Friday, April 5, 2019

Antennas for medium distance HF communication

While radio amateurs are known for their interest in DX, or long distance, communications, this isn't always the case. There are sometimes times where you want an antenna suitable for 'in-between' distances. That is distances too far for VHF simplex but still within your own country or state. We're talking roughly fifty to several hundred kilometres here. As often required for local club nets, emergency communications, field days and plain old ragchewing.

The first thing to think about is the frequency you're going to use. Particularly in this (low) period of the solar cycle, the frequency must be low enough to support communication over the desired distance. Though you may hear signals on higher frequencies closer investigation will find that everything heard will be either very local or a long way away - say over 1000 km. There'll be a big gap where signals skip over and you will not easily be able to reach - at least unless you try a different (lower) frequency or different time of day (eg daytime rather than night).

The highest frequency that will support ionospheric propagation is called the maximum usable frequency (MUF). Signals higher than that will just go out to space. Signals lower than that will reflect back to earth at some distant point (say 1000 or 2000 km away), skipping over closer in places. MUF is markedly affected by the solar cycle with MUF frequencies depressed at the moment due to the solar low. MUF is also lower at night than during the day.  Frequencies just below the MUF are good for long distances but not for the closer-in communication that we're after.

Below the MUF is the critical frequency. This is the highest frequency that signals will reflect back to earth at a 90 degree angle. This is important because a signal at or below the critical frequency will enable communication over intermediate distances after where the ground-wave cuts out. Whereas signals above the critical frequency but below the MUF will have a large patch of ground at intermediate distances from the transmitter (say from fifty to several hundred kilometres) where it is not heard.

What this means is that to reliably cover these distances you're going to need antennas for the lower HF bands.

During the day this means 7 MHz, 5 MHz (if you have it) and possibly 3.5 MHz. While 7 MHz is a good band there are times when the critical frequency drops below 7 MHz so you'll be thankful that you can operate on bands lower than this even if your antenna is limited.

At night 3.5 MHz mainly but also 1.8 MHz if you can manage it. This is because there are times when the critical frequency drops below 3.5 MHz. That, married with the longer groundwave coverage of 1.8 MHz, makes the latter better for some distances under some conditions. Whereas signals on 3.5 MHz will be weak and watery and often interfered with by stronger signals from further away.

As for the antenna, you want something that efficiently radiates a signal at high angles. That is straight up (90 degrees to the ground) and adjacent angles. A (more or less) horizontal wire antenna will fill that requirement well. For instance a half wavelength dipole, inverted vee, G5RV or end-fed half wavelength wire are all good choices.  A low inverted L with most of the wire horizontal should also be OK.

As for height, 6 to 12 metres above the ground is fine. Those low heights (being roughly a quarter wavelength or less above the ground) will make the antenna poor for low angle DX but good for the high radiation angles that we want. Another bonus that's often useful is that at these heights a horizontal dipole becomes more or less omnidirectional so its orientation can suit your available land. 

Contrary to some of what you see written there I've noticed no benefit with very low heights (also see the W8JI article below). And in a suburban setting you may receive more interference with the antenna closer to house wiring and appliances. 

Conditions change between day and night. And there are intermediate times like mornings or afternoons that give better results on (say) 3.5 MHz than in the middle of the day. Having two or preferably three lower HF bands available with the one antenna is a bonus. Multiband options include dipoles fed with open wire line (at least 1/3 wavelength on the lowest frequency), trap dipoles or end-fed wires with a suitable antenna coupler.

You may already have something suitable up already. If not they're easy to make and erect. A lot of jargon has crept in to the ham language. Just remember that wherever you see reference to NVIS (near-vertical incidence skywave) antennas think of a low dipole or similar. Nothing special or mysterious at all.

Anyway,  here are some ideas for suitable low HF / high angle antennas:

* VK4ION field portable dipole
* DX Engineering NVIS antenna (pdf)
* K5AXN gain NVIS antenna
* W8JI on very low NVIS antennas
* NVIS antenna design

PS: Want more practical antenna ideas? Consider this selection of antenna books. They are affiliate links meaning that I receive a small commission (at no extra cost to you) if you decide to purchase.


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