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Author Topic: 300 ohm Feedline Placement  (Read 1520 times)
W4OEQ
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Posts: 137




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« on: December 29, 2012, 08:46:53 PM »

Will there be detrimental effects if my 450 ohm balanced feed-line comes down the support tree trunk and then runs on the ground for 20-30 feet before entering my shack?  In my situation, it is not possible to run the feed-line directly (through the air) from the center of the dipole to the shack window entry point. Tnx.  Tom, W4OEQ
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WB6BYU
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Posts: 13020




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« Reply #1 on: December 29, 2012, 08:54:53 PM »

Yes.

The electromagnetic field surrounds a parallel conductor feedline, instead of being
contained within the shield as is the case with coax.  (You can get or make such
feedline with a shield, but that changes the characteristic impedance and the
losses.)

Running it down the tree trunk isn't too bad - there will be a little additional loss,
but probably not enough to worry about.  But the ground is a major problem, as
it is quite lossy when a significant amount of the field extends into it.

If you can keep the feedline spaced 2 to 5 times the wire spacing away from the
ground it will improve matters:  you may be able to do this with a large diameter
drain pipe and some spacers to keep the feedline centered in the pipe.  (Such
a construction could even be buried.)  If that isn't practical, you could perhaps
dig a ditch and suspend the feedline over the top of it somehow - flush with
the ground, but spaced from the dirt.  Another method is to tack it to a wood
fence.
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K7KBN
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Posts: 2763




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« Reply #2 on: December 29, 2012, 08:55:52 PM »

Quite probably there will be problems.  Try it and see if it works.  Remember animals and people who might be passing near...
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73
Pat K7KBN
CWO4 USNR Ret.
K0ZN
Member

Posts: 1531




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« Reply #3 on: December 29, 2012, 09:25:44 PM »


You will seriously degrade the system by laying ladderline on the ground.  I was experimenting with an antenna a couple of years
ago, and before I got it up into the final position, there was about 20 ft. of the ladderline laying on DRY grass. I figured that dry grass would not
make too much difference. WRONG ! The effect was to "swamp" the system with loss to the point the tuner tuning was very broad.
i.e. it was acting like a partial dummy load.  The system also showed an odd ball impedance at the input to the ladderline.
Once the antenna was up in the air and the ladderline was off the ground, the tuning changed significantly and signal reports improved significantly.

The cold reality is that parallel conductor transmission lines MUST be reasonably kept away from conductive areas/objects
and/or earth ground as a minimum, several times the conductor spacing. That is just the laws of electrical physics; they are what they are.
My experience was that laying it on the ground made a LOT of difference (as in "bad").

Remember, the energy field is AROUND that line. If you introduce lossy earth into that field it WILL absorb some of the energy and/or
change the characteristic impedance of the line.

The above said:  if you live in a very dry desert area where "earth" is dry sand, it may not be too much of an issue. However,
if your soil is moist and conductive, it will make a lot of difference.

If your ONLY possible option is to run line on the ground, I suspect the best bet is to put a good, high quality balun (designed for high SWR/high voltage service) at the base of the tree, then run a very good, high quality, low loss coax into the shack.  Can you make small stand off's out of PVC pipe and elevate the line about 12 to 24"
off the ground.

Bottomline:   laying ladderline or other parallel conductor transmission line on the ground is a bad idea.

73,  K0ZN
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W4OEQ
Member

Posts: 137




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« Reply #4 on: December 30, 2012, 01:09:19 PM »

Thanks for comments.  You all confirmed what I suspected.  I have tried "Plan B" in which I used either 300, 450, or 600 ohm feed line to a junction box at the base of the tree and included either a 4:1 or 1:1 balun at that point.  I connected high quality coax to the balun and ran the coax across the 20-30 feet and up to enter the shack window (second story of my home).  This system worked for most bands but had some "hot spot" frequencies that caused all kinds of problems for the MFJ manual tuner.  I understand that others have recommended that the run of coax should be no more than 3 to 6 feet from the window to the equipment inside the shack.  This would mean that I would have to run the open wire feeder line directly from the center of the dipole to the shack window (second story of the house).  This configuration would not permit me to easily disconnect the feed-line when stormy weather is forecast.  I prefer to have a disconnect point at the base of the tree. 

I am concluding that the only practical solution is to have a balun at the center of the antenna and run coax to the junction point at the base of the tree.  A second run of coax would go to the second story shack window.  I would then have to either confine my operation to a single band dipole (40/15 being an exception) or use a fan dipole.

Comments?  73,  Tom, W4OEQ

 
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WB6BYU
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Posts: 13020




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« Reply #5 on: December 30, 2012, 01:36:19 PM »

You may be able to choose a length of feedline between the antenna and the
coax that gives a better match and reduces losses in the coax.
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W4OEQ
Member

Posts: 137




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« Reply #6 on: December 30, 2012, 04:47:47 PM »

Yes, I can adjust the balanced feed-line to a small degree -- perhaps +/- three feet.  If the junction at the base of the tree is to be convenient, it has to be somewhere between  my knees and my head.   I would not want to create a situation whereby I would need a ladder each time I want to disconnect the antenna.  The unanswered question here is, "Why is the length of coax between the balun and the shack an important electrical factor?"  I know there is probably a technical explanation, but I do not know the answer.  73,  Tom, W4OEQ
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KH2G
Member

Posts: 238




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« Reply #7 on: December 31, 2012, 01:44:00 PM »

Assuming the coax is not in the rf field and acting like a counterpoise, it is not a problem. The balanced line can be somewhat critical in length when it approaches 1/4 wave at the operating frequency. Recall the old days when we used a separate section at the feed point to get our match (Impedance transformation)  Some have set up relays to kick in or out different lengths when long hauls are involved. It becomes worh the effort due to having almost no loss on a balanced line compared to coax.
Regards
Dick KH2G
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WB6BYU
Member

Posts: 13020




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« Reply #8 on: December 31, 2012, 03:07:30 PM »

Quote from: W4OEQ

"Why is the length of coax between the balun and the shack an important electrical factor?" 



When a feedline is NOT matched to the load, the impedance varies along the feedline.
It's actually rare in modern ham usage for an antenna to be matched to open wire line or
twinlead.  (The most common example is a folded dipole fed with 300 ohm line).

If you are feeding a single antenna (such as a dipole) on multiple bands, that means that
not only does the impedance vary from band to band, but it also varies with the feedline
length.  It is often worthwhile looking for a combination of wire and feedline lengths that
give a relatively benign impedance on multiple bands (examples are the G5RV or ZS6BKW
antennas.)  Otherwise we end up with bands where the SWR on the coax section so very
high, and much of our RF gets dissipated as heat in the coax before it reaches the antenna.
(Like the G5RV when used on 10m.)

We can take a specific example to illustrate the point:  let's say we have an 80m doublet
and we are using it on 40m with Wireman 551 ladder line.  Depending on the wire size,
the feedpoint impedance might be 3000 ohms or so.  We can use VK1OD's online transmission
line loss calculator to examine the results:  http://vk1od.net/calc/tl/tllc.php

With no ladder line, the SWR would be 3000/50 = 60 : 1.  That's not too good.  If we add
20' of ladder line the impedance at the bottom is now 75-j250 ohms, and the SWR has dropped
to only 18 : 1.  If instead we use 50' of ladder line the impedance is 170+j540, for an SWR of
38 : 1.  But a length of 31 feet gives us 55+j1 ohms, so the SWR in 50 ohm coax is 1.1 : 1.

Which of those will have the lowest loss in the coax?

And it gets more complicated because what the tuner in the shack sees is the impedance
at the end of the ladder line transformed through the coax:  that initial 3000 ohm impedance
if connected to the 50 ohm coax means the impedance the tuner sees could be 3000 ohms,
or 1.2 ohms, or somewhere between those values with some amount of reactance (either
positive or negative), depending on the coax length.

That's why it generally works best to arrange for the impedance at the end of the ladder line
to be a low impedance point, as that usually corresponds to minimum SWR on the coax, lower
losses, and less extreme impedances for the tuner to match.  One way to do this on multiple
bands is to use a relay to switch extra lengths of ladder line in and out - these can run up
and down the tree trunk along with the main feeder (as long as they aren't too close
together.)

Not that you can't get it to work otherwise, but remember that, just because your tuner
can match the impedance presented by the coax, that doesn't mean that your RF is
actually making it to the antenna.  It isn't difficult to waste 50% or more of your power
heating the coax even when the tuner can find a match.
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W4OEQ
Member

Posts: 137




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« Reply #9 on: December 31, 2012, 05:25:05 PM »

Very helpful explanation of the technical issues.  I think my fan dipole fed with coax looks better and better.  73, Tom, W4OEQ
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