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Author Topic: 80 meter dipole issues revisted  (Read 1310 times)
N5XO
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Posts: 104




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« on: April 09, 2010, 04:46:57 AM »

I am still dealing with 80 meter issues. We discussed this awhile back and I went over everyones thoughts and comments, but still dealing with issues.

Original problem was a home made dipole with ladder line, dropping down to a MFJ 4:1 balun {1,500 watt rated} going to an MFJ 989D tuner and then to an SB221 amp and FT 2000D.

When I tuned up and and ran just the radio barefoot at 200 watts it worked fine, but when I tried to load up the amp on the antenna it would not give me more than 300 to 400 watts, going to a dummy load it would push 1000 plus.

Since never could resolve the problem, I changed things up. Purchased a dipole for 80 meters, put it up on the tower {apex 50ft, ends about 15ft, it's coax fed {no balun} back to the shack and goes into the MFJ tuner and now is running on my henry 3KD classic, tuned into a dummy load I get over 1400 watts on 80, going to the antenna I tune the amp and drive it up to 400 to 500 watts no problem, bring it up over that and the SWR meter jumps to full scale, retune SWR a bit and it helps but then can't get more than 400 watts.

I have no problem with my 40 meter dipole, my beams, etc.

The only common device from the first issue to this one is the radio and the tuner. I hear no arcing, etc from the tuner....but could it not be handeling the load right or am I missing something.

So what the heck?
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AD4U
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Posts: 2150




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« Reply #1 on: April 09, 2010, 05:29:30 AM »

Something is loose or arcing.  How does it work without the "tuner".  By that I mean completely remove the "tuner" and connect the coax PL-259 directly to the back of the amp.

Dick  AD4U
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KA5N
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Posts: 4380




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« Reply #2 on: April 09, 2010, 05:43:54 AM »

A coax fed dipole should be a pretty good match for 50 to 70 Ohms without a tuner.  However the bandwidth of an 80 dipole won't cover the whole 75/80 meter band. So if you try to cram a lot of power into the dipole where the SWR isn't very good you won't get good results.  Also if you use a tuner with a coax fed antenna you may lose a lot of power in the coax.
A SB201 will probably require some added load capacitors to get the amp to load properlyon 80 meters.  
A 4 to 1 voltage balun is usually a poor choice.  
Best results would be a open wire feed and a good tuner with an internal current 1:1 balun.  Even then you may have to change feeder length (by adding lengths) to avoid problem causing feedline lengths.
Good Luck
Allen
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K1BXI
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Posts: 812




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« Reply #3 on: April 09, 2010, 05:43:59 AM »

Have you tried it with out the tuner in line?

Since this is a coax fed dipole you should be able to eliminate the antenna tuner as long as you are near the resonate frequency of that dipole. In fact the pi network output tuning of the 3KD should give you a wide tuning range with that coax fed dipole on 80 meters.

John

(Whoops Dick, I see you have already asked him that...I need to type faster.)
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K1BXI
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Posts: 812




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« Reply #4 on: April 09, 2010, 05:56:48 AM »

I really need to take a speed typing course, Alan beat me to the punch too.

At least we all have the same idea. But I wonder what the coax is (RG- 213 or better hopefully) and would it be possible for him to disconnect the coax at the antenna end and put the dummy load out there and see what the result would be.

John
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KI4SDY
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Posts: 1452




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« Reply #5 on: April 09, 2010, 06:38:47 AM »

The problem is in the tuner, if all the feedline connections are good and the original ladder line used was properly isolated from nearby metal.
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K4SAV
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Posts: 1786




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« Reply #6 on: April 09, 2010, 07:38:18 AM »

"going to the antenna I tune the amp and drive it up to 400 to 500 watts no problem, bring it up over that and the SWR meter jumps to full scale,..."

That should be a clue for you.  SWR doesn't "jump up" unless something is arcing over and causing that to happen.

You didn't purchase one of those dipoles with the lightning protector in the balun at the feedpoint did you?  Or one of those dipoles with the unsealed balun that fills with water.

Jerry, K4SAV
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AH6RR
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Posts: 803




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« Reply #7 on: April 09, 2010, 09:17:33 AM »

How old is the coax? What kind is it? It sounds like the coax is waterlogged or has a sharp bend that caused the center conductor to migrate closer to the sheild and is fine at the 400-500W range but when the power level goes over that it causes a short due to arcing in the coax it self.

That is just a thought but when the SWR jumps like that is a indication of arcing/short. From the information you provided it sounds like the coax. I had some RG-8X that did the very same thing 100W was fine but when it went over 250W it showed a short I went to LMR-400 and have not had a problem since.

Roland AH6RR
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N5XO
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Posts: 104




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« Reply #8 on: April 09, 2010, 09:59:39 AM »

The current antenna is a G5RV styled antenna 102 ft long.

I'm feeding it with new LMR400 {I consider this minimum cable period} through an MFJ 989D tuner.

With out the tuner it is showing 3.1 SWR at 3.823 on the FT2000D, but on the tuner meter it is showing 1.9

Go figure
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KB2DO
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Posts: 53




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« Reply #9 on: April 09, 2010, 10:12:57 AM »

Hi,

you are missing something'

The tuner goes between the rig and the amp.

73,
robbie
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AJ4MJ
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Posts: 48




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« Reply #10 on: April 09, 2010, 10:24:54 AM »

Original problem was a home made dipole with ladder line, dropping down to a MFJ 4:1 balun {1,500 watt rated} going to an MFJ 989D tuner and then to an SB221 amp and FT 2000D.

When I tuned up and and ran just the radio barefoot at 200 watts it worked fine, but when I tried to load up the amp on the antenna it would not give me more than 300 to 400 watts, going to a dummy load it would push 1000 plus.


Sounds like you were saturating the 4:1 balun.  See the "saturation" section here: http://www.radioworks.com/nbalun.html

If you are running a 135 foot dipole on 80 meters, I would omit the 4:1 balun.  It's likely to push your resistance very low which your tuner would have to try and make up for.  If you can cut your ladder line length to a multiple of a half wavelength, then you will need little or no "tuning".

73 de AJ4MJ
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AH6RR
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Posts: 803




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« Reply #11 on: April 09, 2010, 10:51:41 AM »

That antenna is too short for 75/80 meters it need to be 123.4 feet long The G5RV is a 20 meter antenna I know people use them for a multi-band antenna but they are desinged for 20. It sounds also like the Made From Junk tuner is also not working right, that is a big discrepency in SWR between radio and Tuner meters. I would start with a known good SWR/Power meter and check antenna with the tuner out of line(compleaty remove the tuner dont use the by-pass). Then with the tuner in line and if there is a any change then look for signs of arcing inside the tuner (including band swtiches) and for proper grounding of the tuner componets they have very poor quality costruction for the most part.

Roland AH6RR
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K1BXI
Member

Posts: 812




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« Reply #12 on: April 09, 2010, 11:06:42 AM »

"The tuner goes between the rig and the amp."

No Robbie, the tuner is always the last thing before the feed line. Between the amp and the antenna. I think he knows this, and so should you.

Greg....the G5RV is not exactly what we think of as an 80 meter dipole. I use one with a tuner and it will work fine on 80. So I am not knocking the G5RV.

I take it when you said "The current antenna is a G5RV 'styled' antenna 102 ft long." that you made this yourself. What is the ladder line portion of your feed line, 450 ohm or 300 ohm line and exactly how long is it. Is the ladder line connected directly to the LMR400, or are you using some sort of "balun" there? Something that you may have used with your other antennas.

I ask, because your resonate frequency seems to be a bit high for that antenna. The low point is usually around 3.700. Give or take a little.

Depending on many things , feed line length, frequency, antenna tuning network etc there can be some high RF voltages present at some point in the system that you don't get with your resistive dummy load.

John
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WA3SKN
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Posts: 5420




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« Reply #13 on: April 09, 2010, 01:10:24 PM »

The feed point of a typical mounted dipole is about 50 ohms, and fed with 50 ohm coax will not present a problem.
The feed point of your "inverted Vee dipole" with the ends low will also be close to 50 ohms.  But with ladder line (about 400 ohms chacteristic impedance) the impedance will vary with the line length, going up to about 3000 ohms around 1/4 wavelength away (ball park figure) before dropping back to 50 ohms at the 1/2 wavelength point... your 4:1 balun is designed for about 200 ohms.  You can force the match with the tuner, but I'll guess that the core (if any) is saturating, causing your problem.
How long in length is your feedline?  You might be able to vary it's length to get the impedance closer to desired value!

-Mike.
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WB6BYU
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Posts: 12978




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« Reply #14 on: April 09, 2010, 01:29:42 PM »

So the only items in common between the two situations are the rig and
the tuner.  That narrows down the possibilities.

What is the minimum SWR on your dipole when you tune across the band?
Can you run the amp into it without needing the tuner at some frequency?
That would determine whether it is a problem with the tuner (arcing or
saturating at high power) or the rig (picking up stray RF and reducing the
output power.)  In fact, if you put a power meter (perhaps the tuner) in
line between the amp and the radio, you can see whether the rig is
reducing the output power or not.

Do you have the ALC cable connected between the amp and the rig?
Do you have any other cables (external audio input, band switching, etc.)
connected to the rig?  Those ports (or even the mic cable) might not be
well enough bypassed - I've seen similar problems a couple of times due to
cables for digital modes.

Are you using the power meter in your tuner to check the output from the
amp,or its own internal meter?  If the former, you might have a problem
in the metering circuit that prevents it from reading over 400 watts or so,
even if the amp is putting out more than that.
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