Carl,
Interesting comments that have made me rethink my (probably bad)
assumptions.
My assumption is that anything other than a 1:1 SWR was somehow causing more
wear the transmitter - kind of like running your car in the wrong gear.
However, continuing with the car analogy; even though my car has 5 speeds
and I'm usually in fifth when on the highway, it doesn't really hurt the car
to run it on the highway in fourth - the only real negative is increased
fuel consumption. Since power consumption really an issue with the Pegaus,
maybe you're right. Does having a higher SWR cause any problems with the
transmitted signal, such as spectral purity? Does a higher SWR increase the
possibility of causing RFI?
The only problem remaining for, then, is how can my software detect that the
tuner was unable to tune the antenna. For example, say that since the last
time I used it, something failed on my antenna. I hit the tune button and
now it's unable to find a match, but the only indication it gives is a high
SWR reading, which it already had been. I'd really like to be able to have a
'high SWR' indication after the tuning operation - the auto tuners in most
other rigs do. The LDG AT-11MP (external tuner) has a line that indicates
when the SWR is below 1.5.
Looking at my numbers, which probably aren't terribly accurate, it looks
like one possibility would be to look at the Pegasus SWR after the tuning
and use band specific values as the high SWR cutoff. Assuming that I want
to alert the operator to 'real' SWRs over 2.0:1, I could use 2.0 on 160,
80 and 40; 2.5 on 30; 1.8 on 20; 1.5 on 17; 2.5 on 15; and 3.0 on 12 and
10.
One additional problem with measuring SWR on the Pegasus is the resolution
of the forward and reflected power readings is only to the nearest unit,
which I assume is calibrated to watts. With 100 watts forward power,
readings of 1,2,3,4 and 5 reflected correspond to SWRs of 1.2, 1.3, 1.4, 1.5
and 1.6. However, with 20 watts out (a typical reduced power setting for
tuning) the same 1,2,3,4 and 5 correspond to 1.6, 1.9, 2.3, 2.6 and 3.0.
73,
Mark
----- Original Message -----
From: "Carl Moreschi" <n4py@earthlink.net>
To: "tentec" <tentec@contesting.com>
Sent: Tuesday, January 08, 2002 18:49 PM
Subject: Re: [TenTec] Pegasus Internal Tuner
> Jerry,
>
> I think you missed Mark's last post. He proved rather conclusively that
the
> problem
> is an impedance "bump" in the internal tuner itself. With the internal
> tuner removed,
> he got excellent agreement.
>
> Just think about it. With the internal tuner in bypass, there has to be
> relays that are
> bypassing the tuner circuit. It is almost impossible for these relays to
> not present an
> impedance bump, especially at the higher frerquencies. Now since the
> pegasus does
> not cut back power with SWR, the next experiment would be to measure power
> output externally and see if the internal tuner really is hurting
anything.
> I suspect
> it really hurts nothing.
>
> Carl Moreschi N4PY
> Franklinton, NC
> ----- Original Message -----
> From: "Dr. Gerald N. Johnson, electrical engineer" <geraldj@isunet.net>
> To: "Mark Erbaugh" <mark@microenh.com>
> Cc: "Dwayne Kincaid (LDG)" <ldg@ldgelectronics.com>;
<ditsnbits@tentec.com>;
> "Ken Brookner" <kenb@brookner.com>; <tentec@contesting.com>
> Sent: Tuesday, January 08, 2002 11:19 PM
> Subject: Re: [TenTec] Pegasus Internal Tuner
>
>
> > Sometimes a SWR meter shows a match on its load side but if it couples
> > tightly to the line, the SWR on its input isn't matched. That is
> > probably what is making the Pegasus show higher SWR readings than the
> > coupler in the tuner. In most ham style couplers, the coupling is
> > greater (and thus the effect on input match) is greater the higher the
> > frequency. Sometimes if you can pick a line length between the
> > transmitter and the SWR bridge (standard Bird technique) that is a total
> > of a half wave long including the line length of the SWR bridge you can
> > minimize that effect, though that's tough with 9 HF ham bands.
> >
> > The big question is whether the input impedance of the tuner plus its
> > swr bridge is within what the Pegasus will supply full power to or if it
> > causes a power reduction from mismatch. If the Pegasus won't do full
> > power through the tuner's swr bridge, its time to scrap that swr bridge
> > and find one that has less of an adverse effect on the swr. E.g. one
> > with smaller coupling coefficients, that may need some more DC gain to
> > run the automated tuner's logic. Though the smaller signal the gets to
> > the detector diodes the poorer they work without a lot of external help.
> > Maybe the reflected side needs an RF amplifier before the detector to
> > make up for loose coupling to the transmission line.
> >
> > Then maybe the swr bridge is just sloppily made and that makes it upset
> > the SWR on its input.
> >
> > Your antenna analyzer and the Pegasus agree that the SWR bridge in the
> > tuner is the culprit. It probably can be fixed.
> >
> > 73, Jerry, K0CQ
> >
> > --
> > Entire content copyright Dr. Gerald N. Johnson. Reproduction by
> > permission only.
> > _______________________________________________
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> > TenTec@contesting.com
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>
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