Tube Linear into 2:1 Load

[flintstone mop]

Ok Folks
I'm gonna throw this out here. I have two linears, an Ameritron with 3 811A's and the main station with a Ten Tec Titan 425, 2 3CX800A7's.
I tune for max power in CW and reduce power for AM and not exceed the PEP of the amplifier. Everything is fine until I connect to the antenna which is almost 2:1 SWR.
Antenna is 70 feet in the air and I cannot re-tune for better SWR. I don't have $500 at this time for a crane truck with a bucket. Wudda a tuner save the day??
What do I do now? Leave the linear alone and let it fly? How can I get that critical tuning done accurately while radiating RF on the imperfect antenna??

Thanks for any thoughts. 40M was pretty nice on Thanksgiving day around 3:00pm

Fred

[n2bc]

Are you tuning into a dummy load then switching to the antenna?   If so, you'll have to readjust the loading at least.  Tune and load will also change when you lower the drive.

I don't know about the Ameritron but I had a TT Titan.  It will operate quite nicely into a 2:1 load, but it must be tuned into that load.  Watch the grid current on those 3CX800s as you adj the loading.  That little red overdrive LED should be in the shape of a dollar sign, because the more it's on the sooner you going to need new 3CX800s.

Another approach would be to insert additional lengths of coax into the feedline to see the effect on SWR - no leaving the ground at all.

73, Bill  N2BC

[AB2EZ]

Fred

The tube linear does not care if it is working into a perfect 50 ohm load, or an SWR of 2:1... provided you tweak up the tuning and loading after you switch from the dummy load to the real antenna.

Using a tuner is not necessary when the SWR (looking into the antenna feedline) is 2:1. The only reason to use a tuner in this situation would be if you had some components between the linear amplifier's output and the antenna's feedline that work better when looking into a perfect 50 ohm load (for example, a low pass filter at the output of the linear might do a slightly better job if it were looking into the 50 ohm input of a tuner, rather than directly into the 2:1 SWR of the feedline). If the amplifier output is connected directly to the antenna's feedline, a tuner would only duplicate the functionality of the amplifier's output tank circuit.

With the real antenna attached, the linear amplifier should be tuned, with peak input applied (not carrier level input), to peak the output (which should be the same as dipping the plate current)... and the loading should be adjusted to get the desired peak output power with minimum input power.

Proper loading of the linear amplifier occurs when it is loaded at peak input/output... not at carrier level input/output. With proper loading, the efficiency of the amplifier at peak output level will be around 60%, and the efficiency of the amplifier at carrier level will be around 30%. If the peak output of the amplifier is 600 watts, then the output at carrier will be around 150 watts. At peak output, the plate dissipation will be 400 watts. At carrier level, the plate dissipation will be 350 watts.

With or without a tuner (again, a tuner is not necessary), there is a small increase in the loss of the coax feedline as a result of the SWR being 2:1.

Best regards
Stu   

 

[WQ9E]

Fred,

Great advice from the others and I would only add that many vintage wattmeters are not truly directional types (i.e. the popular Heathkit HM-102 style) so unless your meter is a good directional variety the reading will indicate incorrectly high into a higher SWR.  Be careful that you are actually tuning for rated output (reaching the expected plate current with proper drive is a good clue that you are there) and keep the loading heavy and as you were warned watch the grid current.

Rodger WQ9E

[AF9J]

Hi Fred,

Like the others said.  Tune the Titan into a dummy load first, then use the matching network, and retune until the Titan's meter readings are happy.  Most matcing networks in decent amps will handle 2:1 or in some cases, 2.5:1 for an SWR.  If your SWR is much higher, you're going to need a tuner to keep your amp happy.

73,
Ellen - AF9J

[k7yoo]

The main limitation may be in the voltage handling ability of the loading cap. If your antenna reactance is such that your peak power output is at a higher RF voltage than the cap is rated for you are going to arc. Do some simple ohms law calculations and look at the RF current and voltage for 50 ohms. My RF ammeter reads around 5.5 amps at legal limit CW output into a 50 ohm load. I monitor the rf voltage with a scope and can really tell if I move away from 50 ohms impedance as the voltage soars pretty quickly. I run a short hi-Q antenna and the impedance goes high. If you are running a simple dipole and it is "long" you may have no problem, other than running out of loading cap C.
In theory the only limit is the cap voltage handling ability (high voltage) and the amount of C available for low impedance (high current) situations.
Be very careful of any devices downsteam on the feedline that may arc over as these sudden impedance burps are what will eat up 3cx800's (and 8877's) I learned this the hard way. loose or wet coax junctions, splices,etc. are real killers.
Skip

[flintstone mop]

Hello
Some very interesting replies, thank you.
I think I'll take a simple route and add coax to get the SWR reduced. Adjusting the loading or the plate tuning in the AM carrier mode will/should/might throw all of the PEP capability down the chute.
With the higher SWR the power readings will not make any sense, as the watt meter wants to see 50 ohms. I am using a nice Bird Wattmeter here.
My overload $ light rarely lights on my Titan. I didn't know that it was putting death nails into the expensive finals.
I'm not worried about the SWR as the losses at HF is not worth worrying about. A 10:1 SWR would be the time to worry. It's just the careful tuning needed for the linear to handle the PEP without splatter or clipping. I'll investigate Stu's approach for re-tuning while on the air.
Thank you all.
Fred

[KD6VXI]

There is always the way it was done before most of us where born, tune for the dip in plate current.

Honestly, tune for max PEP as indicated on your Bird.  Then, reduce the carrier to approximately one fifth of your PEP output, if using no insane processing.

Your Bird will work fine for tuning purposes.  You just won't have a perfect indication of true PEP output.  However, you should know how much drive it takes to get X amount of watts PEP output when the output is resonant, so it isn't rocket science to ensure your damn close.

The plate circuit will be able to match to 1, 2 even 3 to 1.  The problems arise when you figure out what your circulating current is in the pi circuit L..... 

Adding coaxial cable does nothing for the match, unless your building a transformer.

Readjusting the amplifier to be resonant into the 2 to 1 swr won't throw your PEP capabilities off, since your actually resonating the output circuit to the load.  Resonating it into a dummy load, then putting it into a reactive antenna without touching up the output will.  Think about it.  You tune the plate circuit to resonance, and load the plate circuit to your load. 

--Shane

Hello
Some very interesting replies, thank you.
I think I'll take a simple route and add coax to get the SWR reduced. Adjusting the loading or the plate tuning in the AM carrier mode will/should/might throw all of the PEP capability down the chute.
With the higher SWR the power readings will not make any sense, as the watt meter wants to see 50 ohms. I am using a nice Bird Wattmeter here.
My overload $ light rarely lights on my Titan. I didn't know that it was putting death nails into the expensive finals.
I'm not worried about the SWR as the losses at HF is not worth worrying about. A 10:1 SWR would be the time to worry. It's just the careful tuning needed for the linear to handle the PEP without splatter or clipping. I'll investigate Stu's approach for re-tuning while on the air.
Thank you all.
Fred

[Steve - WB3HUZ]

Nailed it! Just tune and load the thing as normal. As long as there is no arcing or SWR foldback/protection circuitry, you are golden.

[flintstone mop]

I get the picture. Thanks. If the bands aren't real busy, tune off about 6khz, and quickly re-tune into the reactive antenna. It should just be a few quick tweeks, being the amp has already been tuned.
I agree this is not rocket science.
I'll have a power adjust wired into the Elamc using a pot. and an NPN transistor to control the screen volts in the 6146, an ER article for reducing power in Rangers and others like the Elmac, so they can drive a linear. There was another approach where the screen and plate volts were reduced. I didn't like that hookup. Might change the loading of the modulator too much.
The Kenwood/Titan station is no problem for power adjusting. One of a few good things about a plastic radio, besides the digital readout.

Fred

[WU2D]

Speaking of Linears, I was using a tabletop linear which has 4 811's today with my ARC-2 XCVR. This radio puts out 25 Watts and you can reduce the coupling so the carrier on the amplifier comes back to 100 Watts or so. It seems to run fine at this power level.

I have a homebrew matchit that I use on the output of these military radios which do not always load well into 50 Ohm antennas. It has high Q and acts as a bandpass filter, improving harmonic rejection and image rejection on RX.

It is nothing but a simple unbalanced tuner right out of the handbook. The idea is to handle unbalanced to unbalanced from input/output SWR's up to 5:1. It usually will work slightly better in one direction that the other depending on the mismatch so try both ways.

As an experiment I measured the output of the barefoot ARC-2 into a dummy load and then with the tuner:

Barefoot : 28 Watts
With tuner inline: 35 Watts
Into amplifier: 210 Watts
With the tuner in between ARC2 and Amplifier: 290W

These simple linears can benefit from a tuned circuit match on their input.

Mike WU2D

[flintstone mop]

I'm feeling good today!!!!
Successfully added a power reduction circuit to the Elmac from an ER article, thanks to Harry KT4AE. It uses a 500k pot. and an NPN high voltage transistor. I can reduce the power from the Elmac, without disturbing the plate tune or loading, down to 2-3 watts. I need about 15 watts to drive the AL811 amp.
It was a nice rainy day project that took about 3 hrs.

Fred