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Author Topic: Feeding HF Tribander with open/ladder line  (Read 3985 times)
VK5DO
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« on: September 03, 2011, 12:03:31 AM »

I've got a 10, 15 & 20m tribander that was originally sold to be directly coax fed via a 1:1 balun at the middle of the DE.   Suppose I want to feed this thing with open/ladder wire...... 

Ignoring the mechanics of keeping my feed wire clear of other metal for a moment, what effects with regards to tuning/operation of the Tribander am I likely to encounter with an open wire feed?  I'm assuming the antenna lengths/tuning will be quite different to a coax & balun feed and will the traps work the same? Should I expect any benefits using open wire feed other than just a lower loss feeder?  More bandwidth maybe??

Is it just a dumb idea?

Thoughts?

Dene
VK5DO
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K2DC
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« Reply #1 on: September 03, 2011, 03:22:53 AM »

Dene,

   I wouldn't expect any benefits other than reduced feed line loss.  There should be no change in element lengths and the traps will function just as they do now.  But you would need to replace the 1:1 balun at the feed point with a 6:1 for 300 ohm line or a 9:1 for 450 ohm line.  And then there's the complexity of getting the feed line into the shack with perhaps a transition to coax and another balun. 

    Unless you have an extremely long coax run to the antenna, you're only looking at perhaps a couple of dB of coax loss on the upper bands (assuming the antenna is well matched), certainly less than an S-unit.  If you need two 9:1 baluns, the losses from them might be the same as you're seeing with coax now.  All in all, it doesn't seem worth the effort to me.

73,

Don, K2DC
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W8JI
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« Reply #2 on: September 03, 2011, 03:58:22 AM »

I've got a 10, 15 & 20m tribander that was originally sold to be directly coax fed via a 1:1 balun at the middle of the DE.   Suppose I want to feed this thing with open/ladder wire...... 

Ignoring the mechanics of keeping my feed wire clear of other metal for a moment, what effects with regards to tuning/operation of the Tribander am I likely to encounter with an open wire feed?  I'm assuming the antenna lengths/tuning will be quite different to a coax & balun feed and will the traps work the same? Should I expect any benefits using open wire feed other than just a lower loss feeder?  More bandwidth maybe??

Is it just a dumb idea?

Thoughts?

Dene
VK5DO


Why would you think a mismatched open wire line has less loss than a matched coaxial line of good quality?

Did you actually look at the numbers, or are you (like most people do) just assuming open wire line always has very low loss compared to coax?

If I have 75 feet of open wire line feeding an 80 meter half wave dipole, it can have more loss on 80 than the same dipole fed with fairly small coax. You have to be careful with assumptions.  :-)


73 Tom

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WB6BYU
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« Reply #3 on: September 03, 2011, 06:47:32 AM »

Using VK1OD's handy calculator here:

http://www.vk1od.net/calc/tl/tllc.php

The efficiency for 100' of RG-213 on 15m is 81%.  The same length of Wireman 554 window line gives 84%
with a 50 ohm load.  Some of the commercial open wire lines are actually a bit worse - to minimize the
copper losses at low impedances you need to use larger wires.  So unless you raise the feedpoint impedance
of the yagi, any improvement probably won't be worth the effort. 

You can use a balanced transformer to step the antenna impedance up to something closer to that of
the feedline, but it gets tricky.  Many commercial 4 : 1 baluns that are designed to cover the whole
HF band don't give reliable transformation ratios on the higher bands:

http://www.cebik.com/content/a10/trans/41balun.html

Presuming you could get it to work properly, even a 4 : 1 step-up would cut the feedline losses about
in half.  (Higher impedances are more difficult to work reliably in broadband transmission-line transformers.)


But to your original question, you wouldn't need to change the antenna dimensions at all.


With regards to tuning in the shack, the longer the feedline and the higher the SWR, the narrower
the operating bandwidth before you have to readjust the tuner.


If you REALLY need to do this over a long distance, I'd suggest a 4 : 1 step-up transformer at the antenna
and a 4-wire 200 ohm transmission line using relatively large diameter wire.  The feedline descriptions and
equations are in LaPorte's Radio Antenna Engineering.  He also gives an example of a log taper
matching section to increase the impedance to 600 ohms.  All this was standard procedure back in the
days of the big antenna installations at Riverhead and Rocky Point, where feedlines might be close to
1 km long.
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W5CPT
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« Reply #4 on: September 03, 2011, 10:00:21 AM »

If you had a long run from the shack to the tower you might consider this:

http://www.dxengineering.com/Parts.asp?ID=3585&PLID=313&SecID=158&DeptID={7C0A8FE1-F72C-4346-916E-8AA93CD2A66B}&PartNo=DXE-BAL450-H10-B-P

You can run coax from the rig to it and then coax up the tower.

But as far as feeding the antenna itself with ladder or open line, it is more trouble than it is worth.

Clint - W5CPT -
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N2EY
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« Reply #5 on: September 03, 2011, 10:54:56 AM »

The only way I could see this being worth doing is if the whole system were carefully engineered so that the line/matcher losses were lower than with coax. That's a big order!

Some things to consider:

1) The typical trap tribander has a low feedpoint Z on all bands. This means a considerable SWR with any practical parallel line using wire.

2) The low loss figures cited for "ladder line" are usually for real open-wire line made from large copper conductors spaced a few inches by low-loss spacers every few feet. The "Twin-Lead-with-holes" commonly sold today has higher losses, particularly when wet.

3) The mechanical troubles of feeding a rotatable array with parallel line are considerable. The Ancient Ones devised many ingenious solutions, but all were a lot more complex and troublesome than a simple loop of good coax.

4) I'm a big fan of ladder line, but it's not magic.

--

The big question is: what benefits do you think would occur?

73 de Jim, N2EY
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W4VR
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« Reply #6 on: September 03, 2011, 12:46:11 PM »

A simple answer is "no benefits."  Not worth messing with.
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K0ZN
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Posts: 1774




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« Reply #7 on: September 03, 2011, 06:54:12 PM »

99% chance is that nothing will be "better" unless you have a run of a couple of hundred meters of transmission line, then you may avoid a little loss.

Without using proper ratio baluns on both ends of the ladderline, you would have to use an antenna Tuner and SWR bandwidth would likely be much LESS.

The "cost and hassle"  vs.  benefit ratio is very high.

Bottomline: use a very good grade of coax and then go operate and have fun.

73
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K1DA
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« Reply #8 on: September 07, 2011, 06:42:55 PM »

  One of the benefits of parallel line is the multi band capability.  With a trapped antenna you don't need that.  Sure, you might get a 20/15/10 tribander to accept power on 17 or 12 with parallel line  but it's not likely to work  as intended on those bands. .   One thing about parallel line, though, it is not a fussy about placement carrying  HF (as opposed to UHF TV for example) as you might be led to believe.   After running some ladder line to a dipole in through an open window, I relocated it so that it ran over a metal gutter, along the ground, and into the house through a hole which also contains a run of RG 213.  The difference in tuning was very slight.  Field strength (on one of my BEST antenna measurment devices, a microampmeter and diode connected to a whip) was unchanged. 
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W8JI
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« Reply #9 on: September 08, 2011, 04:46:03 AM »

We have to be careful assuming, or telling others, that one situation where poor installation techniques caused no apparent problems means poor installation techniques will not cause other people problems.

We also have to be careful to not dismiss how things like feedlines work just because we might not have noticed how they work in one particular case.

I've seen TV twinlead taped to tower legs that seemed to be OK, and other cases where it just killed the signal when laying on damp wood. That doesn't mean, based on a case where someone got lucky, the way to properly install twinlead is overrated. :-)


  One of the benefits of parallel line is the multi band capability.  With a trapped antenna you don't need that.  Sure, you might get a 20/15/10 tribander to accept power on 17 or 12 with parallel line  but it's not likely to work  as intended on those bands. .   One thing about parallel line, though, it is not a fussy about placement carrying  HF (as opposed to UHF TV for example) as you might be led to believe.   After running some ladder line to a dipole in through an open window, I relocated it so that it ran over a metal gutter, along the ground, and into the house through a hole which also contains a run of RG 213.  The difference in tuning was very slight.  Field strength (on one of my BEST antenna measurment devices, a microampmeter and diode connected to a whip) was unchanged. 
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