the antenna better and it seems that maybe an inductive shunt at the base of the antenna to ground would help. I don’t have the equipment to analyze the antenna and the shunt reactance. I made a 9 turn coil 1″ in diameter and 1″ long using n0. 12 awg thnn wire. I installed the coil at the base of the antenna and now the best swr that I can get is 1.8:1. So is there a way that I can set up the coil and antenna using only an swr meter?…
After 50 responses, none of the online experts have offered a direct answer or explanation.
The coil inductance is too low, try a solenoid of 13 turns, 40mm diameter and 40mm length.
An antenna of this type at minimum VSWR will have a feed point impedance of near zero reactance and resistance equal to 50 divided by the measured VSWR, so in this case 39Ω. Continue reading Shunt matching a loaded HF whip with just a VSWR meter
Last update: 1st November, 2017, 8:19 PM
A correspondent wrote suggesting that he had seen online NEC patterns showing a 30″ square small untuned loop to have a gain of around 10dBi, more than 30dB better than given by Calculate small loop Antenna Factor.
Firstly, lets describe a loop for study, a square diamond with sides of 760mm (30″) of 2mm diameter copper fed in one corner at 7.1MHz.
Calculate small loop Antenna Factor
Calculate small loop Antenna Factor models a small loop in free space (therefore does not include ground losses).
Above is the calculator result, the key figures are Antenna Factor 31.75dB and Gain -44.5dBi.
An NEC-4.2 model was constructed with external excitation (1V/m) incident on the loop which has a 50+j0Ω load inserted at the feed point to represent the receiver load.
Here is the model source.
CM Small square untuned loop
CM 1. Plane wave excitation
CM Owen Duffy
CM Note: rotations might not work properly in various NEC-2 versions, beware of segment size issues in NEC-2.
GW 1 5 -0.38 0 -0.38 0.38 0 -0.38 0.001
GM 1 3 0 90 0 0 0 0 1
GM 0 0 0 90 0 0 0 2 1
LD 5 0 0 0 58000000
LD 4 1 1 1 50 0
EX 1 1 1 0 45 0 0 0 0 0
FR 0 0 0 0 7.1 0
The key result to be extracted from the model run is the current in the 50Ω resistor in segment 1 of wire 1. The magnitude of the current is 5.1204E-04, so the voltage developed in the resistor V=5.1074-04*50=0.02554V. Antenna Factor is the ratio of the E field excitation to the terminal voltage of the receiver, so in dB it is 20*log(1/0.02554) =31.83 dB/m.
The NEC model’s 31.83 dB/m is close to the calculator prediction of 31.75dB/m, but includes the benefit of the lossy ground reflection . Continue reading Small untuned loop for receiving – NEC model
Last update: 19th May, 2017, 7:56 PM