Tom and all
Thanks for the responses.
I was under the assumption that I would find a 50 ohm tap on the tower but with
a lot of reactance requiring a cap that would tune out the X but leave the 50
ohm resistive value in place.
I knew my tap was too low at 46' when I saw less than 50 ohms with the variable
cap in place. Same thing when I tapped the tower at 90' and I saw the lowest R
at 68 ohms with the 160pf only partially meshed.
With all of the information presented in this thread it appears my best bet is
to tap the tower at the 67' level while playing with the larger (1050pf)
variable in series to see what my results are. I have never used the larger cap
with the tap at 90' or 67'...only the smaller 160pf variable.
More experimentation tomorrow if I don't burn out the Skyneedle motor with all
the up and down.
Thanks and details to follow.
Carl AG6X
Sent from my iPhone
> On Feb 14, 2014, at 6:01 PM, "Tom W8JI" <w8ji@w8ji.com> wrote:
>
> Hi Carl,
>
> It sounds like you are trying to find 50 ohms on the tower without any series
> capacitor by looking at R and X. I would not try to do that. The reactance
> puts you out of range on the MFJ bridge. You are down to a few bits
> difference between data points the PIC needs in the MFJ.
>
> Look at this below. You said:
>
> <<<<seen at the other levels too as long as I brought the R down with a
> variable cap. Yesterday, with the gamma arm at the 46' level (and 240 ohms
> on the MFJ) I was able to put the big variable inline to bring the reading to
> 24 ohms with a TRUE X=0. With a 22 ohm to 50 ohm UNUN, I saw 1.3:1 Vswr on
> the output of the UNUN. I worked a W2 in NJ and a W4 in Florida with just
> the 1000D. BUT...again...I'm bringing the R down with the capacitor...not
> finding 50 ohms anywhere on the tower>>>>
>
> Stop trying to find 50 ohms without the capacitor!!!!!
>
> Right now at 46 ft you were at 24 ohms with the capacitor. That should tell
> you and everyone on this reflector :-) that you are tapped too low now!
>
> Let's look at this in simple terms. Here is what you said:
>
> <<<<<When I had the gamma arm mounted at the 90' level. I was able to put my
> baby variable 160pf inline to bring the 425 ohm impedance down to about 60
> ohms and the antenna heard very well; especially on the 1700 KHz broadcast
> band, with a 2.4:1 Vswr. Similar results could be seen at the other levels
> too as long as I brought the R down with a variable cap. >>>>>
>
> That is NORMAL. You will always need the capacitor. Always. The only way to
> eliminate the capacitor is to saw your Yagi antenna off the tower so the
> tower moves above 2 MHz. Then you will probably find a 50j0 tap without any
> capacitor.
>
> You also might use a large skirt, but why??????
>
> Just use a capacitor!!!
>
> If you are trying to eliminate the capacitor, you will have a lot of work to
> do.
>
> 73 Tom
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