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Author Topic: Tuner won't tune 80 m dipole on 80 m.  (Read 3826 times)
N4MFA
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Posts: 7




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« on: July 08, 2013, 06:08:33 PM »

I have an 80 m dipole fed with ladder line through a just purchased Dentron Super tuner. All bands will match up with nice sharp SWR dips EXCEPT 80 m. The SWR meter dip on this band is very shallow and hard to see. Is this normal?
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KH2G
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Posts: 261




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« Reply #1 on: July 08, 2013, 07:51:44 PM »

I would think so as the antenna is tuned for 80 so the tuner should not even be needed.
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WB6BYU
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« Reply #2 on: July 08, 2013, 08:36:58 PM »

How long is your ladder line?  That makes a big difference in the impedance that the tuner
has to match.  If the feedpoint impedance of the antenna is close to 50 ohms on 80m and
the line is 1/4 wavelength long, the tuner could have to match 4000 ohms.  If the length
is 1/2 wavelength it would have to match 50 ohms.  (If you have a 4 : 1 balun on the output
then the impedances would be 1/4 of that.)  Either way, that is a wide range of impedances
to match:  I would expect the Dentron to handle it, but some other tuners (including the
revered Johnson Matchbox) have a more limited matching range.  And for intermediate
feedline lengths there can be significant amounts of reactance.

But here's a test:  when you it tuned as well as you can, is either capacitor at one end
or the other of its range?  If not, there likely is some other combination of settings that
will give you a better match.  Sometimes you don't get a good dip until at least two
of the controls are close to the proper settings - this can take some experimenting to
find the right settings.
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W5DXP
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« Reply #3 on: July 09, 2013, 06:20:43 AM »

I have an 80 m dipole fed with ladder line ...

The problem with such an antenna is that the average length of the ladder-line is often around 60 feet which is 1/4WL on 3.69 MHz. If the feedpoint of the antenna is 50 ohms, the SWR on 450 ohm ladder-line will be 9:1. The impedance looking into the ladder-line at the shack end will be 50(SWR)^2 = 4050 ohms (as Dale said) and that is probably your problem. If you make the ladder-line length 1/2WL long, i.e. 120 feet, you will duplicate the antenna feedpoint impedance at the shack end. If one objects to 120 ft. long ladder-line, one is objecting to 1/2WL on 80m. Smiley The advice to use whatever length of ladder-line it takes to reach the shack is not always good advice.

Here's a chart of the optimum lengths for ladder-line for a 130 ft. 80m dipole:

http://www.w5dxp.com/pnts130.gif

The yellow highlighted region are the lengths I use, i.e. 92-110 ft. 100 ft. seems to be an appropriate multi-band length. There is no single length that would be optimum for all 8 HF bands but 100 ft. might work on all bands with a wide-range tuner.
« Last Edit: July 09, 2013, 06:25:56 AM by W5DXP » Logged

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.
K7KBN
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Posts: 2788




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« Reply #4 on: July 09, 2013, 07:25:21 AM »

I think KH2G's idea is worth looking into.  Feed the 80M dipole with some 50-ohm coax and see how it works.  You shouldn't need a tuner if the feedpoint impedance is the same as the feedline's characteristic impedance.
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73
Pat K7KBN
CWO4 USNR Ret.
N4RSS
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Posts: 258




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« Reply #5 on: July 09, 2013, 08:14:11 AM »

I think KH2G's idea is worth looking into.  Feed the 80M dipole with some 50-ohm coax and see how it works.  You shouldn't need a tuner if the feedpoint impedance is the same as the feedline's characteristic impedance.

He's using it on all bands, ladderline is best, just needs to toy with the length
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W5DXP
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« Reply #6 on: July 09, 2013, 08:52:19 AM »

Feed the 80M dipole with some 50-ohm coax and see how it works.  You shouldn't need a tuner if the feedpoint impedance is the same as the feedline's characteristic impedance.

That idea would trade all the other HF bands for single-band 80m operation. I'm not sure he would be satisfied with that "solution".
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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.
WB0FDJ
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Posts: 141




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« Reply #7 on: July 09, 2013, 10:58:03 AM »

I have an 80 m dipole fed with ladder line through a just purchased Dentron Super tuner. All bands will match up with nice sharp SWR dips EXCEPT 80 m. The SWR meter dip on this band is very shallow and hard to see. Is this normal?

What exactly is your SWR reading?

Doc WB0FDJ
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W9IQ
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« Reply #8 on: July 09, 2013, 11:38:09 AM »

I thought the Dentron Super Tuner has a 4:1 balun in place when using the balanced line inputs. If this is the case, then some of the impedance calculations the tuner sees (after the 4:1 balun) should be revised.

- Glenn W9IQ
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N4MFA
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« Reply #9 on: July 09, 2013, 02:39:29 PM »

As near as I can see, without taking down the antenna to measure, the feedline is about 60 or 70 feet long. This would be about 1/4 of a wavelength on the 80 meter band. The halves of the dipole are each almost exactly 66 feet long. I measured the dipole halves before putting up the antenna. So the end at the tuner must be at a high impedance point, right?
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WB6BYU
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« Reply #10 on: July 09, 2013, 04:59:22 PM »

Yes, it will be a high impedance on 80m - as it will be on 40m and 20m (where the feedpoint
impedance is high and the line is near a multiple of 1/2 wavelength.)  Since you can match
the antenna on 40m and 20m, having a high impedance alone may not be the problem. 
(Actually it is easier for most common tuners to match high impedances than low ones.)
However, on either side of resonance the reactances can be high, and you might be
running out of matching range on one or the other variable capacitors.
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N4MFA
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« Reply #11 on: July 09, 2013, 06:49:09 PM »

I'll redo the feedline  (longer) and see if that helps. Thanks to all who replied with suggestions.

Alan
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N4JTE
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Posts: 1155




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« Reply #12 on: July 09, 2013, 10:15:40 PM »

Laddeline, 450 ohm, although, somewhat forgiving on the "other" bands, on the band you are mentioning you are around 9 to 1.
Perhaps a longer feedline would help, Cecil has been there did that hi.
Bob
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W5DXP
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« Reply #13 on: July 10, 2013, 08:29:08 AM »

If (a 4:1 balun) is the case, then some of the impedance calculations the tuner sees (after the 4:1 balun) should be revised.

Yes, and 1/8WL be added to the lengths in my chart. The current maximum points in the chart are the low resistance points. Adding 1/8WL of ladder-line to a current maximum point might be an easier match for some 4:1 baluns as it might result in an impedance in the ballpark of 112+j450 ohms which the 4:1 would transform to 28+j112 ohms, an easy match for the tuner.
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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.
KH2G
Member

Posts: 261




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« Reply #14 on: July 11, 2013, 05:13:50 PM »

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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