eHam Forums => Elmers => Topic started by: KL3HY on May 09, 2012, 08:49:50 AM

Title: First Manual Tuner
Post by: KL3HY on May 09, 2012, 08:49:50 AM
I just picked up a used manual tuner, a Daiwa CNW-518, and I'm having problems getting it to tune adequately on a couple of bands.  For example, on 75m, about 2:1 is about the best it'll do.  Dropping down to 80m though, it has no problems and it tunes up perfectly fine.  40m tunes fine as well, but 20m it'll only do about 2:1.  The antenna I'm using is an 80m OCF dipole with a 4:1 current balun, and feedline of 65 feet of LMR-400UF.

I got this tuner to allow me to use an amp at some point.  My little LDG KT-100 works beautifully with my Kenwood TS-450, but it seems like this Daiwa has a much narrower matching range (if that's the right term) that it's capable of tuning.  When I was considering buying it I looked up the reviews on it here, and they are pretty much glowing with praise.  Of course, that makes me wonder if mine is damaged somehow.  I pulled the case off, and nothing jumps out at me as being obviously wrong.

Is the narrow matching range I describe above normal for this tuner?


Title: RE: First Manual Tuner
Post by: WB2WIK on May 09, 2012, 09:05:56 AM
The LDG is a reversible L-network with thousands of matching combinations.

The Daiwa is a pi-L, not reversible, only rated to matech 25 to 100 Ohm loads below 40m (80m, 160m) and that's not much range, just 2:1.

The output series cap is only 380 pF.  The Daiwa does have a quite limited matching range, even according to its own manual.

I'd use something else, like a wide-range T-network.

Title: RE: First Manual Tuner
Post by: KL3HY on May 09, 2012, 09:32:15 AM
Okay, that's about exactly what I'm seeing.   I probably should've done more research on this before I bought it.  :(

Oh well--it's making me think again about converting my OCF dipole into an OCF cage dipole now.   ;D


Title: RE: First Manual Tuner
Post by: KL3HY on May 09, 2012, 10:30:03 AM
What if I use the Daiwa to get the SWR as low as possible and then use the amp's plate/load adjustments to take it down from there?  Would that work?

I actually have the amp--it's a Dentron gla-1000b.  I haven't done anything other than power it up in standby mode so far.

This is all new to me, so I apologize if my questions are dumb ones.


Title: RE: First Manual Tuner
Post by: WB2WIK on May 09, 2012, 10:34:57 AM
There are no dumb questions, just dumb rules. ;)

The GLA-1000B will handle some mismatch.  If your tuner can get it down to 2:1 or so, the amp will likely handle that okay.

Only problem is "tuning time" with the GLA.  It used sweep tubes which are very intolerant of long tune-ups, and tuning up with lower power (reduced drive) doesn't work.  Can't work, as the TUNE and LOAD settings will be different at full power; so basically, you must tune at full power in order to actually accomplish tuning.  And with the GLA, if you can't get that done within about 10 seconds, the tubes start degrading; so it's one of those things you have to get used to doing quickly.

Title: RE: First Manual Tuner
Post by: WA3SKN on May 09, 2012, 11:02:52 AM
If you question the tuner, remove the antenna and try it with a dummy load. Most any functional antenna tuner should be able to match a 50 ohm dummy load!  If it doesn't, you can blame the tuner!
Then you can move the dummy load to the far end of your coax and test again... bet it still loads OK.
Then you can blame the OCF dipole for not being a close enough match, or blame the tuner for not being able to provide the match... some on the market have a more limited range than others.
Now... are you "locked in" to that OCF dipole or might other antennas work better for you?


Title: RE: First Manual Tuner
Post by: KL3HY on May 09, 2012, 11:26:18 AM
The only thing I think I could probably blame the OCF dipole for is that the legs might be just a touch long.  A friend has an antenna analyzer and I'm going to see if I can bribe him into bringing over.

One thing I forgot to mention but I don't know if it's relevant, is that I have a Dimond X300A 2m/70cm antenna mounted just above the OCF dipole balun.  One of the X300's radials is very close to the short leg of the dipole, and fairly parallel to it.  I do have plans to move the HF antenna to a different mast, but I haven't done it yet.

Title: RE: First Manual Tuner
Post by: K4SAV on May 09, 2012, 12:35:47 PM
In order to get an 80 meter OCF resonant on most of the bands that it operates, it is usually necessary to make it resonant on the low end of 80 meters.  That means the feedpoint impedance will be very high on the high end of 75.  Because of the high SWR on the feedline, there is a big impedance translation that happens in the 50 ohm feedline.  With 65 ft of LMR400, the impedance the tuner sees is probably 7 to 10 ohms.  That would be out of the range of your tuner.  You might be able to make the tuner tune it by changing the length of the feedline.  Lengths of 75 to 90 feet seem to be good.

Jerry, K4SAV

Title: RE: First Manual Tuner
Post by: KL3HY on May 09, 2012, 12:58:16 PM
Jerry--that's interesting because I will probably end up adding some length to the feedline.  Currently I have a 3' Rohn tripod on my single story ranch style house, with a 10' mast, putting both my antennas at about 25' high.  My plans include setting up a Rohn H50 next to one of the eaves and to do that I'll have to add about 20' of feedline.

Title: RE: First Manual Tuner
Post by: KL3HY on May 11, 2012, 08:19:45 AM
I borrowed an antenna analyzer from a friend and I think this Daiwa tuner has a problem.  First I hooked up the analyzer directly to the feedline, and the results were about as I expected.  However, when I placed the Daiwa tuner inline, I noticed that the values displayed by the analyzer didn't change when I moved the tuner dials.  When I accidentally bumped the tuner lightly, I got a big jump in the numbers on the analyzer.  Each time I lightly tapped the tuner, I'd get big swings in the numbers on the analyzer.

I've already had the cover off the tuner just to check for obvious shipping damage, but I think I need to take it off again and go over everything inside very closely.  Anyone have any hints of what I should look for specifically based on the behavior I mention above?


Title: RE: First Manual Tuner
Post by: K8AXW on May 11, 2012, 08:29:36 AM
sumthin loose....cold solder joint....joint with no solder...... as a last resort, hook it up and run some power through it while taping the wires/components with a wooden dowel rod or pencil.  Look for the jump as you do this.

Title: RE: First Manual Tuner
Post by: WB6BYU on May 11, 2012, 08:33:45 AM
Make sure that the coax connectors don't have intermittents in the either the shield
or the center pin.  Often we focus on equipment and don't think about the effects of
the patch cables that connect them.  In my experience the most common problems
in this area are:
- broken or inadequate connection between shield and connector shell
- center conductor not soldered properly to inside of center pin
- inner of coax socket spread slightly, making intermittent contact

Then check over the tuner:
- Make sure you understand the switching, as some have a bypass mode or allow
the connection to a dummy load.
- Look for burn or arc marks on the switch contacts
- Check that the wires are properly connected to the coax fittings
- Turn the knobs through their ranges to make sure that they line up with the markings
on the dial.  It isn't uncommon for a knob to slip, or to be replaced pointing the wrong
way, so that the switch is in a different position than the dial indicates.
- Of course, any loose hardware requires investigation.

Title: RE: First Manual Tuner
Post by: KL3HY on June 08, 2012, 01:25:00 AM
I just wanted to post a followup to this one.  It took me a while but I've got everything figured out, and I feel a little stupid about some things.  For example, I haven't experienced any change in SWR when tapping/knocking on the tuner case as I mentioned earlier--the only thing I can think of to account for what I saw before was that I probably had the transceiver in USB/LSB, and the mic was picking up the sound of the tapping.

Also, with regard to the high SWR on 75m, when tuned up with the amp the SWR is down to about 1.5:1, so that appears to work like I was hoping.

I'm pretty excited now that I have my amp running--I seem to be pushing about 400 watts using this old Dentron GLA-1000B, and I'm very happy with that.  My goal was to be able to turn down my radio and still put out at least 150 watts, and it's doing that with no problem at all.

Thanks again!