On Wed, 16 Mar 2005 12:51:17 GMT, "tom" wrote:

If you really needed to get a 2m rig on the air and you didn't have an swr
meter, do you think you could get way with creating a simple dipole if it
was VERY carefully cut according to the formula, and if you wound your own
impromptu balun out of 5-6, 4" diameter turns of the (RG-8)cable right below
the dipole? Even if the resonant frequency wasn't exactly in the middle of
the 2m band, how much damage to the rig would you be risking? I don't see
how it could do any serious damage, even if the swr was somehow slightly in
excess of 2, or is it just something that is never, ever done --- not
checking the swr first with a meter? According to the antenna books I'm
reading, 1/2 wave dipoles (where each radiating element is 1/4 wave) don't
need fancy matching transformation stuff, the only issue might be RF coming
back through the outer braid and causing the cable to radiate --- thus the
5-turns-on-the-cable balun.



What you're proposing should work fine. For each quarter wave element,
length in feet = 234/frequency
or
length in meters = 71.4/frequency

So each quarter wave element would be around 18 or 19 inches,
depending on where you want to be in the 144-148 mhz range. The
formula should get you close enough to keep from blowing up your rig.

Coiling the coax as you propose shouldn't hurt anything, either. Try
to bring the coax away from the antenna at a 90-degree angle, and
hanging the antenna vertically would help if you're trying to work
mobiles or repeaters.

Also, you might consider a ground plane vertical, attaching five
quarter wave wires to a female coax plug, one vertical wire, with four
radials in the holes on the ground side.

bob
k5qwg

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!

How about the stripped back coax-coax? That's where you peel the outer
conductur back, exposing 19" of the inner conductor, and the 19" piece of
peeled back outer sleeve is the opposing radiator, and the feedline just
continues in a straight line out of the peeled back part. Then you tie or
tape the top to a hook and simply hang it. This avoids the entire issue of
making the feedline approach the feedpoint at a 90 degree angle. What's
your take on this design? I like its simplicity.

--
73
Tom H
VA7FAB

How about the stripped back coax-coax? That's where you peel the outer
conductur back, exposing 19" of the inner conductor, and the 19" piece of
peeled back outer sleeve is the opposing radiator, and the feedline just
continues in a straight line out of the peeled back part. Then you tie or
tape the top to a hook and simply hang it. This avoids the entire issue of
making the feedline approach the feedpoint at a 90 degree angle. What's
your take on this design? I like its simplicity.

A simple "sleeve" dipole. They can certainly work. I suspect that
tuning them can be a bit tricky - the velocity-of-propagation on the
upper, exposed-center-conductor portion is likely to be a bit
different than the velocity down the folded-back braid section.
They're probably a bit prone to RF on the feedline due to coupling
between the end of the folded-back braid, and the braid inside it.

Weather is another issue - rainwater will get into the braid quite easily
and will run back down the coax into your station. [Trust me on
this... I once failed to adequately RTV-waterproof the RG-8X coax at
the feedpoint of a copper-pipe J-pole, my SWR went sky-high after the
first big rain, and I found water and moss (!) inside my N connector.]

Commercial sleeve dipoles are often built of a copper pipe of two or
more diameters, and sometimes have an additional decoupling sleeve
down below the lower radiator section.

A stripped-back-coax sleeve dipole could make a very handy emergency
field antenna, but I don't think I'd depend on it for base-station use.

--
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-