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BC-939 TN-339 - what's it all about, really?




 
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Author Topic: BC-939 TN-339 - what's it all about, really?  (Read 6298 times)
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Patrick J. / KD5OEI
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« on: June 30, 2010, 12:42:58 AM »

What is the advantage of this versus a traditional "T" network tuner?

Looking at the schematic, this tuna is really a variable RF autotransformer followed by an L and a C in series.

coming in at 50 ohms, it goes across a roller coil. The tap then goes to a series LC circuit and then to the antenna.

The L, is one of two roller jobs, and the C is one vacuum cap or another.

This is true in any configuration, 2-10MC, longwire, or 10-20MC.

Is idea just to cancel out the unwanted reactance of the antenna, whether it is a 15FT whip or a longer wire up to maybe 65FT. (personally I am thinking about a 20FT aluminum pole with the roof of the 30x40 shack as a ground plane)

So, for getting for a moment the variable autotransformer, what is to prevent the tuner from working fully to 30MHz, so it would be OK for 80-10? Sure the system will stink on 80 but it's another antenna.

The L is from 0-15 or 0-96uH. Choice of C is 55pF or 12pF.

Having this to work with, couldn't the two L's be put in series for 0-111uH? Would I want to use a variable cap of maybe 0-400pF?

It looks like the input autotransformer adjusts the ohms match, and the L and C cancel the reactance.

Or am I going to catch flak for messing it up?

The three configurations in the drawing show how versatile it is. I'm interested in advice on it because I do not know this kind of tuner very well at all.


* ant tuning uni bc-939 tn-339.gif (39.72 KB, 1863x772 - viewed 1149 times.)
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« Reply #1 on: June 30, 2010, 07:10:24 PM »

Hi, well I have never owned a 939 but I have seen a few and talked to owners--I understand they are intended to turn a relatively small antenna into a usable load so my impression is they don't get used much.  To me, the attraction has more to do with the unit as an exquisite example of vintage military radio art.  Every component is first class for its day.  To some stamp collector, it might look like junk but to us it is something you might never actually use, but look at, the way some folks look at paintings or sculpture. 

Of course it can be used if you have a short antenna you want to match, and there is probably a way to modify it so it can be used as a regular feedline tuner by rearranging the way components are connected but I'd only do things that could be reversed. 
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« Reply #2 on: June 30, 2010, 11:20:47 PM »

Yeah,

What that guy said:
"they are intended to turn a relatively small antenna into a usable load so my impression is they don't get used much. "

End quote

   Now a days the term would be "application specific".  I can load light bulbs
with mine but all that "L" in there and no variable "C" screams "Short Antenna".

   They are pretty though.  I have never used mine for anything other than
holding my 610 to the floor and giving it that classic look...

/Dan

http://users.ameritech.net/dharlan/dans.htm
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ke7trp
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« Reply #3 on: July 01, 2010, 01:31:15 AM »

There is a great article about this tuner in ER.  Look it up or get a copy.  Its one hell of a tuner. It can be used for a short whip or long wire.  Matches the 610 or T368.  I am still looking for one.

C
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« Reply #4 on: July 01, 2010, 02:14:08 AM »

I saw the article on T-368's in the recent issue. I need to look further I suppose.

I had mine for sale but couldn't get a fair price or a useful trade for it so it was not worth letting it go. I would consider trading it for a ruggedly built high powered (KW+) tuner with a wide impedance range but most so-called KW tuners are crap for ruggedness compared to this.

I think it may be possible to reversibly remove the smaller 15uH roller and substitue a 500 or 1000pF 5000V vacuum variable behind the crank so a wide range of improvised antennae could be used. In the ARRL handbook's mobile section there are dirt-simple series type matching networks for the base of the vehicle's antenna, some are L and some are C.

According to the remarks about the smallish series capacitor, that looks like it makes sense to try a larger variable cap. Back to the reactance figures I guess.
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« Reply #5 on: July 01, 2010, 11:29:28 AM »

Lots of hams have modified those over the years.  I think you should pass it on to someone that needs or wants the thing and build something. They are hard to come by.  I was going to use it to load up a vertical with the T3.  You could put that thing in a dog house at the base of just about any vertical you can make and it will match it.

C
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« Reply #6 on: July 01, 2010, 12:06:46 PM »

I had one of those hooked up to my BC-610. It seemed that 44 feet long wire  was the magic number when used  on 75/40 meters. I stopped using mine because of the high RF voltages that were impressed on all my solid state gear such as tape decks.
(Blew a few of their brains out)

I think if they were used as a remote antenna tuner 100 feet from the shack, you'd be ok.  I also found them hard to tune when using with the 610. Probably an antenna analyzer would be best for a pre-tune.
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« Reply #7 on: July 01, 2010, 03:27:41 PM »

As you probably know, 18 MHz is the top end for the BC-610, hence the tuner specs showing the same, but it works above that frequency too.

As said already, the tuner was intended to match mobile whips, so when you know what you are going to be matching, you can minimize the parts count. In this case that means that a good variable capacitor is absent because it wasn't needed for that specific application.

While you could make the tuner usable for a wide variety of applications by adding a vacuum variable and some re-configuration jack strips, it does seem a shame to mess up a rather rare and sought after item by doing so, and I am not sure those parts will fit inside.  But, I sympathize that so often while everyone wants you to keep a rare item original and not modify it, nobody is willing to buy it from you at any price which would allow replacement with something else capable of doing the same task.

Remember that there is no such thing as a 0-to-something variable capacitor. All have minimum capacities, the vacuum variables having the smallest minimum capacity.  Not really an issue in a tuner but becomes important in higher frequency tank circuits.
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Geoff Fors
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« Reply #8 on: July 01, 2010, 06:34:47 PM »

Lately when I have seen one for sale it has been in  the $300 to $350 range.
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« Reply #9 on: July 01, 2010, 07:20:23 PM »

Lately when I have seen one for sale it has been in  the $300 to $350 range.

That's what I thought, but I kept getting lowballed for $150-200, guys saying they can get them for that. OK, well then they should go to those sellers, right? Maybe I'll put it aside and build something. It seems like I don't get enough L in a roller around here. Most of mine are 30uH or less, the plate tank sort.
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