Hello all,
since I was directly asked I will come with some answer - or my perception
of the problem.
1.It is important to answer the question why to stack the antennas. There is
a bunch of answers:
On VHF to obtain more gain for "direct" propagation - not our case, here you
want to stack the antennas for maximum forward gain - i.e. the rules
pronounced by Bill are more-less correct - a little over 1 WL for 1WL boom.
etc. There is quite interesting study on this topic on SM5BSZ page and of
course the "BIBLE" of antenna modeling - www.cebik.com
2.ON HF I am sure that this is only one - less important point, what you
really want to do is to cover as many vertical angles as possible. To
achieve this you need to have the antennas closer.
Again there is not an easy answer. I do not believe modeling will give you
real answers since:
a. I do not know - may be someone much smarter then myself knows - in which
angles in a given period of time you want to radiate !?!?!?
Observation from this year IARU - I was running OL1HQ on 20m SSB. Around my
sunrise there was a peculiar propagation - As I have said here several
times - we do have 2 more-less similar antennas - 6Y (16m boom) + 5Y (14.5m
boom) the first one in 24m, the other on other tower in 52m. The propagation
was so that it looked like as a completely different frequencies. The W6/7+
KH6 comming 59 ! on the high antenna were absolutely not audible on the low
one and the W2,3,4 comming over 59 on the low one were absolutely not
audible on the high one - by absolutely I mean completely NOTHING ! And to
make the thing more complicated you might hear W3 on the high one better
than on the low one and W7 better on the low one ?!?!?!? (by the prefixes i
mean real location confirmed by zone - not the silly vanity calls I
personaly do not like - W5 should be in TX, OK... not in NH or CA !)
b.The influence of real ground on the vertical angles of radiation is
crucial - try to play with different ground models...., terrain slope.....
Can you pronounce all those variables coming into question. I can not and
the difference is HUGE !
3.The amount of energy radiated over the whole space is of course in all
cases the same. I.e. if you radiate the energy somewhere where you do not
want it - it will be "missing" somewhere else.
What we (me + OK1RF) have done was putting the antennas closer. We have had
for ages 6/6/6 on 10m. They were @12/20/28m since 1986. Last spring we have
made an upgrade to this tower and it is now 6/6/6/6 @13/19/25/31m and it is
considerable difference. I can not make direct comparison it is just
"feeling" By modeling we get there just a few 1/10 of dB more - so nothing
but the vertical diagram looks to me much better. OK2RZ have done a lot of
testing on 10m and the results are not easily interpretable. What is good
varies with type of propagation, time of day........
One more thing - do not trust to AO too much. It uses the MININEC 3.02
engine. It will give you just rough answers. The NEC-2 engines will give you
better answers concerning radiation patterns but less precise answers
concerning impedances and element length. It is a question for NEC-4 or it's
competitor EM-professional to get more reasonable answers.
Have I helped ?
73 !
Jiri
OK1RI
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