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Author Topic: SGC Tuner and Doublet  (Read 2708 times)
W7KKK
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Posts: 374




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« on: October 05, 2009, 09:55:39 AM »

I am in a rental, like it or not. I don't have any radio equipment yet as I am just starting up again but I have some used gear on the way but it won't be here for a week+ yet.
I would like to get an antenna up and would like your ideas.
What I do have is an SGC-237 tuner, ant wire, 450 ohm ladder line and some trees.
Now this is a rental, and the trees are on some abandoned property which is vacant land to the rear of my home.
I have no idea how the landlord with react to a doublet but if it's adverse my lease is up so I can move.
I can get some 100' of wire up, center fed with the 450 ohm ladder line. The problem I see is that the property slopes and the average height of the ant going that long is only going to be 25' estimated. I am thinking that's so low it won't be very good on 80 anyway.
I am thinking of trying to avoid the slope and put a shortened version in a couple of trees that are both closer and higher avoiding the slope and gaining elevation. I think I can get it up to 30-40', maybe more and a length of 40-65' overall.
Anyone agree with me that I am wasting time trying to go the 100'?
I am not sure just how talented these little SGC-237 tuners are or what it will allow me to tune with the the proposed shortened antenna.
I am going with this arrangement over a screwdriver mounted on a short mast and tri pod with a counter poise and hoping for the best.
If the landlord objects to the doublet I just go with the screwdriver or I have to move.
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AA5TB
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« Reply #1 on: October 05, 2009, 12:22:12 PM »

I've been using a SGC SG-237 for about ten years now.  I use it with an Inverted-L antenna that is a half wave length long on 40m.  The vertical portion runs about 6 m up along side my chimney and the horizontal portion runs 14 m out to a tree and pulley system that uses a bird feeder for a counter weight.  I only have an 8 ft ground rod for a return.  The antenna has the advantage of being able to work both NVIS and DX.  An extensive radial system should greatly increase the performance on the bands lower then 40 m but that isn't possible in my installation.

I use it on all bands from 160 m through 6 m.  It performs well on 40m where the ground return current is low. In fact it performs much better then it should on that band.  On 160 m and 80 m the system is very lossy due to the lack of a radial system but I still work an amazing amount of stuff on those bands.  On 20 m and above its radiation pattern becomes lobey and sometimes at undesireable angles.  However, it keeps me very active on all bands in my HOA infected neighborhood.  I've even received 160 m awards using QRP power levels in spite of its poor efficiency on that band.

The SG-237 works fine but some antenna lengths on 80 m and 160 m will present problems.  I think it expects to see a low impedance on those bands and as long as the antenna is a 1/4 wave long or less it has no problems.  If you use a feedline between the antenna and the tuner you may have to experiment with the line or antenna lengths to load up on all bands but I don't think you will have any problems.  Be sure to protect the unit from weather exposure.  The gray plastic cover doesn't like the Texas sun or hail.  An inverted plastic trash can from Walmart works fine, at least against the smaller hail and the sun.

Good luck.

73,
Steve - AA5TB
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W7KKK
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« Reply #2 on: October 05, 2009, 12:28:07 PM »

Did you run any shorter doublets or dipoles with it? I am just curious as to what it would tune with say 40 to 60' of wire center fed.
I am planning of feeding mine with a non resonant length of 450 ohm ladder line to any band 10-80 meters.
I would like to keep this thing as short as I can without insulting the landlord if you know what I mean.
I am also aware that propagation is low right now for 10-17 meters and would like to make some contacts when I do get back on the air so I may have to make it longer.
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AA5TB
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« Reply #3 on: October 05, 2009, 01:52:10 PM »

I did run a doublet with the SG-237 once and it was feed with 450 Ohm line.  The antenna was an Inverted-Vee and roughly cut for a half wave length on 40 m.  I don't recall having any tuning issues.  

For your situation the feed line length will play a major part in the impedance that the SGC tuner sees.  There are liable to be some configurations that it won't like.  However, by changing the feed line length a bit you should be able to find an impedance it is happy with.  It really is a cool system.  

I built a control box for mine so that I could 'hold' the tuning in place once it found a match.  This is a good feature to have because on some bands it can start trying to retune again while you're transmitting if the wind blows your antenna, feed line, etc.  See: http://www.aa5tb.com/sgc.html

73,
Steve - AA5TB
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W7KKK
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Posts: 374




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« Reply #4 on: October 05, 2009, 02:46:41 PM »

OK, TNX. I know about the feed line problems that can occur.
I was thinking that if the tuner will tune 40 and 80 with less than normal 1/2 wave wire in the air maybe, just maybe nobody will spot this thing and I will get lucky.
I think this arrangement has to be better than a screwdriver at ground level.
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W7KKK
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Posts: 374




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« Reply #5 on: October 07, 2009, 01:44:53 PM »

Well I got my wire into the air. I could not manage the 100' run, just too many trees in the way. I got about 55'of wire up about 35' is all.
When the used radio gets here I will find out what I can tune with the SGC-237.
If it does not work out I will try a vertical and a counterpoise and see what happens.
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KT8K
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« Reply #6 on: October 14, 2009, 12:46:22 PM »

That antenna should do well enough for you, but additional length might improve efficiency a bit more.  I recommend thin wire such as 18 gauge magnet wire or similar, as it will be almost impossible to spot from 35' below.  I also recommend using longer wires, but let the ends hang down 20' or so.  They'll still be very hard to spot and may both help with tuning on the lower bands and add a vertical component to your signal that will lower the effective radiation angle.  I used a doublet like this for a few years and it worked amazingly well.
Best rx & 73 de kt8k - Tim
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K3AN
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« Reply #7 on: October 16, 2009, 05:37:26 PM »

I use a SGC-230 to tune a delta loop fed with ladder line. I run the coax/power/control lines through a choke balun installed right at the tuner to obtain a little better balance and reduce feedline radiation and pickup. The balance is probably not that good.

However, if you want to be stealthy I would recommend avoiding the ladder line and installing an inverted L configuration. Use #26 black insulated copperweld antenna wire (so-called stealth wire). That wire can be hard to see even when you're looking for it. If you're not looking for it you'll never see it.
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N5YPJ
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« Reply #8 on: November 01, 2009, 10:20:51 AM »

My SGC-237 matched any freq. I am authorized to use to a 280 ft horizontal loop. Problem was first rain it stopped working. Closer examination revealed that a cable got pinched between the covers and made a serious indentation -  a breach in the gasket and the unit got water in it shorting out an IC. It was located up high under a 2 ft roof overhang so it was not directly exposed nor do I think it would withstand much direct exposure. Mine was returned for refund and I'm back to a manual tuner for the time being.
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AA4PB
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« Reply #9 on: November 01, 2009, 11:50:17 AM »

Sounds like SGC messed up in assembling the unit. I've had an SG-237 on my pick up truck for several years with no problem. I have a plastic shield over it to keep it out of direct sunlight but I'm sure it gets plenty soaked when driving down the road in the rain.

My experience has been that direct sunlight (UV rays) over a period of years will deteriorate the SGC housing and it'll start to look like "aligator skin".
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NA0AA
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Posts: 1042




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« Reply #10 on: November 20, 2009, 05:01:33 PM »

I know reading is a real pain in the rear, but with SGC style antenna couplers, it pays dividends to read their on-line application manual - there are a number of GOOD ways to use this unit, most of them do not involve using a feedline beyond the coupler.

For the OP - set up an inverted 'L', as long and as high as you can, put a couple of radials for each band to the ground screw of the coupler [called the counterpoise], and it should match most anything.

The sweet thing about these couplers is that you have a very low SWR on the coax to the shack.
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KE4DRN
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« Reply #11 on: November 20, 2009, 05:34:56 PM »

hi,

good reading in pdf.


http://www.sgcworld.com/Publications/Books/hfguidebook.pdf

73 james
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