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Author Topic: ladder line question  (Read 2626 times)
KM4IY
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Posts: 31




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« on: December 21, 2012, 12:04:57 PM »

this is probably a really dumb question, but i've been inactive for many years and have forgotten more than i probably knew way back then... i've been reading up on ladder line, but i haven't found the answer yet.  i knew someone here would know.

i don't have a lot of room in the yard for antennas, and i'd like to keep the antennas to minimal visibility.   my house has a double chimney in the middle... one of the chimneys is being used, the other isn't.  i've been using the unused one as a conduit for coaxes, as it opens in the basement, next to my shack.

what i'd like to do, is put a dipole over the house, and run ladder line to the dentron tuner in the shack.  if i go down thru the chimney, how will that effect things?  there's no metal nearby... its brick with ceramic lining.

at the bottom of the chimney where it comes into the house, i usually just close it up with a bunch of old towels, to keep the draft from sucking the heat/A.C. out.  is there any chance of a fire sometime?

i've never used ladder line, and can't find any reading on this subject.  if you can direct me to some, i'd appreciate it...

73
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AA4PB
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Posts: 12669




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« Reply #1 on: December 21, 2012, 12:29:15 PM »

It would probably be okay if you rig spacers at top and bottom to keep the ladder line centered in the chimney. If you have other coaxes in the chimney then you want to make sure they are kept to one side, as far away from the ladder line as possible. Rather than old towels to plug the opening, how about using Styrofoam insulation that wouldn't present any fire hazard.
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WB6BYU
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« Reply #2 on: December 21, 2012, 06:51:27 PM »

The electric field extends around ladder line for several times the line spacing.  To the extent
that the field extends into the brick work the losses will be somewhat higher.  (Pure ceramic
isn't as lossy as regular bricks, and wet bricks are quite bad.)  So ideally you would want some
way to keep the ladder line spaced away from the sides of the chimney.  It's not the end of
the world if it touches it in places, but the more it runs close to the bricks the higher the
losses will be.  (If there is carbon soot on the bricks you run the risk of shorting the
feedline.)

If you can center the feedline in the top of the chimney, and perhaps use some sort of weight
to keep it straight down, or perhaps a styrofoam spacer or plastic rods to keep it centered,
then you should be able to make it work.  Some chunks of foam pipe insulation around the
feedline might work, too.

Chance of fire?  Well, how much power are you planning to run?  There can be high voltage
points along the feedline, and if one of those happens to be at a bad spot it can arc.  I've
never had that problem at 100W, but it always is possible, depending on the the antenna
impedance.  Certainly something like bubble wrap or styrofoam would be a better insulator
for sealing the hole, and less likely to arc.  I've also used fiberglass insulation.
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KM4IY
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Posts: 31




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« Reply #3 on: December 22, 2012, 06:00:56 AM »

i've been inactive for many years and have forgotten more than i think i knew... but you guys jogged my memory regarding the proximity of other feedlines... i have several there, so that won't work...

ok, that's what research is for...

on to something else.  i'm looking at what i think is called, or used to be called, a fan antenna...

thank you for your replies!

73
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N4EF
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Posts: 30




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« Reply #4 on: December 22, 2012, 06:43:22 AM »

If it's is easy to run the ladderline, do it! 

I live in a HOA subdivision and have only two small palm trees on my lot, both of them in the front yard.  I have no decent option for an antenna except in the attic. 

My shack is on the second floor with no earth ground nearby.

I run ladderline from an external antenna tuner, to behind an entertainment center with all sorts of wires and cables back there, though the wall, and into the attic to a zig-zag wire "loop" (if you can call it a "loop") up about 20 feet.  There's just no way I can keep ladderline away from other objects, wires, or metal.

It works well.

It also solved an RFI problem that no counterpoise could.

Try it.

Dave
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WA8FOZ
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Posts: 187




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« Reply #5 on: December 22, 2012, 09:55:55 PM »

Quote
It would probably be okay if you rig spacers at top and bottom to keep the ladder line centered in the chimney. If you have other coaxes in the chimney then you want to make sure they are kept to one side, as far away from the ladder line as possible. Rather than old towels to plug the opening, how about using Styrofoam insulation that wouldn't present any fire hazard.

Yep. I did this for a friend many years ago. The feeder went up the chimney, and the dipole ran to adjacent chimneys. Nearly invisible, and it worked reasonably well. It would be better to have no other conductors in the chimney, but if this is not possible keep them as far away as you can.
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N4FBW
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Posts: 34




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« Reply #6 on: December 23, 2012, 12:10:47 AM »

Bringing ladder line into the shack can sometimes be a recipe for RFI in your house. Try it and see how it works.

A lot of folks get a balanced tuner and put it out by the antenna and run coax into the shack. You can do this with an Icom AH-4 and some of the SGC models.
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K0ZN
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Posts: 1531




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« Reply #7 on: December 23, 2012, 10:24:15 PM »

Some good advice/posts on this.  Just remember that with ladderline (or any parallel line) the energy is AROUND the line, NOT "in it" like coax.

With that type of antenna operated on all bands you can have some very high (thousand) voltage on some bands.  The advise to devise some means
to keep the ladderline centered in the chimney is definitely worth heeding. The issue is when it rains..........

I suspect that antenna will work very well for you once you get the mechanical aspects of it conquered.  I you find a band where the tuner is having
difficulty finding a match, you can add (or subtract) about 1/8 to 1/4 wavelength (for that band) to the line to change the impedance load presented
to the tuner. Be sure to keep any extra ladderline neat; do not coil it or leave it in a rats nest. Best plan if generally to cut the line to the actual
length needed.

If your antenna is reasonably well balanced to ground, it is very unlikely you will have RFI. Almost all RFI with ladderline is caused by unbalance being
introduced into the system. The unbalance can come from a poorly designed tuner or balun or having the antenna unbalanced to ground or large conductive
objects. If you use a balun, spend the money for a good, high quality one that is specifically designed to operate into a high SWR load. The average run of the mill
garden variety balun is NOT designed for that service and likely will burn up/fail due to the high SWR with that system. I run a 128 ft. Center Fed Zepp/Dipole/Doublet fed with ladderline and run legal limit on all bands. I have never had any RFI in the shack. I use a Johnson KW Matchbox which has excellent balance.....that may help but I
have also run the system with an ATR-30 and used the internal balun and also had zero problems with RF in the shack.

Lastly, since you have the transmission line coming directly into your shack, I would strongly recommend you make some serious effort to provide a GOOD, low
resistance lightning ground to connect the ladderline to. i.e.  be able to disconnect the ladderline from the rig and connect it to a HEAVY copper cable that goes
to the best ground you can make or find. If you live in an area with thunderstorms and lightning, this will be time and money very well spent.
 Good luck.

73,  K0ZN
« Last Edit: December 23, 2012, 10:38:24 PM by K0ZN » Logged
AC2EU
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Posts: 332


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« Reply #8 on: December 24, 2012, 08:17:45 AM »

I would sty with the coax in a sitaution such as yours.
Besides, you can get very close to the actual antenna impedance by cutting the coax in half wave multiples for the band of interest.  use  (498 X vf)/f
As stated, you can switch in a bit of coax to help with troublesome tuning scenarios on other bands.

Remember, the antenna and the lead-in are a system, change any of the variables and the system characteristics changes.
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WB6BYU
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Posts: 13015




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« Reply #9 on: December 24, 2012, 09:29:59 AM »

Coax is a good choice if you cut separate dipoles for each band, so the SWR is low on the
bands you want to operate.  It's not the best approach for a single wire if that is all you
can string up, unless you add traps or some other way to keep the SWR low.

Twinlead or ladder line is the best solution if you are going to use a single straight wire
antenna on multiple bands, because the losses will be lower.  Coax isn't good in that
case because the losses can be quite high (even if it is cut to a multiple of a half wave.)

Sometimes the situation dictates one approach or the other:  if working on the roof is
difficult or might draw unwanted attention to an antenna there, then put up one wire
however you can and use a tuner.  If access is easy and safe, then it may be worth
taking the extra time to put up and tune individual dipole elements.

You may find that using 300 ohm twinlead is a good solution here because the spacing
between the wires is smaller, which reduces the extent of the external field.  With a
chunk of foam pipe insulation where necessary to maintain spacing away from masonry
and/or other cables, it should work OK.
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WN2C
Member

Posts: 428




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« Reply #10 on: December 25, 2012, 07:49:27 PM »

SGC 237 auto coupler.  Power on the coax to the coupler and a wire is all you need.  The 237 will tune just about anything including the bed springs.
73
Rick wn2c
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W5WSS
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Posts: 1644




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« Reply #11 on: December 25, 2012, 09:26:24 PM »

Reduces the extent of the external fields....so hmmmm just thinking

If

 Designing for minimal characteristic impedance of the twinlead transmission line and

If

One were to properly "design and build" using a pair of round conductors of the proper diameter such as about #14 standard diameter
and velocity factor is known as a function of the dielectric constant of the insulating material for the purpose of avoiding a dc short or opting for open wire and spacers and

 Since we are including a tuner in this discussion

Then it would be possible provided one could accept the quite allot lower  characteristic impedance for open wire line that is spaced tightly.

well below 300ohms

Just some thoughts

A pair of #14 Teflon coated wires held tightly together.and properly installed Routed for the purpose of reducing other conductors deteriorating the external fields pathway should be feasible.....300 ohms is easier though but just some thoughts
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W5DXP
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« Reply #12 on: December 26, 2012, 08:08:40 AM »

A pair of #14 Teflon coated wires held tightly together.and properly installed Routed for the purpose of reducing other conductors deteriorating the external fields pathway should be feasible.....300 ohms is easier though but just some thoughts

One of the low-loss advantages of parallel feedline is that the higher characteristic impedance forces the current to be lower thus reducing the I^2*R losses, the major component loss at HF. When one reduces the distance between wires, one reduces the characteristic impedance and thus increase the I^2*R losses. That negates one of the advantages of parallel feedline. When I first became a ham in the early 50's, 72 ohm parallel was readily available. Turns out it was pretty lossy.
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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.
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