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Author Topic: Ladderline question  (Read 888 times)
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« on: December 21, 2009, 01:27:25 PM »

When bringing ladderline into a shack and connecting it to a tuner, will the ladderline that is inside the shack cause any radiation problems and if so what is the recommended cure for this. Bill
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AA0PO
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« Reply #1 on: December 21, 2009, 03:35:02 PM »

#1  Yes, Ladder line will radiate and possibly cause RF probmens.  Keep it out of the shack and away from equipment (Mic, Power Supply, Radio, Tuner).  

#2  Make a choke at the tuner or transmitter and run some coax outside where you will attach the ladder line thereto.  See earlier answer with the plastic coffee can, etc.

Have some fun when done.  Merry Christmas!

Tom AAØPO
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W5DXP
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« Reply #2 on: December 21, 2009, 04:00:51 PM »

Are there fields surrounding ladder-line? Yes, and that's why it should be kept a few inches away from conductive objects. But ladder-line with balanced currents doesn't radiate into the far field appreciably more than does coax - because the fields cancel in the near field. Common-mode currents will radiate from coax just as well as they do from ladder-line.
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73, Cecil, w5dxp.com
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WA6BJH
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« Reply #3 on: December 21, 2009, 04:11:08 PM »

Before coax was available, it was, of course, necessary to bring balanced feed lines into the shack.  It is still done, and the ARRL Antenna Book has an example of a balanced antenna tuner that has a balanced output to feed a balanced transmission line.  So it must be possible.

Generally, balanced transmission lines do not radiate.  The currents in the individual wires are equal and opposite so the fields that they create cancel.  The field that the current from one wire creates is exactly balanced by the opposite current from the other wire.  

Nothing is perfect, however, so there must be some radiation.  It is small.  Coax is not perfect, either, and there is some radiation.  

The trick to a balanced feed line in the shack is to make sure that it is correctly located.  Don't stick the ladder line through an open metal frame window and then close the window on it and expect it to work correctly.  You can't drape it across the floor like you can with coax.  And you can't let a coil or loop of it hang behind the desk.  

You can bring it in the window through an appropriate panel that keeps the individual wires separated from surrounding things.  There are designs for entrance panels, or you can buy one.  Bring the ladder line directly to the tuner and keep the ladder line away from things it shouldn't be near.

Good luck.

Kerry
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WB6BYU
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« Reply #4 on: December 21, 2009, 04:33:26 PM »

It depends on your antenna.

If the ladder line is properly balanced (as it should be when it
is connected to a balanced antenna) there will be very little
radiation from the line - no more so than from some types of
coax cable.  If the antenna is not properly balanced then
you can have common mode currents on the ladder line, just
as you can have on coax cable.

I've often used twinlead or ladder line to a tuner in the shack.
Sometimes I did this by running the feedline in through the
attic and dropping it through a hole in the ceiling.  I haven't
noticed any more problems with radiation from such feedline
then I normally do with coax.
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W4MLO
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« Reply #5 on: December 21, 2009, 10:53:49 PM »

This is a question that has no absolutely no correct answer except "it might or it might not". I am running 450 Ohm window line into my shack 100 feet from a 135' doublet, to the back of a Heathkit SA-2060A tuner. I regularly operate at 1 Kw PEP. My window line passes  perpendicular within 2 to 5 inches of my shack computer's speaker cables, several USB cables, the DC lines that feed 3 transmitters, the AC lines that feed my power supply lights and amp, the Icom CAT cable, within 6" of my UPS, 6" from the computer case, 2' from my wireless  weather station, UHF and HF coax's, 3 powered external speakers, a MFJ DC distribution strip and is run out of an aluminum storm window frame thru an old vacuum cleaner tip.

Knock on wood but I have yet to have any issues.

All that being said, at my old QTH I had computer issues running 100W on a coax fed antenna.

Don't believe everything you read on this site, there are a lot of folks re-hashing things they have read but never tried.

Try it and see.

73, Milo W4MLO
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K0BG
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« Reply #6 on: December 22, 2009, 06:52:56 AM »

Quote
Don't believe everything you read on this site, there are a lot of folks re-hashing things they have read but never tried.

That's true!

Cecil alluded to common mode current, and rightly so. If there is an appreciable amount, the ladder line will indeed radiate. There is an apparent misunderstanding in some circles that using ladder line is a way around common mode current problems. That is absolutely not the case. In fact, very few installations have immeasurable common mode. So the real issue is, does the level of common mode cause you any problems? There is no clear answer until you actually erect the antenna.
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N9FME
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« Reply #7 on: December 29, 2009, 05:05:40 AM »

For what it's worth, I just installed an inverted L for 160 and it is fed entirely with 450 ^ ladderline.

One side connected to the "L" wire and the other side to an 8' copper rod which is at the base of the tower and about 15' from the radio room.

The ladder line is fed directly through the concrete block of my house and up into the back of my Dentron Super tuner. Everything in the shack is grouned out to the 8' copper rod. I can run legal limit with no interference to any equipment in the shack. three computers, a couple old and new tv's and no noise...Quite honestly, it works better than any other wire antenna that I've ever used..Tunes all bands and handles full power, great reports, super clean audio, no RF on my signal....makes a great multiband antenna too!!
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N9FME
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« Reply #8 on: December 29, 2009, 05:07:46 AM »

forgot to mention.....about 20' feet of ladder line coiled up outside at the ground rod feedpoint before it heads in to the house.....
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WA3SKN
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« Reply #9 on: December 29, 2009, 05:58:41 AM »

The problem of feedline radiation with open wire feedlines is overstated, and caused by imbalances.  Try the ladderline, it is cheap and you will probably like it!
73s.

-Mike.
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K1BXI
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« Reply #10 on: December 29, 2009, 08:09:02 AM »

N9FME......Are you sure you want to coil up 20' of window line near the ground rod? (or any other place) Not exactly the thing to do with parallel feed line.

John
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W5WSS
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« Reply #11 on: December 29, 2009, 08:16:25 AM »

My feed line is 100% inside as is my indoor balanced dipole. The line radiation can be minimized to a measurement that is not problematic to the equipment and is exposure complient. Proper treatment of the routing guidelines that have been long successful/established techniques for balanced lines along with an understanding of how to tune the system for balance to help prevent common mode currents, has been mostly perhaps forgotten since the advent of mono band 52 ohm antennas easily adaptable to 52 ohm coax perhaps eases the percentage of common mode issues but doesnt mean total prevention using coax either. My line is a short length measuring 5 ft that is properly routed straight to the antenna from the tuner and spaced away from anything by 5". I do not suffer common mode invasion to my equipment. located on the second floor of this apartment an rf ground is not possible and as such their exists no common mode even when power wss elevated to see how everything would behave and alas no common mode issues with using balanced line that is 100% indoors hence balance of the system is the primary deterent to common mode problems.
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AA5WG
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« Reply #12 on: December 29, 2009, 09:35:50 AM »

Hi Bill:
Bring the ladder line into the shack and hook it up to your antenna coupler.  I and others have been doing it this way for years.  Older ARRL handbooks (1940's - 1950's and more) provide basic  instructions on how to home brew true balanced antenna couplers that stay in the radio shack.  This is the traditional way in operating multiband random length dipoles.

73,
Chuck - AA5WG
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K9FON
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« Reply #13 on: December 29, 2009, 01:53:58 PM »

I normally run a ladder line fed 260' doublet and i have had the line fed into the shack to the back of the tuner. I have never had any RFI issues at all. However, I did once tie both sides of the 450 ohm line together once to see how it would tune and i did have some RFI.
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N9FME
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« Reply #14 on: December 29, 2009, 03:16:46 PM »

Hi John,
I originally didnt intend to leave it coiled up but after seeing how well it has been working, I really hated to disturb it...

Greg
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