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From: ralph@arising.com.au
Date: 04 Apr 2002
Time: 22:21:49
Remote Name: 152.91.9.49
I own a Barrett 910 Auto tuning antenna and a Barrett 950 transceiver.
The Autotuning antenna has 14 series inductors. They are arranged between a variable ratio transformer, which is connected to the coax from the transceiver, and the base of the whip. The inductors are taken in and out of circuit under the control of a microprocessor which is watching the peak current at the base of the whip and the phase of the r.f. into the inductor feed.
With the Barrett 950 the tuning is accomplished by hunting for zero phase (hence reactance) and then changing the ratio on the variable transformer until a reasonable match is had. It jiggles the inductors relays in order to get into the 2:1 SWR circle. This is a compromise tuner as there are no discrete capacitors, it is not an L-match.
The Barrett 950 is capable of driving its full 100 W peak into a 2:1 SWR, so they get away with this type of tuner.
There are moving parts:
14 relays for the inductors. 2 relays for the variable ratio transformer 1 relay for the attennuator which is placed in circuit when tuning.
The last two of 14 relays are reed-relays because the voltages are very reactive and consequently high at the base of the whip and they rely on the glass body to provide extra insulation above that obtainable with a plastic relay.
My Barrett 910 Auto tuning antenna came with a 1.8 m whip and I have extened mine to 2.8 m and the 910 tunes to 1.810 MHz so I can operate on 160 m whilst mobile. The additional 1 m provides between 1 and 2 S-points improvement in RX and TX signal reports because it decreases the reactance required in the Auto Tuner and hence the losses. (Up to 12 dB is significant!)
Regards,
Ralph