On 12/23/14 8:40 AM, Hans Hammarquist via TowerTalk wrote:
I don't know how important it is but I avoid using equal distance between the
places I tie the cables. Rumors have it that the small impression on the coax
will cause a reflection and if they are all at equidistant the reflexes can add
up a some frequencies to give a high SWR. Rumors or not, I have tie point at
random distances.
I don't think it's likely, it would take a big deformation to get a
significant reflection. (0.01 dB bumps probably aren't worth worrying about)
This does bring to mind a speculative scheme for filtering. Rather than
coax stubs and Ts, you do it in line. There *is* actually a IEEE paper
from the late 50s on this kind of filtering: stubs, inline sections, and
coupled sections. It's the basis of all those cool printed interdigital
and coupled line filters you see in microwave gear.
So, say you're a top band enthusiast, and you have your 3 element Yagi
up a half wavelength (at least) on a *big* tower. Could you dent the
coax (precisely applied nylon cable ties or some such) at specific
points to suppress BC band interference from a strong local transmitter?
Or suppress your own harmonics?
There are places where they do put "precision" dents into the
waveguide: at Deep Space Network, where they have a 400kW S-band
transmitter (at 2-2.1 GHz), even a -30dB reflection is a significant
amount of power (400 Watts). I have heard stories (but not seen in
person) that when they tune the system they use big clamps to deform the
waveguide wall temporarily, followed by strategic application of a big
mallet.
On other systems, you'll see dozens of tuning screws, but that's usually
to adjust a filter's frequency and phase response. At 400kW, the fields
are high enough that a screw sticking in would probably cause a high
voltage breakdown.
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