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Tuning the MFJ SPT500 Super Penetrator

Discussion in 'CB Antennas' started by Turn it to the right, Mar 24, 2015.

  1. Turn it to the right

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    Attempted tuning on 11 meters using MFJ269C. Antenna reactance with existing Beta matching shipped with these new SPT500's is poor. I did manage to get close to 1.3:1 but the bandwidth seemed to narrow. Others on this and other forums have noted the significant difference in the matching network as compared to the original SPT500. I confirmed that rebuilding the entire matching section first out of number 12 solid copper wire to fine tune the measurements. Now I'm able to tune this thing down to 1.1:1 with a feed point resistance that varies from 47ohms on channel 40 and 63 ohms on channel 1. Channel 26 is where it's feed point is 52 ohms with almost no reactance. After the copper is measured I purchased several brass dowel rods the same diameter and used brass sleeves to solder the matching sections together. The bends must be precise and the loops parallel to each other. I solder sleeved number 12 copper wire for the bends which aid greatly in attaching to the base and the driven element. Then using your MFJ269C start by setting the vertical at 22'3", GP elements at 105". Adusting the spacing between the two matching sections about half way up from the feed point will vary the reactance and change the center tuning. You can also tweak the loop spacing too. Not too much, like a 1/16th of an inch. Don't worry about the ground as you may have to lengthen it to get the feed point resistance down. You'll play a few games with the length of the vertical if you want it close to 1.1:1, but using the MFJ269C you'll see how one adjustment affects the other. From this you can dial it right in. GP radials I adjusted but ended up putting them back at 105" as my feed point resistance was only varying by 16 ohms with close to 0 reactance. My IMAX varied by over 22 ohms and was no where near 0 reactance. One other note: when these antenna get wet they will drift down in freq. as a result I've adjusted my center to channel 28/26 to compensate for this dirft. Just for comparison I put my Siro Gainmaster on the same tower using same coax same height. It averaged 1s unit lower on both TX/RX verses using the modified SPT500. Sometimes the gains were greater but never did the Gainmaster come close to the SPT500 either in resonance or feed point resistance or local or DX communications. With the current node at 54 feet or so it really plays and out performs one of the best verticals from Siro. Anyway good luck


     
    #1 Turn it to the right, Mar 24, 2015
    Last edited: Mar 24, 2015
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  2. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Don't you mean 23' 3" TOTAL radiator length? Are you still using the top hat radial rods?
    My old school P500 required a 23' 3" tall radiator w/o top hat rods. That's right at .625 for 27.275MHz above the radial plate.
     
    #2 Needle Bender, Mar 26, 2015
    Last edited: Mar 26, 2015
  3. Turn it to the right

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    Yes still using capacitance hat as it necessary for keeping the feed point resistance fairly consistant. I found anything over 23' on the driven element as measured from top of the bottom insulator could not be tuned for near 0 reactence. Always too inductive as shown by both the MFJ269 and simple channel 1 to 40 SWR spread.
    I did have decent results from 22'9" but that required some fine tuning the ground leg connection on the GP elements as well as extending the GP elements 1.5" to 106.5" to try and keep the feed point resistance down with all that inductance. The MFJ269C is very valuable when doing these adjustments. Any way I decided on centering around 27.300 after watch ing the rain effects and for some above 27.405 operation. My 1.5:1 SWR spread 26.955 to 27.715. Pretty darn good for $10 bucks worth of chinese aluminum.
     
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  4. gamegetter

    gamegetter Well-Known Member

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    Turn it to the right:
    I am no expert, but isn't 1 s unit equivalent to quadrupling the power? And you were able to quadruple your power by going from the Gainmaster to your adjusted and modified Penetrator 500?

    I'm asking to learn, so thanks....
     
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  5. Turn it to the right

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    Kinda getting off the topic. First S-meter readings are relative and not absolute. Maybe a little subjective depending on who's looking. A true 5/8 GP tuned properly should present about 3Dbd of gain. So the EIRP is effectively double the RF applied. Now granted these cheap CB S-meters are supposed to be calibrated for 6Db per division but AGC action and other issues may cause a non-linear meter deflection. The S-meter stated readings where derived from station reports I have a known baseline in signal strength. As far as my recieve there is a noticeable improvement over the Gainmaster both in overall system noise and recieved signals. So allow me to clarify my statement. In my comparisons of the Gainmaster and the modified SPT500. The SPT500 is better by at least 3db in both transmit and recieve based on relative S-meter results. Your results maybe different.
    Not bashing the Gainmaster as I own one. The SPT500 with corrected matching system out performs it based on my real world testing.
     
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  6. gamegetter

    gamegetter Well-Known Member

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    thanks for your reply...

    at what channel or frequency was the antenna resonant on before your modification? you mentioned 1.3:1 as lowest match...

    I am thinking of one possibly in the future for my trial, nothing definite so just trying to learn about it as is done from other's experiences
     
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  7. Turn it to the right

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    Never got it resonant but the feed point resistance was low enough with the given reactance to achieve a 1.3:1 but the bandwidth seem to suffer. Centered about 27.455Mhz is where it was about 1.3:1 VSWR.
    New matching network solves this and now your 1.5:1 VSWR bandwidth is over 600khz with the antenna having near 0 reactance at the center freq of 27.300Mhz.
     
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  8. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    First; 23' 3" was TOTAL length for the driven, including the 9" or so top hat rods, but I removed the top hat and simply extended the radiator to a full 23' 3".
    On mine, X= 0-1 @ 1:1 swr @ 52ohms, OLD SCHOOL P500.
    Unless they screwed up a good thing, should measure the same on the new SPT-500.

    And my Gain Master beat my P500 by 2dB once I got the GM in it's own airspace away from EVERYTHING! Damn fickle antenna, that GM.
     
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  9. Turn it to the right

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    Ok took mine down and tried your measurements. Results were very poor. Antenna at 23' without capacitance hat has too
    much inductive reactance. Now granted my SPT500 has an enlarged matching system so this maybe a possible reason I couldn't duplicate your findings. Anyway I did further tweaks to more match the original design. First one was angling the capacitance hat radials down about 40 degrees. This allowed the driven element to have an even better feed point resistance bandwidth. Also re-clocked the base insulator and soldered the connector lug so the matching network attaches like the original . Also moved the ground plane connection and repositioned like the original design. These mods along with careful retuning has this SPT500 performing as good or better than the original. Now if the Aluminum was the same quality it would be an exact duplicate.
    Ive had a GM up for 2 years prior to my SPT500. Only advantage of the GM is its wide bandwidth and fiberglass skin making it less susceptible to SWR swings durning wet conditions. Down side It has power limitations and it's not very resonant anywhere. So for 11 meter operation the GM is a compromise, but the SPT500 is optimized. Done all the side by side tests and reached the exact opposite conclusions as "Needle Bender"
     
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  10. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Interesting post 'Turn it to the Right', but it left me questioning what you've done to create your "Enlarged matching system".

    My original 1976 edition used right about 40" of aluminum rod for the match and about 14" for the gnd shunt.
    Originally the 283" (23' 7") total height including the top hat rods, (which on mine were 9.5") minus the 13" below the radial plates left a measurement of 270".

    When I add to that 270" half the 2.25" average circumference I get 272.125" which calcs right about 27.108MHz or almost dead center of the old 23 band of 27.110, but my maths could be off a couple 1/100s of an inch. lol

    When I calc for the difference for a .625 between 27.110 (center of old 23 ch band) & 27.185 (center of 40 ch band) I'm only seeing a difference of 3/4".
    Seems then the right overall P500 length for 27.185MHz should be 22' 8.75" incl top hat, if the rest is OEM.
    Why the last 40ch Hy-gains were 22' 6" I have no idea.

    My P500 came with pre-bent 45° downward top hat rods.

    What have MFJ done to change the basic product in terms of the base length & match?
    Anyone have pics of the newest MFJ SPT-500 base and match?
    Are the SPT-500 top hat rods still 9.5" like mine from 1976?

    As far as the Gain Master performance, I'm wondering just how much the coax length comes into play, and whether or not being fed at a current or voltage node helps or hurts.
     
  11. Turn it to the right

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    Ok on the math as that's not the overall issue. Ok on the SPT500 from 1976. Now let's talk today's SPT500. First matching network is physically smaller in overall dimensions and attachment to Gnd plane and driven element is slightly different. Bottom insulator is clocked different. Overall resonant length of vertical and Gnd plane elements is similar to the 1976 model as are the top hat radials. They're just not bent at 40-45 degrees as they are straight.
    The matching section I built from brass rods and copper is a very close facsimile of your 1976 beta match. Mine was first made from #12 solid copper to play with the overall dimensions and element spacing. This was in turn applied to the final matching section and then integrated into the antenna. Tuning from this point was very simple. I used a MFJ269C for this final phase. 10 feet off the ground, vertical 22'3" to it tuned to 0 reactance at 27.305Mhz with feed point spread of about 6-8 ohms over the CB band. I found this to be quite good as few GP antennas can maintain this feed point resistance figure. Sirio 857 and others I've tried are good but have more spread. After making the changes to more match the original and bending the top hat radials to about 40 degrees the vertical element can be lengthened to 22'55". The center is about 27.170Mhz where it's 0 reactance and feed point is 53 ohms. Remember this is all at 10 feet. Your ground resistance is going to affect you center freq. as you raise the antenna up. Based on my findings the center freq. went up about 80khz or about 8 channels at 60 feet. SWR is 1.10 on 27.255Mhz at 1600watts. Don't think there's much more I can do as this thing is playing very well and can handle 2 kW all day long. There's plenty of pictures of the old SPT500 and the new on the web so you can compare the differences. It's very obvious MFJ wanted to make it very difficult to use this fine GP on 11 meters!!
    One last word on the Gainmaster from Sirio......compromised power handling for bandwidth, not resonant anywhere. I have made higher power handling coaxial capacitor made from RG393 for the Gainmaster but you have to take the entire antenna apart to retrofit. It does work as mine handles 1.5kW with no issues.
     
    #11 Turn it to the right, Apr 27, 2015
    Last edited: Apr 27, 2015
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  12. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Is the 22' 3" / 22' 5.5" which you mentioned measured from the radials up, or the entire radiator length, incl the 10.5" or so below the radials?
     
  13. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    NB, as reported to me by a couple of other NSP500 operators...my dimensions show overall length of 264" inches, 22' feet from the feed point base to the top hat that has 4 x 10" inch wires. The tip tunable wire #1 = 40.5" inches was reported for the CB band.

    The radials are 105" inches, and base assembly wire #5 is 72" inches.

    My experience with Internet reports trying to secure accurate antenna dimensions are sketchy at best and I'm not even sure about these dimensions, so these measurements may vary from your results.
     
    #13 Marconi, Apr 30, 2015
    Last edited: Apr 30, 2015
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  14. Needle Bender

    Needle Bender ...he thinks it's funny that I stepped in it

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    Hi Marconi, I was just looking back over some old posts and came across this.
    As far as the dimensions for the original 23ch Penetrator 500, the match "wire" or rod was right about 40", not 72"

    The bottom 10" or so of the radiator is what I (& Shockwave) believe to be part of the matching network, finishing an 1/8 wave of what I've come to believe is a type of linear-loaded T-matching network... but I'm not certain on that one.

    The main radiator from the radial plate - up was 260.6" plus 9.5" top hat radials,

    totalling 270.1"

    - plus about 2.55" for average circumference = 272.65"

    then dividing 272.65" by the VF (velocity factor) of aluminum
    in air = 272.65 ÷ .9979 =

    ~273.2" total radiator electrical length.

    ... or a perfect 5/8 (.625) at 27.00xMHz, center of the 26.00-28.00MHz 11m band.
     
  15. Marconi

    Marconi Usually if I can hear em' I can talk to em'.

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    Hey, Needle Bender, read my words again. Do you need a NEW Super Penetrator manual? I can post it for you, just ask. That is the reference I was referring too.

    I don't recall the thread specifically, but I did make a post and those are my words. You just missed what I was referring to, the dimensions for the New HyGain SP500.

    The information I wrote about was claiming what I had heard about the NEW SP500 from a couple of operators that reported to have the NEW SP500 attempted to help me out so I could model the antenna. I don't think they were discussing any of the variety of older versions that HyGain and Telex produced, but the subject might have come up...you know how these threads develop sometimes.

    The 72" base assembly I mentioned was Parts List #5, and has nothing to do with the tip assembly part #1, which I wrote was 40.5" inches for the top adjustable part. I also noted that length was told me to be the tuned length for the CB band.

    The manual shows the overall height is 22' feet = 264" inches, the manual shows this length from the base with the SO239 to the tip of the antenna...not including the 4 x 10" inch Top Hat rods, and these numbers were confirmed to me. I estimated a 3" inches offset to get the exposed lengths for a total overall length of 264" = 22' feet.

    You and Shockwave may have figured out the bottom 12" inches is part of the matching, and I might not disagree, but IMO the electrical length has little to do with constructing the kit unless you were in mode to modify. The kit is based on its physical dimensions, and the VF has nothing to do with building the antenna the way the kit intends for CB.
     

    Attached Files:

    #15 Marconi, Dec 12, 2016
    Last edited: Dec 12, 2016

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