[grc] Mounting and spacing of 2 bay OMB MP 1 antennas?

[al davis]

On Thu, 11 Aug 2016 18:19:16 -0400
Danny via grc <grc at maillist.peak.org> wrote:
> We’re working with a property owner that is very concerned about the visual
> impact of us putting a 2 bay OMB MP 1 antenna system on their roof cupola.

You saw what WLPP is using.

https://www.facebook.com/wlpp102.9fm/photos/a.891652214227515.1073741828.880376565355080/951022641623805/?type=3&theater

It's a cheapo 50 foot telescoping mast, a consumer product.

> Wind and icing aren’t serious concerns, so how skinny a mounting pole can we
> use and how far apart do we need to space the bays and how far above the
> cupola roof? The latter is stone tiles with metal trim.

Above the roof ... figure 20 feet above where people are.
In the WLPP picture, there's about 5 feet inside in the attic above the
top floor.

Antenna gain vs spacing ...

The gain of a stacked array is primarily proportional to the length of
the array, with about 3/4 wave spacing a little better, giving the
highest gain per occupied tower space.  About .95 wave (a little less
than 1) gives the highest gain per bay.

For LPFM, I wouldn't worry about gain to this extent.  The bigger
concern is meeting a second-adjacent waiver.  For this, usually again
3/4 wave usually works best.  If your primary concern is straight down,
nulling toward others on the same tower, half wave might be better.



> 
>  
> 
> I read elsewhere that as the spacing changes, so will the gain of the
> antenna system. That running it full wave, we will get higher gain (unity,
> or close to it) , but that most of that energy is going "up" rather than
> horizontally. And if we decrease the antenna spacing, the pattern "squashes"
> towards the horizon, resulting in less energy going straight up toward space
> and more toward the horizon.

Not quite.  Actually, as the spacing increases the pattern squashes.

The gain of an antenna is measured at the horizon.  All antennas
radiate essentially all of the power put in to them.  "Gain" is created
by taking power away from some undesired direction and adding it in a
desired direction.  It's kind of like what lenses and mirrors do to
light.

The real impact of the spacing is that there is an interference pattern
with lobes and nulls formed by the multiple point sources.  By adjusting
spacing, you can fine-tune that pattern to your benefit.

Some specifics ...  with half-wave spacing, there is a null straight
down.  This is usually considered to be good, signal increases as you
travel out, with a max at 30 degrees depression.

With full-wave spacing, there is a maximum straight down.  This is
usually not considered to be good, but it is not as bad as you might
think because a single bay has a donut pattern, with a null straight
down, but there is a maximum at 60 degrees which may be troublesome.

3/4 wave spacing gives a null at 45 degrees depression.  Combined with
the single bay null down, it gives a bigger quiet zone.



Beyond that ...  There is some difference based on the design of each
bay.  You have a choice of a "ring-stub" or "twin-v" design, and
probably see little reason to choose one over the other.  After
installing both, I now have a fairly strong preference for the twin-V
design.  It tends to be more broadband, and there is less downward
radiation, better null straight down.  With the ring-stub, the vertical
part (stub) has a good donut pattern, but the horizontal part (ring)
doesn't have so much of a null straight down.  The bandwidth difference
is significant.  It is possible to make a twin-v broad enough that some
makers sell a single broadband model that is good enough for the whole
band without tuning.  Tuned, they can hold well enough that even with
ice there is not a problem.  WLPP's PCP ring-stub, which looks like a
similar design to the OMB, is noticeably detuned when it rains. This
last winter, we had no ice on the antenna (but lots of ice at higher
elevation in the mountains). I shudder to think how bad it will detune
when there is real ice.  I would not recommend another one of these.

Having commented on antenna quality, of the ones I have installed, the
best I have seen so far is from PSI.  Their LPFM antenna is all welded
construction, as good as I would expect for a big-bucks full power
antenna, but lighter and thinner, and the cost is competitive.  Of all
the LPFM antennas I have used, this is the only one I consider to be
good enough that I would pay a professional tower crew to install, and
have confidence that they won't need to climb again next year to fix
it.  But then, I haven't seen all brands so others I am not familiar
with might be as good or better.

Another possible antenna choice might be the "Dominator".  It's
basically a 5/8 wave vertical whip.  If you want cheap, light, and
unobtrusive visually, this might be a good choice.  The downside is
that it's vertical polarized only.