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.

On Wed, 16 Mar 2005 16:39:22 GMT, "tom" wrote:

I haven't bought it yet --- it'll be a IC-2200H. They (swr meters) might
not cost much, but it's more than I'll have. I just went through a $300
nightmare with a used Yaesu VX5 that supposedly worked perfectly untill 4
days afetr I got it when the Tx final just overheated and crapped out. So
the all the money I spent on 7-10amp ps is basically wasted too. So, please
don't recommend buying anything used because I just want to get on-air, and
I only have another $400 or so and I want to make sure I can do it with
that. If I have to wait anothe rmonth to get stuff like an swr meter, fine.


The Icom IC-2200H most definitely has SWR protection. That radio is
only $230 from places like HRO. If you have $400 to spend, you should
have plenty to get on the air.

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!

I forgot to mention that's in canadian..

--
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.

--
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!

How about if I put the stripped-back coax antenna inside a waterproof
plastic bag that went over the top of it? I think that would certainly help
keep it dry, but would the plastic bag create any kind of mutual-inductance
problems, or does that only happen with conductors or materials with hi
dialectric coefficients?
I remember reading that a the larger the diameter of the radiating element,
the slower the propogation velocity, so I agree with you about how it might
screw up the normal calculations --- but by how much, though? Given that a
vhf dipole has an 11mhz wide band within which the swr is below 2, does it
matter if the resonant frequency is off even if its off by a mhz or two?
If I wound the feedline itself into an 8 turn, impromptu balun and slid a
ferrite bead on the feedline just below the end of the dipole part as a
further barrier to return RF, wouldn't that basically eliminate any
significant return current issues?
Am I missing anything, or can I basically count on a setup like this one to
not stress my final? If not, why not?
--
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

In article 0P0_d.698179$Xk.229269@pd7tw3no, tom wrote:

How about if I put the stripped-back coax antenna inside a waterproof
plastic bag that went over the top of it?

The bag would have to be 24" long. You need to keep not only the very
top of the stripped section dry, but the whole area where the braid
comes out of the stripped insulation and is then turned around
downwards.

I think that would certainly help
keep it dry, but would the plastic bag create any kind of mutual-inductance
problems, or does that only happen with conductors or materials with hi
dialectric coefficients?

A thin plastic bag would have little effect on the antenna's
characteristics - the layer of plastic dielectric is too thin to
change the behavior significantly.

The bag would probably make the antenna rather more prone to flap
around in the breeze, and that'd probably cause much more of an effect
than the bag itself would.

I remember reading that a the larger the diameter of the radiating element,
the slower the propogation velocity, so I agree with you about how it might
screw up the normal calculations --- but by how much, though?

At a wild guess, a few percent.

Given that a
vhf dipole has an 11mhz wide band within which the swr is below 2, does it
matter if the resonant frequency is off even if its off by a mhz or two?

Can you cite the source of that figure? Seems a trifle wider than I'm
used to seeing.

If I wound the feedline itself into an 8 turn, impromptu balun and slid a
ferrite bead on the feedline just below the end of the dipole part as a
further barrier to return RF, wouldn't that basically eliminate any
significant return current issues?

Possibly so. Quite honestly, a small amount of RF on the feedline is
not likely to cause you any problems. It'll alter the antenna's
pattern somewhat but since you're dealing with a wide-pattern omni I
doubt that you'll notice any significant change in the real-world
performance.

Am I missing anything, or can I basically count on a setup like this one to
not stress my final? If not, why not?

Maybe yes, maybe no.

Issue 1 is how well the antenna actually tunes up on your frequencies
of choice, which will depend on a lot of factors which are difficult
to predict in advance. For example, whether the folded-back braid is
stretched downwards along the coax insulation, and how tightly, will
probably alter the effective length of the sleeve by as much as an
inch or so and could significantly shift the SWR. Nearby objects, the
length of the feedline (if there's RF on the outside of the coax),
etc. can also affect the tuning. It's a lot easier to try, measure,
cut, and try again than it is to predict the behavior with the desired
degree of precision.

Issue 2 is how well your radio deals with higher-SWR conditions. Some
rigs seem to have active load-mismatch-detection circuitry and an
active power-reduction capability. Others seem to depend on having
rugged, conservatively-rated RF power transistors, operated well below
their safety margins, which are capable of standing a lot of abuse.
Others just emit blue smoke and give up :-(

Hence, it's likely to depend on your rig and your antenna.

Frankly, if you're concerned about the risk of damage, I would
encourage you to [1] build a design such as a ground plane which uses
relatively rigid parts which won't move around during use and thus
won't be likely to de-tune, and [2] find somebody who has a VHF-rated
antenna analyser, and borrow it for the half-hour needed to trim your
antenna to a low SWR. I'd think that any ham club with a couple of
dozen active members, is likely to contain at least one member who
owns an MFJ 259 or similar and is willing to Elmer you on a simple
antenna construction-and-tuneup project.

--
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!