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Author Topic: Antenna Tuner for Boat Anchors  (Read 1036 times)
AC0FA
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Posts: 298




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« on: March 05, 2007, 08:55:44 AM »

I have a 1965 Swan 350 with about 125w out.

It is rated for an antenna of up to 300 ohms.
I have a 120 foot long attic mounted dipole, Center fed with 300 ohm twin lead.
It also uses LMR 400 wound 10 times around a coke bottle for a ballun near the shack.

I was using a tunner with my solid state rig and the antenna seems to resonate resonably well on 20 40 and 80 meters. SWR less than 6:1 without the tuner.

If I used this antenna with my Plain Swan 350. Would I also need to use an antenna tuner or can the tuning circut of the swan handel it.

Some times the terminology in these old owners manuals makes it dificult for me to get the big picture.  

Any help would be great.
Erik AC0FA    

       
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KE4VBG
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« Reply #1 on: March 05, 2007, 07:51:02 PM »

I'd use the tuner. An swr of 6:1 means you wouldn't be radiating much out of the antenna. I'd hook the LMR (or other 52 ohm coax) straight to the dipole then to the tuner, without the balun.  
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WB2WIK
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« Reply #2 on: March 06, 2007, 09:05:26 AM »

This post leaves me with more questions than answers.

First, how do you use an LMR400 coaxial choke balun on 300 Ohm twin lead?  That, alone, is kind of weird.

Normally, you'd use 300 Ohm twin lead from the antenna directly to the BALANCED output terminals on the antenna tuner, and then use the tuner to provide a 50 Ohm match for the Swan.  That way, you could tune the Swan into a dummy load, set it for max output into 50 Ohms, and then just leave it alone (don't re-tune); if the tuner is adjusted properly, it will provide the same load to the Swan as the dummy load did, so you'd be "done," until you change bands or make a large frequency change within a band.

The BALANCED output terminals of a tuner can be created a few different ways: In a differential tuner, you actually tune both sides of the line (nothing is grounded); in a link-coupled tuner like the old Johnson Matchboxes and a few products that recently came to market from MFJ, the tuner output is balanced by design and intended for direct connection to twin lead; or, in a single-ended tuner (most are "T"-matches or "L" matches), you actually tune the coaxial line that connects to the station transceiver and the tuner employs an internal BALUN, usually having a 4:1 (or higher) impedance ratio, to provide balanced outputs to the twin lead.

Whichever way it does it, it will probably work.

I don't see any reason for the LMR400 choke balun at all, and can't even figure how it would work if it's connected to twin lead...

WB2WIK/6
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AC0FA
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« Reply #3 on: March 06, 2007, 10:33:09 AM »

I no longer have an antenna tunner to use for my 120 foot long dipole. Will it be necessary for me to use a tuner with my 1965 swan 350 tube rig?  
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WB2WIK
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« Reply #4 on: March 06, 2007, 02:30:10 PM »

I would say, "Yes."  You will need a tuner.

The SW-350 pi-network is for *unbalanced* line, not twin lead, and might optimistically transfer power into a 300 Ohm unbalanced line (coax, not twin lead), which represents SWR = 6.0.

However, the SWR on a 120' doublet used on some bands will have SWR *MUCH* higher than 6.0.  For example, a center-fed 120' doublet, fed with 300 Ohm twin lead, has a calculated SWR of >13:1 on 40 meters!  That's likely not within range of the Swan, and will require a tuner with an internal balun to provide connection points for the twin lead.

WB2WIK/6
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K3JVB
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« Reply #5 on: March 18, 2007, 02:31:28 PM »

I have been that route with my Swan station.

I found a nice Swan ST-2 tuner. It is 3k rated, and an inductance type. I used it with 450 ohm with various configurated antennas. It also has a four to one balun.

It will tune almost anything, but I mainly use it now with a coax feed line, and a fan dipole. Mosly to get a little more band width on 75-80 meters.

It will also tune 160.
good luck
JohnB
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W7FRS
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Posts: 210




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« Reply #6 on: March 21, 2007, 12:31:30 PM »

What are you using to measure an swr or 6:1?  I need to get one of those.

The best tuner would be at the antenna.

I've used a cheapo mfj 949d on my swan 500 for 75 meters at the output of the radio.  Sure, it's pushing the limit, but I haven't had to through this tuner away and pull another one down off the shelf.  Oh; can you do short tuneups?  Then again, you said 350, didn't you?

I tune the tuner with an antenna analyzer.  Then I tune the radio into the tuner.  The 949 will display an 8:1 swr.  I intend that it never will.

My prefered method is to use a solid state rig (into the tuner) when I check into the net on 75.

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