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5/8 wave 2 meter antenna



 
 
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  #1  
Old March 14th 05, 02:26 PM
john doe
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Default 5/8 wave 2 meter antenna

Can anyone point me to a good site for construction a 5/8 wave 2M antenna???

I tried home-brewing one already with poor results .... a conductor up a
piece of PVC .... same conductor wound into a coil at the bottome
connected in series to the center of the coax. 4 1/4 wave radials
connected to outer conductor.

Thanks,

-- Rob
ka2pbt
  #2  
Old March 14th 05, 07:09 PM
Dave Platt
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Posts: n/a
Default

Can anyone point me to a good site for construction a 5/8 wave 2M antenna???

I tried home-brewing one already with poor results .... a conductor up a
piece of PVC .... same conductor wound into a coil at the bottome
connected in series to the center of the coax. 4 1/4 wave radials
connected to outer conductor.


I believe there's a project in the ARRL Handbook which shows how to
convert a Radio Shack CB whip antenna to a 2-meter 5/8-wave - it has
the details for the matching coil that is required.

http://www.arrl.org/tis/info/pdf/8009022.pdf has another QST article
which shows how to construct one from scratch, using a technique that
doesn't require a matching coil (the matching inductor is made from a
stub).

I haven't tried either of these myself.

With regard to the version you made - did you account for the loading
effect of the PVC when you measured and trimmed the length of your
radiator?

--
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!
  #3  
Old March 14th 05, 10:22 PM
[email protected]
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Posts: n/a
Default




With regard to the version you made - did you account for the loading
effect of the PVC when you measured and trimmed the length of your
radiator?



Hi Dave, I was just about to recommend the QST article, but you beat
me. A few comments on why his antenna did not perform: As you stated,
the PVC will change the input Z of the radiator. Just about anything
that is too close to the radiator will change things.
According to EZNEC, a 5/8 radiator with 1/4 radials 5 WL above ground
is 80-j300. As such, with a single series inductor matching section,
the best 50 ohm match is 1.6:1. This could get worse or better
depending on the antenna's environment. It will also change if you
slope the radials.
There is an article in the ARRL Antenna Compendium #1 pp.101 that
deals with 5/8 antennas. It basically explains why a 5/8, depending on
the application, is NOT the best performing radiator.
Gary N4AST

  #4  
Old March 14th 05, 11:04 PM
Dave Platt
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Posts: n/a
Default

In article .com,
wrote:

Hi Dave, I was just about to recommend the QST article, but you beat
me. A few comments on why his antenna did not perform: As you stated,
the PVC will change the input Z of the radiator. Just about anything
that is too close to the radiator will change things.


On second reading of that QST article, I admit to being a bit curious.
The K4LPQ version (with a shorted inductive stub inside the radiator,
soldered to it) is clear enough. However, the W9WQ version using a
longer, open-circuited inductive stub wire isn't a straightforward
translation of this, because there's no soldered (or other DC)
connection between the radiator and anything else!

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?

[Regretfully it seems likely that both W9WQ and K4LPQ are now silent
keys, so I can't ask for advice from the horses' mouths.]

According to EZNEC, a 5/8 radiator with 1/4 radials 5 WL above ground
is 80-j300. As such, with a single series inductor matching section,
the best 50 ohm match is 1.6:1. This could get worse or better
depending on the antenna's environment. It will also change if you
slope the radials.


I've seen a number of 5/8-wave antenna designs which deal with this
issue by using something other than a simple series coil. The
commonest approach seems to be to use a coil which is connected in
shunt between the radiator and the ground plane, with the "hot" side
of the coax being fed to a point tapped partway up on the coil. This
approach transforms the radiator impedance down to the 50 ohms needed
to match the coax, and also provides the series inductance needed to
cancel out the reactance. It also provides DC grounding for the
radiator.

http://www.fluxfm.nl/schema/5-8%20go...20radialen.PDF is one
such design. It requires some amount of tooling (e.g. to lathe down
the plastic parts to the specified configuration) but I suspect that a
version could be homebrewed up using simpler materials and methods.
The photos show the way to build the tapped matching coil assembly,
and could probably be adapted to other coil construction methods. I
believe the ARRL Handbook article which adapts a Radio Shack CB mobile
antenna uses a similar tapped coil.

Another approach might be to change the length of the radiator a bit,
to change the resistive part of the feedpoint impedance from 80 ohms
down closer to 50 ohms, and modify the coil to suit. I haven't run
any simulations to see how much change in the radiator length would be
required, and what this change would do to the antenna's gain pattern.

There is an article in the ARRL Antenna Compendium #1 pp.101 that
deals with 5/8 antennas. It basically explains why a 5/8, depending on
the application, is NOT the best performing radiator.


I'll have to look it up if I can find a copy of that edition to see
what they have to say. I agree, in some cases the horizon-directed
gain of a 5/8 isn't what you want. For in-city and in-the-hills
mobile use, a 1/4-wave might give more reliable performance, precisely
because its RF energy is more broadly directed.

--
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!
  #5  
Old March 14th 05, 11:46 PM
[email protected]
Guest
 
Posts: n/a
Default


Dave Platt wrote:
In article .com,
wrote:

Hi Dave, I was just about to recommend the QST article, but you beat
me. A few comments on why his antenna did not perform: As you

stated,
the PVC will change the input Z of the radiator. Just about

anything
that is too close to the radiator will change things.


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

curious.
The K4LPQ version (with a shorted inductive stub inside the radiator,
soldered to it) is clear enough. However, the W9WQ version using a
longer, open-circuited inductive stub wire isn't a straightforward
translation of this, because there's no soldered (or other DC)
connection between the radiator and anything else!

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?

[Regretfully it seems likely that both W9WQ and K4LPQ are now silent
keys, so I can't ask for advice from the horses' mouths.]

According to EZNEC, a 5/8 radiator with 1/4 radials 5 WL above

ground
is 80-j300. As such, with a single series inductor matching

section,
the best 50 ohm match is 1.6:1. This could get worse or better
depending on the antenna's environment. It will also change if you
slope the radials.


I've seen a number of 5/8-wave antenna designs which deal with this
issue by using something other than a simple series coil. The
commonest approach seems to be to use a coil which is connected in
shunt between the radiator and the ground plane, with the "hot" side
of the coax being fed to a point tapped partway up on the coil. This
approach transforms the radiator impedance down to the 50 ohms needed
to match the coax, and also provides the series inductance needed to
cancel out the reactance. It also provides DC grounding for the
radiator.

http://www.fluxfm.nl/schema/5-8%20go...20radialen.PDF is one
such design. It requires some amount of tooling (e.g. to lathe down
the plastic parts to the specified configuration) but I suspect that

a
version could be homebrewed up using simpler materials and methods.
The photos show the way to build the tapped matching coil assembly,
and could probably be adapted to other coil construction methods. I
believe the ARRL Handbook article which adapts a Radio Shack CB

mobile
antenna uses a similar tapped coil.

Another approach might be to change the length of the radiator a bit,
to change the resistive part of the feedpoint impedance from 80 ohms
down closer to 50 ohms, and modify the coil to suit. I haven't run
any simulations to see how much change in the radiator length would

be
required, and what this change would do to the antenna's gain

pattern.

There is an article in the ARRL Antenna Compendium #1 pp.101 that
deals with 5/8 antennas. It basically explains why a 5/8, depending

on
the application, is NOT the best performing radiator.


I'll have to look it up if I can find a copy of that edition to see
what they have to say. I agree, in some cases the horizon-directed
gain of a 5/8 isn't what you want. For in-city and in-the-hills
mobile use, a 1/4-wave might give more reliable performance,

precisely
because its RF energy is more broadly directed.

--
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!


Hi Dave, I'm sure you are reading it right, will look and see if I can
add anything later.
Gary N4AST

  #6  
Old March 15th 05, 02:05 AM
john doe
Guest
 
Posts: n/a
Default

No I didn't account for the PVC ...... but I made it longer than my design
and then trimmed past my design length and it never seemed to make a
difference. Considering my inductor was also formed around the PVC that was
probably way off too.....

I modeled the whole thing with nec first. This is what my input file looked
like:
http://mywebpages.comcast.net/ka2pbt/2M-5-8wave-dat.txt

and this is what my output looked like:
http://mywebpages.comcast.net/ka2pbt/2M-5-8wave-out.txt

Construction was a radiator 58.5 inches long fed along a 1/2" sched-40,
inductor was 19 turns around the same sched-40 2 inches long. The were 4
radials at 90 deg each 19.38"

Any comments welcome ....

Thanks,

-- de ka2pbt

"Dave Platt" wrote in message
...
Can anyone point me to a good site for construction a 5/8 wave 2M
antenna???

I tried home-brewing one already with poor results .... a conductor up a
piece of PVC .... same conductor wound into a coil at the bottome
connected in series to the center of the coax. 4 1/4 wave radials
connected to outer conductor.


I believe there's a project in the ARRL Handbook which shows how to
convert a Radio Shack CB whip antenna to a 2-meter 5/8-wave - it has
the details for the matching coil that is required.

http://www.arrl.org/tis/info/pdf/8009022.pdf has another QST article
which shows how to construct one from scratch, using a technique that
doesn't require a matching coil (the matching inductor is made from a
stub).

I haven't tried either of these myself.

With regard to the version you made - did you account for the loading
effect of the PVC when you measured and trimmed the length of your
radiator?

--
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!



  #7  
Old March 15th 05, 02:53 AM
Dave Platt
Guest
 
Posts: n/a
Default

In article ,
john doe wrote:

No I didn't account for the PVC ...... but I made it longer than my design
and then trimmed past my design length and it never seemed to make a
difference. Considering my inductor was also formed around the PVC that was
probably way off too.....

I modeled the whole thing with nec first. This is what my input file looked
like:
http://mywebpages.comcast.net/ka2pbt/2M-5-8wave-dat.txt

and this is what my output looked like:
http://mywebpages.comcast.net/ka2pbt/2M-5-8wave-out.txt

Construction was a radiator 58.5 inches long fed along a 1/2" sched-40,
inductor was 19 turns around the same sched-40 2 inches long. The were 4
radials at 90 deg each 19.38"


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.

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.

The PDF to which I posted a link earlier today uses a coil of only
10.5 turns, spread out over a distance of about 2.5" on a 3/4" form.
That's about .6 uH or j550 ohms... and it's a shunt-fed design so the
actual series inductance (above the tap point) is even lower.

--
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!
  #8  
Old March 15th 05, 03:54 AM
Buck
Guest
 
Posts: n/a
Default

On Mon, 14 Mar 2005 14:26:57 GMT, "john doe"
wrote:

Can anyone point me to a good site for construction a 5/8 wave 2M antenna???

I tried home-brewing one already with poor results .... a conductor up a
piece of PVC .... same conductor wound into a coil at the bottome
connected in series to the center of the coax. 4 1/4 wave radials
connected to outer conductor.

Thanks,

-- Rob
ka2pbt



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


--
73 for now
Buck
N4PGW
  #9  
Old March 15th 05, 05:55 AM
[email protected]
Guest
 
Posts: n/a
Default

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

Probably, but seems easier to to use a 5 turn coil at
the base...But I agree...A 5/8 GP is not a very good 2m
antenna. Better off building a high quality 1/4 wave GP.
Seriously... By high quality, I mean instead of 3-4 sloped
radials, use 6-8 radials, and even better , use a 2nd set
for decoupling. You can also build "sleeve" versions.
5/8's are ok on HF, where the angles are not critical, and on
10m, will usually be the best choice. But on 2m, they are usually
lame.
But, don't take my word. Build a 5/8 GP, and then build a 1/4
GP, and see which is best. I bet the 1/4 wave wins. For a
5/8 to work well on VHF/UHF, it needs to be a collinear dual
5/8 design, or at the least, use sloped 5/8 or 3/4 wave radials.
5/8's with 1/4 wave radials have pretty lousy patterns for 2m use.
MK

  #10  
Old March 15th 05, 08:14 PM
john doe
Guest
 
Posts: n/a
Default

On Tue, 15 Mar 2005 02:53:22 +0000, Dave Platt 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. 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.

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?????
 




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