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Author Topic: Auto antenna tuner capability limitations?  (Read 624 times)
WA8JNM
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Posts: 175




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« on: March 21, 2005, 06:44:15 PM »

I am recently back to ham radio after a 20 year absence..and thus have not had experience with auto tuners built into radios.

The antenna I installed is a 135 foot doublet, fed with ladder line to a balan in the shack, that I am trying to run on 80-10 meters.

My problem: every so often, my ICOM 765Pro2 can't tune it on 40 meters, or, less frequently, on 20. Then, the next day, all is fine again. One day the swr on 40 with the autotuner is fine (close to 1:1); the next day it won't load at all.

My guess: the antenna's "normal" swr is barely within the auto tuner's capabilities (maybe 3:1?), so the tuner can handle it. Then, altered weather conditions slightly increase the swr to a point that now is just beyond the tuner's capabilities, so it won't tune it at all.  Would that explain the drastic and unpredictable change?

By the way, the center point feed of the antenna is within 30 foot of the 2nd floor shack entrance, so I have a fair amount of the 100 feet(?)of ladder line draped (not coiled) around a shingled roof.  Snow sometimes covers the feedline, so I thought that may explain the swr shift that I speculate may be the problem.

Does this make any sense to the Elmers out there?  Any comments, criticisms, or suggestions for a new hobby?

Dave
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KZ1X
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« Reply #1 on: March 21, 2005, 06:57:09 PM »

As you suspect, having the ladder line's transmission-line characteristics 'spoiled' is probably causing most of your problem.

Is there a way to run the ladder line into the shack?  Get rid of the coax, and put the balun right at the rig.  You may still have a little trouble on 40 meters, but I'll wager the whole setup will work much better overall.
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K3AN
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« Reply #2 on: March 21, 2005, 06:58:56 PM »

Yes, moisture can affect the characteristics of the ladder line, which would change the impedance seen at the transmitter end of your antenna system. With the system you describe, SWRs are likely to be high on some or all bands. But good ladder line won't have much loss at high SWR, at least when it's dry. So the high SWR is only an issue in that it prevents your tuner from achieving a match.

I don't think any autotuner could tune such a system on all bands without requiring adding or removing lengths of feedline, depending on the band you want to operate on.

Why you have as much as 100 feet of the ladder line lying on the roof of your house?
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WA8JNM
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« Reply #3 on: March 21, 2005, 07:49:01 PM »

Thanks for the quick responses.

The ladderline does run into the shack, where it connects to the balan, then a 4 foot piece of coax runs from the balan to the rig.

As for the feedline on the roof, I thought I understood that the length of the feedline that came with this pre-manufactured antenna should not be significantly shortened.  Perhaps I'm wrong about that.  If I am wrong, I could lop off as much as 60 feet of the lead. Would that make the antenna more difficult to tune?

Thanks again.
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N4ZOU
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« Reply #4 on: March 21, 2005, 08:06:23 PM »

 Ladder line must be kept at least it own width away from any other surfaces. Having conductors or non-conductive surfaces close to the ladder line will upset the current balance that forms a magnetic field around the conductors in the ladder line. You can go through conductive or non-conductive surfaces as long as the balance is preserved. A good example is the balanced feed through insulators on the rear of an antenna tuner. In almost all cases these feed through insulators go through a sheet of aluminum forming the antenna tuner case. Balance is preserved even going through a conductive surface. The same balance is required when using standoff insulators to hold the ladder line away from surfaces. I use electric fence insulators with 1-inch width 450-ohm ladder line with no imbalance problems. The nail-up insulators are 1.25 inches in height and when placed in the window between the ladder line conductors even when using the supplied metal nail the balance is preserved with no loss. Snow will produce wide changes in SWR and will impose significant loss. There's not much you can do about it except for it to melt or you go and clean it off. Getting the ladder line wet as in getting rained on will also cause problems. It's not as bad as when using 300-ohm twin lead with solid insulation between the conductors and about all you can do is operating with the knowledge you have some additional changes is SWR and losses. Here is a link you might want to check out. http://www.qsl.net/w5dxp/notuner.htm
It details making an 80 through 10-meter dipole with variable length feed line system with an SWR of 2:1 or less except in the lower part of the 80 meter band. This system along with your internal automatic antenna tuner will take care of all the amateur HF bands. I did one of these antenna systems myself for 160 meters and up. It requires doubling the antenna element and feed line length and adding an additional 32-foot length of feed line to the ladder line length selector. If you have the room it does make a good 160-meter and up antenna.
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WA6BFH
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« Reply #5 on: March 21, 2005, 08:25:06 PM »

Damn, wouldn’t it be nice if radio worked more like some sort of “DC” communications circuit? I mean, isn’t it just a pain sorting out all of these reactance problems!

I’m thinking of just going to a 5 Watt nominal CB set-up, or maybe just FRS. Then I won’t have to worry about {or thinking that I could be} doing anything about not being able to make contact!

I mean really, who wants to genuinely understand and be able to fix or deal with RF problems anyway!
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N5LXI
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« Reply #6 on: March 22, 2005, 12:23:25 AM »

I also use a 75-m dipole fed with ladder line on a 756 Pro-II. To tune 40-meters use an MFJ-914 AutoTuner Extender. It adds some capacitance to the feed line. It works great! I use a LDG balun in front of everything.

joel / N5LXI
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AA4PB
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« Reply #7 on: March 22, 2005, 04:18:27 AM »

The built in tuner on the PRO-II (and most other rigs) is not really designed to handle the impedance ranges that you will see with an all band doublet fed with open wire line. These tuners are designed to handle the slight mismatch of a resonant antenna like a dipole being operated near the band edges. I'd turn off the tuner in the PRO-II and use an external tuner designed to handle a much wider range of impedances.
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K0BG
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« Reply #8 on: March 22, 2005, 06:21:52 AM »

You could also use a remotely mounted AH-4 with the ProII fed with coax. You might need to make an extension cable for the power and control, however. If you want to see how this is done, visit here: http://www.hamoperator.com

Alan, KØBG
www.k0bg.com
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N0XE
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Posts: 197




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« Reply #9 on: March 22, 2005, 07:22:59 AM »

According to Icom America in Washington State, the internal tuner in all their radios should only be used for slight mismatches at the band edges and where SWR should not exceed 3 to 1. They were  never designed to be used as a wide range matching device, although many hams do it every day.  I was told by the Chief Technician that they repair a lot of radios due to this. Get a nice external tuner that can handle the wide ranges that may be needed with a non resonate antenna system, not only will it work better but can also be used with  other radios as well and is a real plus especially if you ever decide to run an amp, the internal tuner will not do you much good there except maybe help tune the input if your amplifier does not have tuned input and all the newer ones do so unless you have an older amp even that is not needed. A  good quality external 2 KW should be a main stay in the shack unless you are only a Qrp op or plan to run pretty much resonate antennas forever. If you are positive you will never run an amp then a good 300 watt model will do.   73 N0XE
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W5DXP
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« Reply #10 on: March 22, 2005, 08:46:39 PM »

Most everything you need to know about that antenna can be had on my web page at http://www.qsl.net/notuner.htm  You need to be fairly close to a current maximum point on the feedline for the 756PRO to be able to match it. As you can see from the graphs, 100' of feedline is a pretty good fixed length. I personally can vary my feedline length from 88 ft. to 120 ft. I rarely use the tuner on my IC-756PRO because I can vary my feedline to the nearest foot. 73, Cecil, W5DXP
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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.
W5DXP
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« Reply #11 on: March 23, 2005, 07:43:56 AM »

Darn, I left my call letters out of the above URL. It should be:  http://www.qsl.net/w5dxp/notuner.htm
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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.
KB4QAA
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« Reply #12 on: April 04, 2005, 08:53:40 PM »

Jim at Radio Works sells a small box that can add a bit of inductance/capacitance? (whatever) to help an autotuner match the far edges.  It only has one knob and about 8 settings.  Just crank till you get a good match.
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