Hummm...Should be about .336 mh for a grounded coil.
About .182 mh for a insulated coil. In the real
world will usually amount to about 5 turns of coil
average on say a .5 to 1 inch form. Trim coil for
best match. MK

wrote:

Hummm...Should be about .336 mh for a grounded coil.
About .182 mh for a insulated coil. In the real
world will usually amount to about 5 turns of coil
average on say a .5 to 1 inch form. Trim coil for
best match. MK


mH = 10e-3 Henry
uH = 10e-6 Henry

73, ac6xg

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!

Dave Platt wrote:
In article .com,
wrote:


On second reading of that QST article, I admit to being a bit
curious.

I infer that in the W9WQ version, the stub wire is performing two
functions at once - it's adding a series inductance, and it's also
coupling the RF out onto the radiator in a capacitive fashion. This
would imply that the stub needs to provide a bit more inductive
reactance than in the K4LPQ shorted-coax version, with some of this
reactance cancelling out the radiator's -j300 and the rest cancelling
out whatever amount of capacitive reactance exists between the stub
wire and the radiator.

Am I reading this right, or am I missing something?

I took a look at the Smith Chart, and for impedances in this region
of the chart, series L and parallel C is not the way to get a match. In
my version, 80-j300, you need 4.7pf shunt C and .15uH series L. I have
no idea how Fig. 1C in the article managed to get a good match with the
single insulated wire up the middle of the radiator.
I seem to recall a tri-band beam (TA-33 jr.?) that used this type of
matching. Must work, so I guess I am missing something.
Gary N4AST

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!

Can the 5/8 wave antenna be matched with a tuning stub such as the
J-Pole uses?


--
73 for now
Buck
N4PGW

Yes, A 5/8 radiator with an input Z of 80-j300 with series 19cm of 450
ohm line will be about 55-j0 ohms. Of course all of this is highly
dependent on the antenna environment on 2m.
Gary N4AST

Am I reading this right, or am I missing something?

I took a look at the Smith Chart, and for impedances in this region
of the chart, series L and parallel C is not the way to get a match. In
my version, 80-j300, you need 4.7pf shunt C and .15uH series L. I have
no idea how Fig. 1C in the article managed to get a good match with the
single insulated wire up the middle of the radiator.

It's possible he isn't creating a full match (with an L network) in
this case. He might just be cancelling out the negative reactance,
using a combination of (series L from the stub, and a bit of series C
from the capacitive coupling between stub-feed and radiator), and not
bothering with a shunt component at all. This would, perhaps, result
in an 80+0j feedpoint impedance and about a 1.6 SWR at the feedpoint,
which would probably end up significantly lower at the other end of
the feedline due to feedline losses.

Or, there might be something stranger going on, with the stub giving a
bit of shunt C to ground (in the PL-259), some parallel L/C inside the
radiator, and six other bits of odd voodoo.

The author says that it ought to be possible to get down to below
1.5:1 on the repeater portion of the band... this suggests that the
design isn't one which "tries" to achieve a true 1:1 match. The WA-2
and similar 5/8-wave antennas using a tapped coil seem to be able to
get down arbitrarily close to 1:1 at their best.

Beats me. Almost makes me want to try building one just to measure it
out and see how well it can work. On the other hand, given the
comments by Cebik and others about the somewhat illusory nature of the
gain advantage of a 5/8-wave, I may just stick with J-poles and
quarterwave ground planes.

--
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!

Ok,

I have not followed all of this thread.

One of the pdf's shows a shunt tapped inductor
as the base loading.

This is an auto-transformer match.

The entire inductor is set to cancel the capacitive reactance
of the aerial, then the coax feed is tapped to the 50R point,
The result is 50R J0.


For 5/8 aerials you may want to look at a physical short radiator
with a capacity hat to lower the angle of radiation.



wrote in message
ups.com...




Can the 5/8 wave antenna be matched with a tuning stub such as the
J-Pole uses?


--
73 for now
Buck
N4PGW

Yes, A 5/8 radiator with an input Z of 80-j300 with series 19cm of 450
ohm line will be about 55-j0 ohms. Of course all of this is highly
dependent on the antenna environment on 2m.
Gary N4AST

"Dave Platt"
... On the other hand, given the comments by Cebik and others
about the somewhat illusory nature of the gain advantage
of a 5/8-wave, I may just stick with J-poles and quarterwave
ground planes.
_______________

Just to note that for AM broadcast verticals, the FCC requires a certain
antenna "efficiency" for various classes of stations, in terms of the
minimum ground wave field strength produced per kilowatt of input power to
the radiator.

The FCC field strength minimum cannot be met by "Class A" stations
(basically the 50kW-ers) using a 1/4-wave vertical radiator. At least a
1/2-wave radiator is needed in most cases.

The most common radiator height for Class A non-directional AM broadcast
stations operating at 50kW day and night is 195°. A radiator height of 225°
(5/8 wave) maximizes ground wave field strength at a given power, but also
produces a high-angle lobe that can interfere with the ground wave during
night-time operation -- so rarely is used by AM broadcast stations. The
ground wave field strength difference between 195° and 225° radiators is
fairly small.

RF
Former staff engineer, WJR Detroit -- (Class A, 760kHz)

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!