Instead of asking what does it take to match this, the question should
be, why does this not already give a good match. Using your updated
information, and assuming some kind of guy wire system that is broken
up, I get resonance at 1.89 MHz with an SWR of 1.03 with no matching
network.
Measurements with the antenna analyzers, you agreed aren't accurate
because of excessive RF. You didn't say what you measured with an SWR
meter and some power, other than nothing matched. The one measurement
you gave was with 75 ft of coax. That was enough to indicate that
something out of the ordinary is seriously affecting the antenna. This
was only an SWR measurement so it is impossible to back the R and J
components out of this. If that was resonance (unlikely but possibly
close) then the antenna impedance is either 130 or 19 ohms, most likely
the lower one.
So apparently my model does not agree with what you measured, but I have
no idea how far off it is. Guy wire lengths can change the answer a lot
as well as other towers that are close. You didn't answer either of
those questions either. For example, in my model, if only one insulator
is used per guy wire, the resonant frequency moves to 1.78 MHz (minimum
SWR 2.6), which happens to be what you measured, but I'm only guessing
at guy wire lengths. I'm willing to help (or at least try), but I have
no information to work with.
You can experimentally find a network to match this, but something is
significantly affecting the antenna, and most likely it will effect the
radiation pattern, or the efficiency of the antenna. How that pattern
will be affected is unknown at this point.
Jerry, K4SAV
AB5MM wrote:
>There seems to be a bit of confusion as to the tower height and the
>tower insulator location. It's probably in the way I explained it in the
>first post. Sorry about that.
>
>A. Starting at ground level is the concrete base.
>
>B. Out of the concrete base is a piece of Rohn 25G, upside down,
>sticking up 4 ft.-3 inches above the concrete. (At approximately 6
>inches above the concrete is the ground radial buss bar bolted directly
>to two tower legs.)
>
>C. Next is the insulator(s) and they are 4 inches in height. The
>insulators are made of high density (LGQ) fiberglass rod.
>
>D. Stacked on top of the insulators is 120 ft.-6 inches of tower. The
>top section is a Rohn "flat top" with a 2" bearing. The bearing was
>included to accommodate the addition of a stinger if we needed it.
>
>E. The guys are broken with the standard 5.5 inch (ANSI 54-3) porcelain
>insulators.
>
>Almost all matching was attempted directly at the tower feed point. This
>consisted of a 15 inch RG8 pigtail, PL259 on one end and split at the
>opposite end for 5 inches. The center conductor attached to the tower
>just above the insulator, with the shield attaching to the base section
>just below the insulator. All the ground radials come to a common
>aluminum buss bar securely attached to the bottom base section 6" above
>the concrete.
>
>The equipment used for trying to obtain a reasonable match are as follows;
>- RF signal source: MFJ 249, MFJ 259B and a CW exciter with variable rf
>output, 0 to 30 watts, 1.00 MHz to 4.50 MHz.
>- VSWR measurement: The two MFJ analyzers. Two Bird meters, one meter
>with 50w slug (fwd) and the second with 5 watt slug. (rev) and finally a
>Yaesu YS-60 pwr/swr meter.
> Only once, did I try loading the vertical from the shack. I used a
>75ft. piece of Andrew LDF4 heliax and my Yaesu. Poor results... the swr
>foldback killed the output.
>
>The matches tried are;
>Low pass "L"... series inductor 0 to 32uh with 25 to 1000pF capacitor
>shunt to ground.
>High pass "L"... series cap 250 to 1800pF with above inductor shunt to
>ground.
>High pass "T"... series 72 to1200pF cap, same variable inductor to
>ground, and then another 75 to1200pF cap.
>Low pass "Pi"... series variable inductor with two caps on either side,
>both are 572 to 1700pF.
>?? ... series cap 72 to 1200pF
>?? ... variable inductor, one end to ground, the other
>end to tower with RF in on the variable tap.
>?? ... variable inductor, one end to tower, the other
>end floating, RF in on variable tap.
>?? ... parallel inductor and 72 to 1200pF cap, in
>series with center conductor.
>
>All variations of the above mentioned schemes have failed in one way or
>another. The ones that did marginally work have too high of a Q and are
>extremely narrow in band width.
>
>As an absolute last ditch effort, I can use a Palstar AT4K antenna
>tuner. I personally don't like to use tuners and would much rather use a
>resonant antenna. A tuner is just a 'cheat' and one more thing that can
>fail at the most inopportune time. Nuf said abt tuners!
>
>Well, that is our version of the "old collage try" and we seem to have
>failed somewhat. Maybe it's time to put back up the inverted L's and dipole.
>
>Best regards and 73,
>
>Steve AB5MM
>
>
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>
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>
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