John S wrote:
On 8/11/2015 6:18 PM, John S wrote:
On 8/9/2015 4:24 PM, Wayne wrote:
"John S" wrote in message ...
On 8/9/2015 12:23 PM, Wayne wrote:
I was looking at a MFJ remote antenna tuner, and it is specified as
being able to match 12 to 1600 ohms.
What does that mean? At 12 ohms the SWR would be about 4.2:1 and at
1600 ohms a SWR of about 32:1.
The 12 to 1600 ohms is the RANGE that the tuner is capable of
matching. Forget SWR because it has no meaning at this point.
That's a bit of what I was getting at. I have another tuner rated for
4:1 SWR, and I assume that means it will match all the way around the 4
circle.
I don't know what to do with a 12 to 1600 ohm spec.
If one started from 1600 ohms around a Smith chart, the circle would
intersect at the other side at about 0.64 ohms.
You are not analyzing this correctly. If you are really comfortable
with a Smith chart, put a capacitor across the 1600 ohms and then put
an inductor in series with the results. By adjusting the values, you
can achieve a 50 ohm output.
Likewise starting at 12 ohms and going halfway around the chart would
end up at 210 ohms, not 1600.
This is similar to the above, but now you want a series impedance and
a shunt impedance following it.
I was just showing that the 12 and 1600 are not on the same circle.
How does one use the specs if the antenna to be matched has a complex
impedance? Look at the internal tuner component ranges?
That is a good question. Do you want to buy the tuner and tear it down
to explore the component ranges, or would it be better to ask the
designers? You should ask them to define the type (complex impedance)
of the load to which their specs apply.
I suspect the designers have a gag order from the marketing guys.
I found the following on the MFJ site for the MFJ-998RT. See page 2 at
this link:
http://www.mfjenterprises.com/Downlo...df&company=mfj
?Impedance matching range: 12 to 1600 ohms
?SWR matching range: up to 4:1 for 50 ohms and upto 32:1 for 50 ohms
?Minimum power for tuning: 5 watts
?Maximum power while tuning: 100 watts with foldback, 20 watts without
foldback
?RF power limit: 1500 watts SSB/CW
?Frequency range: 1.8 to 30 MHz continuous coverage
?Frequency counter accuracy: ?1 kHz across HF bands
?Capacitance range: 0 to 3926 pF nominal (256 values) on input side 0 to
976 pF nominal (64 values) on output side
?Inductance range: 0 to 24.28 ?H nominal (256 values)
I think their minimums are not realistic. I would not know how to
achieve 0pF with a 3926pF variable cap, for example. Also, why 3926
rather than 3900 or even 3920? It isn't all that easy to measure 6pF out
of 3920pF. I'm thinking that somebody just threw some numbers at the
spec sheet.
BUT! It is better than nothing and may help to make some reasonable
judgements.
The parts in a modern autotuner are not variable, they are a bunch of
fixed components switched in and out by relays.
The 3926 pF is the value of ALL the capacitors in parallel and 0 pF
would be a straight connection.
--
Jim Pennino