40m Inter-G
Understanding the Limits

Two Ways a Signal Travels

On 40 metres (7MHz), a signal can reach another station in one of two ways. Groundwave hugs the surface of the earth and is reliable out to around 150km over typical UK terrain, but degrades rapidly beyond that. Beyond groundwave range, you're dependent on skywave — the signal bouncing off the ionosphere and returning to earth at a distance. But skywave on 7MHz only works if the ionosphere is charged enough: specifically, if the Critical Frequency (CF) is above 7MHz.

When the CF drops below 7MHz, the ionosphere simply passes the signal straight through rather than reflecting it back. The result is a dead zone — stations too far for groundwave, but not reachable by skywave either.

A comparison with usa

To understand why inter-G on 40m is so conditional, it helps to compare the scale of the UK against the USA. American operators can work coast-to-coast on 40m even in mediocre conditions, because the distances involved are enormous:

🇺🇸 USA — INTER-STATE

🇬🇧 UK — TYPICAL INTER-G PATHS

Beyond groundwave, UK to UK paths fall into one of three categories. The short ones — Plymouth–Bristol and Lincoln–Manchester at 160–170km — sit right at the edge of 7MHz groundwave range. On flat terrain with a decent antenna they may just about be workable, but it is marginal. The mid-range paths — Cardiff–Newcastle and Norwich–Carlisle at 390–450km — are well beyond groundwave and squarely in the dead zone whenever the CF drops below 7MHz: too far for groundwave, too close for skywave. Only at the extremes of the UK — paths approaching 1,000km or more, like John o'Groats to Truro — do distances start to enter genuinely viable 40m skywave territory, and even then only when the CF is above 7MHz. Compare that to the US examples: even New York to Florida at ~1,600km sits comfortably within the 40m skywave footprint on almost any day of the solar cycle. US operators take reliable inter-state contacts on 40m for granted. For UK operators, it is a privilege that depends entirely on the ionosphere cooperating.

Note: There are some excellent videos on portable operations from US based radio amateurs on YouTube, however, UK radio amateurs should be careful about what conclusions they draw from these videos. The situation is very different in the UK. Many of inter-state QSO they are making are the equivelant for working Rome or Madrid - or further - from London.

It is important to emphasise that when the CF is comfortably above 7MHz — as it regularly is during periods of good solar conditions — those mid-range inter-G paths of 300–500km become some of the most reliable contacts on the band. A low dipole and modest power will produce solid NVIS skywave that makes 40m feel alive with strong signals throughout the UK. The problem is not that these contacts are impossible as the cycle changes — it is that they are conditional, and as the solar cycle declines that condition is met less and less frequently. Don't be afraid to try 40m, equally don't be surprised if it's a struggle working other UK stations.

Radiation Angle Matters

Different antennas radiate at different angles, and that angle determines which propagation mode you favour. A vertical radiates at a low angle to the horizon — great for long-distance DX and groundwave, but it tends to shoot the signal over the heads of stations at medium distances. A horizontal dipole mounted low (below around half a wavelength) does the opposite: it radiates mostly upward, which is ideal for NVIS and short-to-medium skip in the 300–900km range.

A vertical is therefore a poor choice for inter-G work on a marginal day — its low-angle radiation skips clean over nearby stations rather than returning to earth at short distances. Geometry beats antenna gain every time. That said, antenna choice can only do so much: if the CF simply isn't there, no antenna will bridge the gap.

Declining Conditions & Other Bands

It is worth remembering that 40m has been an excellent inter-G band throughout the recent solar maximum — when the CF is high, it is genuinely one of the best bands available for UK-wide working, with reliable NVIS contacts across the full length of the country. The concern is not with 40m itself, but with the direction of travel of the solar cycle. As we move down the slope of the cycle, the trend will be for the Critical Frequency to increasingly fall below 7MHz during daylight hours, making those inter-G contacts on 40m many have become accustomed to as a regular occurence progressively less reliable on a day-to-day basis.

When this happens, the obvious alternative is 80m, which supports NVIS propagation at lower ionospheric frequencies and covers short-to-medium UK distances well — but this comes with real practical barriers, particularly for portable operators where a resonant 80m dipole stretches to around 40 metres, and for home stations with small gardens where even a compromise loaded antenna struggles to be efficient. Activators may be tempted to try an 80m mobile-whip Hamstick but are likely to be disappointed with the performance from what is a very compromised antenna both in terms of efficiency and bandwidth (typically 10-15kHz). Two antennas worth considering are:

60m can be a very useful band for inter-G communication when the CF is below 7MHz but above 5.3MHz, however, 60m is not without its own challenges when it comes to being used for activations, not least that the band is only available to those with a Full UK Licence and therefore making an entire activation on 60m alone is likely to be challenging. The UK frequency allocation is also a little complex and requires proper understanding in order not to transmit out of band.

20m might seem like another option when conditions are marginal on 40m, but the logic doesn't hold for short inter-G paths. If the CF has dropped below 7MHz, the MUF for a typical 200–400km inter-G hop will also be well below 14MHz — 20m simply won't support those distances. The MUF for much longer paths might still reach 14MHz, but that is of no use to operators trying to work stations a few hundred kilometres away. 14MHz is fundamentally a DX band, and inter-G on it is patchy even in good conditions. The same can be said for 17m and 15m

Don't forget VHF and UHF. 6m can be particularly effective during periods of Sporadic E, especially if you are able to operate SSB with a beam antenna, although event a vertical like a simple colinear or Slim Jim will often be effective when the band is open. 2m and 70cm are also handy with 2m SSB very effective from elevated sites particularly during enhanced VHF conditions. Augmenting an almost complete HF activations with a few local 2m or 70cm FM QSO is a tactic a number of successful activators employ.

The honest conclusion is that there is no perfect substitute for 40m NVIS when it's working well, often 80m will be the best alternative if inter-G QSOs are the goal.

Operators looking for reliable inter-G contacts throughout daylight hours as the cycle declines will need to be increasingly flexible with their band choices, accept that some days nothing will work consistently, and perhaps adjust their expectations accordingly - it's certainly not all doom and gloom, but a different mindset is required.

The M0ICR Solar Console

Before heading out for an activation — or settling in for a session from home — it is worth taking a few minutes to check current propagation conditions. The M0ICR Solar Console at m0icr.com/solar provides a live dashboard of the key parameters that determine whether 40m inter-G is likely to be productive.

The two numbers to focus on are the Critical Frequency (foF2) and the Kp index. If the CF is above 7MHz and Kp is 2 or below, conditions for inter-G on 40m are likely to be good. If the CF is below 7MHz, expect the dead zone problem described above. If Kp is elevated (3 or above, and especially 4+), geomagnetic disturbance can absorb or scatter skywave signals even when the CF looks adequate — both numbers matter.

Practical Tips for UKBOTA Activators

The Key Factors

Improvise ◆ Adapt ◆ Overcome