Most “600 ohm” open-line is not actually 600 ohms. I remember going through all
the math of open line back in my Fields and Waves class in college. It needs
spacing from other metallic objects in order to maintain its characteristics. I
also remember learning directly from W1FB and W1ICP as to its benefits and how
to handle it. I’m truly a believer in open-wire line (not so much for ladder
line, although I use it). They affirmed that spacing is important in order not
just to maintain characteristics, but to prevent signal loss and especially
radiation. It needs to retain balance for that, something that my friend K9YC
will tell you is hard to do, especially with antennas the way they are.
I would space the lines as far apart as possible on those supports. Several
feet should do. If possible route them differently, since they will couple if
run in parallel for long lengths. Be sure to twist the open-wire along the way.
Parallel lines have not just “connections” to each other, but to ground and
anything else along the way as well. If you look at power lines (60 Hz), on
long distance runs they are transposed every so often so that each of the lines
has more or less equal ground exposure. Open-wire is only two wires, but the
twist will have a similar effect, equalizing exposure to other elements in the
vicinity. You also might take some hints from the telephone pole users and put
T-arms on your supports so that you can separate the open-wire runs
horizontally as well as vertically. This will allow you to place more runs on
the existing poles.
As K9YC likes to point out, everything is important; do your homework and make
sure it is done right.
73!
Jack, W6FB
> On Mar 16, 2019, at 3:43 PM, K9MA <k9ma@sdellington.us> wrote:
>
> I think Holger means crosstalk, rather than distortion. The critical issue
> will be whether the feedline crosstalk is greater than that from the
> antennas. There may be ways to estimate the feedline crosstalk, but I don’t
> know without doing some research. However, it might be worthwhile spacing the
> wires for less than 600 Ohms as that will reduce crosstalk for a given
> distance between feedlines. The tradeoff will be slightly more loss due to
> more spacers.
>
> 73,
> Scott K9MA
>
> ----------
>
> Scott Ellington
>
> --- via iPad
>
>> On Mar 16, 2019, at 11:48 AM, Jim Brown <k9yc@audiosystemsgroup.com> wrote:
>>
>>> On 3/15/2019 7:08 PM, Holger Hannemann wrote:
>>> Does anyone has experience with potential distortion when operating parallel
>>> running open feeder lines?
>>
>> Distortion requires a non-linear element in the signal path. Transmission
>> lines are inherently linear. There IS the possibility of distortion in a
>> ferrite core device used to transform impedance at either end of the line.
>> Air core transformers would not have this problem.
>>
>> I think you're right to be concerned about crosstalk between multiple lines
>> running parallel to each other for these long distances, which will be in
>> addition to the crosstalk between antennas. In any event, you will need
>> serious filtering for amplifier harmonics just as with any other
>> multi-transmitter station.
>>
>> 73, Jim K9YC
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