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Date: Mon, 10 Jun 2002 12:36:47 +0100
To: rsgb_lf_group@blacksheep.org
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Subject: Re: LF: DJ
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Dear Mal, LF Group,

At 18:58 09/06/2002 +0100, you wrote:
>Has any Decca  Jeep owner considered using ferrite cores for PA coils 
>instead of altering the existing poor quality paxolin air wound devices. 
>The originals in spite of being wound with litz wire are poor specimens, 
>not even good quality at the time, when ceramic formers and better were 
>available and widely used in the USA.

The unloaded Q of the Decca coils is pretty good at about 300 - the loaded 
Q is quite low at about 5 or 6. So the loss in the tank circuits is only 
about 2% or 0.1dB, not very significant. The paxolin has negligible loss at 
136kHz, and cutting and drilling ceramic formers would be very hard work, 
as well as making no difference. If you really feel it necessary , most of 
the loss in the tank circuit is copper loss, ie. due to the RF resistance 
of the litz wire; the only ways to significantly increase the unloaded Q 
would be to increase the size of the coils and wire.

>Using the original coils the inductance when measured in the case and out 
>of the case varies by several Uh and therefore difficult to resonate/align 
>properly...
>Rewind the coils lower in frequency by a few khz than is required  and use 
>a large copper ring in the core of the coils for precise resonance, 
>alternatively wind the coil higher in frequency than required and use an 
>appropriate ferrite core to adjust for precision alignment.
>The best approach is to discard the original coils and rewind new coils on 
>ferrite cores ...

The tuning process does not require great precision due to the low loaded Q 
- getting the frequency within a kHz or so is fine, and once it is done it 
never seems to need readjustment. Fine tuning can be done with the coils 
in-situ by moving the end few tuns up and down the former. It isn't easy to 
find ferrite cores that would reduce the loss significantly and are big 
enough not to saturate. If you do decide to use ferrite cores, the coupling 
coefficient between primary and link windings will change, so it will be 
necessary to optimise the turns ratios for the guard circuit to work properly.

>These Jeeps are big things, about the same size as the Marconi Jeep, and a 
>quarter the size of the Willis Jeep. A better solution is to build your 
>own micro jeep, more power and a fraction of the size as the Decca.

It is certainly possible to make a smaller 1.2kW PA, which is fine for CW. 
But a 2 way CW contact only requires about 25% duty cycle from the TX, 
which is not very demanding. Using the PSK and FSK modes in a beacon mode 
requires 100% key down operation for hours on end, after which you will 
certainly appreciate the generous heatsinking of the Decca, especially if 
the antenna is blowing about and going in and out of tune the whole time!

Cheers, Jim Moritz
73 de M0BMU