The AM Forum

Is it possible to use a relay based autotuner with a vintage transmitter provided the output level of the transmitter is kept low enough not to overload the tuner during tuning? Would the rapid swings in SWR as the autotuner searches for a match create a problem for the transmitter?

What's the transmitter? Transmitter output: link, pi-network, or something weird?

One possible way would be to utilize a power divider circuit using a dummy load and light bulb for TPOs of 150W or less. For tuning, the transmitter output would be split between the dummy load and one lead of a standard 120V incandescent bulb. The other lead from the bulb would go to the tuner.

The dummy load should be rated to handle the TPO. The bulb's wattage rating would be chosen based on how much power the tuner needed. A wattmeter can help with that selection. Utilizing a porcelain lamp socket makes wiring easy. An SPDT knife switch can be used to add the dummy load or bypass the bulb if all lead lengths are kept short.

When in use, the transmitter would see a fairly steady load so you could even do your own tuning while the auto tuner is working.

I've tried this successfully with a Ranger and Valiant.

Potentially something like a Viking II or Gonset G-76.

The pi output of the tube xmitter would be a lot better for the tuning excursions.

I've used an icom clone tuner on a system like that, once.

It worked.

Just be aware, for a 100 Watt xmitter, your going to want a 600 Watt tuna, minimum.  More likely a legal limit if you're using non resonant antennas.

--Shane
KD6VXI

The direct answer is "no". Vacuum tube amplifiers tend to be more forgiving of varying loads than solid state amplifiers, particularly at reduced output. In the case of high mismatches, it's the device dissipation rating that matters, and that's less challenged at low power.

Loading the PA screen voltage down or reducing drive levels are two more easy ways to obtain low power for tuners.

The short answer is, no problem.  I use (and highly recommend) the MFJ 998 auto tuner and run it on a Kenwood TW590sg and an the 590 going into an Ameritron AL82.  I bring the power up from 0 to no more than 100 watts and !CLICK! it's there.

What kind of antenna are you proposing to load into?

Agree.  I use the 998RT with a Globe Champion and a GK 500A without problems.  I use it from 3.635 to 4 Megacycles and 160 meters.  It has been flawless for the past 5 years.  I also use it with my IC 7300 and AL 1200.  Tune at low power and switch to high.

The tuner may become confused if there is instability in the transmitter due to the many weird loads tried.  Once the thing has memorized the loads there won't be any issues.

Hi!

Just a Quick comment.
Some ATU's throws in a 6 dB pad during tuning, this will reduce SWR swing at the TX.

  That is a great idea! A 6 db pad inline makes the SWR extreme at the Tx 2:1 maximum while taking the output from a 100w Tx down to 25w when matched.

Jim
Wd5JKO

I really appreciate everyone's insight. I ask because I have an Elecraft KAT500 tuner and it'd be nice to only have one tuner for the entire station. I don't plan on running more than 100W carrier and so that should be within its capabilities. I actually asked Elecraft if there were any concerns with the power ratings when being used with more intensive modes such as AM and they said that it should be fine.

One thing you need to be careful of is that the tuna also expects a 50 ohm load on its INPUT side.  A vintage xmitter can present a wide range of impedance at its output which can give false SWR readings from its internal bridge.  Tune the rig up into a 50 ohm dummy load first, then switch to the tuna.  That way your rig will be close to 50 ohms at its output, and the SWR bridge in the tuna won't get confused.

I'm kind of suspicious that the tuna can handle full carrier AM.  I'm further wondering why you'd even want to use it since a pi-output can match a very wide range of impedance, generally speaking.

The Viking II pi-network can handle impedance's anywhere from roughly 50 ohms to 600 ohms.
With many of these old pi-network output boatanchor rigs, you can load up metal bed springs (which I did), random length of wire strung around the back yard over bushes, the vegetable garden, laying on the ground (which I did), and even strings of Xmas tree lights (which I did) without a tuner.

Or did I miss the answer?