Well, if those 4 discrete tap points are the only ones that you have
available, Carl, you likely won't find one that give that gives you 50 ohms
real. So the best that you can do is pick the one that falls closest to 50
ohms real when you tune out the series inductive reactance with the
capacitor. Even if the 90' point gives you 68 ohms real, that's not too bad
- about 1.4:1 VSWR. If the 67' tap gives you less than 50 ohms real, you
could add a shunt C back to GND at the base of the tower the and readjust
the series C to only cancel a portion of the inductive reactance, leaving
the remainder to work against the shunt C to form an L-network to transform
the real part up to 50 ohms. By alternating back and forth adjusting the
series and shunt capacitors, you should be able to bring the feedpoint VSWR
to 1:1. Note that whatever the tap point, if the real part of the impedance
is LESS than 50 ohms, a shunt C will be required.
GL,
73,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Carl
Braun
Sent: Friday, February 14, 2014 11:52 PM
To: Tom W8JI
Cc: 160
Subject: Re: Topband: Still in search of resonance
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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