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Author Topic: General tuner question  (Read 942 times)
KA3VEZ
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Posts: 92




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« on: July 09, 2011, 05:45:49 PM »

Hello all.  I have a basic question for tuners.   I made a simple 40m dipole and cut it for the SSB portion of the band using the 468 / freq.  After shorting another 9 inches on both sides I got it to work.  Question is now if I had a tuner and wanted to goto the CW portion on 40m would the tuner be able to tune down there or is it better to just make another dipole for the that portion of the band? Thanks for any ideas.


-Kirk, KA3VEZ


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KI4AX
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Posts: 113




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« Reply #1 on: July 09, 2011, 06:12:34 PM »

Have you tried the antenna at the cw portion of the band to see what the VSWR is? As long as it is below 2.0 it would probably be OK. Otherwise just get a tuner and tune it in at the frequency you want to operate on. Once you get it "tuned in," at the frequency you want, make a note of the settings on the tuner so that the next time you want to operate that frequency, or close to it, you can set the tuner up before you start transmitting; its easier on the finals that way. You will a have small amount of loss in the tuner but it should be very minimal. If this small amount loss bothers you then put up a second wire... but difference will, most likely, not even be noticeable.

Dan KI4AX
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KA3VEZ
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Posts: 92




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« Reply #2 on: July 09, 2011, 06:19:17 PM »

Yes I did try and the SWR was well over 1.5 and I don't want to push it over that limit if I can help it.

~Kirk, KA3VEZ
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W8JI
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« Reply #3 on: July 09, 2011, 07:04:15 PM »

Yes I did try and the SWR was well over 1.5 and I don't want to push it over that limit if I can help it.

~Kirk, KA3VEZ

Kirk,

40 meter dipoles are normally able to cover the entire band with acceptable SWR. You need to cut it for the middle of the range.

Maybe you are being overly fussy about SWR??

What frequency did you cut it for, and what was the SWR there? What size wire is the antenna made from, and how high is it?
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W8JX
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« Reply #4 on: July 09, 2011, 07:06:50 PM »

Yes I did try and the SWR was well over 1.5 and I don't want to push it over that limit if I can help it.

~Kirk, KA3VEZ

Actually it will not hurt a solid state radio but it will cause it to throttle back to protect itself. If radio has a built in tuner, 3 to 1 or a bit more is not a problem. Old radios with tube finals had a Pi L network in output that could match a 3 or 4 to 1 SWR to finals with no problem.  
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W5FYI
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Posts: 1045




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« Reply #5 on: July 09, 2011, 09:09:34 PM »

I agree with W8JI and the others, a 40M dipole should cover the entire band without needing a tuner. What you could do is attach an alligator-clip jumper to the ends of your existing dipole whenever you want to operate CW at a low SWR. Better yet, solder the additional wire to the dipole and retrim it for the middle of the band.

Low SWR shouldn't be the only goal for pruning antennas. Even a 2:1 SWR can deliver close to 100 percent of your power to the antenna (depending on coax losses and length). And, as KA3VEZ pointed out, most solid-state rigs use a fold-back circuit to throttle back power when SWR becomes too excessive.

To answer your original question, yes--a tuner will be able to present your radio with a 1:1 SWR, but you would still have the same losses in your coax, plus the slight additional loss in the tuner. GL
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K7KBN
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Posts: 2802




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« Reply #6 on: July 09, 2011, 11:37:41 PM »

I remember operating my DX-100B for 4 or 5 years before I ever used an SWR meter to check the antenna.  I'd worked DXCC and all states on 40 CW using a dipole cut to "about" 66 feet overall.  Transmitter loaded well; no balun and no RFI even though my folks' RCA 21" TV (!!!) was sitting about 8 feet away in the next room.

SWR was just over 8:1.  Noted it in my log and kept making DX contact after DX contact.  Eventually when I moved and took it down, I did an actual measurement of the length.  One half was 31 feet; the other half just less than 28.

I miss those days!
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73
Pat K7KBN
CWO4 USNR Ret.
WB6BYU
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Posts: 13243




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« Reply #7 on: July 10, 2011, 05:55:56 PM »

Quote from: KA3VEZ
Yes I did try and the SWR was well over 1.5 and I don't want to push it over that limit if I can help it.


1.5 : 1 shouldn't be any problem for a transmitter.  While there may be some transmitters that start
to throttle back the output at this level because they don't have enough margin, but a reasonable
design should handle at least 2 : 1 without problems.  (Most tube radios will handle a much wider
range than this.)  I can't remember ever having a problem with the operating SWR bandwidth of
a 40m dipole, even one with thin wire:  I just string it up and use it across the band.

But, yes, you can use an antenna tuner to extend the SWR bandwidth of an antenna.  This is
more commonly done on 80m, where the 500kHz represents a much wider percentage bandwidth.

Or, one approach I've used for 80m dipoles is to tie the insulator in a few feet from the end of the
wire and tie it off to the end support with rope in the normal manner.  I put an alligator clip on a
short piece of string (so it is INSULATED from the wire) and I can vary the resonant frequency by
clipping it out the support rope (lowest frequency), back up the antenna wire (highest frequency),
or somewhere in between to tune the dipole exactly where you want it without having to cut the
wire.
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