Need Recommendation for 160M AM Legal Limit Ant Tuner

[ND9B]

I think my current "legal limit SSB/CW" antenna tuner overheats on high power 160M AM. Can anyone recommend a model that can operate full power on 160M?

I have a K7DYY Super Senior transmitter, and I like to operate at 1.880 MHz. My antenna runs an SWR of 2.2:1 at this frequency. (With this transmitter, it is imperative that the SWR stays below 1.5:1.) With a separate SWR meter on both ends of the tuner, I see the tuner input SWR drift up after several minutes of transmitting. The SWR at the output stays steady at 2.2:1. I figure my 160M OCF antenna is not to blame since I'm not seeing an SWR change at the antenna. Note that this is only happening on 160M AM, no problems working 160M SSB. Am I right to suspect the antenna tuner (MFJ -998)?

Bob ND9B

[WD5JKO]

Bob,

   You might find that your line SWR at the feedpoint will vary with feedline length. If you add 30' (1/16 wave) at a time, you will likely see the SWR rise or fall. With luck, some added coax will lower your SWR to 1.5:1 or less, and you won't need any tuner. Knowing the resonant point of the antenna is useful, but hard to gauge unless the feedline is an exact electrical half wavelength. When it isn't an exact electrical half wavelength, even when the antenna is resonant, but not 50 ohms, the line feedpoint will vary with feedline length.

   I think the SS will be happier feeding a low SWR antenna versus a narrow bandwidth tuner. A say this beacuse some of the SS crap outs seem to come from folks running tuners, when some that don't just talk and talk and talk without the dreaded spark storm coming out the fan ventilation holes.

  I remember talking to Bob Peters, W1PE (SK) about this. Bob ended up getting a refund for his "Senior". He always used a tuner. Was it a DC short in the tuner, or was it the narrow bandwidth of a tuner, or was the tuner arcing over on modulation peaks?

Jim
Wd5JKO

[W1ITT]

Looking at an online photo of the innards of the 998 tuner, I see that the inductors are wound on toroidal cores.  It's possible that one or more of them is saturating and heating, and changing value.  Take the top off that tuner and operate for a while.  Then, with the transmitter unkeyed, feel around for warm components.  It's also possible that there's a capacitor that doesn't like the RF current, depending on what side of the Smith Chart is presented to the load side of the tuner.  As mentioned previously, adding some transmission line may help by rotating the impedance around to a part of the chart that the tuner is happier with.  This would be a wonderful excuse to buy one of the inexpensive infrared non-contact thermometers.  After all, we're doing radio science here.
But something sold as a "legal limit" tuner shouldn't be stressed at 2.2 to one SWR if components are chosen well.  The added coax may well be the inexpensive way to skin this kitty.

[kb3ouk]

Definitely sounds like the inductors are heating up, I have a smaller MFJ tuner that I use with a barefoot FT-901 on 160 (the tune is rated at like 300 watts PEP and the 901 only does 30 watts carrier) and it does the same thing, and it even uses air inductors.

[Jim, W5JO]

I have a 998RT at the base of my tower that is feeding an inverted L that measures about 1.9:1 at the feed point  and don't see the problems you mention.  Using a AL-1200 on 160 with full power has not been a problem. 

Are you sure of your installation and integrity of the connections?

[flintstone mop]

FIRST OF ALL

Congrats on trying to get something useful on 160m. Time is ticking, though. We're experiencing nice quiet conditions with the cold WX.
160m is very demanding for attention to good connections and healthy tuner parts. A lot of high voltage and RF current.

The RT version of that 998 might be better, and located at the antenna. Dipole or Vertical?
MFJ claims that it will be ok near legal limit AM. Again, 160m puts a lot of stress on tuner parts.

I own the Dentron 3KA tuner, feeding 600ohm OWL to an inverted VEE. 65 foot apex, 250 feet of wire. Each end is 20 feet from the ground. I also built the famous K1JJ tuner, and one day, I'll take some field strength readings and see which tuner is giving me more juice to the ether.
I have heard a lot of "L's" on the air and they are very effective on 160m. Dipoles 30 feet high and shortened are a disaster.

I do not love nor hate SS amps. But they sure are picky SOB's, when it comes to the final RF output. Protection, protection, protection....poor little things.

An AL1200 is a beast!!! Love that tube. I own an AL1500 and holds court with the best of them.

[ND9B]

On a related subject, how does average AM power compare to average SSB power for the same PEP? I've always heard that AM average power is more than SSB, but by how much? (And let's talk about real voice modulation, not theoretical sine waves.) I've noticed that all the antenna tuners advertising "Legal Limit" quantify that claim with "CW/SSB". Maybe there are no tuners on the market that can do legal limit AM!

Bob ND9B

[W1ITT]

Bob..
I'm thinking that the relationship of average power of voice AM to the PEP power of SSB depends on a number of factors such as percentage of modulation, the operator's voice, and the type and amount of processing used in the audio chain.
Mathematics aside, a fairly simple way to get a rough quantification would be to monitor the RF peaks with a scope lightly coupled to the transmission line. At the same time, observe a wattmeter with an actual meter movement, not a digital unit.  Most of these are damped to some extent that will show an average, at least at voice rates.  In your normal voice, talk the SSB up to a given PEP and observe where the wattmeter tends to hang out most.  Then do the same thing with an AM transmitter, matching the PEP on the scope and see what happens with the wattmeter.  Actually, you might find it easier to start with the AM rig as power levels are more easily controlled in the SSB rig.  Even if not precise, you will get a good approximation of a ratio.
This is all rather quick and dirty expedient methodology.  But human voice is an imprecise thing, and what you are concerned about at the moment is the average power that is converted to heat by anything that happens to be lossy in the tuner.  And heat effects show a thermal lag due to mass, even with the wimpy just-good-enough components in many ham products. 

[DMOD]

Since I prefer switched inductor tuners, I would look at QTH and other sale sites for an Ameritron ART-15, or a Millen 92200.

http://www.isquare.com/millen/manuals/92200-manual.pdf

In my experience, each will take just about any power level up to 3k Watts continuous carrier because of their internal components.

As others have stated, one really needs to look at the overall transmission system and re-evaluate your current set-up.

Phil - AC0OB

[Opcom]

I've blown up and arced my MFK "3KW" tuner many times. no good for legal limit AM. Just say no.