Topband: Beverage Question
jeremy at maris.plus.com
Sun Sep 21 15:34:20 EDT 2014
The way I looked at it was that the equivalent diameter would be a bit less than twice the diameter of the wires plus the spacing. According to the Mil Spec for WD-1A/TT is 0.84mm diameter per conductor spaced 0.84mm (or perhaps 0.72mm if just twice sheath diameter.
That gives around 2.5mm. The effective diameter will be less than this but it helps that the two conductors are twisted together. Can't do the maths for that but I reckon around 2mm diameter or 12 AWG.
The standard formula (see eg http://www.w0btu.com) gives around 470 ohms with 50mm spacing. My quick tests with a 40ft sample of line (changing termination resistance and measuring SWR using an antscope via a 75/450 ohm transformer) show that to be about right for 450 ohm line.
On 20 Sep 2014, at 16:14, Carl wrote:
> The way I understand is that you will be using the WD-1A as individual lines with each having its wires connected in parallel.
> Id start by calculating the resultant wire gauge such as say two #24 in parallel = a #21; I dont know what the fudge factor would be due to wire and jacket insulations in determinining the 6" spaced line impedance. From what Ive read that is too much spacing for best performance due to imbalancing plus not being a bit twisted to cancel the imbalance as with ladderline and conventional WD-1A useage.
> If the reason for change is poor strength and reliability then why not go with parallel galvanized or aluminum fence wire and create some twisting arrangement every 100' so.
> In the more conventional installation I calculated about 93 Ohms for the WD-1A and use a 1:1 ratio for 75 Ohm coax; cant notice any deterioation between that and using 50 Ohm coax with its own transformer. Both work fine to 30M.
>> I want to replace the 450 ohm ladder line for my two DXE 720 ft long
>> bi-directional beverages with two lengths of WD-1/TT field telephone wire.
>> The wires will be secured only at the ends, but will be suported along their
>> length by ceramic insulators mounted on 4x4 posts spaced every 60 ft. The
>> spacing between the parallel wires will be about six inches. Here are my
>> quesetions: First, does anybody know what the likely impedance is of this
>> configuration, or if not, how to measure it? Second, if I keep the DxE
>> matching trnsformers used with the 450 ohm ladder line, will beverage
>> performance be degraded noticeably with the new wures if, say, the impedance
>> of the new line turns out to be 600 ohms or so? In other words, as a
>> practical matter how important to performance (e.g., F/B ratio and
>> low-noise) is it to make sure impedances are matched.
>> Finally, a mechanical question. I want to have a pulley at one end of the
>> parallel wires to equalize the tension in the wires. I've not been able to
>> find a good pulley with a five or six inch diameter, except for nylon
>> pulleys used for clotheslines. Do you think one of these could be used.
>> There will be no motion of the pulley, since it is only used to equalize
>> tension. The field telephone wire is rated at 200 lbs maximum tension, and
>> I'm guessing the actual tension will be about half that.
>> Jim W8ZR
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