[ARC5] SCR-274N Antenna Matching

[Mark K3MSB]

Hi Guys

As reported in a previous post, my matching network for my system is a
series capacitor followed by a 4:1 UNUN.   About 60 pF on 40M, and about
200 pF on 80M.   This works well, but the UNUN was a KW UNUN I have slated
for a 160M Inverted L this winter so I had to find another solution.  And
after all, a KW balun for an SCR-274N is a bit overkill……

So, I inquired of Mr. Stinson where to purchase the small toroids for my
new balun and Dave said he had one and it would be in the mail!  Nice
guy!   When it arrived, I found it was already wound for me!  Super nice
guy!

Dave commented that if the balun is first there are less losses in the
system.  So, using the balun and a 20-230 pF air variable cap salvaged from
an old HT-37 transmitter, I yanked a Bud Box off the pile and with Greenlee
punches and soldering iron in hand, set to work on my “match box”.

Here are my results.

Configuration 1: Balun before series cap then into a Cantenna dummy load:

80M:  Tuning Window at 5.5
Antenna Coupling:  Max (10)
Ep:  540V   Ip:  70 mA  (Did I goof the Ep notation?)
Air variable Capacitor on my matchbox:  12:30 O’clock

40M:  Tuning Window:  6.8
Antenna Coupling:  Slightly less than 10 (maybe 9.5?)
Ep:   480V   Ip:  155 mA
Air variable cap on my matchbox at 9 O’Clock (minimum)

I didn’t note individual output power, but they were low, maybe 10 to 15W
max.

Configuration 2: Series cap before Balun then into a Cantenna dummy load:

80M:  Tuning Window 6.8
Antenna Coupling 7 (nice!)
Ep  480V     Ip 150 mA   Pin:  72W
Power output 28W
Air variable Capacitor on my matchbox:  1 O’clock

40M: Tuning Window 6.2
Antenna Coupling:  Max  10   (not nice!)
Ep 480V     Ip  150 mA    Pin:  72W
Power out:  17W
Air variable Capacitor on my matchbox:  10 O’clock

For the series cap before balun (Configuration 2),  the screen voltage was
around 250 to 270V and the Oscillator voltage was around 180 to 190V.  As
noted above,  Ep is 480V key down. The three voltages are low but that’s
what coming out of the BC-456 using the Dynamotor.  The issue is my 28V
supply is not strong enough as it’s dropping to 24V on key down.
Solution:  build a bigger 28V supply.

So, is seems that the series cap before balun produces better results here
than the balun before the series capacitor.   28W on 80 and 17V on 80 with
an Ep of 480V is OK (maybe the 40M output is a tad low?).

What bothers me is that the Antenna Coupling on 40M is at maxed out.  Since
the air variable cap is not at it’s limits it seems the series capacitance
is not an issue.

Comments?

73 Mark K3MSB