On 9/20/97 9:40, Webster D. Williams at kr4wm@sccoast.net wrote:
>Steve Ellington wrote:
>>
>> >
>>
>> I believe that most MFJ tuners use the T network and TenTec uses a
>> swtichable L network. L networks are know for their low loss while the
>> losses in T networks can be quite high.
>
>We're getting into an area I don't know about- I thought the MFJ used
>the same circuit the ARRL handbooks show as the SPC transmatch, known as the
>PI network? Go ahead, I'm learning!
No, it uses a T network. This is a very common circuit. (I think the SPC
uses the same) One capacitor from the input to the inductor, another cap
from the inductor to the antenna. The other leg of the inductor is
grounded.
The T network can match a wide range of impedances, which is why it is
favored.
>> Once a match is finally made by juggling the knobs and dials, the
>> settings can be recorded for future use.
>
>Yes, this is true, but the roller inductor action in my Ten Tec 229B
>tuner has so much backlash (with respect to the linear scale) that it's
>difficult to reproduce exact inductances.
As one who has used a T-type tuner with a junky indicator (A Murch
UT-2000), I can say that you aren't going to get the spot-on match every
time. Some fine tuning is necessary, especially on 80m. But it certainly
takes less time to crank in the recording settings and tune from there.
>> The balun used in the MFJ is also inferior to TenTec's.
>
>This is probably true. It's been years since I've been inside either, so
>I can't make an educated comment. I prefer glass-fiber-tape covered,
>teflon-insulated-wire toroids for tuner baluns. I don't know what either
>Ten Tec or MFJ is using.
So don't use their baluns. I remote-mounted mine on the side of my house.
Coax comes from the tuner to the balun, then transmitting twinlead to the
antenna.
>> Perhaps pruning the feeder or antenna to make the match easier
>> would result in lower tuner power loss.
>
>I'm using a 75m Delta Loop on all bands, so the feeder length is
>basically
>irrelevant. Loss isn't my problem, the difficulty in determining the
>correct
>"LO" or "HIGH" value of capacitance to match a given frequency is.
I'm in a similar boat. I'm using a 125 foot doublet.
The trick is to start with your lowest band. Go to 80m with the inductor
at maximum inductance, the capacitors at mid-scale. Crank the inductor
for a dip in SWR. Then mess with the caps. Don't touch the inductor again
until you are satisfied with the caps. There's lots of interaction, so
don't try adjusting more than 2 knobs at once. Write down these settings.
Once you have 80m done, jump to 40m. Most likely, you'll use slightly
less capacitance on the higher bands. The technique is the same. Tune for
inductor FIRST for a dip in SWR, then adjust the caps.
My experience is that the cap settings WILL NOT CHANGE for a particular
band. If I tune up for 3.970, then QSY to 3.790, all I have to do is
adjust the inductor a turn or two for a good match. On 40m, the range of
inductor adjustment is quite small. The higher bands won't need
adjustment to cover the whole band (usually)
>One other thing I didn't mention is that the trees I'm using for support
>tend to sway in the wind and grow new leaves. As large as my antenna is,
>and because of it's configuration, this causes appreciable changes in
>impedance, and any particular condition of the antenna is not reproducible
from
>day-to-day.
Take down your antenna and check for loose or poor connections. I have
used the same doublet for 8 years at two different QTHs, and I do NOT see
any drastic changes from season to season. The only time I have seen such
changes is when I've had broken wires at the feedpoint.
Bill Coleman, AA4LR, PP-ASEL Mail: aa4lr@radio.org
Quote: "Not in a thousand years will man ever fly!"
-- Wilbur Wright, 1901
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