My other cohort suggested a ground-plane vertical, too. What's the deal
with ground plane verticals? It seems they are slightly mechanicaly more
complicated than a half wave dipole, but that's fine if there's some
advantage. Are they safer to use without an swr meter?

"tom" wrote in message
news:LeZZd.695607$Xk.128464@pd7tw3no...
My other cohort suggested a ground-plane vertical, too. What's the deal
with ground plane verticals? It seems they are slightly mechanicaly more
complicated than a half wave dipole, but that's fine if there's some
advantage. Are they safer to use without an swr meter?



Make just about any kind of vertical with a vertical section of about 19.5
inches and use either 3 or 4 radials about 20 inches long drooping to around
45 degrees. You can also take two wires about 19 inches long and make a
vertical dipole. Feed the antenna with some rg-58 about 50 feet or more
long. The coax will have enough loss in it the swr as seen at the
transmitter will be low enough as not to worry. While I don't recommend
this as anythihng like an optimal antenna, it will get you on the air. That
is what I put up here when I moved in about a year ago and have not gotten
around to doing anything beter as I don't work 2 meter FM that much.

Yeah, that's what I figure, too. I should have mentioned that even though
the rig is a mobile unit, I'm using it as a base station from my basement
suite, so the antenna doesn't need to be capable of being mounted on a
vehicle. Given that new piece of info, doesn't it make more sense for me to
make a simple, 2-wire dipole temporary antenna than a ground plane? Or are
there some reasons why ground plane antennas are better than dipoles?

--
73
Tom H
VA7FAB

On Wed, 16 Mar 2005 18:33:20 GMT, "tom" wrote:

Yeah, that's what I figure, too. I should have mentioned that even though
the rig is a mobile unit, I'm using it as a base station from my basement
suite, so the antenna doesn't need to be capable of being mounted on a
vehicle. Given that new piece of info, doesn't it make more sense for me to
make a simple, 2-wire dipole temporary antenna than a ground plane? Or are
there some reasons why ground plane antennas are better than dipoles?

One of the main reasons is mounting. The dipole will have to be
strung from the ceiling or something high. The ground plane can be
set on a table, file cabinet, etc. It can also be mounted on a pole
and just set in a corner of the room.

Yeah, that's what I figure, too. I should have mentioned that even though
the rig is a mobile unit, I'm using it as a base station from my basement
suite, so the antenna doesn't need to be capable of being mounted on a
vehicle. Given that new piece of info, doesn't it make more sense for me to
make a simple, 2-wire dipole temporary antenna than a ground plane? Or are
there some reasons why ground plane antennas are better than dipoles?

Getting best performance (pattern) out of a 2-wire dipole usually
requires that you bring the feedline out in a perpendicular fashion
for some distance before you turn it in a downwards direction. This
may or may not be convenient.

You'll also need ways of tensioning or supporting the wires to keep
the dipole fairly straight and vertical, and this also may be somewhat
inconvenient. You'll either need to hang the upper wire from a
support above it, and hang a weight of some sort down below the bottom
wire to keep the antenna taut and keep it from swinging in the wind,
or use fiberglass rods or something like that to support the wires.
In either case, you'll have to have a non-metallic support structure,
and keep the antenna a reasonable distance away from any metal-bearing
portions of the structure (pipes, stucco walls containing chicken
wire, etc.).

A quarter-wave ground plane can be made quite simply from three to
five pieces of copper wire and an SO-239 connector. The
feedline comes down the bottom, and if the feedline is sufficiently
stiff you can probably support the whole antenna just by using plastic
tie-wraps to lash the feedline to a vent pipe on your roof and letting
the feedline bear the (very modest) weight of the antenna. You won't
need an overhead support, you won't need any sort of balun (loop or
otherwise).

The gain pattern of a center-fed half-wave vertical dipole, and a
quarter-wave monopole with two to four radials drooped downwards at 45
degrees, are very very similar. See

http://www.cebik.com/gp/58-2.html

and take a look at the second chart of the radiation patterns - it
compares a vertical dipole with two ground-plane antennas (horizontal
and drooped radials). "In practical terms, the low-angle lobes of
the dipole and the 45° sloping-radial monopole overlap, with the
90°-radial monopole slightly weaker."

So, basically, I think you'll get equivalent performance from a
monopole with a ground plane, and will find the mechanical
arrangements rather easier to manage.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

What is with you and the swr meter? Forget it. Make a dipole and hang it
up.Make a groundplane and hang it up. Get a cheap 1/4wave commercial mobile
whip, cut it 19" long, stick it on a metal bracket and screw it to the house
gutering. Bandwidth is so wide at VHF you won't have a problem. I've never
bothered with an swr meter at VHF. The formulas work.

Brad.

"tom" wrote in message
news:LeZZd.695607$Xk.128464@pd7tw3no...
My other cohort suggested a ground-plane vertical, too. What's the deal
with ground plane verticals? It seems they are slightly mechanicaly more
complicated than a half wave dipole, but that's fine if there's some
advantage. Are they safer to use without an swr meter?

I believe the original poster wanted to know how to create a dipole. If you want to make a dipole
with a vertical configuration, you have to use a method which keeps the radiators vertical and the
feedline horizontal for some part of a wavelength. Try this:

Make a TEE shaped assembly of PVC pipe using a TEE connector, one piece of PVC about one metre long
and two pieces of PVC about 25 centimeters long. The longer pipe mounts horizontally and the
shorter pipes are oriented vertically. Cement all the pieces into the TEE. Strip the outer jacket
off a length of RG58 about 20 inches from the end. Make an opening in the braid next to the outer
jacket and pull the center conductor through so that the braid and center conductor are sepatate.
Push the RG58 into the long pipe and make the center conductor bend up into the upper vertical PVC
and pull the braid down through the lower vertical PVC. If necessary, prune the center conductor
and braid for best SWR, but the SWR should probably be low enough as is.

-ken-

I get it, and then maybe once it's all set, I could put end-caps on the top
and the bottom of the vertical segment to keep it dry. Maybe it would be
good to put in some kind of fastener, too, holding the flexible braid
immobile against the interior of the downward-pointing vertical segment ---
before the installation of the end-caps, obviously.
I like it --- simple but effective.
But does the horizontal segment have to be an entire meter, though? I think
I can remeber seeing those T-like, FM antennas mounted on large towers but
with horizontal segments that were shorter than a meter --- I think they
were slightly shorter than the vertical segments.

--
73
Tom H
VA7FAB

If you really want an effective and simple to build 2M antenna for FM
work, purchase an SO-239, (UHF Female chassis fitting) and a couple
pieces of brass welding rod, or hobby rod.

Using nothing more than pliers and a solder iron you can make a nice
ground plane antenna. Vertical piece cut to about 19 inches, three or
four radials cut to about 20 inches, and bend the ground radials down
about 45 dgreees. This will provide a near perfect 50 ohm match. Simply
screw the PL-259 on your cable on to this antenna and tape it to the
side of a wood or metal pole. Real easy, and real effective.


Ed K7AAT

.. A couple other guys who know what they're talking about are also
recommending that same arrangement. 50 ohms on the cable and 50 ohms on the
antenna --- perfect. No need for a matching device, I like it. But aren't
dipole-like configurations supposed to present 73 ohms of load? Why is a
1/4 wave groundplane only 50 ohms?

--
73
Tom H
VA7FAB