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Author Topic: Ladder Line question  (Read 22133 times)
N4VNV
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« Reply #15 on: November 01, 2012, 07:19:11 AM »

When I used ladder line it transmitted RF just like the antenna wire did. As a result RFI got into my house wiring. It was a nightmare. I feed with coax to a remote auto-tuner at the feed point, work 6 thru 160M bands and everything is hunky-dory.
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WV4L
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« Reply #16 on: November 01, 2012, 08:02:55 AM »

Using coax to an auto tuner at the feed point of the antenna is a workable solution. The caveat to a ladder line/coax combination is to keep the coax short. I use an tuner in the shack that has a 18 foot run of coax to a balun and then the needed length of ladder line to the feedpoint of the antenna. Any losses on the short run of coax from the tuner to the balun are nominal at best. Loss from the balun using ladder line to the feed point-very, very little if any.
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W8JI
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« Reply #17 on: November 01, 2012, 08:55:35 AM »

When I used ladder line it transmitted RF just like the antenna wire did. As a result RFI got into my house wiring. It was a nightmare. I feed with coax to a remote auto-tuner at the feed point, work 6 thru 160M bands and everything is hunky-dory.

If you had significant radiation from the ladder line, you had an installation or design issue in the system. Perhaps a poor balun, or a grossly unbalanced antenna.

Some antenna tuners and some baluns actually force ladder line into UNbalance. A friend of mine had severe RFI, and it turned out to be the balun design he was using. The balun was actually designed in a way that forced gross unbalance!!

That aside, antenna tuners generally do NOT transform common mode. There is a false idea that common mode disappears, or is easier to handle, on the input side of a tuner. You can read about that here:

http://www.w8ji.com/tuner_baluns.htm




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K1DA
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« Reply #18 on: December 04, 2012, 10:14:53 AM »

I run 300 ohm twin lead through a hole which carries two RG 8 leads and a rotor cable.  The 100 foot flat top to which it is  connected works on 80 through 10, including 60 meters.  So far, a pair of 3-500s amp  hasn't caused any problems.  I don't think 200 watts will cause you a problem.  My exciter runs that much.
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W2WDX
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« Reply #19 on: December 05, 2012, 09:06:00 PM »

Most times there numerous factors that lead to RF radiation from ladder-line. Most times its not understanding how it works and installing it poorly.

Some things you should be aware of:

  • Pay attention to the length of the feedline. Resonant lengths are bad and that is dependant also on the type of antenna being fed.
  • Make sure you have true balance in the entire system. This can done buy measuring both legs on an oscilloscope at low power and using the add function to measure for amplitude and phase being equal. You can also use two RF ammeters, one for each leg. Balance in the entire antenna system is crucial.
  • Use a balanced tuner that does not use a balun on the output. Most modern tuners are very poor in this regard.
  • Do not run the feed-line near metal objects. This throws off the balance in the feedline.

If you use balanced feedline properly, and everything is in balance, you will have very little feedline radiation and much, much lower loss over coax. Less than a dB even under a severe mismatch condition. With coax you will have significant losses if the mismatch is even at 2:1; on the order of 5dB or even higher depending on frequency, type of cable, length and amount of mismatch. That pretty much cancels any gain you may have in your antenna.

For instance, take a 100' length of Belden 9913 with a loss figure of about .38dB at around 7mHz, feeding an antenna with 1.4 SWR. In this case you lose about 9% of power to losses in the cable. So 100W in and you radiate 91W from the antenna, the rest is lost. If your SWR climbs to let's say 3, the loss goes up to 13.5%. So 100W in and you radiate about 87W.

Now lets say you use some RG174 with loss figure of about 2.50dB/100'. (Similar to RG-58 with RG8X only slightly better) The power loss is about 45% with an SWR of 3. So 100W in means you radiate about 55W.

Add a zero to the above figures and you see how much you lose on your Killowatt amp. Now these loss figures are at about 40M. The losses go up even higher as you go up in frequency. For instance at 50mhz the loss in RG174 is now at about 6.6dB/100'. That's about 75% loss with a perfect match at the feedpoint. And 9913 gets up to 1.1dB.

Now remember, this is with a tuner. The mismatch between your feedline and antenna still exists, the tuner is only cancelling the reactance at the coax-tuner connection point, not the antenna feedpoint. Basically, with a modern transistorized radio you're putting back the Pi-network not used in your radio, allowing the transistors to run at nominal current levels and not burn up. You are not really correcting the mismatch between the coax and the antenna, you are just tuning the entire system to resonance (not the same thing). Now the same thing mentioned above happens with balanced feeders like ladder-line. But because the loss figures are so low (with losses as low or lower as .02dB at 100'), the actual radiated energy is much much higher even with sever mismatch.

Sidenote: As far as losses in coax goes... I had a friend who was complaining he didn't "get out" as well on his 2m rig as I did, even though we had the same antenna and power levels. So I went to his house to see what the problem was. The difference was I was using LMR600 into my 2m loop array, and he was using RG-8X. We both had fairly good matches at an SWR of about 1.2. However, due to coax losses, my cable loss was about 16% and his was about 60%. Nuff said.

John W2WDX
« Last Edit: December 05, 2012, 10:04:12 PM by W2WDX » Logged

K1DA
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« Reply #20 on: December 10, 2012, 09:17:33 AM »

W2WDX about sums it up.  The materials don't cost much, most tuners have a balun built in for the balanced/50 ohm unbalanced conversion, and a  center fed  wire dipole is a simple antenna.  You can make your own  center insulator. Chances are it will work very well on a number of bands.  If it doesNOT t, you'll find yourself learning a lot about balance/unbalance and impedance transformations, the inpedance of a particular center fed wire length on different bands, what 1/4 wave and half wave lengths of line do to inpedances, and the radiation patterns of a particular length of wire on different bands.  You'll    be the better for it.  If you want plug and play....a  center fed dipole, open wire line  to a remote auto tuner with a proper balun, and hope the tuner likes what it sees. 
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W5WSS
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« Reply #21 on: February 10, 2013, 04:26:03 AM »

The routing of twin balanced transmission line directly flat against any solid mass will shear half the transmission off along the line.

During receive the induced voltage from the antenna to travel back towards the rig is not effected by the proximity of the line to solid mass.

Use coaxial cable for the antenna feedline.

Place a tuner at the antenna or as close as you can to allow the coax to operate in a matched state allowing full delivery, full or almost so delivery of the transmitter power to the load avoiding the high losses of a mismatched line.
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K0ZN
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« Reply #22 on: February 12, 2013, 09:00:35 PM »

Here is why they call it AMATEUR Radio..... you probably need to experiment with your set up to see what works best in your situation. That is pretty common
in this game! Sometimes stuff that looks ugly works quite well and the reverse.....you just gotta try it!

My take would be that if the plastic you mention is NOT a type of plastic that would attenuate RF (You would have to put a small piece in your
microwave and see if it gets warm or not....warm is bad...no temp rise is good!) and the wood is/stays DRY, you likely are OK to run it that way at
lower power levels. This assumes there is no metal/conductor/sheet metal otherwise close to the line. (Close would be < 6" ). You need to know what
material is just behind that wood.

I don't know if it would take or adhere, but you could spray paint coax to a color that matches the siding. I would not do this with ladderline. That way
you could run coax up to an autotuner if you want to go that route.

It is likely that a decent manual tuner to ladderline to a balanced antenna (center fed dipole or loop) would be the most flexible antenna you could
put up in terms of frequency coverage and likely have the fewest problems. That said, I think too many hams try to get something up that will work "all bands" where in reality, sticking with two or maybe three bands in a coax fed situation may yield better results. I would suggest you concentrate your antenna efforts on
40, 30 and 20 M if you are QRP and using CW; that will likely give you the best bang for the buck.

Good luck.  73,  K0ZN
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AC4RD
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« Reply #23 on: February 13, 2013, 04:33:50 PM »

If you use balanced feedline properly, and everything is in balance, you will have very little feedline radiation and much, much lower loss over coax. Less than a dB even under a severe mismatch condition.

John is a wise man.  :-)  There are people who don't like balanced feeder lines, but I sure don't understand why.  You can run it for LONG lengths with very little loss, you can feed an antenna with it and use it for lots of bands with virtually no loss, and it's inexpensive besides. 

Now, yeah, it takes a little work to run it away from metal and things that can interfere with it.  And for some hams, that makes it unworkable.  But if you CAN feed an HF antenna with balanced line (twinlead, window line, 300-ohm line), it's a great solution for most people.
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N0NZG
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Posts: 121




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« Reply #24 on: February 28, 2013, 07:06:39 PM »

In response to your query of running ladder line in very close proximity to vinyl siding.  PVC pipe and vinyl siding in very close proximity to you ladder line will cause some additional loss.  These plastics have metal-based “ plasticizers” in them that I suspect cause the extra losses. If you take an antenna and put it inside a PVC pipe you will see a change in resonate frequency.  This same effect can be seen winding a loading coil on a PVC form vs air or cardboard for a form.  Lastly look at the difference in loss between “TV” type twin lead vs open window ladder line vs home brew ladder line with little spacers every 4 inches or so. The loss goes down as you remove these so called dielectrics from the feed line. This is also why a foam dielectric coax has less loss than a solid core coax. If this was a length critical application your velocity factor will also change.   Food for thought. In your situation I would go with a remote tuner.

73, Jeremy
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W1JKA
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« Reply #25 on: March 01, 2013, 06:09:56 AM »

  First of all there is nothing wrong about starting off into the hobby with QRP, after all most OT hams started off with small one or two tube homebrew qrp power xtmrs.,some stayed qrp like me and many have returned back to it. As K0ZN says you will most likely find 20/30/40m your most used qrp bands.Until such time that you get your ladder  line question resolved and purchase a tuner please consider that many qrp ops both CONUS and DX use simple no tuner frequency cut coax fed attic dipoles with very satisfactory results.
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N0NZG
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« Reply #26 on: March 01, 2013, 11:59:01 AM »

That is so true. I use a tuned 20 meter dipole in the attic with my Bitx20 with great results.  Make the most of what you can as any antenna is better than NO antenna.
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WB6BYU
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« Reply #27 on: March 04, 2013, 11:49:38 AM »

Quote from: N0NZG
...PVC pipe and vinyl siding in very close proximity to you ladder line will cause some additional loss.  These plastics have metal-based “ plasticizers” in them that I suspect cause the extra losses...




Actually, if you look at the physics involved, the feedline losses tend to be
more in the conductor resistance rather than the dielectric characteristics, at
least though the VHF range.

For example, foamed dielectric coax has lower loss than solid polyethylene, but
that is primarily because the conductors have to be larger to maintain the same
characteristic impedance when the dielectric constant of the insulation changes
due to the presence of more air in the mix.  TV twinlead has relatively small
conductors compared to typical ladder line, which accounts for most of the
difference in loss, rather than the dielectric surrounding it.

While it is true that putting a J-pole inside a length of PVC pipe shifts the tuning,
in my experience the change is much less for a normal dipole.  It isn't the antenna
itself that is detuned, but primarily the velocity factor of the matching stub
(especially if the antenna lays against the inside of the pipe rather than hanging
straight down the middle.)  You can test this by inserting the radiator of the J-pole
inside a pipe, measuring how much the SWR changes, then pushing the rest of
the matching stub inside to see the difference.

Open wire line has a relatively high impedance (though it is possible to make
lower impedance lines using multiple wires).  That helps to keep the impedance
high along the length, which minimizes the current, reducing resistive losses.

But when open wire line runs at a high SWR, so there are points of relatively
low impedance with high current, the losses can be quite higher than expected.
For example, using VK1OD's transmission line loss calculator here:

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

The losses in 20' of Wireman 551 on 80m (so a short line on a low frequency
where we expect feedline to be low) is about 0.85dB with a load impedance of
10-j200 ohms, and 1.5dB at 5-j200 ohms.  In fact, this is a convenient tool to
look at the losses in partial wavelengths of feedlines, which don't always behave
as the textbook loss tables might suggest based on SWR.  That's because the
tables assume the loss is averaged over the whole wavelength when operated
at a high SWR, while in practice the lower impedance portions will have the
highest losses, consistent with our observation that loss is more due to conductor
losses rather than dielectric losses.
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