[TowerTalk] Several questions :-)
T A RUSSELL
n4kg@juno.com
Mon, 28 Sep 1998 06:32:48 -0600
N4KG responses interspersed in text.
On Sun, 27 Sep 1998 19:19:08 -0700 "Dan Levin" <djl@andlev.com> writes:
>
>Just hoping that the assembled brain-trust can clarify a couple of
>things.
>
>All of the below pertains to my planned installation of 10, 15, 20 and
>40
>meter coverage on a heavy duty 70 foot crank up with a heavy duty mast
>sticking 15' out on top. All references to Force 12 antennas are for
>example only (although I am leaning that way :-).
>
>1) A well designed small tri-bander (e.g. C3) has about 4.4 db of gain
>on
>each band (yes, I know, plus or minus a bit). A well designed large
>tri-bander (e.g. C31-XR) has about 6 db of gain (ditto). In return
>for that
>1.5 db, you pay about 2x the dollars, and have about 2x the boom
>length.
>But a two stack of small tri-banders, again say two C3's, has about 7
>db of
>gain - at least according to the F12 brochure. In my case, this
>argues
>strongly for the stack - I would get a smaller footprint (shorter
>booms),
>the ability to "spray" in two directions, and more gain than I would
>get
>with the larger antenna. Am I missing something here? Are there any
>significant reasons why I would not want to stack two small
>tri-banders,
>rather than having one large one?
>
You forgot to include the versitility provided by a switchable
stack. Having UPPER / LOWER / BOTH options, you can
better match the waveangles that are propagating to your
target at any given time. And YES, they do vary. During
high sunspot activity and daylight hours, the angles can
get quite high, much higher than most references indicate.
de N4KG
>2) If I am going to stack, then the two antennas will not be perfectly
>aligned. One will be on the mast centered in the tower, and the other
>will
>be on either a TIC or a side-mount - in either case offset
>horizontally from
>the top antenna by at least a foot or two (maybe three). Does this
>cause
>any major problems? Are there any solutions?
>
W2PV addressed this issue (indirectly) in his
YAGI Antenna Book. Basically, there is a very small
change in gain with a phase difference of 30 degrees
or less. This can be translated to 0.08 wavelengths
of displacement. There may be some degradation
of F/B with displacement since cancellation is more
dependent on exact alignment of amplitude and phase.
de N4KG
>3) If stacking tri-banders, I will have to live with a height
>separation
>that is not optimal for all three bands. Does anyone believe that
>this is a
>major problem? Has anyone modeled a two stack of small tri-banders,
>and
>convinced themselves that the separation issue has a major or minor
>effect.
>Remember, I am selecting the stack in part because of the extra db of
>gain -
>if I am going to burn that db because of non-optimal separation, it
>would
>matter. Has anyone stacking tri-banders on a crank-up played games
>with
>lowering the tower to get better separation on 10 or 15 meters -
>modeling
>the net effect of the lower height vs better separation?
>
Your best solution here is to model it, or find someone
who already has done so. (Ask N6BT)
>4) If you have your way, with all of your experience and hindsight,
>what
>would you suggest that I put up? My options (realistically, as I see
>them)
>are:
>
>1) One large tri-bander, either with 40 meters built-in (C4 of some
>type or
>C-36XR), or with a 2 element 40 meter shorty above it. Tri-bander
>alone at
>~80 feet, or tri-bander at 72' and 40 at 82'.
You need high and low antennas to cover ALL of
the angles supported by the ionosphere. The
N6BV studies used antenna heights which place
antenna pattern NULLS within the range of usable
angles. N4KG
>
>2) A two stack of small tri-banders, with a 2 element 40 above the top
>one.
>Tri-banders at say 42' and 72', 40 at 80 feet.
>
I like this option for maximum versitality. Be sure
to include an upper / lower / both switch.
>3) Two large dual band antennas - perhaps a 20/40 at 72' and a 10/15
>at 82'.
>It looks like I could use the EF-240/320 (4 on 20, 2 on 40) and the
>Magnum
>415/410 (4 on 10 and 4 on 15).
>
High band antennas at 80 to 100 ft get creamed by
lower antennas during daytime conditions, especially
with higher sunspot activity. You need BOTH high
and low for full coverage. Remember, the MUF varies
by a factor of approximately 3 to 1 from daytime to night.
It is foolish to expect the same angles to propagate
with that much variation in MUF. de N4KG
>All suggestions appreciated.
>
> ***dan
> Dan Levin, N6BZA
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