2m homebrew antenna advice???
 

Hi, I am putting together a 2m and also an antenna for marine use, 156megs.
Can anyone here please give me some advice on which of these two I am going
to use? Has anyone here had good luck or horror stories, easily tuned or
matched?

I posted the following some time back; it's a little long, but it summarizes
my experiences with what I think is an "extended double Zepp for 2 meters":


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(PTXS). W0PBV. (785) 539-4448
NRA Life Member and Certified Instructor (Home Firearm Safety, Rifle, Pistol)