Must 300 ohm feedline be kept completely clear of contact w anything?
 

How critical is it to mount twinlead feedline (300 ohm TV ribbon type) so that it doesn't come into contact with anything? Obviously, with bare ladder-line type feeders it's absolutely essential (to prevent shorting!), but what about the insulated type of 300 ohm twinlead --- can you have it run next to wood, say, or even metal as long as there is no conduction between the inner conductors and anything metal that might cut through the insulation? I've heard that Home Depot has a newer type of 300 ohm twinlead that is oval, almost circular in cross section because it is foam filled and thus if it were to be lying on top of something, or next to something, there would be an extra 'cushion' between the current carrying wires inside and anything outside the wires so would that mean it would be OK to run this type feedline up against the wall of a house? Or would it also be absolutely essential to somehow mount it so that it is free and clear of anything?
73 from Canada
VA7FAB

Must 300 ohm feedline be kept completely clear of contact w anything?
 

Thomas anonymous wrote in
:


How critical is it to mount twinlead feedline (300 ohm TV ribbon type)
so that it doesn't come into contact with anything? Obviously, with

You don't want unbalanced coupling between one side of the feedline and
something else. That would unbalance the feedline. You MAY not want the
impedance bump that comes with equal coupling on both sides, though that
might be tolerable if you're tuning the line anyway. And you don't want
lossy coupling of any kind.

--
Dave Oldridge+
ICQ 1800667
VA7CZ

Must 300 ohm feedline be kept completely clear of contact w anything?
 

Dave Oldridge wrote:
Thomas anonymous wrote in
:


How critical is it to mount twinlead feedline (300 ohm TV ribbon type)
so that it doesn't come into contact with anything? Obviously, with


You don't want unbalanced coupling between one side of the feedline and
something else. That would unbalance the feedline. You MAY not want the
impedance bump that comes with equal coupling on both sides, though that
might be tolerable if you're tuning the line anyway. And you don't want
lossy coupling of any kind.

In general there is a magnetic/electric field between the conductors in
twinlead feed line. If this field is in contact with a metal or other
conductive surface it will distort the impedance of the wire. Twinlead
with the round surfaces between the conductors will keep the field out
of the way of any surface the twinlead is resting on. This helps to
maintain the constant 300 ohm impedance of the twinlead. That is why tv
leadin uses standoffs to restrain the wire from the antenna to the
receiver/transmitter.
There is nothing wrong with using twinlead. It's cheap, light weight and
easy to install.

Dave WD9BDZ

Must 300 ohm feedline be kept completely clear of contact w anything?
 

David G. Nagel wrote:
. . .
There is nothing wrong with using twinlead. It's cheap, light weight and
easy to install.

When wet, it can become much lossier than RG-58. See
http://www.eznec.com/Amateur/Article...Feed_Lines.pdf.

Roy Lewallen, W7EL

Must 300 ohm feedline be kept completely clear of contact w anything?
 

Roy Lewallen wrote:
David G. Nagel wrote:
. . .

There is nothing wrong with using twinlead. It's cheap, light weight
and easy to install.


When wet, it can become much lossier than RG-58. See
http://www.eznec.com/Amateur/Article...Feed_Lines.pdf.

Roy Lewallen, W7EL


Roy;

True. Remember I said I don't use the stuff so I forgot that little
tidbit. I also try not to use RG-58 for anything other than a mobile
antenna and I am trying to get away from that also.


Dave WD9BDZ

Well, that's what you call a definitive answer, hi. I live on the Canadian Wet Coast, near Vancouver --- you could say moisture is sometimes a factor around here ...
:)

Thx and 73 from Canada

Must 300 ohm feedline be kept completely clear of contact w anything?
 

"Thomas anonymous" wrote in
message . ..

Well, that's what you call a definitive answer, hi. I live on the
Canadian Wet Coast, near Vancouver --- you could say moisture is
sometimes a factor around here ...
:)

Thx and 73 from Canada


--
Thomas anonymous

Thomas,

Belden made a product called "Shielded Perm-ohm" 300 ohm shielded twin
lead. It apparently still exists, but not with that name. Go to
http://www.fairradio.com/wirean.htm and search for it. The
product is now called "City Color," according to that web page.

Some years ago I used it to pass perfect TV signals through a 15-foot
conduit
out of my attic . (An earlier attempt with regular twinlead was a
disaster.) I
believe the shielded twin lead has slightly less loss than coax.

Must 300 ohm feedline be kept completely clear of contact w anything?
 

In general there is a magnetic/electric field between the conductors in
twinlead feed line.
Dave WD9BDZ

Dave,

Just to clarify, is the 'magnetic/electric field' to which you refer
the same as saying 'electromagnetic field'? I am just trying to
clarify whether it is just how you expressed it or if you are making a
distinctive difference.

Thanks
Buck

--
73 for now
Buck
N4PGW

Must 300 ohm feedline be kept completely clear of contact w anything?
 

Buck wrote:
In general there is a magnetic/electric field between the conductors in
twinlead feed line.
Dave WD9BDZ


Dave,

Just to clarify, is the 'magnetic/electric field' to which you refer
the same as saying 'electromagnetic field'? I am just trying to
clarify whether it is just how you expressed it or if you are making a
distinctive difference.

Thanks
Buck

Buck

You are correct in your interpretation of my stumbling words.
The main thing is close proximity to another conductive material will
affect the transmission of energy on twin lead.
I hope that I have helped you. Amateur radio is a fascinating field of
endeavor. Your education does not ever stop.

Dave

Must 300 ohm feedline be kept completely clear of contact w anything?
 

On Sun, 16 Apr 2006 21:38:18 -0500, "David G. Nagel"
wrote:

Buck wrote:
In general there is a magnetic/electric field between the conductors in
twinlead feed line.
Dave WD9BDZ


Dave,

Just to clarify, is the 'magnetic/electric field' to which you refer
the same as saying 'electromagnetic field'? I am just trying to
clarify whether it is just how you expressed it or if you are making a
distinctive difference.

Thanks
Buck

Buck

You are correct in your interpretation of my stumbling words.
The main thing is close proximity to another conductive material will
affect the transmission of energy on twin lead.
I hope that I have helped you. Amateur radio is a fascinating field of
endeavor. Your education does not ever stop.

Dave

Thanks, and yes, the education continues.


--
73 for now
Buck
N4PGW

Must 300 ohm feedline be kept completely clear of contact w anything?
 

On Sun, 16 Apr 2006 21:55:45 -0400, Buck wrote:

I'm using a random length loop, probably about 200 feet fed with
450 ohm ladder line. The ladder line hangs about 18 inches from
my tower and I get good reports from all over. My advice, if you
have and idea, try it, if it works great. If not, back to the drawing
board

Jim WU7G


In general there is a magnetic/electric field between the conductors in
twinlead feed line.
Dave WD9BDZ

Dave,

Just to clarify, is the 'magnetic/electric field' to which you refer
the same as saying 'electromagnetic field'? I am just trying to
clarify whether it is just how you expressed it or if you are making a
distinctive difference.

Thanks
Buck

I agree, however in my current situation I really can't keep going up and down to the roof because I'm renting a suite in the basement and I'm already pushing things with all the feedlines and isopole VHF antennas already up there. The landlord is a really decent guy and has been totally reasonable --- so far. If I keep wanting to do more and more he's going to feel obligated to say no sooner or later.

There's another issue, though, a more important one --- I moved the feedpoint to the SW corner of the roof (pic at http://members.shaw.ca/cyberhun/ham.htm) and used coax for the feedline. But when I was up there, I was so freaked out by the height that I forgot to waterproof the termination --- but I'm not going back up there til I can figure a way to do it more securely. It doesn't matter how many guys are holding the bottom of the ladder. If it starts to slide one way or the other, there's too much leverage for anyone to stop it. Water damage be damned, a fall from that height could be deadly.

Must 300 ohm feedline be kept completely clear of contact w anything?
 

A much simpler antenna is possible at your QTH, based on iteration #4.

Feed your 110 feet long antenna in the center with 600 ohms open wire
tuned feeders [or 450 ohm ladder line ... however, 450 ohm line is more
lossy in bad weather]. You can twist the ladder line at 1 turn every two
feet, or the open wire line 1 turn in 4 feet, to accommodate any
imbalances cause by nearby objects.

If your MFJ 949E does not have a balanced output then use a short
section of coax, 2 to 3 feet is PLENTY, to connect between the MFJ949E
and a MFJ 912 Balun, and then to the tuned feeders.

This is a very low loss setup that should work the following bands with
good to very good antenna efficiency: 80/75, 60, 40, 30, 20, 17, 15, 12
and 10 meters. [My setup, very similar, has a maximum of 1.4 dB loss
across this set of bands using 150 feet of feedline ... 1/4 of an S unit.]

NOTE: The G5RV is in reality a single band antenna!! It is designed for
20 meters where it performs admirably! It can be tuned to work other
bands. However the losses in the coax section can become moderately
high. The CF Doublet with tuned feeders has much lower feedline losses.

Thomas anonymous wrote:
I agree, however in my current situation I really can't keep going up
and down to the roof because I'm renting a suite in the basement and
I'm already pushing things with all the feedlines and isopole VHF
antennas already up there. The landlord is a really decent guy and has
been totally reasonable --- so far. If I keep wanting to do more and
more he's going to feel obligated to say no sooner or later.

There's another issue, though, a more important one --- I moved the
feedpoint to the SW corner of the roof (pic at
http://members.shaw.ca/cyberhun/ham.htm) and used coax for the
feedline. But when I was up there, I was so freaked out by the height
that I forgot to waterproof the termination --- but I'm not going back
up there til I can figure a way to do it more securely. It doesn't
matter how many guys are holding the bottom of the ladder. If it
starts to slide one way or the other, there's too much leverage for
anyone to stop it. Water damage be damned, a fall from that height
could be deadly.