## Antenna half power bandwidth and Q, concept and experimental validation

Many antennas can be represented near their series resonance as a series RLC circuit, and in many cases R changes very slowly with frequency compared to X. This provides a convenient and good approximation for the behavior of the antenna impedance in terms of a simple linear circuit.

## Series resonant circuit

The response of a simple series resonant RLC circuit is well established, when driven by a constant voltage source the current is maximum where Xl=Xc (known as resonance) and falls away above and below that frequency. In fact the normalised shape of that response was known as the Universal Resonance Curve and used widely before more modern computational tools made it redundant.

Above is a chart of the Universal Resonance Curve from (Terman 1955). The chart refers to “cycles”, the unit for frequency before Hertz was adopted, and yes, these fundamental concepts are very old. Continue reading Antenna half power bandwidth and Q, concept and experimental validation

## G3CWI’s ground wave tests Jul 2017 using WSPRlite

Richard (G3CWI) published an interesting blog article Comparison of groundwave performance of Small Transmitting Loop and Quarterwave GP summarising a recent WSPR test on 40m over 20km distance.

## 100% efficient tx and rx antenna systems

Ground wave suffers attenuation due to two key components:

1. dispersion of energy as the wave spreads out from the source; and
2. absorption of energy in heating the soil.

Item (1) is simply inverse square law effect, and Norton provides us with several approximations for estimating (2) from Sommerfields work.

Calculate efficiency of vertically polarised antenna from far field strength uses Norton’s f5 approximation for ground wave attenuation.

Above is a calculation for a 100% efficient transmitter. (The trick to getting this is to leave the measured field strength field empty and the calculator will insert the value that gives 100% efficiency.)

So the next question is what ambient noise level might we expect in a rural setting on 40m. Continue reading G3CWI’s ground wave tests Jul 2017 using WSPRlite

## STL propaganda indeed: QW vertical – dipole – STL model pattern comparison

STL propaganda indeed: dipole – STL pattern comparison compared the patterns of a Inverted V dipole and STL, both configurations typical of SOTA deployments.

Seeing some pretty wild extrapolations to a vertical quarter wave with elevated radials, again typical of SOTA deployment, this article presents a comparison of all three using NEC-4.2 models.

See STL propaganda indeed and STL propaganda indeed: dipole – STL pattern comparison for details of the models for the STL and dipole.

The QW vertical is modelled using 2mm dia copper wire for vertical and radials, the radials are elevated 0.5m over ‘average ground’ (σ=0.005, εr=13).

Bear in mind that these are models that are based on some assumptions like ground parameters for example, and results may be different for other scenarios. Likewise, the results at 20m cannot simply be extrapolated to other bands, and practical modes of propagation utilised vary from band to band.

## Key differences

### Polarisation

Polarisation is a significant difference. Vertical ground waves are attenuated more slowly than horizontal waves, though ground wave propagation is not so commonly exploited on 20m due to its very short range. Because vertical ground waves are attenuated more slowly, a vertical polarised receiving antenna is likely to capture more ‘local’ noise that a horizontal one, but in SOTA context, local noise is not such an issue on mountain tops.

The QW vertical is vertical polarisation.

The STL is vertical polarisation.

The Inverted V dipole is horizontally polarised broadside to the dipole, and tends to vertical polarisation off the ends.

Radiation pattern is a 3 dimensional characteristic, often selectively plotted in two dimensions in the most favorable plane… which is fair enough but the reader needs to keep in mind the bigger three dimensional characteristic as it applies to their own application.

The radiation patterns of the antennas are quite different, the vertical is omnidirectional in azimuth whereas the others are not. So, it is challenging to produce a single general figure of merit comparing all antennas.

Above is a comparison of gain in the plane of maximum gain of the STL and dipole.  Continue reading STL propaganda indeed: QW vertical – dipole – STL model pattern comparison

## STL propaganda indeed: dipole – STL pattern comparison

At STL propaganda indeed a realistic model was developed of the Chameleon P-Loop2 on 20m, similar to that used in the experiment Comparing the performance of an inverted vee dipole with a small transmitting loop on 20m.

This article presents NEC-4.2 derived radiation patterns for both the loop and Inverted V Dipole used for the experiment using data published in the experiment writeup.

## The effect of radiation pattern

The original experiment cited at the start compared WSPR signals received by a number of stations at moderate distance, and a key parameter becomes not so much the maximum gain of the two antennas compared but the gain at the relevant path elevations and the higher dipole will tend to have relatively better gain at lower elevation than the lower STL, so that further disadvantages the STL in the test scenario. This factor would be additional to the relative maximum gain of both antennas.

That is not to suggest that the test somehow set out to disadvantage the STL, both antennas were quite typical of SOTA deployments and the relative performance over moderate distance paths is highly relevant to that application.

## Patterns from the models

The following patterns are from an NEC model that tries to capture realistic values for significant loss elements that affect the gain of each of the antennas.

The major lobe axis is shown above, and the difference in the patterns varies a little with elevation. at 45° elevation the difference is 6.11–7.72=13.8dB. It should be no surprise to an open mind that  Richard concluded there as an advantage of 12.77dB to the dipole his experiment.  Continue reading STL propaganda indeed: dipole – STL pattern comparison

## STL propaganda indeed

Recent postings to List your favorite SOTA antenna on QRZ.com referred to an experiment Comparing the performance of an inverted vee dipole with a small transmitting loop on 20m as propaganda.

The experiment above states The manufacturer of the loop gives a calculated efficiency of 39.754% at 14174 kHz. This is very similar to that claimed by Chameleon of their P-Loop2, so it will be used as a study example.

## Experimental method

Taking the Chameleon P-Loop2 on 20m as the study example,

1. build a model in the AA5TB spreadsheet;
2. build an NEC-4.2 model in free space and reconcile it with AA5TB;
3. build an NEC-4.2 model in proximity of ground compare it with AA5TB;
4. build an NEC-4.2 model in proximity of ground and add realistic estimates for conductor and capacitor loss and compare it with AA5TB;
5. compare the NEC-4.2 model in proximity of ground with realistic losses to Chameleons published VSWR curve.

Chameleon gives the following table on their website.

## Step1: build AA5TB model

An extract from the AA5TB model using Chameleon’s stated dimensions. The efficiency figure reconciles exactly.  Continue reading STL propaganda indeed

## 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.

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

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

## 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.