Looking for an omni-direction FM Transmitter antenna...
 

Hello all,

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

Any thoughts would be commended.

Sincerely,

Patrick Cambre

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.


There are very few omni-directional antennas that also have gain. Are you using
this for a transmitter or a receiver? If for a receiver, you also need some
bandwidth, unless you want to concentrate on a small segment of the 10mHz band.
For a part 15 transmitter, bear in mind there are some restrictions on antenna
size.

I've gone through the same excercise on the 2m Amateur Radio band, in an effort
to have as wide a coverage as possible without resorting to a directional beam.

My solution, which might also work for you, was a colinear array of three
sections. You can find design information on the web or in the ARRL Antenna
Handbook. For 100mHz, a 3 element colinear would be about 8 feet high. You
could also use a J-Pole with colinear section which would only be about 6 feet
high. The colinear requires a matching coaxial balun at the feedpoint, the
J-Pole has a built-in matching stub and may be fed directly with 50-ohm coax.
The disadvantage of these antennas is that they are fairly narrow band. You
just can't get something for nothing these days. :-)

If you can't find colinear or J-pole information on the web, but this sort of
antenna looks interesting to you, drop me a line and I will direct you to
specific sources of information.

Doug Moore

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.


There are very few omni-directional antennas that also have gain. Are you using
this for a transmitter or a receiver? If for a receiver, you also need some
bandwidth, unless you want to concentrate on a small segment of the 10mHz band.
For a part 15 transmitter, bear in mind there are some restrictions on antenna
size.

I've gone through the same excercise on the 2m Amateur Radio band, in an effort
to have as wide a coverage as possible without resorting to a directional beam.

My solution, which might also work for you, was a colinear array of three
sections. You can find design information on the web or in the ARRL Antenna
Handbook. For 100mHz, a 3 element colinear would be about 8 feet high. You
could also use a J-Pole with colinear section which would only be about 6 feet
high. The colinear requires a matching coaxial balun at the feedpoint, the
J-Pole has a built-in matching stub and may be fed directly with 50-ohm coax.
The disadvantage of these antennas is that they are fairly narrow band. You
just can't get something for nothing these days. :-)

If you can't find colinear or J-pole information on the web, but this sort of
antenna looks interesting to you, drop me a line and I will direct you to
specific sources of information.

Doug Moore

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

The following is a little long....

I've never made any bandwith measurements, but I've made and used several
copper-pipe vertical collinear 2-meter antennas which have served satis-
factorily (i.e., they have withstood ice-storms, I "get out", etc.) for
many years. I don't recall where I got the original design, and, having
just stepped outside and measured one, I'm surprized at how much my
vertical element length differs from that described on page 156 of my old
(1972) "Radio Amateur's VHF Manual", but I heartily recommend them for
their ease of construction, ease of mounting, and ease of matching:

X Y Z (see sketches below)
VHF Manual # 1: 0.475L 0.25L small
# 2: 0.64L 0.11L small ("small" is never defined!)
where L (Lambda) = wavelength

Note that the total lengths of both designs is the same: about three
half-wavelengths. The second design is an "extended double-Zepp" and
supposedly gives slightly increased gained and lowered radiation angle.

The VHF manual suggests using balanced transmission line or coax and a
balun, but I just use coax tapped on the horizontal part at the minimum-
SWR point found by trial and error. (Mentally, I think of the impedance
at the shorted end of the horizontal section as being zero ohms and the
impedance at the open end being in the neighborhood of 4000 ohms and I
start tapping at an appropriate distance between for 50-ohm coax!)

Assuming 146 Mhz and calculating L = 11200/Freq (MHz) = 76.7 inches
X Y Z
VHF Manual # 1: 36.4" 19.2 small
# 2: 49.1 8.4 small
Mine: 42.5" 19.5" 2.5"
where L (Lambda) = wavelength

I've made at least four of these things; basically, ONE 10-foot length
of half-inch copper and four matching elbows should come out just right
(allowing about a half-inch each direction for each joint, this takes
2x19+2x41=120 inches of pipe). But when I couldn't get the SWR down as
far as I felt it should go (about 1.5:1), I extended both vertical
sections with a few more inches of pipe.

My first modification substituted two T's for the two elbows at the open
end and added a three-foot wooden dowel (with a just-fit diameter)
extending about a foot and a half into each vertical section and crossing
the "open" end for additional support.

I mounted my first one (#1 below) by (essentially) drilling a couple of
holes through the short vertical pipe and nailing it to the wall! One
could also lay U-shaped bracket(s) across the shorted end and screw them
to the wall or clamp them to a tower. But some other designs (two of
which use FOUR T's and NO elbows, and the third uses FIVE T's) give
better mounting possibilities:

With design #2, the two horizontal copper-pipe stubs may either be
inserted into holes drilled in a board and pinned by driving nails
through the edges of the board into the stubs, or the holes may be
drilled slightly UNDERsize in a pole and the stubs just driven into
the holes.

Design #3 just gives more room for nails or clamps.

There is a little problem with wind trying to swing the antennas around
the shorted end like a gate on a hinge, so design #4 adds a FIFTH T
CROSSWISE to the plane of the rest of the copper and eliminates all
swinging tendencies!

#1 #2 #3 #4
| | | |
| | | |
|X | | |
| TAP | pinned | | o
| Y / | v | | | |
+------+--+ +---------+-+- +---------+ +---------+
/Z| | | |
+------+--+ +---------+-+- +---------+ +---------+
| Y | ^ | | | |
| | pinned | |
|X | | |
| | | |
| | | |

--Myron, W0PBV.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

The following is a little long....

I've never made any bandwith measurements, but I've made and used several
copper-pipe vertical collinear 2-meter antennas which have served satis-
factorily (i.e., they have withstood ice-storms, I "get out", etc.) for
many years. I don't recall where I got the original design, and, having
just stepped outside and measured one, I'm surprized at how much my
vertical element length differs from that described on page 156 of my old
(1972) "Radio Amateur's VHF Manual", but I heartily recommend them for
their ease of construction, ease of mounting, and ease of matching:

X Y Z (see sketches below)
VHF Manual # 1: 0.475L 0.25L small
# 2: 0.64L 0.11L small ("small" is never defined!)
where L (Lambda) = wavelength

Note that the total lengths of both designs is the same: about three
half-wavelengths. The second design is an "extended double-Zepp" and
supposedly gives slightly increased gained and lowered radiation angle.

The VHF manual suggests using balanced transmission line or coax and a
balun, but I just use coax tapped on the horizontal part at the minimum-
SWR point found by trial and error. (Mentally, I think of the impedance
at the shorted end of the horizontal section as being zero ohms and the
impedance at the open end being in the neighborhood of 4000 ohms and I
start tapping at an appropriate distance between for 50-ohm coax!)

Assuming 146 Mhz and calculating L = 11200/Freq (MHz) = 76.7 inches
X Y Z
VHF Manual # 1: 36.4" 19.2 small
# 2: 49.1 8.4 small
Mine: 42.5" 19.5" 2.5"
where L (Lambda) = wavelength

I've made at least four of these things; basically, ONE 10-foot length
of half-inch copper and four matching elbows should come out just right
(allowing about a half-inch each direction for each joint, this takes
2x19+2x41=120 inches of pipe). But when I couldn't get the SWR down as
far as I felt it should go (about 1.5:1), I extended both vertical
sections with a few more inches of pipe.

My first modification substituted two T's for the two elbows at the open
end and added a three-foot wooden dowel (with a just-fit diameter)
extending about a foot and a half into each vertical section and crossing
the "open" end for additional support.

I mounted my first one (#1 below) by (essentially) drilling a couple of
holes through the short vertical pipe and nailing it to the wall! One
could also lay U-shaped bracket(s) across the shorted end and screw them
to the wall or clamp them to a tower. But some other designs (two of
which use FOUR T's and NO elbows, and the third uses FIVE T's) give
better mounting possibilities:

With design #2, the two horizontal copper-pipe stubs may either be
inserted into holes drilled in a board and pinned by driving nails
through the edges of the board into the stubs, or the holes may be
drilled slightly UNDERsize in a pole and the stubs just driven into
the holes.

Design #3 just gives more room for nails or clamps.

There is a little problem with wind trying to swing the antennas around
the shorted end like a gate on a hinge, so design #4 adds a FIFTH T
CROSSWISE to the plane of the rest of the copper and eliminates all
swinging tendencies!

#1 #2 #3 #4
| | | |
| | | |
|X | | |
| TAP | pinned | | o
| Y / | v | | | |
+------+--+ +---------+-+- +---------+ +---------+
/Z| | | |
+------+--+ +---------+-+- +---------+ +---------+
| Y | ^ | | | |
| | pinned | |
|X | | |
| | | |
| | | |

--Myron, W0PBV.
--
Five boxes preserve our freedoms: soap, ballot, witness, jury, and cartridge
PhD EE (retired). "Barbershop" tenor. CDL(PTX). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)

As Doug mentioned, it's not so easy to get 20% (20MHz centered at
98MHz) bandwidth and gain and "omnidirectionality". Of course, it
can't be truely omni and have gain: best you can hope for is omni in
the horizontal plane.

That said, the next issue is "best all around". What exactly do you
mean by that? How much gain is "good"?

If I were doing it and needed it to cover the whole 20MHz, I'd stack
however many full-wave vertical doublets I could manage and feed them
with a harness to keep the feeds in-phase. I'd separate them
vertically by enough to keep the mutual coupling moderately low and
perhaps experiment with that (or model it) to optimize the phase of
the mutual coupling. I'd probably make the doublets with fairly large
diameter tubing, or use a "bowtie" arrangement, to keep bandwidth
high.

If I was concerned with only one frequency, or a narrow band, I'd use
a coaxial collinear because it's relatively easy to build and mount
and feed and get working properly (IF you understand how it works).
That's assuming I wanted to use at least four elements; fewer than
that and it's probably not worth the effort.

But also beware that the vertical collinears will give you vertical
polarization and that may not be what you want!

Cheers,
Tom

(Patrick) wrote in message . com...
Hello all,

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

Any thoughts would be commended.

Sincerely,

Patrick Cambre

As Doug mentioned, it's not so easy to get 20% (20MHz centered at
98MHz) bandwidth and gain and "omnidirectionality". Of course, it
can't be truely omni and have gain: best you can hope for is omni in
the horizontal plane.

That said, the next issue is "best all around". What exactly do you
mean by that? How much gain is "good"?

If I were doing it and needed it to cover the whole 20MHz, I'd stack
however many full-wave vertical doublets I could manage and feed them
with a harness to keep the feeds in-phase. I'd separate them
vertically by enough to keep the mutual coupling moderately low and
perhaps experiment with that (or model it) to optimize the phase of
the mutual coupling. I'd probably make the doublets with fairly large
diameter tubing, or use a "bowtie" arrangement, to keep bandwidth
high.

If I was concerned with only one frequency, or a narrow band, I'd use
a coaxial collinear because it's relatively easy to build and mount
and feed and get working properly (IF you understand how it works).
That's assuming I wanted to use at least four elements; fewer than
that and it's probably not worth the effort.

But also beware that the vertical collinears will give you vertical
polarization and that may not be what you want!

Cheers,
Tom

(Patrick) wrote in message . com...
Hello all,

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

Any thoughts would be commended.

Sincerely,

Patrick Cambre

In article ,
(Patrick) writes:

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

Any thoughts would be commended.

FM broadcast is usually horizontally polarized. You can borrow from
the small general aviation "rudder" antenna for the VHF 108-137 MHz
civil aviation band.

That one is two half-wave whips at right angles in the horizontal
plane on the plane's vertical stabilizer. Feed it like a half-wave
dipole. Impedance will be somewhere near 75 Ohms; in aircraft
installations that depends on the vertical stabilizer structure.
Has a fairly good omnidirectional pattern.

It's easy to scale up slightly in size to the FM band. Transmission
levels beyond Part 15 regulations is illegal.

Len Anderson
retired (from regular hours) electronic engineer person

In article ,
(Patrick) writes:

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

Any thoughts would be commended.

FM broadcast is usually horizontally polarized. You can borrow from
the small general aviation "rudder" antenna for the VHF 108-137 MHz
civil aviation band.

That one is two half-wave whips at right angles in the horizontal
plane on the plane's vertical stabilizer. Feed it like a half-wave
dipole. Impedance will be somewhere near 75 Ohms; in aircraft
installations that depends on the vertical stabilizer structure.
Has a fairly good omnidirectional pattern.

It's easy to scale up slightly in size to the FM band. Transmission
levels beyond Part 15 regulations is illegal.

Len Anderson
retired (from regular hours) electronic engineer person

I highly recommend a J-Pole antenna for FM. I have used them on 2m and 70cm
without disappointment. Even with 200mW I can hit repeaters some distance
away. They can be scratch made from copper tubing (if you can solder your
water pipes you can make this one). I am thinking of one for 6M to run
52.525.

They are 5/8 wave and you get a respectable gain. Just make sure to feed it
with RG8 or 9913.

You can compute your requirements and see a model at
http://www.packetradio.com/jpol.htm.
Hope this helps. E-mail me if you have any questions.

"Patrick" wrote in message
om...
Hello all,

I would like to build the best all around omni-directional antenna,
that would have a good gain, in the FM broadcast band.

Any thoughts would be commended.

Sincerely,

Patrick Cambre