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