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

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.

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.

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

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

--
73
Tom H
VA7FAB

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