I'm Limited To An Antenna On The Front Of My House. Out My Window I Made A Simple !/4 wave Verticle Dipole. Would A 1/2 Wave Simply Be Twice The Length, Using 462/f Mhz, And Would There Be An Advantage With A 1/2 Wave Over a 1/4 Wave ?
Thanks

A quarter wave vertical needs quarter wave radials at 90 degrees to the
vertical.

A half wave vertical does not need radials as it is complete but needs fed
at the center with the coax running out 90 degrees to the dipole (as far as
possible)

For formulas see URL:
http://www.geocities.com/Augusta/7117/antenna.html

--
One Watt

To steal ideas from one person is plagiarism;
to steal from many is research.
-- Comedian Steven Wright


"WolfMan" wrote in message
t...
I'm Limited To An Antenna On The Front Of My House. Out My Window I Made A
Simple !/4 wave Verticle Dipole. Would A 1/2 Wave Simply Be Twice The
Length, Using 462/f Mhz, And Would There Be An Advantage With A 1/2 Wave
Over a 1/4 Wave ?
Thanks

You need a full half wave to be a real "dipole". one quarter wave on each
side of center. I can't imagine what you have now.

--
Steve N, K,9;d, c. i My email has no u's.


"WolfMan" wrote in message
t...
I'm Limited To An Antenna On The Front Of My House. Out My Window I Made A
Simple !/4 wave Verticle Dipole. Would A 1/2 Wave Simply Be Twice The
Length, Using 462/f Mhz, And Would There Be An Advantage With A 1/2 Wave
Over a 1/4 Wave ?
Thanks

If you built a dipole, then it was (most likely) not a quarter-wave - but a
half-wave
a j-pole may be a good choice given the situation that it needs to be 'out
the window' and is uhf
or, for that matter, a coaxial half-wave dipole. (strip and fold back
1/4-wave of braid - and weatherproof it)

"WolfMan" wrote in message
t...
I'm Limited To An Antenna On The Front Of My House. Out My Window I Made A
Simple !/4 wave Verticle Dipole. Would A 1/2 Wave Simply Be Twice The
Length, Using 462/f Mhz, And Would There Be An Advantage With A 1/2 Wave
Over a 1/4 Wave ?
Thanks


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WolfMan wrote:
"And would there be an advantage with a 1/2-wave over a 1/4-wave?"

Short answer: Yes.

A dipole is directive. It has nulls at its ends and concentrates
response perpendicular to the wire. Even a very short, almost vanishing
dipole has these directional characteristics to some extent.

The theoretical isotropic antenna is the only truly nondirectional,
gainless antenna. Directive gain is the power intensity that is radiated
in the perferred direction versus the power intensity averaged in all
directions (1955 Terman, page 871).

Directive gain of an elementary doublet (dipole) is 1.5 as compared with
an isotropic. The gain of a resonant 1/2-wave is only 1.64, which is
only some better than a very short elementary doublet.

Gain and directivity are only part of the antenna story. Radiation
resistance and loss resistance appear as series components of a load
resistance. The larger the radiation resistance is as compared with loss
resistance, the more efficient is the antenna.

Radiation resistance of a short dipole is almost proportional to the
square of its length. So, it rised rapidly as the antenna gets longer in
terms of wavelength for the too short antenna. More radiation resistance
is usually better because efficiency rises.

Kraus says in his 1950 edition on page 137 that a dipole of 0.1
wavelength has a radiation resistance of 7.9 ohms. If the dipole is only
0,01 wavelength, its radiation resistance is 0.08 ohms. This agrees with
the length squared formula.

On page 146, Kraus says that the radiation resistance of an ordinary
1/2-wave dipole in free space is 73 ohms. This will be more efficient
than an antenna about 1/2 the size in nearly all cases.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
. . .
Kraus says in his 1950 edition on page 137 that a dipole of 0.1
wavelength has a radiation resistance of 7.9 ohms. If the dipole is only
0,01 wavelength, its radiation resistance is 0.08 ohms. This agrees with
the length squared formula.

On page 146, Kraus says that the radiation resistance of an ordinary
1/2-wave dipole in free space is 73 ohms. This will be more efficient
than an antenna about 1/2 the size in nearly all cases.

The dipole Kraus is analyzing (shown in his fig. 5-1 on p. 127) isn't
what we'd normally call an "ordinary dipole". As this figure and the
accompanying text show, Kraus' short dipole is heavily end loaded to
cause the current to be uniform along its length. This increases its
radiation resistance by exactly a factor of four over that of a short
unloaded dipole (a plain straight piece of wire fed in the center),
whose current drops uniformly from the center to a value of zero at the
ends.

The equation for a short unloaded dipole can be found in the same
edition of Kraus on p. 262 as eq. 10-63, R11 = 5 * (beta * L)^2. The
quantity in parentheses (beta * L) equals 2 * pi * the dipole length in
wavelengths, so for a 0.1 wavelength dipole, it gives the radiation
resistance as just a smidge under 2 ohms. (EZNEC gives just over 2 ohms
for example model Dipole1.ez with the frequency changed to 60 MHz to
make the antenna 0.1 wavelength long. Remember that the Kraus formula is
an approximation, and for an infinitesimally thin antenna.)

This makes Richard's observation about efficiency even more true.

Roy Lewallen, W7EL

Roy, W7EL wrote:
"The dipole Kraus is analyzing (as shown in his fig. 5-1 on page 127)
isn`t what we`d normally call an ordinary dipole."

Yes. Roy is right. On page 136, Kraus says:
"Let us now calculate the radiation resistance of the short dipole of
Fig. 5-1."

The dipole of Fig. 5-1 has its ends loaded with capacitive hats. The
result of the very short, in terms of wavelength, dipole and its
capacitive loading are to cause the same current throughout the dipole.
This is not the case in an ordinary dipole where current drops to near
zero at its ends.

My mistake was searching for numbers that would give an idea of how
radiation resistance varies as an antenna drops in length below resonant
length, and not reading the whole chapter.

As Roy says, on page 262 Kraus writes: When the length L is small,
(10-62) reduces very nearly to R11 = 5(beta L)squared ohms.

Best regards, Richard Harrison, KB5WZI

Hal,

Your idea using the J-Pole is probabaly the best solution.
1. No radials needed
2. Easy to match using ANY type of feedline
3. Total length is about 3/4 wavelength (about 52") including matching
section
4. Easy to contruct

We found this URL that may give you an idea regarding contruction.

http://w4zt.com/jpole/

John

Hal Rosser wrote:
If you built a dipole, then it was (most likely) not a quarter-wave - but a
half-wave
a j-pole may be a good choice given the situation that it needs to be 'out
the window' and is uhf
or, for that matter, a coaxial half-wave dipole. (strip and fold back
1/4-wave of braid - and weatherproof it)

"WolfMan" wrote in message
t...

I'm Limited To An Antenna On The Front Of My House. Out My Window I Made A

Simple !/4 wave Verticle Dipole. Would A 1/2 Wave Simply Be Twice The
Length, Using 462/f Mhz, And Would There Be An Advantage With A 1/2 Wave
Over a 1/4 Wave ?

Thanks



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Stay away from the coaxial dipoles. They are junk. The vynil used on the
ouside of coax is unsuitabe for use as the dielectric of an antenna element.
While there are good ways of making this antenna the 'strip the coax and
fold back the sheild' method is not one of them.

"Hal Rosser" wrote in message
. ..
If you built a dipole, then it was (most likely) not a quarter-wave - but
a
half-wave
a j-pole may be a good choice given the situation that it needs to be 'out
the window' and is uhf
or, for that matter, a coaxial half-wave dipole. (strip and fold back
1/4-wave of braid - and weatherproof it)

"WolfMan" wrote in message
t...
I'm Limited To An Antenna On The Front Of My House. Out My Window I Made
A
Simple !/4 wave Verticle Dipole. Would A 1/2 Wave Simply Be Twice The
Length, Using 462/f Mhz, And Would There Be An Advantage With A 1/2 Wave
Over a 1/4 Wave ?
Thanks


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Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.770 / Virus Database: 517 - Release Date: 9/27/2004

Hi John and others,

Interesting topic.. I also agree with the j-pole. Not to bend any
eyebrows.
(I also have 300ohm 2M j-pole designs web-posted )

The only comment is the use of 300ohm twinlead for antenna construction.
300ohm twinlead comes in several different variations and the velocity
factors are all over the place. So, results vary!! I've answered my fair
share of e-mails in the past 5 years, trying to explain this problem to
other new Hams. This can really discourage someone that tries to build a
first time antenna with the wrong type wire.

Stuff to look for:

Don't use the "thick stuff": also called the "Heavy Duty" with a heavy thick
brown plactic covering. It is very difficult to cut and almost impossible
to work with.

Do not use the "foam type": with foam covering the wire, between the wire
and the outside black plastic casing.

Look for the "thin stuff": and the flatter the better.

Another thought is to move to a different construction material with a
little more consistency: 450ohm ladderline, Solid copper type.
Ref: http://home.comcast.net/~buck0/ll_coolj.html

Hope this helps others!
73
Steve KB1DIG


"John Steffes" wrote in message
...
Hal,

Your idea using the J-Pole is probabaly the best solution.
1. No radials needed
2. Easy to match using ANY type of feedline
3. Total length is about 3/4 wavelength (about 52") including matching
section
4. Easy to contruct

We found this URL that may give you an idea regarding contruction.

http://w4zt.com/jpole/

John

Hal Rosser wrote:
If you built a dipole, then it was (most likely) not a quarter-wave -
but a
half-wave
a j-pole may be a good choice given the situation that it needs to be
'out
the window' and is uhf
or, for that matter, a coaxial half-wave dipole. (strip and fold back
1/4-wave of braid - and weatherproof it)

"WolfMan" wrote in message
t...

I'm Limited To An Antenna On The Front Of My House. Out My Window I Made
A

Simple !/4 wave Verticle Dipole. Would A 1/2 Wave Simply Be Twice The
Length, Using 462/f Mhz, And Would There Be An Advantage With A 1/2 Wave
Over a 1/4 Wave ?

Thanks



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