Down Tilt on Slot Antenna
 

Is there anyway to down tilt the signal on horizontally polarized omni slot
antenna? I would like to add 1-3 degrees of downtilt.

Matt

In article ,
"Matt" wrote:

Is there anyway to down tilt the signal on horizontally polarized omni slot
antenna? I would like to add 1-3 degrees of downtilt.

Matt



Mount the antenna upside down.......



Me

If it's a single element, how in the world will you ever measure it
accurately enough to KNOW what the up-or-down tilt is? That is, the
directionality of a simple half-wave dipole just isn't enough to matter
when you get down to a couple of degrees. In freespace you MIGHT be
able to measure things that closely if you were extremely careful, but
over practical ground, I don't believe you have any hope.

If it's multiple vertically-stacked elements, then arrange the feed so
their currents are shifted in phase a small amount from element to
element. A back-of-the-envelope picture and some simple trig should
tell you just how much phase shift you should have between adjacent
elements.

Cheers,
Tom

I'd also think that physically tilting the antenna that much might not
be too much of a trick, and it should give you the tilt you need. As Tom
says, it must have a tremendously narrow elevation pattern if you're
going to see any significant change with that small amount of tilt.

Roy Lewallen, W7EL

K7ITM wrote:
If it's a single element, how in the world will you ever measure it
accurately enough to KNOW what the up-or-down tilt is? That is, the
directionality of a simple half-wave dipole just isn't enough to matter
when you get down to a couple of degrees. In freespace you MIGHT be
able to measure things that closely if you were extremely careful, but
over practical ground, I don't believe you have any hope.

If it's multiple vertically-stacked elements, then arrange the feed so
their currents are shifted in phase a small amount from element to
element. A back-of-the-envelope picture and some simple trig should
tell you just how much phase shift you should have between adjacent
elements.

Cheers,
Tom

Well, let me just post this at the bottom. RCA made a line of VHF TV transmitting antennas which were a slot arrays.
The only one I managed to get close to before it became airborne and clamped a thousand or more feet in the air was for
channel 9. It was 80 feet long (2 - forty foot sections) and was constructed exactly as Tom, K7ITM suggested, the phase
of the feed of each element was progressively changed, in this case, to effect 1.5° of downtilt. I would suggest
contacting RCA TV antenna engineering for their design parameters, but they seem to have gone somewhere.



--
Crazy George
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"Matt" wrote in message ...
Is there anyway to down tilt the signal on horizontally polarized omni slot
antenna? I would like to add 1-3 degrees of downtilt.

Matt

On Mon, 21 Mar 2005 21:04:47 -0600, Crazy George wrote:

Well, let me just post this at the bottom. RCA made a line of VHF TV transmitting antennas which were a slot arrays.
The only one I managed to get close to before it became airborne and clamped a thousand or more feet in the air was for
channel 9. It was 80 feet long (2 - forty foot sections) and was constructed exactly as Tom, K7ITM suggested, the phase
of the feed of each element was progressively changed, in this case, to effect 1.5° of downtilt. I would suggest
contacting RCA TV antenna engineering for their design parameters, but they seem to have gone somewhere.



--
Crazy George
The attglobal.net address is a SPAM trap. Please change that part to: attdotbiz properly formatted.
"Matt" wrote in message ...
Is there anyway to down tilt the signal on horizontally polarized omni slot
antenna? I would like to add 1-3 degrees of downtilt.

Matt



We had 1 degree of down-tilt on the RCA Slot antenna for Channel 11.
Unfortunately I got the task of Replacing Slot covers on the antenna which
at the time was 15 years old. The antenna was retired 2 years ago, it was
33 years old...

I was 21, and stupid enough to climb a 1000' tower.



--
Korbin Dallas
The name was changed to protect the guilty.

"Crazy George" wrote:
Well, let me just post this at the bottom. RCA made a line of
VHF TV transmitting antennas which were a slot arrays.
The only one I managed to get close to before it became airborne
and clamped a thousand or more feet in the air was for
channel 9. It was 80 feet long (2 - forty foot sections) and was
constructed exactly as Tom, K7ITM suggested, the phase
of the feed of each element was progressively changed,
in this case, to effect 1.50 of downtilt.
_______________

The antenna described above is the RCA "Traveling Wave" design. Without
beam tilt for the situation described, it has an omni, horizontal plane RMS
gain of ~17.2X (~12.4dBd) . The beam width within the 3dB points on its
elevation pattern is about 3.2 °.

If this antenna was placed with its radiation center 1,500 feet above smooth
earth, the radio horizon would be 0.59° below the horizontal plane, so using
beam tilt would raise field strength there. The 1.5° value stated above
exceeds that, but there may have been good reasons for it.

Adding beam tilt to an antenna with a very broad elevation pattern, and
installed on a short tower is a waste of time and money, however.

RF (RCA Broadcast Field Engineer, 1965-1980)

Visit http://rfry.org for FM transmission system papers.

Thanks for checking in, Richard.

Please refresh my admittedly fuzzy memory from 40+ years ago. Did they (RCA) not adjust the phase delay by using
different lengths of dielectric in the inside between adjacent sets of slots? Allowing one size (per channel) of
radiator, but allowing selection of downtilt after machining of the exterior. Or was that Dielectric's design? And,
that particular antenna never worked to expectations, and I always said I thought it had too much tilt. And indeed it
had an extremely strong signal on the ground near the tower. Much more than any other installation I ever visited.

P1-9-1324 back then.

--
Crazy George
The attglobal.net address is a SPAM trap. Please change that part to: attdotbiz properly formatted.

The interior of the VHF TW antenna essentially is a 50 ohm center conductor
against the ID of the pylon. Beam tilt is done by the location of the slots
on the pylon. The useful part of the elevation pattern of these antennas
is made very smooth (no sharp nulls), and it produces fairly uniform fields
near the ground from close to the tower base on out toward the radio
horizon.

The antenna was designed by RCA (Gibbsboro), and the design and production
facilities for it were acquired from them by Dielectric when RCA folded in
the mid-1980s.

As for the effects of beam tilt, Figure 1 in paper #9 at http://rfry.com
gives a graphical representation and discussion of what can be expected.

P1-8-3246 (1950s).

RF

"Richard Fry" wrote
As for the effects of beam tilt, Figure 1 in paper #9 at http://rfry.com
gives a graphical representation and discussion of what can be expected.
_______

Sorry, the * CORRECT * URL that should show above is http://rfry.org .

RF