N2RM, W3LPL Stacks

[D. Leeson]

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 W3SPL 
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