Do you perhaps have another tower close to this one? By placing another
tower 100 ft away from this one and tuning it to the right frequency in
EZNEC, the feedpoint impedance changes from 43 ohms to 25 ohms.
Although the impedance you will measure at the end of 75 ft of coax will
be closer to 120 ohms (at a different frequency). This will change the
radiation pattern also.
>I would like to extend a hardy "Thanks" to each of you that responded to
>my plea for help with matching the 120'-6" 160m Rohn 25 Vertical. It's
>still not working like I think it should, so let me tell you what we've
>managed to accomplish to date.
>1. We now have 40ea. 125' long ground radials installed. Another 20 will
>be added this weekend for a total of 60. I think I'm going to say that's
>enough for now. NOTE: Going from 22 to 40 ground radials made little
>2. Both the MFJ-249 and MFJ-259B are virtually useless at these
>frequencies. Too much rf from the BC band. This alone would have saved
>us hours in trying various matching networks, not to mention the number
>of "Pepcid Complete" ingested.
>3. In sweeping the tower from 1.600 MHz to 2.500 MHz, we found it's
>natural (no matching other than the 75' of LDF4-50A) resonance at 1.788
>MHz. The lowest swr observed at this frequency was 2.6/1. At 1.800 it is
>3.0/1 and slowly rising to 4.0/1 at 2.000 Mhz. Conventional wisdom tells
>us that the thing is too long (tall). This still brings up the
>question, "How can this be the case when the vertical is only 120 ft.-6
>in. tall to begin with?" What are we missing here guys? The center of
>the band (1.900 mhz) is what we've been shooting for all along.
>4. A high pass "T" network brings the match down flat, but it's Q is
>high enough to only realize a practical band width of ~33kc.
>5. A low pass "L" network works as well and exhibits a little larger
>band width of ~60 kc. Our goal is ~100 kc wide... is this reasonable?
>We really don't know what to do or try next, if anything. Should we be
>satisfied with what we have?
>Press on, nothing is as persistent as persistence itself.
>Tnx & 73,
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