[ARC5] My ARC-5 40 meter station

[Charles]

Thanks for the suggestion. I hadn't realized the BC-456 modulator is indeed 
a screen modulator, whereas the MD-7 is a plate modulator with a screen 
winding! I would be interested to see the output waveform of your 
transmitter producing 15 watts carrier at 100% screen modulation (with a 
stock BC-456 modulator), if it's even capable of 100%. Can you please post a 
scope picture?

I do have to wonder, though, has anyone read Dave Stinson's "ARC-5 Notes", 
to which I posted links yesterday? There is discussion of the various 
matching methods (including the series cap, which is what I'm using), AND 
actual harmonic output measurements of each. Not much difference between 
them for either power output or harmonics, so I went with the simplest, a 
doorknob series cap. There is even room inside the transmitter to mount it 
after removing the unneeded antenna relay behind the top of the front panel.

Rather than add an additional transformer and variable cap, I simply added 
one turn to the 40m output tank, (two turns for the 80m), in series with the 
output variometer link. I can now adjust the loading into 50 ohms from "too 
little" to "too much" via the front panel knob. I'm a firm believer in the 
KISS principle :)

-Charles
WB3JOK/0


-----Original Message----- 
From: Chris Bowne
Sent: Sunday, February 22, 2015 8:14 AM
To: J Mcvey
Cc: Charles ; ARC-5
Subject: Re: [ARC5] My ARC-5 40 meter station

Great work Charles. To get the best loading and max your power output, try 
using the autotransformer matching network that Walt, KJ4KV published in an 
ER in Uniform column many years ago. It works very well, I use it on my 
command sets to get about 50 watts out on CW and 15 watts on AM, using the 
original BC-456 screen modulator and DM-33 dyno.  Consists of a parallel 
tuned circuit with the transmitter fed to a tap on the coil a few turns up 
from the ground end through a 150 pf series cap and the 50 ohm antenna fed 
off of a second coil tap a few turns above the transmitter input tap. I 
threw one of these together using a pi network coil set from a junker Eico 
720 transmitter and a 200 pf transmitting variable.  As I recall the input 
tap is about 5 turns up from the ground end and the antenna tap 3 turns 
above that. Use a cap with relatively wide spacing as the voltage across the 
plates can get quite high at resonance.  I had arcing problems with a narrow 
spaced cap.  You tune for resonance with the autotransformer cap and then 
peak output with the command  set transmitter variable roller inductor. 
With this setup your output should peak with the transmitter coupling 
control at about 2/3 of full travel,  I can't recall the ER issue the 
circuit appeared in but it was around 1989-1990 as part of a series of 
Walt's ERIU column on command sets.

The circuit should also help with transmitter harmonic output suppression.

Sent from my iPhone

> On Feb 21, 2015, at 18:19, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
>
> You get alot more than 8 watts out. The output was designed for low 
> impedance (5-12 ohms ) a few hundred ohm capacitive 5-J200 antenna.Specs 
> say you should exceed 40 watts in CW  and have a 15 watt voice 
> carrier.Perhaps try using a 9:1 "unun"