HF Antennas: Vertical or Horizontal? (2024)

Serge Stroobandt, ON4AA

Copyright 2013–2020, licensed under Creative Commons BY-NC-SA

0. Home
1. Antenna Designs
2. Vertical or Horizontal?

Ifyou can have only one antenna and can chose between avertical HF antenna or arelatively high horizontal HF antenna, go for thehorizontal antenna! Wide‑band active small magnetic receiving loops (not to be confused with tuned magnetic loops) offer, when mounted close to theground, improved noise immunity at reception.

InJanuary 2008, —as little as three months before his lamented dead— L.B.Cebik, W4RNL(SK) published what was going to be thelast entry of his seminal 10–10News series.¹ Itturned out to be aninteresting gain comparison of single element 28.4MHz antennas modelled over various ground types of which asummary is presented here (Table1). His findings seem to fit well with what John Devoldere, ON4UN modelled and published for equivalent antennas on thelower HF bands.² This led me to write up thefollowing synopsis.

… benefit from nearby ground gain

Ataheight of about λ/2, thenearby ground reflection of ahorizontal HF antenna will start to be constructive at interesting take-off angles for long- distance ionospheric contacts. This will provide anet gain over theantenna in free-space.

… suffer from nearby ground loss

This is not thecase with vertical HF antennas. Nearby ground only contributes loss. This even more so when theground forms part of thereturn path of theradiating structure. Even when far-away ground reflections may cause thedirectivity of avertical HF antenna at low take-off angles to be much higher than that of ahorizontal HF antenna, its net gain will still be lower at those angles. This makes thehorizontal HF antenna aclear winner, at least for what transmission is concerned. Note that gain and directivity are not synonyms; gain takes into account losses, directivity does not.

Vertical HF antennas do have their merit though. Atthelower end of theHF spectrum, theλ/2 height requirement for horizontal antennas can become cumbersome (even though horizontal phased arrays have aless stringent minimum height requirement). Avertical HF antenna can get away with aheight of only λ/4. Furthermore, if thereturn conduction current of avertical radiator flows through salt water, losses will be lower. Finally, thedirectivity of avertical HF antenna can be effectively employed at thereception end to cancel out high-angle interference caused by near-by stations. This is why some stations use receive-only phased arrays of verticals on thelow bands.

Polarisation is not really anissue at HF. This is because theionosphere is mainly ananisotropic medium, i.e.it messes up polarisation. However, horizontally polarised antennas are again preferred over noisier verticals because of thefollowing considerations pertaining thepolarisation of noise signals:

1. Earth-to-cloud lightning (QRN) is vertically polarised.
2. Vertically polarised man-made noise (QRM) propagates longer distances over ground than horizontally polarised man-made noise. Consequentially, thecapture area for vertically polarised man-made noise is much larger than that of horizontally polarised man-made noise —actually, by afactor of thedifference in ground wave propagation distance squared!

Antenna input impedances differ between differential and common mode, as well as between different antenna types.³ Thedifferential and common mode input impedance of anantenna can be derived by considering theantenna as atwo‑wire, respectively single‑wire, transmission line (Figure1). Myarticle about common mode chokes explains this in further detail. Antennas with ahigh commonmode inputimpedance (seeTable2) will receive less noise from interfering sources in thenearfield.

Theabove mentioned points about thepolarisation of noise also hold true for wide‑band active small magnetic receiving loops (not to be confused with tuned magnetic loops) when mounted close to theground.

However, there is anadditional reason why active small magnetic receiving loops are so successful at improving noise immunity:

3. Namely, themagnitude of theelectrical field strength \(|\mathbf{E}|\) in thenear field of human made noise sources is often much higher than 376.37 times themagnitude of themagnetic field strength \(|\mathbf{H}|\).

Small magnetic loops with acircumference \(C < \frac{\lambda}{10}\) are not sensitive to themore pronounced electrical field strength emitted by nearby noise sources.

Only plane waves in thefar field exhibit thecharacteristic impedance of free space, which is exactly:

\[Z_0 = \frac{\left|\vec{E}\right|}{\left|\vec{H}\right|} = \sqrt{\frac{\mu_0}{\epsilon_0}} = \mu_0\cdot c_0 \approx 376.73\,\Omega\]

where:
\(c_0 = 299\,792\,458\,\frac{\text{m}}{\text{s}}\): thespeed of light in free space
\(\mu_0 = 4\pi\cdot10^{-7}\frac{\text{H}}{\text{m}}\): thefree space permeability
\(\epsilon_0 = \frac{1}{\mu_0 c_0^2}\): theabsolute permittivity of free space
\(Z_0\): thecharacteristic impedance of free space