Jim,
I was told (as a little child) when they installed the antenna system for
the TV transmitter in Gothenburg, Sweden at the Brudarmossen tower, a tower
I believe is around 1000' tall, they experience a mysterious, high SWR. The
reason was that the cable clamps, holding the feeder coax, were placed by
engineering precision on an equidistant that generated the added "small"
reflections that added up in phase.
Shall you believe the story? I find it very unlikely that they managed to
"hit" the spot but the story is plausible.
... and, yes, you can make a notch filter that way. To be efficient you
will need a long distance to get a deep null.
Hans - N2JFS
____________________________________
From: jimlux@earthlink.net
To: towertalk@contesting.com
Sent: 12/23/2014 12:13:48 P.M. Eastern Standard Time
Subj: Re: [TowerTalk] Fwd: Running feed line and rotor cable together?
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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