In article ,
john doe wrote:

Is that 58.5 inch figure a typo? Your NEC model says 51 inches, and
my quickie spreadsheet calculation says 50.9 inches for a 145 MHz
center of band.

Actually, the 51 you're seeing in the NEC model is probably the number of
segments. One end is at 0,0,36 and the other is at 0,0,94.5; so yes it's
really 58.5 inches.

Whups... my bad.

I came to this number by playing with the model until
the REAL component of the impedance got as close to 50 as I could get it.

Hmmm. What did that do to the pattern? You no longer have a 5/8-wave
antenna. Adding about 8 inches has brought it very close to being a
3/4-wave radiator. As such, it's going to have a substantially lower
amount of towards-the-horizon energy in its pattern, and a big lobe
aiming upwards at roughly 45 degrees above the horizon.

This is the classic problem with running a 2-meter J-pole on 440 -
it'll load up and radiate, but a lot of its radiation is aimed at
airplanes rather than repeaters :-(

I also wonder about the coil - it calculates out to be just over 1
microHenry, or about j910 ohms at 145 MHz. That seems like quite a
bit too much, based on jgboyles's posting earlier today indicating a
feedpoint Z of about 80-j300.

My model comes up with a feedpoint impedance of 5.4485E+01-j2.8560E+03

So I tried to build a coil with an inductive reactance to cancel that .. I
came up with 3.13 microhenries.

Is my model way off?????

I think you might want to take two looks at it:

- Check the radiation pattern. By lengthening it to get a 50-ohm
resistive component in the feedpoint, I suspect you've given up
much of the gain benefit of a true 5/8-wave radiator. You may
actually have less towards-the-horizon power and sensitivity than
you'd get with a 1/4-wave groundplane or a 1/2-wave J-pole.

- Check the formula and actual inductance for your coil.

With so much capacitive reactance from the radiator to cancel out with
the coil, I suspect that you may also find that you've calculated out
an antenna which is going to be rather narrow-banded. Even slight
frequency shifts, or errors in the coil winding (a fraction of a turn)
could leave you with a lot of residual reactance and an unacceptable
SWR.

--
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!

Wow, I didn't even consider the pattern ....

Thanks for all the advice ... I'll have another go at it and let you know
how I make out.

-- ka2pbt

"Dave Platt" wrote in message
...
In article ,
john doe wrote:

Is that 58.5 inch figure a typo? Your NEC model says 51 inches, and
my quickie spreadsheet calculation says 50.9 inches for a 145 MHz
center of band.

Actually, the 51 you're seeing in the NEC model is probably the number of
segments. One end is at 0,0,36 and the other is at 0,0,94.5; so yes it's
really 58.5 inches.

Whups... my bad.

I came to this number by playing with the model until
the REAL component of the impedance got as close to 50 as I could get it.

Hmmm. What did that do to the pattern? You no longer have a 5/8-wave
antenna. Adding about 8 inches has brought it very close to being a
3/4-wave radiator. As such, it's going to have a substantially lower
amount of towards-the-horizon energy in its pattern, and a big lobe
aiming upwards at roughly 45 degrees above the horizon.

This is the classic problem with running a 2-meter J-pole on 440 -
it'll load up and radiate, but a lot of its radiation is aimed at
airplanes rather than repeaters :-(

I also wonder about the coil - it calculates out to be just over 1
microHenry, or about j910 ohms at 145 MHz. That seems like quite a
bit too much, based on jgboyles's posting earlier today indicating a
feedpoint Z of about 80-j300.

My model comes up with a feedpoint impedance of 5.4485E+01-j2.8560E+03

So I tried to build a coil with an inductive reactance to cancel that .. I
came up with 3.13 microhenries.

Is my model way off?????

I think you might want to take two looks at it:

- Check the radiation pattern. By lengthening it to get a 50-ohm
resistive component in the feedpoint, I suspect you've given up
much of the gain benefit of a true 5/8-wave radiator. You may
actually have less towards-the-horizon power and sensitivity than
you'd get with a 1/4-wave groundplane or a 1/2-wave J-pole.

- Check the formula and actual inductance for your coil.

With so much capacitive reactance from the radiator to cancel out with
the coil, I suspect that you may also find that you've calculated out
an antenna which is going to be rather narrow-banded. Even slight
frequency shifts, or errors in the coil winding (a fraction of a turn)
could leave you with a lot of residual reactance and an unacceptable
SWR.

--
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!

Interesting .... I changed my number of segments in NEC from 51 to 501 and I
got an impedance more in line with 80-j300 ..... I'll go re-work my coil
now

de ka2pbt


"Dave Platt" wrote in message
...
In article ,
john doe wrote:

Is that 58.5 inch figure a typo? Your NEC model says 51 inches, and
my quickie spreadsheet calculation says 50.9 inches for a 145 MHz
center of band.

Actually, the 51 you're seeing in the NEC model is probably the number of
segments. One end is at 0,0,36 and the other is at 0,0,94.5; so yes it's
really 58.5 inches.

Whups... my bad.

I came to this number by playing with the model until
the REAL component of the impedance got as close to 50 as I could get it.

Hmmm. What did that do to the pattern? You no longer have a 5/8-wave
antenna. Adding about 8 inches has brought it very close to being a
3/4-wave radiator. As such, it's going to have a substantially lower
amount of towards-the-horizon energy in its pattern, and a big lobe
aiming upwards at roughly 45 degrees above the horizon.

This is the classic problem with running a 2-meter J-pole on 440 -
it'll load up and radiate, but a lot of its radiation is aimed at
airplanes rather than repeaters :-(

I also wonder about the coil - it calculates out to be just over 1
microHenry, or about j910 ohms at 145 MHz. That seems like quite a
bit too much, based on jgboyles's posting earlier today indicating a
feedpoint Z of about 80-j300.

My model comes up with a feedpoint impedance of 5.4485E+01-j2.8560E+03

So I tried to build a coil with an inductive reactance to cancel that .. I
came up with 3.13 microhenries.

Is my model way off?????

I think you might want to take two looks at it:

- Check the radiation pattern. By lengthening it to get a 50-ohm
resistive component in the feedpoint, I suspect you've given up
much of the gain benefit of a true 5/8-wave radiator. You may
actually have less towards-the-horizon power and sensitivity than
you'd get with a 1/4-wave groundplane or a 1/2-wave J-pole.

- Check the formula and actual inductance for your coil.

With so much capacitive reactance from the radiator to cancel out with
the coil, I suspect that you may also find that you've calculated out
an antenna which is going to be rather narrow-banded. Even slight
frequency shifts, or errors in the coil winding (a fraction of a turn)
could leave you with a lot of residual reactance and an unacceptable
SWR.

--
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 found that increasing the number of segments had a significant change
in the input Z. The material I read on 5/8 antennas indicated the real
part of the Z was near 50 ohms. I could not get that result until I
increased the number of segments. Guess it is cause 146 MHZ antennas
are a good bit shorter than 3.5 MHZ antennas, and any small deviation
such as lenght, or # of segments will change the end results.
Gary N4AST

A source is spread out over an entire segment. So when you change the
number of segments, you change both the length and the effective
position of the source. When the source is at the bottom of a quarter
wavelength radiator, small changes in source position don't make much
difference in the impedance seen by the source. However, when the
antenna approaches a half wavelength, the source impedance changes quite
dramatically with source position. Consequently, you'll see substantial
changes in reported source impedance with segmentation in that case.
This might or might not be the cause of what you're seeing. As an
experiment, you might try moving the source up one segment and see how
big a difference it makes.

Whenever the result is very sensitive to small changes in the model, you
shouldn't expect a real antenna to come out exactly like the model
predicts, since small differences between the model and real antenna
will likewise cause significant differences.

The absolute length doesn't matter -- a 146 MHz antenna will be no more
or less sensitive to the same amount of change (in terms of percentage
of the antenna size or of the wavelength) than a 3.5 MHz antenna if both
are proportioned the same. In fact, 146 MHz antennas are typically
considerably fatter in terms of wavelength than 3.5 MHz antennas, and
this makes them less sensitive to small changes.

Roy Lewallen, W7EL

wrote:
I found that increasing the number of segments had a significant change
in the input Z. The material I read on 5/8 antennas indicated the real
part of the Z was near 50 ohms. I could not get that result until I
increased the number of segments. Guess it is cause 146 MHZ antennas
are a good bit shorter than 3.5 MHZ antennas, and any small deviation
such as lenght, or # of segments will change the end results.
Gary N4AST