On Jun 20, 8:04*am, "Pete Bertini" wrote:
Just curious if this has ever been done? *I'm thinking about
putting up a 90 foot dipole feed with homemade open wire
line. *I'd like to bring it into the shack using paralled runs
of LMR-400 cable, since the final 25 feet is via 3" electrical
conduit that also has rotor and other cables.

I believe the parallel cables with give me a 100 ohm impedance.
The open wire will be using #10 with homemade spreaders,
I'm going to try for 600 ohms at the feedpoint. I was wondering
if tapering the spacing on the feedline would give me a smoother
impedance where the open wire, arrestor, and twin coax arrange-
meet?

Pete k1zjh

Based on my personal experience trying to replace the inside shack
portion of 600 ohm open feed line with two parallel pieces of RG-213
about 8 ft. long. The coax sections will effectively kill the use of
your dipole on any of the higher frequencies. As I remember, the
antenna was usable up through 10 meters , but after putting in the
coax, I couldn't tune the system above 20 meters.

My antenna is a full sized 160 meter horizontal loop in the form of a
trapazoid, almost a square, up at 35 ft. Fed in one corner with 600
ohm open wire,125 ft. long. The feedline comes into the house (mfg
home with aluminum siding) using two Birnbach feed through insulators.
Then to a DPDT antenna switch to ground the system during thunder
storms, and finally about 6 ft of 600 ohm feed line to the balun in
the Dentron MT-3000 tuner.

I think the problem with using the double coax is the very large
capacitance it adds to the feed line, effectively becoming a low pass
filter. Could be mistaken about the cause, but not the symptoms. I
have actually used the system to make some local 6 meter contacts.

Try it anyway, but be prepared to go all the way to the shack with
your 600 ohm feed line.

Paul, KD7HB

" wrote in news:6d1a078a-e5f4-
:

....
I think the problem with using the double coax is the very large
capacitance it adds to the feed line, effectively becoming a low pass
filter. Could be mistaken about the cause, but not the symptoms. I
have actually used the system to make some local 6 meter contacts.

The coax is, and always is a transmission line, and at the length you
described cannot be approximated well as a shunt capacitance.

What you have is a cascade of two line sections, one of say 600 ohms,
then one of 100 ohms, and each is probably operating with standing waves,
so there is impedance transformation.

To illustrate, lets say your feedpoint at 3.6MHz with a certain loop
antenna was 100+j0, and you had say 30m (100') of 600 ohm open wire, the
impedance looking into that would be around 240-j675. If you feed that
with say 6m (20') of LMR400 twin, then input Z would be around 60+j260
and loss would be about 40%.

The synthesised shielded pair is relatively lossy, and low Zo. Most
people use this configuration thinking that the shielding prevents
external fields from common mode current, but they are quite wrong. See
http://www.vk1od.net/transmissionline/stcm/index.htm .

Though the traditional approach has been to use a 4:1 voltage balun at
the rig to feed these things, there is good argument to use a 1:1
Guanella balun (current balun), and it can be located outside the shack
and inboard shield effectively grounded to deal with common mode current.
You still need to minimise the length of coax operated at high VSWR, and
it would not be necessarily absurd to think about low loss coax.

Approximating coax as a shunt capacitance might be reasonably accurate
for some applications at audio frequencies, but it is probably not for
most RF applications.

Owen

"Owen Duffy" wrote in message
...
" wrote in news:6d1a078a-e5f4-
:

...
I think the problem with using the double coax is the very large
capacitance it adds to the feed line, effectively becoming a low pass
filter. Could be mistaken about the cause, but not the symptoms. I
have actually used the system to make some local 6 meter contacts.

The coax is, and always is a transmission line, and at the length you
described cannot be approximated well as a shunt capacitance.

What you have is a cascade of two line sections, one of say 600 ohms,
then one of 100 ohms, and each is probably operating with standing waves,
so there is impedance transformation.

To illustrate, lets say your feedpoint at 3.6MHz with a certain loop
antenna was 100+j0, and you had say 30m (100') of 600 ohm open wire, the
impedance looking into that would be around 240-j675. If you feed that
with say 6m (20') of LMR400 twin, then input Z would be around 60+j260
and loss would be about 40%.

The synthesised shielded pair is relatively lossy, and low Zo. Most
people use this configuration thinking that the shielding prevents
external fields from common mode current, but they are quite wrong. See
http://www.vk1od.net/transmissionline/stcm/index.htm .

Though the traditional approach has been to use a 4:1 voltage balun at
the rig to feed these things, there is good argument to use a 1:1
Guanella balun (current balun), and it can be located outside the shack
and inboard shield effectively grounded to deal with common mode current.
You still need to minimise the length of coax operated at high VSWR, and
it would not be necessarily absurd to think about low loss coax.

Approximating coax as a shunt capacitance might be reasonably accurate
for some applications at audio frequencies, but it is probably not for
most RF applications.

Owen

Owen, my thinking was to make a transmission line with a characteristic
impedance that gradually changed from 600 ohms down to as close to
200 ohms as practical, just to avoid the impedance bump at the junction
of the open wire and shielded balance line. I've found a few
references that state the taper should follow a log response; and also
if the line needs to be a wavelength or longer pretty much dashes that idea.
It would be a lot easier to construct a line with charateristic impedance of
400 to 600 ohms, and deal with narrow spacing issue for only a relatively
short portion of the entire span.

I didn't expect the shielded cable to be immune to common reradiation
problems, but that issue could be dealt with separately if it was a
problem. Pete

"Owen Duffy" wrote in message
...
" wrote in news:6d1a078a-e5f4-
:

...
Though the traditional approach has been to use a 4:1 voltage balun at
the rig to feed these things, there is good argument to use a 1:1
Guanella balun (current balun), and it can be located outside the shack
and inboard shield effectively grounded to deal with common mode current.
You still need to minimise the length of coax operated at high VSWR, and
it would not be necessarily absurd to think about low loss coax.

Owen

Okay, then it might make sense to do this: Plan II

instead of using the balun is in the tuner, there is no reason
why I couldn't mount another 4:1 balun outdoors, and use a short run
of low loss coax back through the conduit into the operating room.

That would be about 15 feet of LMR-400 cable. I could go to 1/2 heliax
but would there be any benefit??

So, I would have balanced open-wire feedline from the dipole to
the 4:1 balun followed by a 1:1 common-mode RF choke with
the requiste RF ground for decoupling,
followed by the short run of low loss coax going back to a
unbalanced ant. port on the tuner. I'd still have coax in the mix,
but the 4:1 balun should hopefully tame the high impedance
excursions across the antenna operating range to
reduce the coax's losses at those points?

Otherwise it is going to be hard to get balanced line into the shack,
unless I pull it through the PVC conduit with the other coax cables.

Pete

"Pete Bertini" wrote in newsiwTn.64499
:

So, I would have balanced open-wire feedline from the dipole to
the 4:1 balun followed by a 1:1 common-mode RF choke with


See my article entitled "Is a 4:1 balun a good choice for use with an ATU
on HF?" at http://vk1od.net/blog/?p=987 .

It is difficult to answer the question about the LDF without knowing the
impedances involved. I used TLLC ( http://www.vk1od.net/calc/tl/tllc.php )
to work the examples I gave, you could do the same if you know the
feedpoint impedance etc.

If you want to solve the cases for an arbitrary two wire line, try
http://www.vk1od.net/calc/tl/twllc.htm .

Owen

"Pete Bertini" wrote in
news
....
So, I would have balanced open-wire feedline from the dipole to
the 4:1 balun followed by a 1:1 common-mode RF choke with

I should have mentioned that 4:1 baluns integrated into ATUs are most often
voltage baluns.

When operated near a voltage maximum with high standing waves, they can be
very lossy. Sometimes it is claimed that they 'tame' difficult antennas
better than current baluns, which is often due to their internal loss. The
problem as such is not the balun, it is the extreme load presented by a
very poor antenna, and the lossy balun is a poor (grossly inefficient)
circumvention.

So, when a 4:1 voltage balun allows a match where a 1:1 current balun
doesn't, the problem is probably the antenna, not the balun.

Owen

That's pretty much what I've been reading on various websites.
At least some of the mystery is being cleared up A fan dipole
is starting to look better; at least I can control the feedpoint
impedance to within reason and keep a decent pattern on
the bands.

Pete

"Owen Duffy" wrote in message
...
"Pete Bertini" wrote in
news
...
So, I would have balanced open-wire feedline from the dipole to
the 4:1 balun followed by a 1:1 common-mode RF choke with

I should have mentioned that 4:1 baluns integrated into ATUs are most
often
voltage baluns.

When operated near a voltage maximum with high standing waves, they can be
very lossy. Sometimes it is claimed that they 'tame' difficult antennas
better than current baluns, which is often due to their internal loss. The
problem as such is not the balun, it is the extreme load presented by a
very poor antenna, and the lossy balun is a poor (grossly inefficient)
circumvention.

So, when a 4:1 voltage balun allows a match where a 1:1 current balun
doesn't, the problem is probably the antenna, not the balun.

Owen

On Jun 20, 1:33*pm, Owen Duffy wrote:
" wrote in news:6d1a078a-e5f4-
:

...

I think the problem with using the double coax is the very large
capacitance it adds to the feed line, effectively becoming a low pass
filter. Could be mistaken about the cause, but not the symptoms. I
have actually used the system to make some local 6 meter contacts.

The coax is, and always is a transmission line, and at the length you
described cannot be approximated well as a shunt capacitance.

What you have is a cascade of two line sections, one of say 600 ohms,
then one of 100 ohms, and each is probably operating with standing waves,
so there is impedance transformation.

To illustrate, lets say your feedpoint at 3.6MHz with a certain loop
antenna was 100+j0, and you had say 30m (100') of 600 ohm open wire, the
impedance looking into that would be around 240-j675. If you feed that
with say 6m (20') of LMR400 twin, then input Z would be around 60+j260
and loss would be about 40%.

The synthesised shielded pair is relatively lossy, and low Zo. Most
people use this configuration thinking that the shielding prevents
external fields from common mode current, but they are quite wrong. Seehttp://www.vk1od.net/transmissionline/stcm/index.htm.

Though the traditional approach has been to use a 4:1 voltage balun at
the rig to feed these things, there is good argument to use a 1:1
Guanella balun (current balun), and it can be located outside the shack
and inboard shield effectively grounded to deal with common mode current.
You still need to minimise the length of coax operated at high VSWR, and
it would not be necessarily absurd to think about low loss coax.

Approximating coax as a shunt capacitance might be reasonably accurate
for some applications at audio frequencies, but it is probably not for
most RF applications.

Owen

Thanks, Owen. I made up my test after reading the june 2008 QST
story.

Paul, KD7HB

Looks like most of the information I needed was already
written up on the DX Engineering webpages. I'll have to
go do a bit more reading before committing to anything.

Thanks for all the comments so far....

Pete