[TowerTalk] Fwd: Running feed line and rotor cable together?

[Jim Lux]

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.