Extrapolating VSWR of a simple series resonant antenna

An online expert helped recently helped his Small Transmitting Loop (STL) disciples with:

Also remember that the bandwidth given by the calculators is the half power point. That’s equivalent to an SWR of about 4.3 at the ends.

Whats that?

Most STL, and lots of other resonant antenna systems exhibit a classic VSWR curve being that of a approximatly constant resistance in series with an ideal capacitor and inductor.

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Above is that classic VSWR curve.  Continue reading Extrapolating VSWR of a simple series resonant antenna

Finding the inductance of the outside of LDF4-50A

There are applications for estimating the inductance of the outside of LDF4-50A at radio frequencies.

For the purpose of calculating the inductance, the geometric mean radius is appropriate. This article offers two methods for estimating the geometric mean diameter (GMD) of the conductor.

Above a section of LDF4-50A.

 

Above is a magnified view of the profile, it is corrugated copper outer conductor with a shallow but not quite symmetric profile.  Continue reading Finding the inductance of the outside of LDF4-50A

Loss components in an NEC model of a Small Transmitting Loop

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 (0.005,13);
  • 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

Exploiting waveguide mode of the loop conductor in a small transmitting loop

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

Estimating the voltage impressed on the tuning capacitor of a small transmitting loop

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

A QRP small transmitting loop evaluation

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

Precise RF small transmitting loop

Precise RF have announced two small transmitting loops for amateur radio, this article looks at the Precise High Gain Loop.

Description

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

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

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