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Author Topic: 20m "random" wire length?  (Read 4694 times)
KB4QAA
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Posts: 2260




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« Reply #15 on: May 13, 2013, 07:53:04 PM »

As long as you your tuner can match the load, that is all that matters.

When the tuner "matches the load", the antenna system is resonant by IEEE definition. The random length of wire winds up being one component in a resonant antenna system just as the caps and coils in the tuner are components in the resonant antenna system. Such can be proven by using a grid dip meter on the random length of antenna wire after the tuner is properly adjusted. Assuming the receiver has a 50 ohm input impedance, the GDO will indicate a resonant dip very close to the transmitter frequency after the tuner has achieved a match.

http://www.w5dxp.com/OWT1.htm
A wonderful discussion of antenna matching.  I don't believe anything I have said disagrees!  Cheers, bill
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WB0KSL
Member

Posts: 94




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« Reply #16 on: May 13, 2013, 07:59:04 PM »

Quote from: W6BYU

While some of us prefer to use wire lengths that are multiples of 1/2 wavelength
because they are easy to match with simple tuners, either homebrew or commercial.


Agreed, otherwise the end fed half wave would not work out so well :-)
I think that MOST of the commercially available remote tuners, such as the SGC and others, get a bit finicky trying to feed a half wave.  The web page tables are intended to help pick a wire length that avoids that problem on all bands.  If one has a tuner that handles matching a half wave cheerfully, on all bands of interest, that clearly obviates the need for the "magic" length tables.  Sadly, my SGC-237 is not among them.  As to performance, clearly efficiency, pattern, gain are affected by the length of the random wire, but as I see the purpose of the tables of suggested lengths, it is in the light of ease in matching, not performance in the traditional sense.  If i want to work, say, 30 meters, but my tuner won't handle the match because the random wire is a half wave or multiple, then I have no "performance" at all on that band.  I see it as a matching issue only.

73 de wb0ksl
John

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W5DXP
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Posts: 3540


WWW

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« Reply #17 on: May 14, 2013, 07:18:14 AM »

I don't believe anything I have said disagrees!

I apologize if I appeared to disagree with you. I was just expanding the thread to the other part of the antenna system that tends to get ignored. Lots of good things happen between the tuner and the antenna when we "make the transmitter happy". If the antenna is happy when it is radiating the maximum amount of RF, we might even say, "a tuner also makes the antenna happy". Smiley
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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.
NU9J
Member

Posts: 109




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« Reply #18 on: May 14, 2013, 09:28:47 PM »

Well that was interesting  Wink

To conclude the story, I ended up running a 16.5' horizontal piece of magnet wire (invisible btw) from the end-fed port of the tuner out the window with an identical piece of wire connected to the ground post running inside. It tuned without trouble to have 0 reflected power. I made a contact in Cuba on 20m bpsk-31 with 20 W tonight with a 59 report (though, that is admittedly not so reliable). The setup also seems to tune very well on 17m and 10m, but has a lot of trouble on 12m. But hey, that's much more than I asked for!
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~Philip
WB6BYU
Member

Posts: 13010




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« Reply #19 on: May 15, 2013, 08:17:37 AM »

Glad you got it to work!

Adding extra quarter wave wires on the indoor side for each band will make it
easier to tune and reduce some of the possible quirks.
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