Amateur Radio (G3TXQ) - A note on the vertical stacking of HF antennas
I was asked a question recently about the benefits of stacking two HF antennas vertically. The questioner had the facilities to mount a Hexbeam at any height up to 65ft, and a second Hexbeam at a fixed height of 25ft; he wondered what advantage would accrue from adding the second antenna and whether it was worth the investment of time and money.
Initially we might think that adding a second antenna will deliver the 3dB improvement predicted by simple theory; we would be wrong! I modelled the situation using EZNEC and was surprised to find that adding the second beam at 25ft actually detracted from the DX performance of the single antenna at 65ft. A quick look at the elevation patterns of the two antennas shows why:
The left-hand diagram shows the elevation performance of a 20m Hexbeam mounted at a height of 65ft. It radiates its maximum signal of 10.65dBi at a take-off angle of 15°; at lower take-off angles the signal strength reduces abruptly, and by 5° (typical of the angles needed for long distance ionospheric propagation) the signal is only 5.01dBi.
The right-hand diagram shows the performance of the same antenna mounted at 25ft. The maximum signal now occurs at a take-off angle of 39°, and by 5° it is only -4.5dBi: that's nearly 10dB worse than when it was up at 65ft! What should we expect if we now use a combination of two antennas at these heights? The diagram below reveals the answer:
The red trace is the performance delivered by the two-antenna array; for comparison, the performance of the single antenna at 65ft is show by the blue trace. We note that:
- Maximum radiation of the two-beam array is 0.8dB greater than the single antenna, but it occurs at the slightly higher take-off angle of 17°
- At 5° take-off angle the signal is 4.89dBi - fractionally worse than the single antenna
- The secondary lobe at 50° has been suppressed, but in its place a smaller lobe has appeared close to the overhead position
It should not surprise us that if we take half of our transmitter power and feed it to an antenna with a 39° take-off angle we will not improve the low-angle performance. We might think that lifting the lower antenna would help: it does, but only marginally, because although the take-off angle is better the antenna spacing is worse. It turns out that, with the upper antenna fixed at 65ft, there is nowhere we can put the lower antenna without being worse off at low take-off angles! Further modelling shows that we would need to mount the upper antenna at 85ft and place the lower antenna at 40ft before we get anywhere close to a 3dB advantage over the single antenna at 65ft! Even then we are only 0.5dB better off than by using the 85ft antenna alone.
Conclusion: don't expect that if you stack two HF antennas vertically you will automatically see a 3dB improvement in performance .... you may even be making things worse!