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Author Topic: Tuner location  (Read 1001 times)
KB3FFH
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Posts: 162




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« on: February 09, 2012, 08:28:57 PM »

Is there a big differance between a tuner at a dipole base as opposed to one 100ft away at the transceiver? Thanks Bill
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N3OX
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« Reply #1 on: February 09, 2012, 08:44:40 PM »

Bill, it depends on the bands you're trying to run and so forth, but generally, yes, there is a difference and it can be a big one.

For example, if you try to use a coax fed 80m dipole on 40m, the impedance on 40m will be really high, the SWR on the coax will be terrible, and there will be a large increase in the loss in the coax.  I have a 60 foot vertical here that has a homebrew tuner at the base.  It's resonant around 3800kHz.  I have a nice L network that puts it on 40m:

http://n3ox.net/projects/stepperswitch/40m_match_lg.jpg

Once I tried to tune it with the tuner in the shack instead.  I have about 90 feet of coax between the shack and the base of the antenna.  My matching network switch had a "bypass" position, so I toggled back and forth between the tuner in the shack and the L network at the base of the antenna.  The difference as measured on some broadcast carriers on 40m was about 10dB.  90% of my power was being lost in the coax when I used the shack tuner instead of the L network at the base!

To prove a point about this sort of thing I spent the evening down on 40m CW working with the shack tuner anyway.  Made some nice contacts, including breaking a pileup for a station in Gambia, but all that proved was that ten watts radiated from a vertical on 40m can make some nice contacts.

Feeding a half wave vertical directly with coax (or direct coax feed of a full wave doublet like an 80m dipole on 40m) is among the WORST things you can do in terms of extra feedline loss.  The only thing worse than a high impedance resonance is an antenna that's much SHORTER than a quarter wave.  Like if you try to use a coax fed 80m dipole on 160m you might see 15-20dB loss!

But it all depends on the exact antenna impedance on each band you intend to use it on.  Using a shack tuner to "stretch" the bandwidth of an 80m dipole across the band is no problem.  The overall system performance also depends on the power you can run... if you have an amp and shack tuner and you lose 70% of your power in the feedline, you might still come out stronger than a 100W autotuner at the antenna...

Generally putting any matching device at the feedpoint is best if you need to use coax.
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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
N4JTE
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Posts: 1158




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« Reply #2 on: February 09, 2012, 09:26:40 PM »

A single band dipole needs no tuner, nature of the design, unless wanting full band coverage, still no big deal with tuner in the shack.
Bob
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WV4L
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« Reply #3 on: February 10, 2012, 08:23:47 AM »


As Dan pointed out.
If you are feeding with coax. Tuner at the base for multiband use. If you want multiband use and the tuner in the shack feed with ladder line.
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NA0AA
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« Reply #4 on: February 10, 2012, 11:37:52 AM »

The theory is that by locating the tuner remotely at the antenna, the SWR on the feedline from the tuner [or coupler] is the lowest possible, therefore reducing the losses on that portion of line to the nominal 'matched' loss of that feedline.

Just pulling numbers out of the air for the purposes of illustration, say your coax has a loss at 10 MHz of 0.8 dB per 100 feet when matched, and a loss at say 10:1, of 5 dB per 100'

Your feedline is 150 feet.  A matched feedlines loss at that length would be 1.2 dB.  An un-matched feedlines loss would be 7.5 dB for the same length.  Pretty easy to see that your power losses of 6.3 dB makes a pretty good arguement for remote couplers.

Given this advantage, it's pretty easy to see the advantages of a remote coupler, alhough I think it's going to be a while before high-power handling remote couplers will be as commonplace as in-shack tuners.
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KW6LA
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Posts: 93




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« Reply #5 on: February 10, 2012, 04:37:34 PM »

There is a lot of matching that can be done at the feed point instead of the tuner. My Alpha delta 40/ 80 dipole had a bad match, so I made a helical hair pin to match 50 ohms.
That said, it was 4 feet over my 2 story home and not a text book install.  Sometimes you are stuck with Bum antenna systems ! Good Luck.
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