[MRCA] Giant leap backwards

Tue Aug 8 17:20:16 EDT 2023

Hi Steve

That's what I used to do to match my transmitters to a 50 ohm line.   Now I
use an old air variable cap from an HT-37 with several rotors removed.

Mark K3MSB

On Tue, Aug 8, 2023, 3:57 PM sbjohnston--- via MRCA <mrca at mailman.qth.net>
wrote:

> On Command Set transmitters that I wanted to use with a 50 antenna, I've
> had good luck placing a 50 - 100 pF transmitting capacitor in series with
> the antenna output.  For these already-converted rigs where someone already
> mounted an SO-239 in place of the wire connection terminal, I installed
> this capacitor right behind the panel in series with the coax connector.
>
> With power from a Heathkit HP-23 supply (my usual experimenting supply
> with much higher B+ than was originally used) I get more than 80 watts of
> good CW out.  Very satisfying.
>
> Again, these were "restorations" of already-modified transmitters - not
> NOS specimens.
>
>
> Steve WD8DAS
>
> sbjohnston at aol.com
> http://www.wd8das.net/
> http://af4k-crystals.com/
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>   1. Re: Giant leap backwards (Christopher Bowne)
>
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>
> Message: 1
> Date: Tue, 8 Aug 2023 15:29:26 -0400
> From: Christopher Bowne <aj1g at sbcglobal.net>
> To: Ray Fantini <RAFANTINI at salisbury.edu>
> Cc: mrca at mailman.qth.net, MMRCG at groups.io
> Subject: Re: [MRCA] Giant leap backwards
> Message-ID: <1B8FE5E0-875D-4D4A-98CC-916537CAD610 at sbcglobal.net>
> Content-Type: text/plain; charset="utf-8"
>
> Good on ya Ray.  As you?re probably aware, the Command Set transmitters
> were designed to work into short wire antennas in the aircraft with
> relatively low fees point impedances especially at low frequencies such as
> the 3-4 Mc range of the T-19.  They can be loaded to put out 40-50 watts in
> CW with a simple matching network, increasing the number of turns on the
> output coupling coil on the transmitter plate tank coil, or using a 4:1
> balun in reverse with the low
> impedance end connected to the transmitter and the high impedance side
> connected to the 50 ohm feed line. My favorite method has been to use a
> matching network that Walt Hutchens published in ER?s Electric
> Radio in Uniform around 1989.  Consists of a
> parallel LC resonant circuit with the bottom end grounded, the transmitter
> fed to a rap about 3 turns above the bottom of the coil, with a nominal 150
> pf cap in series  between the transmitter output and the tap on the coil.
> The 50 ohm antenna feedline is  connected to another tap about 3 more turns
> above the transmitter feed tap.  Of course both the transmitter chassis and
> the feed line shield are both connected to the bottom of the coil.
> Essentially a high Q tunable resonant auto-transformer, will certainly
> provide some suppression of 2nd harmonic energy.  I still have an OO card
> for out of band 7 Mc emissions from my first forays onto 80 meters with a
> T-19 back in high school 55 years ago when operating with a dipole
> connected directly to the transmitter output!
>
> I made my KJ4KV matching circuit with an junk box Eico 720 pi network
> coil, the full coil resonated nicely with a 3-4 Mc Command Set plate tuning
> cap.  For 40 meters, a clip lead jumper shorts out the large low frequency
> section of the coil.  Be sure to use a fairly wide spaced variable cap, I
> had occasional arcing even with the  Command Set variable cap.
>
> The matching network also works well to match TCS transmitters to a 50 ohm
> antenna.  They are also designed to work into low impedance short antennas
> at low frequencies.
>
> When you get comfortable with CW, check into the Old Military CW Net on
> Sunday nights at 2030 Eastern on 3558 kHz!
>
> 73 de Chris, AJ1G
>
> Sent from my iPhone
>
> > On Aug 8, 2023, at 14:01, Ray Fantini <RAFANTINI at salisbury.edu> wrote:
> >
> > ?
> > When I was out at the Hamvention picked up a AN/PRC-74 with all its
> accessories, that included a CW key. So, I have decided that the next great
> challenge in life will be mastering CW and maybe using the 74 to check into
> the pack net this year at Gilbert (MRCA). It makes way too much sense to
> just set the 74 up with a power supply and practice that way so I decided
> to build up a CW operating position. Common sense would dictate using a
> more modern transceiver like my URC-94 or the big rack that?s used for RTTY
> but no, all that is to simple, the solution was to build up an entirely new
> operating position and use something that would be way over complicated.  I
> decided to use the 1938 HRO RAS along with a T-19/ARC-5 type transmitter.
> The power supply for the transmitter came from an old military sweep
> generator and provides 450 volts for the plates of the 1625 and a regulated
> 200 volt source for the MO tube in the transmitter and with keeping all the
> original tank can generate
>   a whopping fifteen watts into fifty ohms. The panel provides a place to
> turn AC power on and off to the transmitter, a fat multi contact switch
> that works as a T/R relay, key line interlock and spot switch that allows
> the MO to receive B+ but not the PA and a high voltage relay is located on
> the back of the panel that is controlled by the key to provide HV and B+ to
> the transmitter. Now all I have to do is to develop my ability of being
> able to receive code to some level of proficiency.
> >
> > Ray F/KA3EKH
> >
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