‘What Basic HF Aerial’ ‘What Basic HF Aerial’
It’s been a while since we have covered HF aerials, so it’s high time we re-visited the topic. Also, recent changes to the licensing structure have led to a whole new group of radio amateurs becoming active on the HF bands for the first time.
Roger started with a few of the basics of HF propagation, including the properties of the different layers of the ionosphere, such as frequency versus absorption and reflection for each layer. The relationship between vertical critical frequency and Maximum Usable Frequency (MUF) was explained. MUF being approximately 2.5X the critical frequency.
The longest possible ‘single & multiple hop’ propagation is achieved with the signal hitting the highest ionised layer at the shallowest possible angle. For this reason, the radiation angle of the antenna is very important, which led us neatly onto the basic types and characteristics of various antennas.
Horizontal aerials are affected by the ground, which acts as a reflector. In the typical back garden environment, this means that they radiate at a high angle with a short skip distance. Fine for working around Europe, but not well suited for DX operation. Beams are an option and to keep the radiation angle nice and low they should be located 3 – 5 wavelengths above the ground – totally impractical for the lower HF bands ! Single element verticals are a simple option, giving omni-directional low angle radiation.
A few practical aerials were explained, including horizontal centre-fed dipoles, trap-dipoles, end-loaded dipoles and end fed wires (such as the Inverted ‘L’). For practical reasons, these antennas can be ‘dog-legged’ to fit into the average suburban back garden.
As most modern transceivers are designed to work into a 50 ohm resistive load, matching is particularly important. Either the antenna can be designed for a correct match, or a suitable ATU will need be used.
It’s also possible to check the level of mis-match with an Antenna Analyzer. Various commercial devices are available from manufacturers such as MFJ and Autek. Everyone had heard of MFJ, but no one seemed to be familiar with Autek. Details of this range can be found at autekresearch.com/index.htm
Roger covered some of the basic designs of matched antennas, such as wires for single bands with ¼ wave resonant lengths and including the effect of length versus diameter on bandwidth. Harmonically related dipoles could be used. For instance, a dipole constructed for use on the 7MHz band would be a reasonable match at it’s third harmonic at 21MHz. Alternatively, a ‘Nest of Dipoles’ could be used. That is several dipoles all fed at one point.
Many modern transceivers have a build-in matching unit, but this is usually only suitable at an SWR of no greater than 3:1. An external ATU will be necessary in most cases.
Practical common sense advice was offered, such as keeping the antenna clear of earthed objects such as trees and buildings. Feeding inverted ‘L’ antennas from the far end, well clear of the house. EMC considerations were covered, such as keeping the antenna well away from house wiring, TV aerials and burglar alarm wiring etc.
On the subject of planning permission, Roger drew our attention to an RSGB booklet, which is free to members. The ‘4 year rule’ was mentioned. Basically, if your antenna has been up to 4 years and no one has complained, you shouldn’t have any problems and don’t apply for permission for what you’ve already got !
All in all it was an interesting evening of basic common sense advice and information. No complicated mathematics or theories, just a bit of encouragement to either start or carry on experimenting with elevated wire.
We finished off with a short Q&A session and then let Roger escape back to deepest Hertfordshire. I’m sure all those there present enjoyed the meeting and would like to thank Roger for coming along.
