[Lowfer] Loop Coupling Xfmr

Wed, 31 Jul 2002 22:05:22 -0400

Bill

I suspect that you are right and the unbiased collector/emitters hanging on the
transformer will generate little if any problem with signal levels we currently
encounter. "High power" 136 kHz operation from relatively close neighbors may be
another matter!

If you find a suitable DPDT instead of the SPDT just configure it as a transfer
relay and completely remove the power amplifier from the circuit during receive.
You can use the TX supply voltage to operate the DPDT just the same as you had
planned for the SPDT. Just a thought.

Roger on coupling between antennas. Interestingly, my 5050 TX loop injects 14 dB
of signal and noise (at resonance) into my box loop that is mounted in the same
plane about 25 feet away! A clip lead across the variable capaitor fixes that.
More than once I've forgotten to remove it when firing up the transmitter,
though. Probably a good thing I'm not running a kilowatt!

Jay

Bill Ashlock wrote:

> Jay,
>
> Your concern about diode action in the signal path is well taken. Maybe a
> SPDT relay swinging the coax input to the output of the two transistor
> final, when receiving, would be effective? The coil could be energized by
> the Vcc and would automatically connect the coax to the input of the final
> when the transmitter is keyed. This scheme has no diodes in the direct
> signal path except for the shunting effect of collectors of the grounded
> emitter complementary output stage. The transistors are biased fully off
> when there is no Vcc and should not effect the low Z signal off the
> transformer. I had to select this type of stage a couple of years ago when I
> began using the dual loop setup (which BTW, I haven't connected in about 6
> months). There's about 6vp-p of signal pick-up by the other 90deg loop and
> any loading of this signal lowers the Q of the one that is being driven.
>
> Bill
>
> >I'd keep an eye open for distortion products caused by those non biased
> >junctions. A double pole transfer relay at the antenna would completely
> >remove
> >the final during transmit. A single pole double throw relay with the arm to
> >the
> >transformer, one connection to the transmitter output and the other
> >connection
> >to the separate receive feedline back to the shack. Separate feedlines for
> >transmit and receive offers the advantage of leaving the
> >tranmitter/feedline
> >chain intact while being able to do whatever you want to the receive line -
> >like
> >adding filters, step attenuators, splitters and the like. That has been the
> >optimum way to do it for years on vhf, uhf and the microwave bands. May
> >just be
> >the hot ticket on LF as well!
> >
> >Jay Rusgrove, W1VD
> >
> >Bill Ashlock wrote:
> >
> > > Mike,
> > >
> > > Since you are running with a Vcc of only 14.5v the primarary inductance
> >can
> > > be lower than 400uh. At 1w RL = (Voutp-p)^2/8 = 24.5 ohms for the
> >effective
> > > load on the transmitter. You want to have the transformer's primary XL
> >at
> > > least 5 times this or 122 ohms, right? At 185k that would be 105uh. So
> >if
> > > the primary measures more than 105uh with 7 turns you are OK.
> > >
> > > I think I may have come up with a way to receive and transmit on the
> >same
> > > loop but still have the final at the base of the loop. If you turn off
> >the
> > > Vcc supply the signal may just pass through the final transistors in
> >reverse
> > > with some attenuation. Have some more tricks in mind using a negative
> >bias
> > > on the coax if this doesn't work. According to Jay he has to attenuate
> >the
> > > signal off his transformer by ~20db to prevent receiver overload, so
> >there
> > > is plenty of signal to give up. I'll try this at home and let you know
> >how
> > > it works.
> > >
> > > Bill
> > >
> > > >From: "WE0H" <[email protected]>
> > > >Yea, I was playing around with the turn's ratio and removed one to try
> >it
> > > >and later realized that the inductance would be way under 400uh now.
> > > >Whoops.
> > > >I'll fix it tomorrow and will try for 9 turns. It is at 7 turns now. If
> >9
> > > >is
> > > >worse, I'll have to do a double pass with the loop wire and double the
> > > >other
> > > >side as well.
> > >
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> >--
> >Jay Rusgrove
> >Advanced Receiver Research
> >Tel: (860)485-0310
> >FAX: (860)485-0311
> >www.advancedreceiver.com
> >E mail: [email protected]
> >
> >
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