[NLRS] Stacking distances for loopers

[Ken Boston]

Glen;

A few years ago, I had the opportunity to check the interaction between
my car roof and a 144 halo antenna, with a network analyzer, while
varying the spacing.  I cannot remember to many of the details, but do
remember that when I was very close to the roof, which put the metal
surface within the near field of the antenna, the antenna was detuned,
and performance was degraded.  If you have any type of SWR indicator for
the bands you mention, you can see this if you set up to read the match,
set the match with the antenna clear of everything, and then move it
into the position, close to the metal roof that you suspect will degrade
the performance.  Once you move the antenna out of this region, but
before reaching the recommended spacing called out in the antenna
literature, you will find the antennas will work, but may experience
some pattern skew, and need to still have to 'diddle' a little with the
match.  Near field calculation for 144 was; Lambda/2*pi. This worked out
to about 13 inches at 144, and indeed, the halo was hard to match when
within this distance of the roof.  I ended up with my halo at about
24-30 inches off the roof, where it seemed to work OK.  I am not sure if
this will be more critical with a high Q antenna like a looper, but I
would guess that it would. 

Kenneth L Boston PE
W9GA
 
 
 
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-----Original Message-----
From: nlrs-bounces at mailman.qth.net [mailto:nlrs-bounces at mailman.qth.net]
On Behalf Of Glen Overby
Sent: Thursday, May 24, 2007 8:50 PM
To: nlrs at mailman.qth.net
Subject: [NLRS] Stacking distances for loopers




When preping for the recent SBMS 2ghz+ contest, I got looking at the
stacking
distance specs for Directive's loop yagis.  I've been ignoring these
specs and
put the antennas directly on my roof rack (about 4" from the metal
roof).
It's easy, but lately I've been wondering how bad that has hurt me?
Directive's page on stacking dis-simmar antennas talks about how much
you can
squish antennas together, but not about possibly cutting off the bottom
portion of an antenna's aperature.

Here's the list of stacking distances:

                        Band    #       Boom    Beam    Stacking
Distance
                                Ele     Length  Width   E(Horiz)H (Vert)
Directive 3318LYRM      902     18      72"     30      24      21 1/2  
Directive 2324LYRM      1296    24      72"     27      21 3/4  19 1/4  
Directive 1345LY        2304    45      81"     16      15      13      
Directive 945LY         3456    45      60"     16      9.8     9

Directive's advice on stacking antennas says to use half of the stacking
distance for an antenna when calculating stacking distance between
dis-simmilar antennas.

So, my car roof is definately in the aperature of all of these, but how
much
does that matter?

Glen, kc0iyt.

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