Vertical antenna designs & radials
Some years ago, the original manufacturer of Butternut antennas (they’re now made by DX Engineering) had a few ‘Tech Note’ pdf files on their website. In particular, there were two entitled “Dirty little secrets”. These attempted to explain how vertical antennas work and how good SWR might mean you have a bad antenna. These are a really interesting read and include a few diagrams from the ARRL Antenna Handbook to back up the analysis.
The first document took a swipe at competitor designs, some of which may still be on the market. The second includes thoughts on traps and the design of the Butternut HF9V. One thing to note though – the documents don’t take radiation pattern into account. Some antennas may radiate better at a low angle – which is better for long-distance DX.
Here’s a brief summary of the two documents.
The first document criticizes certain claims made by other manufacturers of vertical antennas. It argues that some antenna designs then being advertised make unrealistic promises about performance.
Key points from part 1:
1) One manufacturer claimed their antenna had “no earth loss”, which the author says is impossible since earth losses come from RF passing through soil, not the antenna itself. Proper radial systems help reduce earth losses per ARRL guidelines.
2) Another antenna was marketed as being a halfwave on multiple bands despite its short height (22.5ft), making this unlikely except perhaps around 21MHz. Does a quarter-wave perform better when centre-fed and called a halfwave?
3) The author provides methods to calculate antenna efficiency based on radiation resistance vs total feedpoint impedance. Knowing these values allows estimating how much power is actually radiated vs lost due to various factors like ground conductivity.
4) Examples are given showing how the “no-radial” design likely has high loss resistance reducing efficiency compared to what would be expected for a proper quarter/half wave antenna system with sufficient radials installed according to accepted practices.
In summary, while the specific technical details may require more expertise to fully validate, the overall message seems to be that some vertical antenna products exaggerate capabilities and downplay importance of things like radial systems in order to market them effectively, even if actual performance suffers. The article aims to educate readers so they can make informed decisions rather than relying solely on marketing claims.
In part 2:
1) The author discusses how traps and loading coils add loss resistance which reduces efficiency in multi-band trap antennas. Traps block current flow at specific frequencies but also introduce losses.
2) For maximum efficiency, as much of the antenna should radiate as possible without unnecessary traps/loads. But many designs use multiple traps to cover many bands even if it hurts performance.
3) Some argue convenience outweighs efficiency sacrifices when using these inefficient multiband trap antennas. But there are ways to build efficient single band or limited band verticals instead.
4) Loading coil placement matters – base loading puts high currents through the coil reducing radiation resistance. Center loading improves this somewhat by allowing lower portion to carry higher currents. End loading has its own issues.
5) The Butternut HF9V-X design minimizes losses by only having 4 tuned circuits total acting on 6 bands (one acts like a trap). Most of the radiator is always active. This maximizes radiation resistance compared to other “trap” designs for similar size.
6) Many positive testimonials highlight excellent real-world performance despite modest radial systems due to overall efficient design minimizing losses vs competitors with inferior designs prioritizing convenience over performance.
In summary, while multi-band trap antennas may seem convenient, they often sacrifice efficiency from excessive lossy components. Better results can be achieved with fewer traps/loads and more of the antenna actively radiating. Thoughtful engineering focused on maximizing radiation resistance leads to better performing antennas according to both analysis here and field reports.
The document summary was principally prepared by an IBM Granite 4.0 AI installed on a home PC, but there were a few human tweaks to improve accuracy.
