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1.5 KW PEP Gain-Master Modification

Discussion in 'CB Antennas' started by Shockwave, May 6, 2011.

  1. Shockwave

    Shockwave Sr. Member

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    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.
     

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    Last edited: May 6, 2011

  2. Robb

    Robb Yup

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    Several of us have wondered why the RF choke on the bottom uses so many wraps.
    Do you have any thoughts/insights to this?
    IS it more of a matching device - as well as a acting as an RF choke?
    Just wondering . . .
     
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  3. Shockwave

    Shockwave Sr. Member

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    I believe the answer to this question has to do with the impedance of the RF where the choke is inserted. This choke is electrically placed at the end of a balanced element where the impedance is higher then it would be at a typical feed point. In order to give the RF voltage a place to drop across, this choke must be of higher impedance then normally found with coaxial chokes at this frequency.

    The choke is not responsible for matching the antenna. This is the job of the matching stub and coaxial capacitor. Although, without the choke, it would be impossible to achieve a match because the mast and coax would radiate heavily. Remove more then one wrap off this choke and the VSWR starts climbing as the coax and mast become part of the antenna.
     
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  4. bob85

    bob85 Supporting Member

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    nice work shockwave, thanks for the internal pics(y)
     
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  5. Shockwave

    Shockwave Sr. Member

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    Thanks Bob. I like this antenna and thought I'd put a little effort into making it meet more peoples requirements.
     
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  6. Marconi

    Marconi Supporting Member

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    Very interesting revelation. That cap device looks like a miniture version of a gamma match to me.

    SW, what is the continuity accross this device? It looks like it should be showing an open circuit, like a gamma fed device.

    I guess the pig tail, where the shield is shown, in the manual, to be shorted to the center conductor, is why we see a short at the coax connector, right?
     
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  7. i got questions...... :unsure:

    so the impedance is very high on one end of the choke (antenna end) but its 50 ohms at the other end (feed-point) ? what is causing the impedance difference ?

    if the impedance of the antenna is so high , how does it tune to 50 ohms ?

    so you're saying the impedance of an antenna effects how many wraps are needed for a coax choke ...... its not just the intended frequency/bands used ?
     
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  8. Shockwave

    Shockwave Sr. Member

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    Booty Monster,

    The easiest way to understand this is that there are two currents involved with the orange coax in the Gain Master. We have the normal transmission mode current that is carrying RF to the top black wire. This current is contained within the cable and is a 50 ohm impedance. The second current is the important one in this case and deals with the current on the outside of the orange coax.

    I'm not sure if you would call the second current antenna mode currents or CMC because it has similarities to both. In any event, when the RF reaches the top end of the orange coax, it travels back down the outside of the braid as a radiated current. The further this current travels towards the base of the antenna, the higher its impedance is because current is dropping as voltage increases.

    It's not so much the driving impedance of the antenna that determines the impedance of the choke required. It's the impedance of the RF current you are trying to present an open circuit to with the choke that matters. Just because the antenna impedance is 50 ohms does not mean common mode currents on the braid will be the same.

    Marconi,

    This capacitor does have similarities with a gamma match in that the cap is formed in the shape of a tube. The differences are there is no tap to the main radiator to vary the inductive tap point and the value of the cap is not so easily adjusted. If size was not an issue here a gamma match could be used. The DC short in this antenna is provided entirely by the lower shorted coaxial tuning stub as you suspected.
     
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  9. W5LZ

    W5LZ Crotchety Old Bastard

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    Impedance and coaxial chokes.
    That reactance/impedance of the -coil- only affects the outside of the feed line, not the inside. The length of the coax used to make that coil may affect the antenna system's SWR, but only if the antenna isn't a 50 ohm load to start with. That's a matter of -degree- though. A small change in SWR doesn't amount to anything, a large one does. Nothing is ever exactly right, so some changes have to be expected, small ones.
    The benefit of using a choke is to remove/reduce the unwanted current flow on the outside of the coax, that's all it's going to (or should) do.
    How many turns for that coil? Good question, and it's frequency/band dependent. Just something to keep in mind is that 'too many' turns is 'better' than 'not enough turns, but don't carry that to extremes. The size/shape of that coil also can make a big difference because it affects the amount if inductance present (on the -outside- of the feed line!). There's a string of formulas you can use to 'figure' that inductive affect, or, you can do it the 'easier' way and just try different size/shapes till you're satisfied with the results.
    - 'Doc
     
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  10. Marconi

    Marconi Supporting Member

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    BM, Bob would be another source to talk to about CMC.

    I think the difference that needs to be understood here, is that the impedance inside the coax is not part of the issue. The choke, if working correctly, generates a high impedance common mode current field on the exterior of the coax, and it is this high impedance field that helps minimize the CMC's from continuing to flow beyond that point along the feed line.

    I may get some argument on this one, but I also consider this choking field condition also helps prevent these currents from flowing on the mast as well, including anything else conductive and attached at that point in the system.

    IMO this is why the Gain Master works so well. If it didn't, that setup probably wouldn't be worth a hoot based on my model of an end fed 5/8 wave dipole with a grounded mast. I think SW is right on point, describing the GM choke, and where it's located in the system...as compared to other more traditional installations needing or using a choke.

    Note: I also agree with 'Doc on his post as well.
     
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  11. Shockwave

    Shockwave Sr. Member

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    Because the shield of the coax is connected to the mast ground after the choke at the SO-239 connector, the choke isolates the mast as well as the coax from CMC currents flowing below its location.
     
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  12. AudioShockwav

    AudioShockwav Extraterrestrial Admin Staff Member

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    Good Info.
    Stick.


    73
    Jeff
     
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  13. Highlander_821

    Highlander_821 Amour d'Ecosse

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    Thanks so much for posting this. Great work!(y)
     
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  14. loosecannon

    loosecannon break on through

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    my question is could this coaxial capacitor be made with some delrin tubing and some brass tubing, sort of like is inside the antron/imax antennas.

    if so, this could make tuning easier as it can be made adjustable.

    just a thought.
    LC
     
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  15. Shockwave

    Shockwave Sr. Member

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    I'm sure you could do this but the main problem is there is very little room inside the fiberglass radome where this cap is located. You have about 1/2 inch inside diameter and if the goal is making the cap handle as much power as possible in this size, I think you're better off with Teflon as the dielectric material. The nice thing about using the RG-393 cable is that you don't have to tune it. As long as the braid is cut the same length as the stock RG-303 cap, you have the same pf value. Changing the diameter of the conductors, insulator spacing between them or the velocity factor of the insulation and the length of the cap will have to be tuned to the correct value.
     
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