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Author Topic: Tuner won't tune 80 m dipole on 80 m.  (Read 4563 times)
W5DXP
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« Reply #15 on: July 12, 2013, 08:14:01 AM »

If the antenna is already matched, why would you expect to see a dip?

The antenna is ~50 ohms and the feedline is ~450 ohms with a 9:1 SWR. That's NOT a matched system. Ladder-line fed dipoles are not flat systems like coax fed dipoles are. I assumed he meant that the SWR did not dip to an acceptable value.
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K8JD
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« Reply #16 on: July 15, 2013, 03:53:54 PM »

I put up an EDZ for 20M Last spring. It's a just shy of 90 ft long dipole and happens to be about 1/2 wave on 60M and I can tune it  easily thru 150 ft of bal line and an MFJ-974 balanced line tuner.
Have worked Hawaii and Europe with it on 60M. Also works fine on 40M where it's about a  3/4 wave dipole /

.
Maybe I misunderstood the original question. He said all is well on the other bands but was questioning a shallow to no dip on 80 and I still say - If the antenna is already matched, why would you expect to see a dip? 80 Mtr antenna on 80 is usually pretty flat.
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73...John
SKCC 1395T, FISTS 3853
Official US Taxpayer
GW3OQK
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« Reply #17 on: July 16, 2013, 01:02:01 AM »

Last week I put up a doublet with 36m top and 18m of 450 ohm ladder line. (13m high) It tuned up on 5 MHz to 28 MHz, but not on 3.5. Neither my Z-Match or VC300 would tune it. An extra 15m of 450 ohm line made no difference.

Finally my old home brewer's instinct made me think there was not enough coil for 80m in the Z-Match so I added 2 turns.  Now it tunes up perfectly, with the tuning capacitor approaching max.

The reason I chose this size antenna was to get lobes at 45 degrees on 20m and up if I could. My garden runs E-W and the best DX directions from UK seem to be in the quadrants, not N-S.
 
Andrew
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W5DXP
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« Reply #18 on: July 16, 2013, 06:53:13 AM »

Last week I put up a doublet with 36m top and 18m of 450 ohm ladder line. (13m high) It tuned up on 5 MHz to 28 MHz, but not on 3.5.

EZNEC says the impedance looking into the ladder-line on 3.8 MHz is 821+j1563 ohms. If you doubled the length of the ladder-line to 36m, the impedance looking into the ladder-line would be 53-j98 ohms, much easier to match. If you increased the length of the ladder-line to 38.5m, you would have a nearly perfect match to 50 ohms requiring no tuner at all. Of course, 38.5m (1/2WL) of ladder-line may be hard to manage on 80m but 1/2WL is much more manageable on 40m.

A 1/2WL dipole fed with 1/2WL of ladder-line is a resonant system requiring no tuner. For instance, a 65 foot dipole fed with 61.5 ft. of ladder-line would be a resonant system on 7.2 MHz, requiring no tuner.
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KD8IIC
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« Reply #19 on: July 16, 2013, 10:01:23 PM »

 I found good information on the DX Engineering web site in their balun tech notes section about tuning open feed lines and came away with this very good advice from W8JI ; Divide lowest op freq into 123, then multiply by velocity factor of lead in, .91 or whatever.This will give you the first Odd 1/8th Wavelength Multiple.The other lead in lengths to use are mutiples of 3-5-7-9 etc. Never an even number...This will get you in the ballpark easier than the trim and try method.Saved me a lot of trouble.  73
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GW3OQK
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« Reply #20 on: July 17, 2013, 05:41:02 AM »

My point was answering the subject. My "Tuner won't tune dipole", so I modified the Tuner. Now its tuning perfectly!

Andrew
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W5DXP
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« Reply #21 on: July 17, 2013, 11:43:57 AM »

Now its tuning perfectly!

There's no such thing as a perfect antenna tuner. Smiley
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