W6NL on Stacks
Dave Leeson, W6NL
June 2, 2000
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Reprinted from the CQ-Contest reflector archives, November, 1992.
The recent discussion of differences among stations such as N2RM and W3LPL cries out for some experimental confirmation.
There are three issues relating to stacked antennas at HF. Two are static (array gain over ground, pattern fill over ground) and one is dynamic (diversity effect of aperture size on fading statistics). Naturally, integrity of implimentation (cable loss, aiming errors, etc.) is important, as are modulation characteristics in the SSB case.
It's been my impression, backed by some informal experience, that not much array gain is realized over ground, because the requirement for matching phase and amplitude is not met at all radiation angles. In any event, even
the free-space gain of 3 dB by doubling the number of array antennas may not
account for the apparent perceived difference in signal levels mentioned by GM/ECO.
Pattern fill over ground, which can be thought of as a form of static space diversity, raises the mean signal level by eliminating elevation angle nulls.
This couples the antenna system effectively to any active single- or multi-hop ionspheric modes and reduces fading from shifting levels of the various active modes (angles). The far-end received signal strength statistics also depend on the degree of coupling of the far-end receiving antenna to the same modes, which implies you'd want to illuminate all the multi-hop angles that might be important to a range of far-end receiving antenna heights. It also means that comparisons at one station might not correlate with those at another.
Operating a stack intended to improve stateside 40m coverage, I've noticed that the phasing is not especially critical even though the upper-lower-both comparisons always seem to favor the stack. This could imply that the biggest benefit from a large stack is the reduction of average fading because of space diversity, which would raise the far-end perceived signal level. As an example, in a mobile system with 60 dB fades, combining 8 antennas separated by at least the correlation distance of the fading will result in a maximum combined fade of only 3 dB. The distance between N2RM and W3LPL certainly exceeds the correlation distance for HF fading, so some extended-time measurements would be needed to confirm the anecdotal impressions (which are usually right if repeated over any reasonable time period).
An interesting experiment would be to record far-end signal strength of an unmodulated carrier from a physically large array (say a 4-stack) compared to the same power level at the same time from a physically small (single) antenna. The question of terrain topology and ground electrical parameters can be removed if both transmitting antennas are at the same site, and it's possible that a rapid-switching measurement would suffice to determine the source of the difference among small and large stacks. A measureable difference in fade statistics would confirm the conjecture that diversity is an important factor. This has been mentioned in connection with, for example, large wire antennas like rhombics. Capturing the data is the hard part; you can use the threshold effect of receiver manual rf gain control to infer the fading statistics. If the answer is that space diversity provides the major benefit, it's not necessarily so that vertical stacking is the only way to get it. I'd be interested to know more.
73, Dave W6QHS
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