Tuning the MFJ SPT500 Super Penetrator

[Ranch55]

Wow. Were you running a lot of power? Im running a stock radio. If not then i will use your method. Nothing written in stone yet and im in no hurry. Im trying to "get it right the first time ".

Yes, I was running power ...... 1kw plus ......
If you keep it at or below about 700 ~ 800 watts, I do believe that it will work out just fine for you.

 

[old goat 321]

Yes, I was running power ...... 1kw plus ......
If you keep it at or below about 700 ~ 800 watts, I do believe that it will work out just fine for you.

The heat shrink tubing im looking at has a dielectric strength of 500v/mil. Its inexpensive and easy to use. If it keeps the swr normal that will be fine for my needs. But a question ranch. Should i run it past the upper standoff and screw the standoff into it? Or should i butt into the standoff? Same question for the bottom of the radial plates?

 

[Ranch55]

The heat shrink tubing im looking at has a dielectric strength of 500v/mil. Its inexpensive and easy to use. If it keeps the swr normal that will be fine for my needs. But a question ranch. Should i run it past the upper standoff and screw the standoff into it? Or should i butt into the standoff? Same question for the bottom of the radial plates?

I think that on the bottom, I would run it about 2 1/2" down from the bracket.....
On the top, either way you mention should be fine. If you run it above the standoff, I would seal the hole with some Marine Grade GOOP ......

 

[Divebomber]

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

Turn it to the right, I have the new Penetrator 500 and I would like to know more details of how much longer or shorter you made your hairpin match? I considering bending some new rods but not attaching them at the top above the standoff and making a sliding,locking bar between them for easy adjustment. Also interested in how much longer you made the ground shunt?
Thanks, 73 Divebomber

 

[W2EKG]

It's shorter than the original.

 

[W2EKG]

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