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Author Topic: Full wave loop; ugly balun or ?  (Read 4770 times)
KJ6HYC
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Posts: 103




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« Reply #15 on: December 08, 2012, 05:33:31 AM »

I have read this thread with great interest, as I have a tuned 80M loop feed w/ladder line. My setup is Balanced tuner into 83ft 14ga ladder line feeding directly a tuned 80M loop. My ant. analizer measurement at the feed point on desired freq. w/transmittion line disconnected is 82 ohms impedence, SWR =1.9. 
What I have gleemed out of this thread is, to get the best efficency I should match the feed line to the loop at the feed point. My 450 Ladder line is heavy gauge and have read that it comes in at a lower inpedence of about 410 ohms.  410 Ohms Divided by 82 ohms = 5 to 1 ratio.  I haven't seen a 5:1 balun, a 4:1 is the closest.
My questions are; do I have it right? if so, should the 4:1 balun be voltage or current?

Thanks;
Wayne - KJ6HYC
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W5DXP
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« Reply #16 on: December 08, 2012, 07:15:43 AM »

My questions are; do I have it right? if so, should the 4:1 balun be voltage or current?

You are worrying about negligible losses. There is generally no reason to try to match 83 ft. of ladder-line on 80m. But let's use a transmission line calculator to be sure.

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

Using ARRL generic 450 ohm ladder-line, the loss in the mismatched line is 0.1 dB and that is not enough to worry about. What you may need to worry about is the impedance presented in the shack which is 220-j545 ohms and outside of the range of most built-in auto-tuners. If you can match that impedance through a 1:1 choke, don't worry about matching to the ladder-line impedance. It's like worrying about being hit by lightning when one is a thousand times more likely to die in a vehicle accident.
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WB6BYU
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« Reply #17 on: December 08, 2012, 07:41:23 AM »

Quote from: KJ6HYC

What I have gleemed out of this thread is, to get the best efficency I should match the feed line to the loop at the feed point.



In that case I haven't made myself clear enough.

As Cecil said, given that you are using ladder line (the ARRL numbers understate losses -
the Wireman 551 - 554 numbers in VK1OD's calculator are probably more accurate) then
the losses will be low anyway, even for multi-band use, assuming you are using a good
tuner in the shack.  In that case the loop doesn't have to be exactly resonant and the
antenna will still work well.  With your balanced tuner, you have a very good multiband
system and there isn't much you can do to improve performance (without changing
to a different antenna or getting it higher.)

Where the matching is important is if you are using coax feed to the loop, since a high
SWR will increase the losses to a larger extent. 


The truth is, if the loop is close to resonant on 80m, then just about any sort of coax
feed is fine for casual operation.  With 100' of RG-58 your losses are less than 2dB.
In many cases that is good enough.  The real issue is when the loop isn't close to
resonance (sometimes you have to just put up a loop where it fits without the
luxury of pruning it) and/or for multiband operation.  In those cases, ladder line
feed is generally going to be more efficient.  The 4 : 1 balun approach provides
a practical alternative for multiband operation at good efficiency with direct coax
feed to the antenna as long as you can prune the wire to keep a reasonable SWR.
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KJ6HYC
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Posts: 103




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« Reply #18 on: December 08, 2012, 12:04:40 PM »

Thanks, I wasn't aware of the line loss calculator, used it and found I was loosing only 2%.  So now I will put my effort into rasing the loops height.

73
Wayne KJ6HYC
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