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1.5 KW PEP Gain-Master Modification

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  #33  
Old 05-11-2011, 12:23 PM
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That might prove to be an interesting development if he joins.
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  #34  
Old 05-26-2011, 11:55 AM
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I just got mine and cut away the shrink tubing to look at it. seems it might not be strong enough without ading some kind of support line above the rg393 modafication. I have some small twine ill epoxy there to take some of the load baring

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  #35  
Old 12-12-2011, 09:50 AM
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Originally Posted by Shockwave View Post
So far the only legitimate complaint about the Gain Master has been its power handling capacity. Its high tech design does deserve credit for the gain it produces on the horizon. Originally they were advertised at 1 KW for short periods. After some failures in the first production runs this has been derated to 500 watts PEP or 125 watts AM carrier. I really wanted this antenna to be able to handle the full 1500 watt PEP on 10 meters and......it can be done!

The weak link is the series capacitor feeding the top element. The stock antenna uses a short piece of special coax trimmed to the correct length. Ever wonder why some people report blowing the cap up with just a couple hundred watts while others insist the can put 1 KW into it? This all depends on how clean the end of the coaxial capacitor was cut when it was tuned. There have been some improvements in this area since the first production runs.

If it was a perfectly clean and flush cut it may have an RF breakdown voltage around 1500 volts. If there are any imperfections in the cut causing the end to not be concentric or any of the braid to come unraveled, the breakdown voltage can fall well below 1000 volts. Unfortunately there are no commercially made capacitors of the correct value that can fit in this fiberglass radome and the reason this part must be custom fabricated.

The stock capacitor is made from RG-303 coax cable. It just so happens that RG-393 coax has the same velocity factor, and capacitance per foot as RG-303 but it has well over twice the RF breakdown voltage at 5000 volts. This makes RG-393 the perfect Teflon coax cable to fabricate the custom upgraded capacitor from. The bad news is it's very expensive costing over $100 for a new 10 foot section to experiment with and I'm not selling any.

If you want to try this modification on your own, the most important thing is how you prep the open end of the coaxial capacitor. You'll notice the braid is pulled back over the Teflon by about a 1/4 inch to prevent arcing. What is not so clear is that the center conductor inside the coax has also been pulled back inside the Teflon by the same distance. The hole in the end was filled with clear silicone and plugged with a piece of Teflon cut from the insulation to completely captivate the center conductor in Teflon.

The length of the new capacitor is determined by the length of the braid on the stock cap. The distance the braid overlaps the center conductor determines the capacitance value. The new cap will have a longer Teflon center but the length of the outside braid should be identical to the stock one. I unwrapped the braid and formed it to fold under the board to solder flat. Using a Dremmel tool I cut the bottom of the board through the center of the two holes the wires originally went through so the new braid could be soldered in the exact same position. The top black wire is soldered to the top braid on the cap since it does not fit well inside the fiberglass when the connection is at the bottom.

The copper trace on the bottom of the board is spaced too close to ground and can arc. This is why the center conductor are soldered on the top of the board. I Used clear silicon to insulate this connection from the board and seal it. The unused copper pad on the bottom of the board was removed to prevent arcing. The lower circuit board that connects the matching stub to the two orange cable also needs its center pad modified in the same way. All three center conductor should be pulled from the board and soldered on top with the old pad removed.

Seal both boards in heat shrink tubing that has glue inside and you should be good for 1500 watts PEP in intermittent communications duty cycle. I wouldn't go past this point due to the orange coax being the next weak link in the chain. It's average continuous duty cycle ratings indicate it should have no problem at 1500 watts PEP for intermittent communications service. Beyond this and you risk melting the coax or the connector in the center of the harness. Using larger coax prevents you from wrapping enough turns around the base to form the choke.

Almost forgot: The length of the top black wire must be shortened by the length of the braid on the capacitor that was added in series with it. It is the difference between soldering the black wire to the board or soldering it to the end of the braid so it fits inside the antenna. Trim it with a VSWR meter checking the bandwidth ends to insure you get it where you want it without passing the point.
hey shockwave would rg213 work to raise the power handling if cut to the correct length for capacitence? if it will how much power?
why not also do the 2 foot shorted piece?

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  #36  
Old 12-16-2011, 09:01 AM
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i looked up the voltage for rg213 and its 5000v. the same as 393 but its a little higher capacitense so the 24 inch shunt piece would need to be shortened to about 22.9 inches and the 3 inch or so series cap on top would be a tiny bit shorter but i dont think its enough that i would need to lengthen the top wire.
i dont see why 213 couldnt be used for the whole thing except the single conductor wire on top. trail and error for the bottom choke will make it a chalenge

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  #37  
Old 12-20-2011, 01:38 AM
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You need RG-393 for its Teflon insulation. It's not a power issue, it's a voltage issue and this type of insulation makes a big difference in RF breakdown voltage. I didn't replace the coax used in the tuning stub because it didn't burn up on me yet. I don't suspect it will either since the voltage at this point is lower then what is across the cap.

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  #38  
Old 12-20-2011, 01:51 AM
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Quote:
Originally Posted by Shockwave View Post
You need RG-393 for its Teflon insulation. It's not a power issue, it's a voltage issue and this type of insulation makes a big difference in RF breakdown voltage. I didn't replace the coax used in the tuning stub because it didn't burn up on me yet. I don't suspect it will either since the voltage at this point is lower then what is across the cap.
both 393 and 213 are 5000 volts rated so wouldnt that work? do you know what the impedence and vf is of the red coax they use for the rest of the gainmaster?

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  #39  
Old 01-03-2012, 05:01 PM
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Default Well Done!

Nice findings Shockwave.
I took my Sirio Gaimaster down at the end of summer to put a 5 element Jogunn yagi on the tower.
I checked it over and other than the RF Choke red coax getting bleeched by the sun (Sirio are using more colour fast dyes they say now), mine is as good as when I first install it and conducted the Youtube Reviews.
Thanks for sharing this upgrade with us, I'll make one and try it sometime.

73 Dave

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  #40  
Old 01-04-2012, 04:02 AM
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Quote:
Originally Posted by Oggy View Post
Nice findings Shockwave.
I took my Sirio Gaimaster down at the end of summer to put a 5 element Jogunn yagi on the tower.
I checked it over and other than the RF Choke red coax getting bleeched by the sun (Sirio are using more colour fast dyes they say now), mine is as good as when I first install it and conducted the Youtube Reviews.
Thanks for sharing this upgrade with us, I'll make one and try it sometime.

73 Dave
do you know how to contact them? my had smashed coax in the box and it was missing the bubble wrap to protect it plus the box was all torn and taped up and looked like it had been opened- maybe returned and resold by h&y so i want to get all new coax from sirio and the newest one with the heat shrink tubing over the metal cap

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