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1-10 of 10 messages
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Radio Works Line Isolator & AutoTuner
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by KB0GU on August 25, 2003
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Interesting, I just installed two T-4 Line Isolators due to ground loop problems in the shack. The first I installed with a double male connector right at the back of the Kenwood TS870 and a coax to the input on the amplifier. The second isolator is on a double male connector, actually 90' then double male connector on the output of the amplifier and a coax to the tuner. This cleared up some interference on 20 meters at high power. Now when I attempt to tune up the amplifier on 30 meters using the 40 meter band setting on the amplifier (which I have done successfully in the past) my auto tuner in the TS870 cannot find a low SWR to the input of the amplifier. Prior to putting the line isolator on and now if I take it back out of the line between the radio and the input of the amplifier it would quickly find a 1.2:1 SWR and I could load the amplifier to the legal limit for 30 meters. With the line isolator in the auto tuner thrashes around and never achieves less than a 3:1. If I place a short coax approx. 6 feet long out of the radio then the line isolator then another coax to the amplifier input it will come closer to finding the match to the input of the amp but not quite. Am I missing something here?
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RE: Radio Works Line Isolator & AutoTuner
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by WB2WIK on August 25, 2003
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The input SWR of the amp is probably quite high on 30m, and the isolator may be totally saturating (it's a ferrite device) and making the situation much worse. These isolators are intended for use where the line is well matched.
Hmmm...an amp on 30m? The TS870 runs close to the "legal limit" on 30m by itself :0)
WB2WIK/6
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RE: Radio Works Line Isolator & AutoTuner
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by N1JAO on August 25, 2003
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I was thinking the same thing- your limited to 200 watts on 30 meters. Check this link out from the ARRL- it is the band plan, and power limits.http://www.arrl.org/FandES/field/regulations/Hambands_color.pdf
That being said, I also use the T4s, and found the same thing with 200 watts on my MKV- but adding short jumpers fixed the situation.
Robert
N1JAO
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RE: Radio Works Line Isolator & AutoTuner
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by AC5E on August 25, 2003
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Running power on 30 is a definite no-no, but you probably have one of those situations where your coax is close to an odd number of electrical quarter waves on 30.
That turns the last quarter wave of coax into a "matching transformer," and the transformation ratio can change drastically in just a few inches of coax. That often turns a "matching transformer" into a mismatching transformer.
A jumper, ideally something around 8 feet or 1/8th wave on 30, should bring your coax to close to an even number of quarter waves - an integer number of half waves - and give you a rig end SWR that's close to the SWR on the antenna itself. If the SWR is still high, you need an antenna that's a closer match to the feedline.
73 Pete Allen AC5E
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RE: Radio Works Line Isolator & AutoTuner
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by KB0GU on August 25, 2003
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I understand the legal limit for this band and would run 200 watts or less into any antenna. But, when tune up occurs it is into dummy load for maximum output and then throttle back to limit power or less for linearity. The isolator saturates when the auto tuner is seeking its match? I thought the power output was about 5 to 10 watts in the auto tune mode?
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RE: Radio Works Line Isolator & AutoTuner
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by AC5E on August 25, 2003
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Well, looking at the dozen or so amps around the shack, not one has a tuned input for 30 Meters. 40, yes and a low SWR into the amp input. 20, same thing. 17, the 20M input is usually broadbanded enough to work satisfactorily.
But 30 - no, because it's too easy to let the power output slip up on you, to the point of illegality. Amps have to be type approved before they can be sold, and I suspect the FCC would not approve an amp whose input would be anywhere close to 50 ohms on 30 Meters.
73 Pete Allen AC5E
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RE: Radio Works Line Isolator & AutoTuner
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by W8JI on August 26, 2003
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Isolators are just beads over coax. The only thing that affects the beads is current along the shield of the cable, called common mode current. SWR does not set the level of common mode current, so SWR would have no effect on how hot beads or "saturated" beads get when the beads are in a line between a tuner and radio.
Typically beads will overheat long before they reach saturation, unless you are running a short duty cycle or pulse transmitter. But I doubt you have that problem between pieces of gear. You might have thermal drift in something in the antenna.
The last place you want an "isolator" is in the shack between pieces of interconnected equipment. You really want every chassis of every piece of gear at the operating table at the same electrical potential, not with chokes on the interconnecting cables that would let the chassis float independently.
Ground everything in the shack together with short leads, and use "isolators" (which are really choke or current baluns) at the exit point of the shack if necessary!!!
The tuner should NOT be sensitive to RF. You may be trying to correct for a design problem in the tuner. You also could have something in the antenna arcing or changing value with heat. Tuning a bad antenna can be a real problem.
73 Tom
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RE: Radio Works Line Isolator & AutoTuner
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by W8JI on August 26, 2003
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By the way, NEVER expect to be able to use an auto tuner in the rig to tune the input of an amplifier.
The input impedance of most amplifiers varys quite a bit with drive power as well as amplifier loading. That will generally "confuse" an auto tuner.
73 Tom
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RE: Radio Works Line Isolator & AutoTuner
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by KB0GU on August 26, 2003
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Hmmm, interesting responses. Thank you one and all for taking the time to reply. I also e-mailed Radio Works and got an explanation which was helpful. I have learned that I must be more precise in my posts to avoid oblique responses. Legal limit for 30 meters is 200 watts hence I stated, "legal limit for 30 meters", all equipment is bonded with short grounds straps to a common ground on the tuner and the tuner is grounded by buss to multiple ground rods and a radial field burried in the yard, the nearest ground rod is within 5 feet of the operating position, the remainder 8 of them surround the foundation of the house and are all inter-connected by ground wire used by electrical companies. The auto tuner in the transceiver effected a match to the input of the amplifier when the amp was placed in the 40 meter band and produced a maximum of 500 watts in the amplifier's SSB position and 300 watts in the CW position, much lower than the potential of the amplifiers rated 1.5kw ability, since the input network designed for 40 meters is lossy on 30 meters and limits the amount of drive power actually applied to the tubes even though the auto tuner made the radio happy with the presented load prior to installation of T-4 line isolators by Radio Works. The auto tuner refuses to find this same match when the line isolator is placed in the existing coax line to the amp with no further changes to that line except the line isolator and its male to male adapter. Adding an additional hunk of coax to this line improved the situation but did not correct it to pre-line isolator state. The antenna is not intermittent as the test was conducted with the dummy load instead of the antenna. Who would transmit illegal power output on any band? The output of the Kenwood TS-870 is 5 or 10 watts when the auto tune is activated and the transceiver attempts to find a low SWR and then shuts off, failing to find acceptable SWR it thrashes until an alarm sounds and you deactivate the auto tune mode. I think saturation is out of the question with that low of power. No heat is exhibited by the isolator under any power level even the 100 watt ability of the transceiver. So, the answer? The line isolator presents a delay in the phase of the signal travelling to the amplifier effectively lengthening that coax electrically. The isolator also introduces a block on any RF flowing on the outside of the coax which should not affect the ability of the rig to auto tune within its range. The dynamic fluctuation of the tubes input under differing drive levels coupled with a change in the coax electrical length due to the isolator appears to gang up and be the culpruit. Perhaps the rig is now truly seeing this dynamic fluctuation much better with the isolator in place and associated electrical length of the coax and hence it can't find a match as it keeps moving just outside of the lock ability's reach where before the coax length without the isolator was just right and allowed this lock to happen. Sounds reasonable to me.
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RE: Radio Works Line Isolator & AutoTuner
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by WB2WIK on August 27, 2003
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KB0GU, I don't have a TS-870S, but possibly it has the same feature as my TS-850S, which is:
-You can use the internal tuner in a non-automatic mode, by selecting a menu function (discussed in the operator's manual) and then using the M.CH tuning knob to adjust the tuner.
This often provides a good match to weird loads that the auto-tuner just doesn't like. Once you get used to using this function, it takes only a few seconds to find a match that the tuner algorithm will not find.
I've used the manual override of the AT function many times over the years with my TS-850S/AT and it works well.
Possibly that will help!
(Still, to run an amplifier to gain 3dB on 30m seems silly to me -- I like 30m and while I usually run 100W, I find half the ops are running QRPp, and I can easily work anything I can hear when using 100W.)
73!
Steve, WB2WIK/6
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