impromptu dipole
 

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.

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.


First, remember that most local 2-meter activity is going to be
vertically polarized. That means the dipole would have to be held by
one end with the other end down. A very simple antenna for 2-meters
is the J-Pole. There are numerous sites giving the dimensions for one
made out of common TV twin-lead. You can also make one out of 1/2"
copper pipe soldered together. These are very simple antennas that
are inexpensive to build and will outperform the dipole you are
considering.

Dick - W6CCD

Thanks, that's usefull info --- but what about the swr issue? If a half
wave dipole is tuned to a certain frequency and has an swr of 1.05, say, for
the swr to rise to 2 (at VHF wavelengths) you have to go roughly 6mhz up or
5mhz down from that tuned frequency. The 2m band is only 4mhz wide in
total, anyway, so if you measure carefully, are you risking damage to the
rig (also assuming the rig has no swr protective circuitry) if you don't
check the swr with a swr meter? I don't have one, that's why I'm asking.

"tom" wrote in message
news:vNXZd.696711$8l.564220@pd7tw1no...
Thanks, that's usefull info --- but what about the swr issue? ...
....., are you risking damage to the
rig (also assuming the rig has no swr protective circuitry) if you don't
check the swr with a swr meter? I don't have one, that's why I'm asking.

AFAIK all commercial rigs made in the last 15-20 years have protection.

On Wed, 16 Mar 2005 14:55:23 GMT, "tom" wrote:

Thanks, that's usefull info --- but what about the swr issue? If a half
wave dipole is tuned to a certain frequency and has an swr of 1.05, say, for
the swr to rise to 2 (at VHF wavelengths) you have to go roughly 6mhz up or
5mhz down from that tuned frequency. The 2m band is only 4mhz wide in
total, anyway, so if you measure carefully, are you risking damage to the
rig (also assuming the rig has no swr protective circuitry) if you don't
check the swr with a swr meter? I don't have one, that's why I'm asking.

I wouldn't worry about an SWR of 2.0. You could always buy an SWR
meter. They aren't very expensive. Without one you are just going to
be flying blind. Many things can affect SWR besides the antenna
length. Are you sure your radio doesn't have SWR protection? Most
do. What is the make and model?

Dick

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?

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.

"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.

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!

. 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?

The simplest ground-plane-style antenna has its radials sticking out
sideways. It's a monopole. The actual radiator is only half as long
as a dipole, and thus has only half as much radiation resistance, and
the feedpoint impedance is only half as much as that of a dipole.
It's on the order of 35 ohms or so.

If you bend the radials until they point straight down, you've got a
center-fed dipole with the feedline running up the center. Feedpoint
impedance is somewhere around 70 ohms.

Feedpoint resistance of anywhere from 35 to 70 ohms are available via
the obvious compromise - just bend the radials down less (for lower
resistances) or more (for higher resistances).

In most installations, radials bent downwards at an angle of about 45
degrees result in a 50-ohm resonant feedpoint resistance.

--
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!

Yeah, maybe I should just try to get some assistance with an swr meter
rather than endlessly screwing around, wondering how much I'm off resonance,
and all the stress associated with all of that. Maybe, if nothing else, the
shop where I'm buying all my gear might have one.
In any case, thanks for all the advice --- I'm pretty clear on whats what,
now.

--
73
Tom H
VA7FAB

OK, I'm sold --- a groundplane vertical it will be. In fact, I think I'll
go ahead and make it ahead of time so it's already set up and ready to go
when I get the IC2200H.
Thanks again for all the help ...

--
73
Tom H
VA7FAB

On Wed, 16 Mar 2005 22:44:41 GMT, "tom" wrote:

OK, I'm sold --- a groundplane vertical it will be. In fact, I think I'll
go ahead and make it ahead of time so it's already set up and ready to go
when I get the IC2200H.
Thanks again for all the help ...


Seeing that you are getting the rig in the future, you might want to
take your antenna to the shop when you pick up your rig. They will
probably be glad to see that it is tuned for your radio so you don't
ruin it.


--
73 for now
Buck
N4PGW

On Wed, 16 Mar 2005 22:17:47 GMT, "tom" wrote:

Yeah, maybe I should just try to get some assistance with an swr meter
rather than endlessly screwing around, wondering how much I'm off resonance,
and all the stress associated with all of that. Maybe, if nothing else, the
shop where I'm buying all my gear might have one.

If not, http://www.mfjenterprises.com has vhf swr meters in the 35
dollars u.s. range and up. You'll need one eventually...

bob
k5qwg


In any case, thanks for all the advice --- I'm pretty clear on whats what,
now.

They don't cover 2m, I checked.

--
73
Tom H
VA7FAB

Now that's a good idea --- I'll just have to make sure I don't jab anyones
eyes out on the subway (I'm car-less in Canada). I'm trying to imagine just
how bulky it will be, hmmm --- I should be able to manage it, I think.

--
73
Tom H
VA7FAB

On Thu, 17 Mar 2005 02:52:07 GMT, "tom" wrote:

They don't cover 2m, I checked.

You didn't check far enough. The MFJ-812B covers 2-meters and 220.
It lists for $34.95.
http://www.mfjenterprises.com/produc...rodid=MFJ-812B

Dick

You're right --- I only looked at that other site. I'll definitely shop
around for one, but even if I can find one, it'll be $70 with taxes, in
canadian dollars, and it isn't likely that I'll be able to find to begin
with up here, but what the hell --- now I know that there relatively
inexpensive meters I'm further ahead than I was before I knew about them. I
thought they were all at least $150 and up. Thanks.

--
73
Tom H
VA7FAB

tom wrote:
Thanks, that's usefull info --- but what about the swr issue? If a half
wave dipole is tuned to a certain frequency and has an swr of 1.05, say, for
the swr to rise to 2 (at VHF wavelengths) you have to go roughly 6mhz up or
5mhz down from that tuned frequency. The 2m band is only 4mhz wide in
total, anyway, so if you measure carefully, are you risking damage to the
rig (also assuming the rig has no swr protective circuitry) if you don't
check the swr with a swr meter? I don't have one, that's why I'm asking.

Hello, tom. I may be coming in to the discussion a little late.

My IC-V8000 and IC-2720 both have SWR protection circuitry that folds
back the power output if the SWR is too high. The '8000 folds back to
25 Watts and the '2720 goes to 15 on both bands.

If you want the operator's manual for your pending IC-2200 you can
download it from either www.icomamerica.com or www.icom.co.jp (and
probably Icom Canada's web site too, though my first stab at the name
didn't find the site -- icomcanada.com maybe?) and read all about its
features before you get the radio in your hands. I wouldn't be at all
surprised to find it'll fold back to a usable power level if the antenna
is less than perfect.

HTH

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 just checked the owners maual, and it does have protection circuitry.

--
73
Tom H
VA7FAB

Now that's a good idea --- I'll just have to make sure I don't jab
anyones eyes out on the subway (I'm car-less in Canada). I'm trying
to imagine just how bulky it will be, hmmm --- I should be able to
manage it, I think.


1/8" brass welding rod is soft. If I were you, I'd cut the elements long
enough to turn the ends into little loops, for eye protection.


Ed K7AAT

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

On 17-Mar-2005, "tom" wrote:

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.

Yes, I omitted those refinements as sort of obvious.

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.

I chose the one meter length arbitrarily so that it would be about a half wave. The idea is that
the feedline should be kept at right angles to the radiator for some distance. A coil of coax where
the feedling exits the PVC might be a good idea to reduce coupling.

73,
-ken-

If the idea of the coiled-feedline balun is to prevent the outer braid from
turning into a radiating element, wouldn't it be preferable to position the
coils in the feedline next to the antenna feedpoint instead of at the
opposite end of the horizontal segment? Or would there be harmfull,
inductive coupling to the the coils if they were that close to the
feedpoint?
Also, if one were to use a ferrite-bead type balun (not the type that the
feedline is wrapped around) instead of a coiled-feedline type balun, do you
think it would it be necessary to have electrical contact between the
ferrite-bead and the outer braid (necessitating stripping the outer
insulation)?


--
73
Tom H
VA7FAB

Tom, you are making this waaaay too complicated. Just solder up a
ground plane on an SO-239, hook a coax to it and start transmitting.
You don't even need welding rod. I have used coat hanger wire in the
past. Forget about baluns, beads and inductive coupling. We're not
launching a spacecraft here. Keep it simple.

Dick - W6CCD

On Fri, 18 Mar 2005 20:17:24 GMT, "tom" wrote:

If the idea of the coiled-feedline balun is to prevent the outer braid from
turning into a radiating element, wouldn't it be preferable to position the
coils in the feedline next to the antenna feedpoint instead of at the
opposite end of the horizontal segment? Or would there be harmfull,
inductive coupling to the the coils if they were that close to the
feedpoint?
Also, if one were to use a ferrite-bead type balun (not the type that the
feedline is wrapped around) instead of a coiled-feedline type balun, do you
think it would it be necessary to have electrical contact between the
ferrite-bead and the outer braid (necessitating stripping the outer
insulation)?

Check with your local club and see if someone has an swr meter you can
borrow. Most clubs are happy to help out.


Jeff
KBØGVI

I just got the rig all set up a few minutes ago with a vertical
groundplane --- a design I chose because it obviated the need for any kind
of matching due to it's intrinsic, unbalanced 50 ohms impedance. Works like
a charm too, thanks to everyone for all the excellent advice, its very much
appreciated.

--
73
Tom
VA7FAB
Skype Name:
va7fab_tom_in_vancouver