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Author Topic: How low to adjust whip SWR before using tuner?  (Read 792 times)
KD6KWZ
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Posts: 276




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« on: November 02, 2011, 09:30:31 PM »

I'm tinkering with a Wilson FGT-5 Sliver antenna for 10 meters. I don't want to junk it for possible future 12 meter use.
I'm trying different length whips on the end to try for the best SWR without using a tuner. How low an SWR do I need
to get the SWR on the whip only, so that most of the power is not lost in the tuner?
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K7KBN
Member

Posts: 2813




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« Reply #1 on: November 02, 2011, 10:33:26 PM »

Get it low enough so it's in the range of the "tuner".  You'll likely lose more power in the feedline than in the "tuner".
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73
Pat K7KBN
CWO4 USNR Ret.
N8WWM
Member

Posts: 26




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« Reply #2 on: November 02, 2011, 11:16:45 PM »

If you want to use it on 12 meters try getting a stiff piece of wire and clipping it on to the existing whip. You probably will get great results with adding 6 inches or so to the overall length. Experiment!

Wire is cheaper than a new 12 meter antenna. If you find that it does not work, fine. However, I would bet if you started with a foot of wire being slid up and down the whip until you found the right length for good SWR on 12 meters, you will find the sweet spot.

Use 2 small tubing clamps to loosely hold the wire to the whip...when you find the sweet spot on 12 meters just mark the clamps and wires appropriately. Then take it all off for 10 meters and put it back on for 12.

73 Doug
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WB6BYU
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Posts: 13337




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« Reply #3 on: November 03, 2011, 10:00:46 AM »

I'd just replace the whip with a length of stiff copper wire and trim it for minimum SWR, then
look for a whip of about the same length.

How low the SWR has to be depends on the tuner - some will only match up to 3 : 1, and you
should be able to get it under that.  (In fact, you should be able to get it low enough that you
don't even need a tuner, unless it is a quirky antenna.)

Unfortunately, tuner losses depend on the IMPEDANCE, not the SWR.  Losses generally will be
lower with a 500 ohm load than with a 5 ohm load, even though both are an SWR of 10 : 1.
You have the additional problem of minimum capacitance and stray reactances on 10m that
mean a tuner designed for high efficiency on 160m and 80m may not be as efficient on 10m.

But, as mentioned before, the highest losses likely will be in the feedline rather than in the
tuner.  If you get the SWR below 3 : 1 just about any tuner should match it efficiently,
and even 5 : 1 probably is OK if the tuner has enough matching range.  But if you can
tune it down to 1.5 : 1 then you will reduce the coax losses and eliminate the tuner losses
because you won't need to use it.


Edited to add:  another way to lower the resonant frequency is to add an alligator clip,
possibly with a short length of wire attached, to the existing whip (presuming it is metal
rather than fiberglass.)  Slide the clip up and down the element for minimum SWR.
« Last Edit: November 03, 2011, 10:02:18 AM by WB6BYU » Logged
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