It was a bust. :-(

I used copper pipe -- 1-inch for the bottom quarter-wave element and
1/2-inch for the upper quarter-wave element. The elements are 8 feet, 1 5/8
inches long and at the feed point, the space between the elements is about
three inches.

I put a piece of thickwall PVC inside the lower element as an insulator (per
Cecil's advice) and slid RG-8 through the PVC to the feed point. I attached
the coax shield to the 1-inch pipe and the center conductor to the 1/2-inch
pipe.

This afternoon, a ham pal of mine used his MFJ antenna analyzer (model?) to
test the sleeve. The VSWR was never better than 2.8:1 and exceeded 3:1 over
much of the intended range of operation. He suggested than we pull the coax
out and reconnect it as a more conventional dipole. Good idea. The VSWR
dropped to a 1.3:1 best and didn't exceed 1.5:1 over the entire 10m band.
As long as I keep the coax a few inches from the lower dipole element,
everything's fine. Oh ... don't lean it up against the orange tree,
either.

I think we might have extracted more information with his analyzer had we
not been getting close to being in the dark. If the antenna is inductive,
feeding it through a cap might pull the VSWR down to 1:1 at best. I know
from my J-poles that this is the case -- not sure if it applies to copper
pipe dipoles.

I have very few ideas for correcting the original sleeve design. Feeding one
or both elements through capacitors comes to mind. Resizing would not seem
to be appropriate, since the poor VSWR of the sleeve was still better at 10m
than anywhere else.

73,
"Sal"
(really KD6VKW, anxiously awaiting Saturday morning, to present my CSCE's at
the local VE session.)

"Sal M. Onella" wrote in message
...
It was a bust. :-(

I used copper pipe -- 1-inch for the bottom quarter-wave element and
1/2-inch for the upper quarter-wave element. The elements are 8 feet, 1
5/8
inches long and at the feed point, the space between the elements is about
three inches.

I put a piece of thickwall PVC inside the lower element as an insulator
(per
Cecil's advice) and slid RG-8 through the PVC to the feed point. I
attached
the coax shield to the 1-inch pipe and the center conductor to the
1/2-inch
pipe.

This afternoon, a ham pal of mine used his MFJ antenna analyzer (model?)
to
test the sleeve. The VSWR was never better than 2.8:1 and exceeded 3:1
over
much of the intended range of operation. He suggested than we pull the
coax
out and reconnect it as a more conventional dipole. Good idea. The VSWR
dropped to a 1.3:1 best and didn't exceed 1.5:1 over the entire 10m band.
As long as I keep the coax a few inches from the lower dipole element,
everything's fine. Oh ... don't lean it up against the orange tree,
either.

I think we might have extracted more information with his analyzer had we
not been getting close to being in the dark. If the antenna is inductive,
feeding it through a cap might pull the VSWR down to 1:1 at best. I know
from my J-poles that this is the case -- not sure if it applies to copper
pipe dipoles.

I have very few ideas for correcting the original sleeve design. Feeding
one
or both elements through capacitors comes to mind. Resizing would not
seem
to be appropriate, since the poor VSWR of the sleeve was still better at
10m
than anywhere else.

73,
"Sal"
(really KD6VKW, anxiously awaiting Saturday morning, to present my CSCE's
at
the local VE session.)


Ive built a couple of these and from my experience the bottom pipe is not
large enough, maybe 2.5 inches using rg58 coax. PVC may not be the best
insulator for this either. Styrofoam is very good. Even then the lower
section will have to be somewhat shorter than you would expect. Since you
are experimenting try some RG58 with your existing antenna and mkae the
change from PVC to styrofoam making the changes one at a time so you can see
the individual effects. Also sweep the antena to find out its actuall
resoant frequency.
Jimmie

Sal M. Onella wrote:
I have very few ideas for correcting the original sleeve design.

What are the resonant frequencies and feedpoint
resistances for the two types of feeds?

There's no rule that says the antenna must be
50 ohms at resonance. A 2.8:1 SWR could just be
an indication that the feedpoint impedance at
resonance is 18 + j0 ohms.

Did you do anything to choke the common-mode
currents on the feedline? If not, those currents
can upset the SWR meter reading.
--
73, Cecil, http://www.qsl.net/w5dxp

On Feb 22, 12:16 am, "Sal M. Onella"
wrote:
It was a bust. :-(

They can be quirky..And the decoupling is pretty critical. I always
preferred
a base fed half wave for those reasons. You still need to decouple for
best
performance, but there are no "coax in the way" issues, etc.. I use a
simple
"gamma loop" feed. IE: single turn coil, and a cap if needed.
Sometimes
you don't need the cap, but if you do, 30-50 pf is about the usual
value for
10m. I make those from a short length of coax.. I'm basically copying
the
feed system of the usual cushcraft ringos.. BTW, cushcraft sells a 10m
ringo if one doesn't want to build one, or have the tubing.
MK

"Jimmie D" wrote in message
...


snip

Ive built a couple of these and from my experience the bottom pipe is not
large enough, maybe 2.5 inches using rg58 coax. PVC may not be the best
insulator for this either. Styrofoam is very good. Even then the lower
section will have to be somewhat shorter than you would expect. Since you
are experimenting try some RG58 with your existing antenna and mkae the
change from PVC to styrofoam making the changes one at a time so you can
see
the individual effects. Also sweep the antena to find out its actuall
resoant frequency.
Jimmie

TKS. I will try both of those approaches. BTW, The PVC wasn't chosen for
its electrical characteristics. I used it only as a physical separator to
prevent arc-over. Styrofoam, eh?

The analyzer permitted sweeping, albeit manually. The resonant freq is
where it ought to be and it tests OK with the coax out of the lower section.

"Cecil Moore" wrote in message
.. .
Sal M. Onella wrote:
I have very few ideas for correcting the original sleeve design.

What are the resonant frequencies and feedpoint
resistances for the two types of feeds?

I will need to re-borrow my buddy and his analyzer. We just took readings
but no notes. (Yes, I know better.)

There's no rule that says the antenna must be
50 ohms at resonance. A 2.8:1 SWR could just be
an indication that the feedpoint impedance at
resonance is 18 + j0 ohms.

Agree. However, the venerable Kenwood TS-120 says to keep the VSWR under
1.5:1. So, my first attempt is a bad antenna for the intended purpose.

Did you do anything to choke the common-mode
currents on the feedline? If not, those currents
can upset the SWR meter reading.

No. That was careless of me. Four turns of the coax if I remember
correctly from an earlier discussion here. Thus, the first attempt may not
be a bad antenna but a bad measurement.

wrote in message
ups.com...
On Feb 22, 12:16 am, "Sal M. Onella"
wrote:
It was a bust. :-(

They can be quirky..And the decoupling is pretty critical. I always
preferred
a base fed half wave for those reasons. You still need to decouple for
best
performance, but there are no "coax in the way" issues, etc.. I use a
simple
"gamma loop" feed. IE: single turn coil, and a cap if needed.
Sometimes
you don't need the cap, but if you do, 30-50 pf is about the usual
value for
10m. I make those from a short length of coax.. I'm basically copying
the
feed system of the usual cushcraft ringos.. BTW, cushcraft sells a 10m
ringo if one doesn't want to build one, or have the tubing.

I will try decoupling and also try that feed.

I have several commercial antennas but I tend toward building them myelf. I
learned the hard way about the three types of copper pipe, K, L, & M. Until
I got home and hit google.com, I didn't know to shop for the more economical
Type M and paid premium $$$ for Type L. I don't want to think what Type K
would have cost.

"Sal"

On Feb 23, 12:40 am, "Sal M. Onella"
wrote:
wrote in message

ups.com...





On Feb 22, 12:16 am, "Sal M. Onella"
wrote:
It was a bust. :-(

They can be quirky..And the decoupling is pretty critical. I always
preferred
a base fed half wave for those reasons. You still need to decouple for
best
performance, but there are no "coax in the way" issues, etc.. I use a
simple
"gamma loop" feed. IE: single turn coil, and a cap if needed.
Sometimes
you don't need the cap, but if you do, 30-50 pf is about the usual
value for
10m. I make those from a short length of coax.. I'm basically copying
the
feed system of the usual cushcraft ringos.. BTW, cushcraft sells a 10m
ringo if one doesn't want to build one, or have the tubing.

I will try decoupling and also try that feed.

I have several commercial antennas but I tend toward building them myelf. I
learned the hard way about the three types of copper pipe, K, L, & M. Until
I got home and hit google.com, I didn't know to shop for the more economical
Type M and paid premium $$$ for Type L. I don't want to think what Type K
would have cost.

"Sal"- Hide quoted text -

- Show quoted text -

Sal, I would like to know how it works with the PVC as I have tried it
on exactly this type of antenna and it worked horribly. Apparently the
PVC I was using has some really lousy dielectric qualities, This stuff
would melt in a microwave while other PVC pipes do not.. I understand
that all PVC is not created equally and that some may be OK. To get
mine to work I used larger pipe, smaller coax, and a different
insulating material but my original was identical to your first
attempt. Unfortunately for me I made all the changes at once and
really dont know which fixed the problem except that the PVC was an
issue. I am very interested in finding whether or not just adding the
choke fixed the problem because this is also something I forgot to do
on my intial attempt. Second attempt had one of those chokes from
"Wireman" made of a length of coax and ferrite beads. One thing that
concerned me even after the antenna was working is the the SWR
appeared to be a little too good. It was 1.2:1 in the middle of 10
meters and never got ave 1.7:1 even at the band edges. I thought that
this is a little too good to be true..

Jimmie

"JIMMIE" wrote in message
oups.com...


snip

One thing that
concerned me even after the antenna was working is the SWR
appeared to be a little too good. It was 1.2:1 in the middle of 10
meters and never got ave 1.7:1 even at the band edges. I thought that
this is a little too good to be true..

My concern, too, for the same reason but the readings appear to be
authentic. For check, I tried the modified antenna (coax not running
through the lower element) on a few other bands with very, very low power
and got terrible VSWR readings. I get about a 1.4:1 at the low end of the 10
band, dropping to a 1.1:1 near the top of the band. I believe having big,
fat elements helps.

Due to cable loss, my measured VSWR at the transceiver is a skosh better
than what I would see at the antenna. A nice chart in the ARRL Antenna Book
shows that with 1 dB cable loss (approximate for 100' of RG-8), my 1.4:1
VSWR measured at the radio is actually closer to 1.6 at the antenna. Still a
keeper. I went on the air last night with a few watts and got a great
signal report from a local ham, so the antenna is working.

"Sal"
(KD6VKW /AE)

Sal M. Onella wrote:
"JIMMIE" wrote in message
oups.com...


snip

One thing that
concerned me even after the antenna was working is the SWR
appeared to be a little too good. It was 1.2:1 in the middle of 10
meters and never got ave 1.7:1 even at the band edges. I thought that
this is a little too good to be true..

My concern, too, for the same reason but the readings appear to be
authentic. For check, I tried the modified antenna (coax not running
through the lower element) on a few other bands with very, very low power
and got terrible VSWR readings. I get about a 1.4:1 at the low end of the 10
band, dropping to a 1.1:1 near the top of the band. I believe having big,
fat elements helps.

Due to cable loss, my measured VSWR at the transceiver is a skosh better
than what I would see at the antenna. A nice chart in the ARRL Antenna Book
shows that with 1 dB cable loss (approximate for 100' of RG-8), my 1.4:1
VSWR measured at the radio is actually closer to 1.6 at the antenna. Still a
keeper. I went on the air last night with a few watts and got a great
signal report from a local ham, so the antenna is working.

"Sal"
(KD6VKW /AE)




A year or two ago I was into playing with various monopole designs for
10 meters and greater freqs.

The bazooka I built and liked was a stainless steel whip for the
radiator. The bazooka sleeve was hook to coax and the mast--at the
sleeve where the bottom of the radiator was mounted on a teflon block
insulator. The sleeve was either 1-1/8 or 1-3/8 copper pipe.

The antenna was fed at the top of the sleeve where the radiator exited,
with a 1:1 current balun on a toroid core. The balun was able to handle
100+ watts and I drove it at 100 W.

The swr was below 1.5:1 on most of the ten meter band, I don't believe
it was ever above 2:1, if memory now serves me correct.

The antenna was ok. However, a 1/2 monopole with a gamma feed is what I
finally settled on and still run today. Benefit is that this antenna
requires a minimal counterpoise for excellent performance and radiation
pattern and ease of construction.

Regards,
JS