I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as the
VSWR doesnt change much with the extra coax section.

I did cut the section properly as the velocity factor for the coax is .78%
which gave me 26.825 ft.

Not sure what I am doing wrong or assuming - but looks like I could
use some input.
--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!

On Jan 30, 10:19 pm, wrote:
I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as the
VSWR doesnt change much with the extra coax section.

I did cut the section properly as the velocity factor for the coax is .78%
which gave me 26.825 ft.

Not sure what I am doing wrong or assuming - but looks like I could
use some input.
--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!


What do you mean by "modified"? We will need antenna dimensions,
where the feed point is, overall height, height above ground, is it
straight or bent, etc., in order to give you any advice...

The only immediate comment I can make is that any 'dipole' that is
near 50 ohms at the feed point is going to be several thousand ohms at
double the frequency... A matching section of coax will not help...


denny / k8do

wrote:

I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as the
VSWR doesnt change much with the extra coax section.

I did cut the section properly as the velocity factor for the coax is .78%
which gave me 26.825 ft.

Not sure what I am doing wrong or assuming - but looks like I could
use some input.

Try a half wavelength of coax.

ac6xg

wrote:
Not sure what I am doing wrong or assuming - but looks like I could
use some input.

If there was any way to get a low SWR on a simple
single-wire coax-fed dipole on 75m and 40m at the
same time, half of the ham population would have one.

A soon-to-be-published "Worldradio" article of mine
describes a fixed length dipole with a fixed feedline
length that works well on 40m and 75m without an antenna
tuner.
--
73, Cecil http://www.w5dxp.com

wrote in
newsoidnRiueftdl13YnZ2dnUVZ_rXinZ2d@hawaiiantel. net:

I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as
the VSWR doesnt change much with the extra coax section.

The feedpoint impedance of a full wave dipole is likely to be in the
region of 4000 ohms.

A quarter wave transformer needs to be of Zo (50*4000)^0.5 or 447 ohms.
You will not find a practical coax line with such Zo.

If you for instance used a quarter wave of RG58C/U for a transformer, the
input impedance would be 2 ohms (VSWR=25) (taking into account the 5+dB
of loss in the transformer.

Both the length and the Zo of a quarter wave transformer are important to
its operation.

Does this help to explain the results you observed?

Owen


I did cut the section properly as the velocity factor for the coax is
.78% which gave me 26.825 ft.

Not sure what I am doing wrong or assuming - but looks like I could
use some input.

Jim Kelley wrote:



wrote:

I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as the
VSWR doesnt change much with the extra coax section.

I did cut the section properly as the velocity factor for the coax is
.78% which gave me 26.825 ft.

Not sure what I am doing wrong or assuming - but looks like I could
use some input.

Try a half wavelength of coax.

ac6xg

why a half wave?

--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!

Owen Duffy wrote:

wrote in
newsoidnRiueftdl13YnZ2dnUVZ_rXinZ2d@hawaiiantel. net:

I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as
the VSWR doesnt change much with the extra coax section.

The feedpoint impedance of a full wave dipole is likely to be in the
region of 4000 ohms.

its half wave



A quarter wave transformer needs to be of Zo (50*4000)^0.5 or 447 ohms.
You will not find a practical coax line with such Zo.

If you for instance used a quarter wave of RG58C/U for a transformer, the
input impedance would be 2 ohms (VSWR=25) (taking into account the 5+dB
of loss in the transformer.

Both the length and the Zo of a quarter wave transformer are important to
its operation.

Does this help to explain the results you observed?

maybe I should have asked does my coax tune my antenna???????


Owen


I did cut the section properly as the velocity factor for the coax is
.78% which gave me 26.825 ft.

Not sure what I am doing wrong or assuming - but looks like I could
use some input.

--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!

Denny wrote:

On Jan 30, 10:19 pm, wrote:
I recently built a modified vertical dipole
from wire and using a pole and trees as supports.
I had hoped it would work on 2 bands 75 and 40 but it only
works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1 so I thought I could use a 1/4 coax
matching section 50 ohms but this doesnt solve the problem either as the
VSWR doesnt change much with the extra coax section.

I did cut the section properly as the velocity factor for the coax is
.78% which gave me 26.825 ft.

Not sure what I am doing wrong or assuming - but looks like I could
use some input.
--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!


What do you mean by "modified"?

looks like the letter "H" on its side "normally" but modified
the feed is asymmertical about 5 ft off the gound.

similar to Force 12 Sigma 80


it looks like this

|
|
| abot 56 ft high
|
|
|
|

feed here has 2 loading coils

|
___________|____________ both sides approx 17 ft
|

base insulator here

_|_ base mount

ground

feed point impedance is supposed to be very low - maybe 12 ohms

We will need antenna dimensions,
where the feed point is, overall height, height above ground, is it
straight or bent, etc., in order to give you any advice...

The only immediate comment I can make is that any 'dipole' that is
near 50 ohms at the feed point is going to be several thousand ohms at
double the frequency... A matching section of coax will not help...


denny / k8do

--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!

On Wed, 31 Jan 2007 18:40:56 -1000, wrote:

works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1

it looks like this

|
|
| abot 56 ft high
|
|
|
|

feed here has 2 loading coils

|
___________|____________ both sides approx 17 ft
|

base insulator here

_|_ base mount

ground

feed point impedance is supposed to be very low - maybe 12 ohms

Which gives rise to the question, where did the match at 75 meters
come from? I suspect it comes from the added loading.

Next question, why does it have the high mismatch for 40 meters. The
same suspicion follows logically (as the harmonic relationships follow
logically).

With loading resonance is not length, it is the combination of all
elements. What appears to be a normal (good) 75 meter operation
should also appear to be normal (poor) 40 meter operation. At 75
meters, the antenna is normally Low-Z. At 40 meters, the antenna is
normally Hi-Z. Your SWR measurement confirms the sense of "normal" in
both bands.

73's
Richard Clark, KB7QHC

Richard Clark wrote:

On Wed, 31 Jan 2007 18:40:56 -1000, wrote:

works on one 75 where the VSWR is 1.3:1 and the bandwidth is 85 khz
for the 2:1 points.

On 40 meters the VSWR is over 10:1

it looks like this

|
|
| abot 56 ft high
|
|
|
|

feed here has 2 loading coils

|
___________|____________ both sides approx 17 ft
|

base insulator here

_|_ base mount

ground

feed point impedance is supposed to be very low - maybe 12 ohms

Which gives rise to the question, where did the match at 75 meters
come from? I suspect it comes from the added loading.

Next question, why does it have the high mismatch for 40 meters. The
same suspicion follows logically (as the harmonic relationships follow
logically).

With loading resonance is not length, it is the combination of all
elements. What appears to be a normal (good) 75 meter operation
should also appear to be normal (poor) 40 meter operation. At 75
meters, the antenna is normally Low-Z. At 40 meters, the antenna is
normally Hi-Z. Your SWR measurement confirms the sense of "normal" in
both bands.

73's
Richard Clark, KB7QHC

so whats the solution to making it work on 40? a tuner?
some kind of tuned piece of coax?????????????



--
I SPILLED SPOT REMOVER ON MY DOG..............AND NOW HES GONE!!