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

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!!

On Wed, 31 Jan 2007 20:42:51 -1000, wrote:

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

Read Owen's reply.

73's
Richard Clark, KB7QHC

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

A better description would help. That looks more
like a ground plane antenna than it does a dipole.
Please describe the antenna in more detail.
--
73, Cecil http://www.w5dxp.com

Hmmm, I can't make much of this diagram and the group has decided to
use passive aggressive behavior to "teach you"... You are asking for
help and getting didactics thrown in your face... Shame on them!

I doubt you can feed this antenna on 40 with coax... You will need to
either put a tuner at the feed point, or resort to open wire line and
a tuner at the radio...

My first impulse would be to turn this into a top loaded vertical with
an elevated counterpoise, the vertical self resonant on 40 and loaded
for 80...
Feed it at the bottom of the vertical section with the braid going to
the 17 foot horizontals (some tuning needed)... Use a loading coil
about half way up the vertical for 80 and short it out for 40... The
counterpoise (the 17 foot horizontals as best I can make out) would be
coil loaded at at the center connection of the 2 "radials" so they
are resonant on 80 with a shroting clip for 40... A relay will select
for you, or you can walk out and manually move the clips...

Go to the library there and get an interloan copy of Moxon's "HF
Antennas for all locations"... He has an excellent discussion of
loaded radials/counterpoises, etc... With his help you should see how
to make your antenna play... Any problems drop me an email through
and I will give you a fax number where you can shoot me
a diagram of your antenna and we can discuss it...

cheers ... denny / k8do

Denny wrote:

Hmmm, I can't make much of this diagram and the group has decided to
use passive aggressive behavior to "teach you"... You are asking for
help and getting didactics thrown in your face... Shame on them!

I doubt you can feed this antenna on 40 with coax... You will need to
either put a tuner at the feed point, or resort to open wire line and
a tuner at the radio...

My first impulse would be to turn this into a top loaded vertical with
an elevated counterpoise, the vertical self resonant on 40 and loaded
for 80...
Feed it at the bottom of the vertical section with the braid going to
the 17 foot horizontals (some tuning needed)... Use a loading coil
about half way up the vertical for 80 and short it out for 40... The
counterpoise (the 17 foot horizontals as best I can make out) would be
coil loaded at at the center connection of the 2 "radials" so they
are resonant on 80 with a shroting clip for 40... A relay will select
for you, or you can walk out and manually move the clips...

Go to the library there and get an interloan copy of Moxon's "HF
Antennas for all locations"... He has an excellent discussion of
loaded radials/counterpoises, etc... With his help you should see how
to make your antenna play... Any problems drop me an email through
and I will give you a fax number where you can shoot me
a diagram of your antenna and we can discuss it...

cheers ... denny / k8do


the physical antenna stays as is and works extremely well on 75.
What has me asking the questions is that I was told a 1/4 wave piece of coax
would get it to work on 40.

Maybe I need to run it thru some modeling software and see what actually
does need to happen to get 40 to work.


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

wrote:
the physical antenna stays as is and works extremely well on 75.
What has me asking the questions is that I was told a 1/4 wave piece of coax
would get it to work on 40.

Was that a parallel stub by any chance?
--
73, Cecil http://www.w5dxp.com