[TenTec] TITAN DX

[Dan]

Ah who needs a good ground, it's just one more hole to drill in the wall! :)
To be perfectly honest the only case where I found a ground was needed was on
my old tube type gear.  Would get an rf bite on my old ts 520 if I didn't
ground it. Never bothered with it much on modern stuff.

"George, W5YR" wrote:

> Not necessarily, Bob.
>
> SWR bridges of the conventional Bruene design will read 1:1 ONLY when
> they see a 50-ohm resistive load in the output port. In your 100-ohm
> example, the meter will read 2:1 not 1:1.
>
> The Bruene design "assumes" that the input will be a 50-ohm coax
> connected to a transmitter that wants to see a 50-ohm resistive load.
> The application is such that either the antenna system will present
> approximately a 50-ohm load for the transmitter - about 1:1 on the SWR
> meter - or a tuner will be in the feeder line to force a 50-ohm
> resistive load for the transmitter and hence for the SWR meter.
>
> Even using a Bird 43 for SWR measurement requires that the coax be 50
> ohm. "Equal input and output impedances" just doesn't enter into the
> picture as bridges and power meters are designed for 50-ohm lines.
>
> Your point about common-mode current is well taken, and I know of no
> bridge or analyzer, even the MFJ, that can identify this component and
> deal with it. With our comparatively simple SWR meters and analyzers, it
> is up to us to ensure that no common mode current is present when we
> take readings if we expect much accuracy.
>
> Common-mode current can "mess up" the readings of most analyzers as
> readily as it affects SWR meters. It is perhaps worse with the MFJ with
> its metal case since if you stand there on the ground holding the
> analyzer case in your hands, you are providing a path for the
> common-mode current to ground through your body. This changes the
> common-mode current magnitude and path(s) and can provide results
> different from when the meter case is not "grounded." The AEA/Tempo
> CIA-HF analyzer gets around this by using a very think and heavy plastic
> case.
>
> Common-mode current is the most common cause for "screwy" SWR readings.
> A good way to identify the presence of such current is to watch the SWR
> meter while you move your hand - loosely grasping the coax - along the
> line for several feet. If the meter varies at all, you have common-mode
> current on the outer-braid of the coax. This is a major source of RFI in
> the shack and IS NOT avoided or disposed of by "a good ground."
>
> But, don't get me started on that one!    <:}
>
> Getting rid of common-mode current almost always involves use of one or
> more appropriate current-mode chokes in the feedline, usually one at the
> antenna feedpoint and another at the shack end.
>
> Interesting stuff . . .
>
> 72/73, George   W5YR - the Yellow Rose of Texas      NETXQRP 6
> Fairview, TX   30 mi NE Dallas in Collin county      QRP-L 1373
> Amateur Radio W5YR, in the 55th year and it just keeps getting better!
> Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)
>
> Bob & Linda McGraw K4TAX wrote:
> >
> > George & Steve & co:
> >
> > Most SWR bridges will read 1:1 when equal impedances are on each side of
> > the bridge.  Feed one from a 100 ohm source and terminate the output in
> > 100 ohms and you will have 1:1 SWR indicated.  I think what is missing
> > (actually present) is current flowing on the outer conductor.  Often,
> > simple bridges do not know how to or for that matter even deal with this
> > condition.  One of the MFJ analyzers should give you a reasonably true
> > picture.
> >
> > Feedline Z is another issue and is only part of the equation when
> > feeding a load.  Yep, it's most always complex even when feeding a
> > resistive load equal to the source impedance.
> >
> > 73
> > Bob K4TAX
>
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