Category: Amateur Radio

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

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

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

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

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