Category: Amateur Radio

NEC requires the user to define a model structure as a set of geometry elements. It includes two powerful cards that make definition of the structure simpler and more reliable, they are the GM card for coordinate transformation and GX card for reflecting a structure in coordinate planes.

This tutorial demonstrates the use of these cards to define what might appear to be a fairly complex hypothetical NVIS antenna scenario quite simply, and more importantly, reliably. I say reliably because the logical definition of the model based on similar elements already defined, the more confident the developer can be that they are indeed similarly defined,  the differences are explicit, and that they are properly connected.

Above is a model to explore coupling from a tx antenna to a nearby rx antenna, The scenario contains 52 wire elements which one could naively define using 52 GW cards.

Instead, we will define it with far fewer GW cards and use model symmetry, rotation and translation to define the model. Continue reading NEC GM, GX tutorial

A correspondent questioned the writings of an online expert who opined whilst discussing loaded monopole antennas:

… there is a formula circulating the Internet which states that antenna Q is equal to 360 times the frequency in MHz, divided by the 2:1 VSWR bandwidth in kHz. One has to assume they mean antenna system Q, but that’s not a given. While this formula might give you a comparison between antenna A and antenna B (all else being equal), the actual Q of the antenna (system or otherwise) requires a textbook-full of formulas, and a lot more information than just the 2:1 bandwidth! Fact is, this formula is no more specific than the number of DX contacts a specific antenna garnered.

The formula given is:

Q=360*fc/B(VSWR=2) where fc is the centre frequency.
Continue reading A certain formula for antenna system Q

Introduction

End Fed Half Wave antennas are again very fashionable with hams, accompanied by extraordinary claims and somewhat sparse understanding (the way of modern ham radio).

To add some light I have created a set of NEC 4.2 models of a half wave antenna on 20m to give some insight into the behaviour of a bottom fed vertical half wave over real ground.

This analysis does not consider harmonic operation, antennas are a half wave at 14.2MHz.

Four models are used:

1. 20mHW-VEP – bottom fed vertical above perfect ground;
2. 20mHW-VEA – bottom fed vertical above real ground;
3. 20mHW-VCA – centre fed vertical above real ground (ie ground independent feed);
4. 20mHW-HCA – centre fed horizontal at 5m height above ground;

NEC 4.2 model description:

• 14.2MHz;
• no conductor loss;
• real ground assumed to have conductivity=0.005S, εr=13, of course results are dependent on these values;
• conductors are ~10m long, 20mm diameter;
• bottom fed vertical half wave uses a 10m x 20mm vertical driven ground electrode;
• centre fed vertical is raised 200mm above ground;
• feed line and feed line common mode current are excluded;
• the centre of all antennas is ~5m above ground (real or perfect).

Above are the patterns from the models for discussion. Continue reading End fed half wave – NEC models for 20m