Easy way to make open wire transmission line

[VK4WDM]

There have been quite a number of posts in several threads extolling the virtues of 450 ohm open wire feeder line - low loss, easy to tune across a range of frequencies etc. It can be brought commercially but the best way is to make it yourself.

When I started out in AR we used pieces of wooden dowel boiled in wax, drilled and kept in place with twisted wire. You can use plastic pipe etc but it is still a fiddly business boring all those holes.

Several suppliers (including TTS, TET Emtron and Andrews) are now selling a plastic spreader that makes it very easy, the wire is just clipped in. I made 25m of feeder tonight in about half an hour.

So get rid of that lossy coax and use open wire line preferably coupled to the rig via a balanced coupler (easy to make) or (purists cover your eyes) a 4:1 balun and a short length of coax.

73

Wayne VK4WDM

[VK3RX]

How far apart do they make the wires, Wayne? I can't see that mentioned on the TTS or TET websites.

My doublet with homebrew spreaders (yes, tediously made of irrigation tube ) the wires are 85mm apart or about 3.5".

[VK4WDM]

Hi Damien

The wires are 50mm apart. To my mind that is a bit too close but according to the suppliers it gives a nominal 450 ohms.

73

Wayne VK4WDM

[VK4BEG]

Hi thought I add my construction secrets for open wire feed. Purchase black plastic gutter guard (cheap as chips), most often you can cut in down the center so you get double the length and then simply weave the wire through the holes, you can cable tie lengths together and taper it near the feed point if you wish. I have used this technique and is quite simple and cheap and will last as the gutter guard is UV.
Have fun

[VK5AJL]

The characteristic impedance depends not only on separation but also wire thickness and other factors. For air separation, which is what we are talking about, one approximation is Zo = 276 log ( d / r ) where d is distance between them and r is wire radius. Alternate 120 ln (d / r)

Given required impedance and separation - radius of wire r = d / ( 10 ^ (Zo/276))

Given required impedance and wire radius - distance of separation d = r x 10 ^ (Zo/276)

That is if my mathematics is any good this time of morning. Posted in the interests of information only.

I use a tuner because it has less insertion loss than any impedance matching balun I could find. The tuner does all the impedance matching needed. Since I use a tuner the impedance of the line is a little interest to me so long as its balanced.

[VK3AUU]

If you are going to use a tuner, you don't really need to be very pernickety about what the impedance of the line is. I used to have a half wave on eighty metres fed in the centre with 66 feet of 300 ohm ladder line. That worked OK on all the pre WARC bands with a tuner because there was a voltage point down at the shack end

David
VK3AUU

[VK6XT]

I have used these spacers for a while...wondering who actually makes them? Have become too expensive at some outlets. I find that Altronics " extra heavy duty" 32x0.2 insulated wire is a good fit in the slots ( solid wire extremely difficult to use). Using stranded and insulated wire is good for long life and freedom from the possibility of short circuits. The calculated impedance is 535 Ohms using the 32x0.2 .

[VK4BXI]

There was a Ham in South Africa making them. Worked out at about 25 Rands a packet and very cheap for us. I still have a few packets of them. I will find out if they are still available if there is interest.

Bob

[VK4TI]

""VK4Bxi::There was a Ham in South Africa making them. Worked out at about 25 Rands a packet and very cheap for us. I still have a few packets of them. I will find out if they are still available if there is interest.

Bob ""

25 rand is about $3.00 ? or was the convesion wrong ?

[VK5PJ]

25 rand is about $3.00 ? or was the convesion wrong ?

depends on how many is in a packet?

[VK4BZ]

As a previous poster said, I also use watering system risers, cut to 45mm in length. With mine, I put a thin zip tie (100mm) through the middle of the tube to secure the wires at each end, pull it tight and hey presto. Quick and easy to make and works a treat.

[VK4BXI]

The feeder spacers made locally in ZS are made by ZS6REX but distributed by ZS6BFI of the Highveld branch(ZS6HVB) I have asked for a link and pricing.
Bob (ZS6BXI) VK4BXI