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THE AM BULLETIN BOARD => Technical Forum => Topic started by: KC9LKE on August 04, 2011, 03:47:49 PM



Title: Mounting distance between plate and mod transformers?
Post by: KC9LKE on August 04, 2011, 03:47:49 PM
I looked in the east and west handbooks and couldnít find any practical info on this.

In an attempt to stuff 100 lbs of iron in a 50 lb footprint I run out of room fast, and going vertical is not really possible. Maybe Iím over thinking this but before I start the layout and bolt things down I would like to know my options.

The plate is 2kW, and the mod is 5ooW. They are both E I cores and I plan to keep the cores parallel to each other as viewed from the top. Also I am planning to keep the mod transformer away from the steel sides and back door of the rack cabinet. (noise)
 
Any suggestions on the mounting distance between the plate and modulation transformers?  My concern is coupling between the two.


Thanks

Best Regards
Ted / KC9LKE


Title: Re: Mounting distance between plate and mod transformers?
Post by: W7TFO on August 04, 2011, 04:22:17 PM
Many BC rigs, obviously concerned with noise & hum, mount them within 12" of each other.  Even open frame jobs like Electro Engineering & RCA.

They also set them 90 degrees, core to core, to stave off mutual coupling.

I'd go that paradigm and not worry.  If you do have a bit of hum outside ripple, try a bit of global rectified FB from RF to audio input.

73DG


Title: Re: Mounting distance between plate and mod transformers?
Post by: W1RKW on August 04, 2011, 06:01:44 PM
you could always wrap a tranny in mu-metal.


Title: Re: Mounting distance between plate and mod transformers?
Post by: k4kyv on August 04, 2011, 06:05:31 PM
Or fire up the plate transformer into a load, which could be an RF final running a steady carrier into a dummy load or even a real antenna, or load the transformer with a bank of resistors that will pull the rated current through the transformer at the rated voltage.  Then set the modulation transformer near the plate transformer, in approximately the same physical relationship as they  would be in the rig, and measure the a.c. voltage that appears across the mod transformer windings. If any measured induced a.c. is no more than a couple of percent of the total plate voltage, I wouldn't worry about it.  Try orienting the mod transformer for minimum coupling at the given distance (you will probably find a null point), note the orientation and mount the transformers with the same relative orientation in the actual rig.

I have used the same method for orienting RF coils in a multi-stage transmitter for minimum mutual coupling.


Title: Re: Mounting distance between plate and mod transformers?
Post by: KL7OF on August 04, 2011, 09:53:16 PM
You can mount iron on the sides of a rack cabinet ....if it is strong enough...I have a transmitter with 20 v mod iron and mod choke mounted on the sides of the rack...one on each side ...above the floor mounted HV iron..


Title: Re: Mounting distance between plate and mod transformers?
Post by: WB6NVH on August 04, 2011, 10:21:55 PM
As W7TFO says above, I was always taught to mount them at 90 Degrees to each other to prevent stray coupling when they are not the shielded kind.


Title: Re: Mounting distance between plate and mod transformers?
Post by: k4kyv on August 04, 2011, 11:02:41 PM
Actually, I never heard of a problem of stray coupling with high level transformers. Usually this is a problem associated with low level transformers, such microphone input. But still, I would take precautions like orienting the cores of closely spaced transformers 90į from each other, and keeping them as far apart as possible in the space available.


Title: Re: Mounting distance between plate and mod transformers?
Post by: KC9LKE on August 05, 2011, 02:18:16 PM
Thanks all for the info.

Since I plan to load the plate supply and give it a workout before I go any further that will be the perfect time to verify mutual coupling.  90 degrees as far apart as possible is what Iíll shoot for but it will be a learning experience with load on the plate supply.  I do plan to keep the iron away from the back door, it doesnít close very tight.  The sound of a Johnson 500 power supply when keyed is what Iím also trying to avoid.  So great, Iíll move on and give it a try.

Thanks again

Best Regards
Ted / KC9LKE


Title: Re: Mounting distance between plate and mod transformers?
Post by: KM1H on August 06, 2011, 09:49:19 AM
I would think a well grounded steel plate would eliminate coupling. Or hammer out a mumetal shield from an old scope.

Carl


Title: Re: Mounting distance between plate and mod transformers?
Post by: Tom WA3KLR on August 06, 2011, 10:25:25 AM
A grounded steel plate is just an electrostatic shield and won't do any good for this situation.


Title: Re: Mounting distance between plate and mod transformers?
Post by: k4kyv on August 06, 2011, 12:46:51 PM
I would think a well grounded steel plate would eliminate coupling. Or hammer out a mumetal shield from an old scope.

A grounded steel plate is just an electrostatic shield and won't do any good for this situation.

It will ruin mu-metal to hammer it. Warning decals are posted on some scope tube shields against even letting it drop to the floor.

In their later transformers, UTC cast the cases of their top-of-the-line products with some kind of proprietary material called Hypermalloy that is supposed to make superior magnetic shielding. Interestingly, it is not attracted by a magnet. Light in weight and looks kind of like pewter or nickel.

But I agree that it would be an exercise in futility to try to place a magnetic shield between two transformers.  Better to space them as far apart as possible and experiment with orientation.


Title: Re: Mounting distance between plate and mod transformers?
Post by: KA3ZLR on August 06, 2011, 01:16:25 PM
Hi,

Keep them Perpendicular of one another.

73
Jack
KA3ZLR


Title: Re: Mounting distance between plate and mod transformers?
Post by: KL7OF on August 06, 2011, 05:35:16 PM
In most homebrew transmitters, is coupling really a problem??  In some engineer designed rigs, it doesn't seem like any thought was given to coupling issues...I see where it might be a problem but seldom is...


Title: Re: Mounting distance between plate and mod transformers?
Post by: KE6DF on August 06, 2011, 07:41:52 PM
At the high level signals modulation transformer work at, it would seem unlikely you would get enough coupling to matter.

For a mod transformer operating at 2KV, even 20 volts of induced 60hz signal would only be one percent.

Now if you are talking about coupling between a power transformer and a microphone input transformer then even a millivolt would be a big enough signal to matter by the time it goes through several stages of amplification.

I wonder if anyone here has ever seen coupling between a power transformer and a large modulation transformer become a problem.


Title: Re: Mounting distance between plate and mod transformers?
Post by: WB6NVH on August 06, 2011, 08:02:43 PM
I was thinking about a mic input transformer up above.  Seems logical that a high level transformer wouldn't have that consideration.  Probably pretty hard to go wrong with those no matter what you do...
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