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Author Topic: Dipole Resonace and Receive Performance  (Read 3560 times)
AF6D
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« on: June 28, 2011, 06:28:14 PM »

It appears that my commercially made multi-band fan dipole antenna is low in frequency across the board. The SWR at 14.200 for example is 5:1. It is relatively good at 1.6:1 on 80 and 40 where I work anyway. It was recommend that I cut the elements to get closer to resonance. I have an excellent LDG AT-1000 autotuner that handles the SWR and I wonder:

Does cutting the dipoles closer to resonance actually have an affect on receive, am I being nicer to my tuner by being resonant, both? or it just doesn't matter so much?
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N0SYA
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« Reply #1 on: June 28, 2011, 07:57:06 PM »

Hi
I don't think the rx performance will be any better if one cut the dipole unless one was not using the tuner. The tuner matches the entire system, and with the swr being fairly low to begin with there isn't a lot of loss even sans tuner. Without the tuner the pruning improvement could be seen if you could a/b it somehow but that would be impractical. I would just use the tuner and enjoy.
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If you have a clumsy child, you make them wear a helmet. If you have death prone children, you keep a few clones of them in your lab.
K2OWK
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« Reply #2 on: June 28, 2011, 08:57:37 PM »

Just a note. If I am correct the tuner matches the transmitters output so it sees a good VSWR. It does not match the antenna or correct the antenna for off frequency operation or high VSWR. If your antennas has 5 to 1 VSWR it will still have a 5 to 1 VSWR after you set your tuner. If you have two SWR meters with one reading the transmitter output before the tuner and one after the tuner you will find that the one before the tuner will read the low VSWR that the tuner is set for the other will read the high VSWR of the antenna. What I am saying is the tuner matches the transmitters impedance not the antennas VSWR.

73s
K2OWK
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AF6D
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« Reply #3 on: June 28, 2011, 09:33:53 PM »

Thank you. I appreciate your replies. I am familiar with the purpose of a tuner but was really asking about reeiver performance as a result of resonance. This isn't VHF where an inch would make a difference. I suppose the question is how much loss in power I have through the tuner. I'll try an experiment after my nap with a dummy load and measure PO. I'll maure the output at the amp into a dummy load and then I'll attach another SWR meter and measure PO to the antenna and compare notes. I've notcied that the meters in a LDG often read differently than stand-alone meters.

This antenna has worked great for several years. I miss my SteppIR but a guy has to do what he can do when leasing. The SWR was never an issue but I have a Live Oak tree that has grown within feet of the feed point. This antenna has always been close the roof of the house (20 feet or so at the feed point, but most of the wire isn't.) Conclusion: a bushy tree! I can either wait for fall or get the tree saw out. Other antennas now nearby are probably a factor as well. I spend most of my time on 80/40 where the SWR without the tuner is 1.6:1 so this isn't a big deal. It really is only because now I know the antenna performs poorly on 20 and above.
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N3OX
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« Reply #4 on: June 28, 2011, 10:18:32 PM »

Thank you. I appreciate your replies. I am familiar with the purpose of a tuner but was really asking about reeiver performance as a result of resonance.

As long as the signal received that makes it through the coax is sufficient to make your noise floor dominated by external noise, it doesn't much matter where the antenna is resonant. 

If it's anywhere near the band of interest that's plenty, and even a lot of off band antennas will hear just fine. Basically, as long as you get a big noise increase when you plug in the antenna, you won't be missing anything.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
WB2WIK
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« Reply #5 on: June 29, 2011, 08:33:39 AM »

Thank you. I appreciate your replies. I am familiar with the purpose of a tuner but was really asking about reeiver performance as a result of resonance.

As long as the signal received that makes it through the coax is sufficient to make your noise floor dominated by external noise, it doesn't much matter where the antenna is resonant. 

If it's anywhere near the band of interest that's plenty, and even a lot of off band antennas will hear just fine. Basically, as long as you get a big noise increase when you plug in the antenna, you won't be missing anything.

I agree with this.

But it's curious that your dipole's so far off on 20m.  Maybe it's parallel and coupling to an aluminum rain gutter or something?  I never found trees to do any real "detuning" with HF antennas.   Then, I never had any metal trees! Cheesy
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AF6D
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« Reply #6 on: June 29, 2011, 03:43:12 PM »

This is the odd part. It has worked since 2008 when I moved down here to Running Springs. I dropped the antenna yesterday and inspoected the coils -- fine as expected because 75/80m operation is fine. But the antenna is low across the board. It is just more noticable at 20m and above. The only change is the large tree next to it has grown quite large. There has always been a 2m and 6m vertical on my upper balcony. We don't have rain cutters up here in snow country and there is minimal wiring in the attic. I chnaged the coax to a new shorter run of LMR-400 just "because" and it of course had no effect. You still heard me on 20m 100 miles away Smiley
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AD4U
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« Reply #7 on: June 30, 2011, 05:39:38 AM »

Simply said, in the "real world" of HF you will not notice any improvement in receive performance of this antenna, if you can some how get the 20M part into resonance.

Dick  AD4U
« Last Edit: June 30, 2011, 07:20:53 AM by AD4U » Logged
WB2WIK
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« Reply #8 on: June 30, 2011, 08:41:54 AM »

This is the odd part. It has worked since 2008 when I moved down here to Running Springs. I dropped the antenna yesterday and inspoected the coils -- fine as expected because 75/80m operation is fine. But the antenna is low across the board. It is just more noticable at 20m and above. The only change is the large tree next to it has grown quite large. There has always been a 2m and 6m vertical on my upper balcony. We don't have rain cutters up here in snow country and there is minimal wiring in the attic. I chnaged the coax to a new shorter run of LMR-400 just "because" and it of course had no effect. You still heard me on 20m 100 miles away Smiley

Yes, I heard you fine.  I think I was hearing you better than you were hearing me.  I have no clue why your resonance dropped lower in frequency.  I'd joke about "wire stretching," but that would really be a joke.
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AF6D
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« Reply #9 on: June 30, 2011, 12:16:57 PM »

I've heard of a bacon stretcher but not a wire stretcher. Do they sell them at HRO?  Wink
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W8JI
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« Reply #10 on: July 03, 2011, 04:48:07 AM »

I've heard of a bacon stretcher but not a wire stretcher. Do they sell them at HRO?  Wink

I stretch wires all the time to remove kinks. Just last week I stretched a run of number 8 solid copper about 80 feet or 100 feet long by about 5 feet  because someone working for me cut it too short.
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AF6D
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« Reply #11 on: July 03, 2011, 02:27:50 PM »

Did you use the basic model or the premium? I looked at the basic model and wasn't impresed with its quality.
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KB4MB
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« Reply #12 on: July 07, 2011, 05:55:52 AM »

Easy way is to listen with and without the tuner.  I don't think it will make a difference.  But at least you will know with real world experience Smiley

However, since your are running coax, you will be losing a bit of power due to the 5:1 in the linewhen you transmit - if it is a long run with smaller coax, it might be a noticable difference.  Nothing to get worked up over, but there is that aspect of it. So if it is convenient, you might want to fix due to that.  Best is to find out where you are resonant and then with a calculator figure out exactly how much to trim (probably a little bit less, just to be safe! Smiley )

Also, sometimes these little things just aren't worth it - your tuner is in line and that is what it is there for - to touch up.  I have a hex-beam that in my installation the SWR on 15m is 1.5:1, and can climb to 1.8:1... this has no practical issue, but since it makes me feel better, occasionally I will switch the manual tuner in line and tune for flat.
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