[MRCA] Giant leap backwards

[Christopher Bowne]

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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