On Tue, 6 Apr 2004 19:29:45 -0400, "Ralph Mowery"
wrote:


"Moody1951" wrote in message
...
Please help me out with the math. What is the length of a 5/8s wavelength
2
meter antenna? I've got something that might be able to be cut to the
freq.

Others have given you the length. They failed to mention that the impedance
is not 50 ohms and you must match that to the coax/rig. You will have to
research that for yourself, but usually a coil is used in series with the
antenna at the base of it.

Also relevant:

The calculated length is close, but factors like material type,
thickness, etc, mean that the actual length could be a little
different. Electrical length is what counts, not physical.

Best bet is to take the calculated length as mentioned earlier, add a
little bit, and cut to that greater length. Then test the antenna with
an SWR meter, trim a little shorter, and test again to optimize SWR
across the band of interest. You will never hit 1:1 (more of a
theoretical goal) but below 1:1.5 across the 2M band is fine.

Start your testing at a low power, until you know the SWR is below
1:2.

Happy trails,
Gary (net.yogi.bear)
------------------------------------------------
at the 51st percentile of ursine intelligence

Gary D. Schwartz, Needham, MA, USA
Please reply to: garyDOTschwartzATpoboxDOTcom

Please help me out with the math. What is the length of a 5/8s wavelength 2
meter antenna? I've got something that might be able to be cut to the freq.

Moody-

Did you realize what a stimulating topic this was going to be? Perhaps I can
help by announcing that there are two slightly different legitimate answers.

First, radio waves travel very close to 300 million meters per second. The two
meter band is roughly at a frequency of 150 million Hertz, so one wavelength
would be 300 million divided by 150 million. I think you get the idea, and can
use your exact frequency to get the exact wavelength. In the case of a 5/8th
wavelength antenna, simply multiply the calculated wavelength by 5/8 (or
0.625). (One inch equals 0.0254 meters.)

To use such an antenna, you must find a way to connect your transmission line
to the antenna and a ground plane, so that it matches. In other words, you
want to transfer the maximum amount of power to the antenna so it can be
radiated, and reflect the minimum amount of power back to the transmitter.

The beauty of a 5/8th wavelength (ground plane) antenna, is that it is
extremely easy to match. Its input impedance is 50 Ohms resistive in series
with a small amount of capacitance. Therefore you need a small amount of
series inductance to match it. Some higher-frequency examples use the
stainless element itself, wound in a small coil near its base.

The other "legitimate" answer is that a given 5/8th wavelength antenna you buy,
may actually be 0.58 wavelength, NOT 0.625. Such antennas are usually called
5/8th because of the confusion between 0.58 and 5/8.

The beauty of a 0.58 wavelength (groundplane) antenna, is that it concentrates
the maximum amount of power towards the horizon, compared to other similarly
constructed antennas (according to my Antennas Professor, 35 years ago). While
it is true that there is a small lobe towards the sky above, the main lobe is
sharper than the lobe of a quarter wave ground plane, and is aimed almost
directly at the horizon. If the vertical element were any longer (say, 0.625
wavelength), the lobe pointed up would increase and sap some of the power from
the lobe towards the horizon. If it were shorter, the main lobe would be
fatter and power would be spread over a larger, slightly higher angle.

A drawback of the 0.58 wavelength antenna is that it is not as easily matched.
Such antennas are often matched with a tapped coil. The bottom of the coil is
connected to ground, the top to the antenna element, and the co-axial
transmission line center conductor is connected to a tap on the coil.

73, Fred, K4DII

"Moody1951" wrote in message
...
Please help me out with the math. What is the length of a 5/8s wavelength
2
meter antenna? I've got something that might be able to be cut to the
freq.

Thanks

Lets assume you want a center frequency of 146 megahertz...

300/146=2.057945m (round it to nearest mm 2.058m)
2.058/8=0.25725m
0.25725x5=1.28625m (round it to nearest mm 1.286m)

Now I have a few questions...

How the ^%$#(*&$ do you get to even own a 2m radio without knowing how to do
this ELEMENTARY calculation? Do you have a licence? Have you learnt
absolutely nothing? This formula is the first one in the book together with
Ohm's Law!

73
Roger ZR3RC

Neat calculator at URL:
http://www.geocities.com/kk5hy/calc.html

You can cut it for hi, lo, or mid band

--
Incognito By Necessity (:-(

If you can't convince them, confuse them.
- - -Harry S Truman




"Moody1951" wrote in message
...
Please help me out with the math. What is the length of a 5/8s wavelength
2
meter antenna? I've got something that might be able to be cut to the
freq.

Thanks

Ok lets start this over again

Hi Moody,

The ansewr to your question, if you intend to use it on the upper portion of
the band (tuned to 146Mhz) would be 48.12 inches.

Aside from your asking antenna question here, also check out the ARRL
Handbook, as you will find the answers to simple questions, others get
annoyed at LOL

also visit the http://www.qsl.net/w4sat/antenna.htm website.

As far as the point you expressed Bob, I agree with you somewhat. Not all
Techs dont know anything. I have spoken to many Advanced/Extra that cant
figure out the length of a dipole for 40m, or how to program the memory in
the 2m radio. Even go as far as "you are not on frequency, and you dont need
to say over on a repeater". Now it makes me made when, me being a Tech needs
to answer these questions

Take in to the account of why many join the ham ranks. Not all are technical
minded people, or not yet atleast

73

Steve KC2GOG


"Moody1951" wrote in message
...
Please help me out with the math. What is the length of a 5/8s wavelength
2
meter antenna? I've got something that might be able to be cut to the
freq.

Thanks

"Moody1951" wrote in message
...
Please help me out with the math. What is the length of a 5/8s wavelength
2
meter antenna? I've got something that might be able to be cut to the
freq.


Others have given you the length. They failed to mention that the impedance
is not 50 ohms and you must match that to the coax/rig. You will have to
research that for yourself, but usually a coil is used in series with the
antenna at the base of it.

Please help me out with the math. What is the length of a 5/8s wavelength 2
meter antenna? I've got something that might be able to be cut to the freq.

Moody-

Did you realize what a stimulating topic this was going to be? Perhaps I can
help by announcing that there are two slightly different legitimate answers.

First, radio waves travel very close to 300 million meters per second. The two
meter band is roughly at a frequency of 150 million Hertz, so one wavelength
would be 300 million divided by 150 million. I think you get the idea, and can
use your exact frequency to get the exact wavelength. In the case of a 5/8th
wavelength antenna, simply multiply the calculated wavelength by 5/8 (or
0.625). (One inch equals 0.0254 meters.)

To use such an antenna, you must find a way to connect your transmission line
to the antenna and a ground plane, so that it matches. In other words, you
want to transfer the maximum amount of power to the antenna so it can be
radiated, and reflect the minimum amount of power back to the transmitter.

The beauty of a 5/8th wavelength (ground plane) antenna, is that it is
extremely easy to match. Its input impedance is 50 Ohms resistive in series
with a small amount of capacitance. Therefore you need a small amount of
series inductance to match it. Some higher-frequency examples use the
stainless element itself, wound in a small coil near its base.

The other "legitimate" answer is that a given 5/8th wavelength antenna you buy,
may actually be 0.58 wavelength, NOT 0.625. Such antennas are usually called
5/8th because of the confusion between 0.58 and 5/8.

The beauty of a 0.58 wavelength (groundplane) antenna, is that it concentrates
the maximum amount of power towards the horizon, compared to other similarly
constructed antennas (according to my Antennas Professor, 35 years ago). While
it is true that there is a small lobe towards the sky above, the main lobe is
sharper than the lobe of a quarter wave ground plane, and is aimed almost
directly at the horizon. If the vertical element were any longer (say, 0.625
wavelength), the lobe pointed up would increase and sap some of the power from
the lobe towards the horizon. If it were shorter, the main lobe would be
fatter and power would be spread over a larger, slightly higher angle.

A drawback of the 0.58 wavelength antenna is that it is not as easily matched.
Such antennas are often matched with a tapped coil. The bottom of the coil is
connected to ground, the top to the antenna element, and the co-axial
transmission line center conductor is connected to a tap on the coil.

73, Fred, K4DII

Moody1951 wrote:
Please help me out with the math. What is the length of a 5/8s wavelength 2
meter antenna? I've got something that might be able to be cut to the freq.

Thanks

Too bad this thread degenerated into a bashing contest.

The length of the antenna is the least of your problems.
You need to figure out how to match it so it will take power.
A web search should turn up some options along with the proper length.
"J-pole" is a place to start. Simple and effective antenna. There are
also loading coil solutions if you're really stuck on 5/8 wave. If you
really want a "good" antenna, search on "coaxial collinear".
mike

--
Return address is VALID.
Bunch of stuff For Sale and Wanted at the link below.
Toshiba & Compaq LiIon Batteries, Test Equipment
Honda CB-125S $800 in PDX
Yaesu FTV901R Transverter, 30pS pulser
Tektronix Concept Books, spot welding head...
http://www.geocities.com/SiliconValley/Monitor/4710/

Go to http://www.hamradio.com/ .
"Quick Search by MODEL #" for SF-2. You will see "1) Manufacturer:
HUSTLER" for $14.95.
Click on "Add to my HRO Cart" and go from there.
Mike's right. For $15 it 'taint worth the trouble to build.
Regards, K3DWW

"mike" wrote in message ...
Moody1951 wrote:
Please help me out with the math. .....

Too bad this thread degenerated into a bashing contest.

The length of the antenna is the least of your problems.
You need to figure out how to match it .....

Go to http://www.hamradio.com/ .
"Quick Search by MODEL #" for SF-2. You will see "1) Manufacturer:
HUSTLER" for $14.95.
Click on "Add to my HRO Cart" and go from there.
Mike's right. For $15 it 'taint worth the trouble to build.
Regards, K3DWW

"mike" wrote in message ...
Moody1951 wrote:
Please help me out with the math. .....

Too bad this thread degenerated into a bashing contest.

The length of the antenna is the least of your problems.
You need to figure out how to match it .....