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Author Topic: Open wire feedline concerns  (Read 1753 times)
WI7C
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Posts: 1




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« on: May 06, 2000, 08:55:11 PM »

I have never used open-wire feedline before (because coax is so much more convenient) but I now want to make a multi-band wire antenna to be used with a tuner with my new FT-847 transceiver. Please, please forgive me for asking some really elementary questions about open-wire feedline.

Since coaxial cable has a center-conductor surrounded by a grounded shield, I can understand how the RF between the transmitter and radiator is 'contained' within the coax - so to speak. But with an open-wire feedline, there is no shielding - just two parallel wires. What prevents this feedline from radiating RF into the shack? Are there any special precautions that must be taken into consideration with respect to computers, pets, children, etc. that could possibly come into close proximity to the open-wire feedline that isn't necessary with coax?

Secondly, is there a preferred method for routing the open-wire feedline through the wall? I have aluminum siding on the house, so I assume the open-wire feedline would have to be kept away from the siding, right?

Any other hints, suggestions, resources, etc. concerning the usage of open-wire feedline would be greatly appreciated. Many thanks.

KC, WI7C
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K0SR
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Posts: 37




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« Reply #1 on: May 06, 2000, 11:28:49 PM »

Open wire feeders won't radiate if you meet two basic requirements.  One, the currents in each wire have to be balanced or equal, and secondly the wires can't be to far apart.  The second requirement is easy to meet,  300 or 450 ohm ladder line you can buy has a spacing that is a small fraction of a wavelength.  The first one can be a little tougher to meet.  Theoritically, if the currents flowing in each wire are equal and opposite, then the resulting magnetic fields will cancel each other and the line won't radiate (but will carry power to the antenna just fine).  This works the same whether you're transmitting or receiving.  In practice, the balance will never be perfect.  Anything that will "un-balance" the currents will upset the operation of the line.  This would include running the line close to metal objects like masts or towers, or bending the line through a tight radius. It will also depend on the antenna.  If you are intending to feed a dipole, and that dipole is nice and straight and both legs are the same length, then you should be in great shape.  If the antenna is unbalanced, this will un-balance the feedline as well.  You might have more trouble with RF in the shack than you would with coax, but now you have an idea of how to deal with it.  The big bonus here is the low line loss you'll have, especially on frequencies where your antenna may not be resonant. Coax doesn't like high SWR, open line is a lot more forgiving.  Oh, and I don't have any good ideas about getting it into the house.  I removed the glass from a "window well" window and replaced it with plexiglass, and just drilled holes through it that had the same spacing as the open wire line.  Good luck with your new antenna !

Steve K0SR
k0sr@uswest.net
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K5LKT
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Posts: 1




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« Reply #2 on: May 15, 2000, 06:13:27 PM »

1 the loss difference will let you hear more and work more. 2 it allows one ant to be easly used multi band.
3. if you use 66 ft or a 40 mtr ant you will get gain approaching a beam in 15 to 35 degree angles from parell on 20 , 15 and 10 as well as new bands. if you dont have a good ground you will need radials on the ground. try it you will like it. k5lkt@aol.com
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W6ASO
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Posts: 2




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« Reply #3 on: June 12, 2000, 01:05:26 AM »

Won't repeat what the others have written but generally agree with them.  One big advantage of the open wire feed line is that you don't have to worry about SWR and can use a tuner (which you will need anyway to match the transmitter to the feedline) and use the antenna for multibands.  Have used open wire feed lines for years with good success.  I use one of the "remote baluns" from Radio Works so I can run  short (15-20 feet) of high quality coax up or through the wall up into the attic or even outside under the eves and connect to the open wire line there.  Hope this helps... 73 and good luck  Chuck
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K0RS
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Posts: 717




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« Reply #4 on: June 20, 2000, 10:22:30 PM »

Before yagis first started to become popular after WW2, most hams fed some variety of wire antenna with open wire trnsmission line.  It was natural then as hams began experimenting with yagis to continue to feed them with the transmission line that they were most familiar with...open wire. I remember seeing schemes in old antenna books showing yagis fed with open wire via some kind of electro-mechanical contraption that would keep the open wire from shorting or becoming tangeld when the beam rotated.  One I remember used two copper "tracks" that encircled the tower.  One side of the the feed line connected to each track.  There were a couple of wipers that rubbed along the tracks that rotated with the beam, presumably maintaining some kind of electrical connection.  Looked very high maintenance.  Coax wasn't in general use then, and usually was only obtainable from military surplus.  Anybody  try feeding a yagi with open wire lately?  Seems like with modern baluns now, it would be a snap.  A long run of low loss open wire from the shack to the tower and just a short coax jumper to the antenna thru an appropriate balun to provide for easy rotation.  Of course one couldn't enjoy the frequency excursions of an open wire fed dipole, but the idea would be to keep transmission line losses low, not use the antenna on multiple bands.
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WB6BYU
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Posts: 13280




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« Reply #5 on: June 22, 2000, 11:59:51 AM »

Here are two approaches I have used:
1)  You can take a lot of liberties with the spacing and "parallel-ness"
of open wire line, particularly on the lower bands (as long as you are
not concerned with the exact impedance.)  In one case, I ran a pair
of wires from the antenna in through an attic vent and stretched them
between nails in the rafters.  Spacing was typically 6" to 18".  Over
the shack I brought the wires together and spliced them into a piece
of 300 ohm twinlead which dropped through a hole in the ceiling to the
rig.  I had no problems running 100 watts.  For a long run outdoors,
I nailed a couple pairs of insulators to fence rails, and stretched
100' or so of wire between them with no spacers.  Spacing was
roughly 8", and I didn't notice much change in SWR due to the wires
blowing around, even in windy weather.

2)  Another way to get through a wall or inside a house is to create
a balanced shielded cable using two lengths of coax cable (which
should be the same length.)   At each end, connect the center
conductors to the open wire line, and solder the shields together.
This makes a 100 ohm balanced, shielded line (or 150 ohms if you
use 75 ohm cable - it really doesn't matter) which can be run
through walls, buried, tied in knots, etc.  True, the loss in this section
will be higher than in the rest of the line, but it seems to be a good
trade-off.  I used RG-8 this way in a 1.5kW installation with no problems.

Good luck!

Dale WB6BYU
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AC4R
Member

Posts: 18




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« Reply #6 on: January 09, 2007, 09:34:53 AM »

I am running homebrew balanced line made of 14 AWG stranded copper wire using 7 in long pieces of pvc electrical conduit drilled one inch from each end for 6 inch spacing. I have the spacers roughly 3 ft apart to ensure the spacing is maintainted to the antenna. The feedline goes into a 176 ft dipole up 30 ft which I use primally on 20, 50, 80, and 160 meters . I use a Palstar AT1KM tuner which tunes flat on all bands and run 400 watts from a AL-811 amp on everything except 160.  No problems with the mic line picking up any RF . I have wood windows in my house and simply drilled two holes thru the window frame to feed the wire into the shack. After the good results I have had running open line on this all band dipole , I am out of the coax business for good except for connecting rigs and amps in the shack. I am sold on balanced line and its benefits far out weigh the problem of getting it inot the shack


AA4FH
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