[ARC5] SCR-274N Antenna Matching

[Ian Wilson]

These are all nicely linear components, so lend themselves well
to SPICE modelling. I did some modelling of the ARC-5 TX that
covers 40m a couple of years back. The results were pretty close
to my very simple capacitor matching efforts.

You should be able to do this with LTspice (= free).

73, ian K3IMW



On Thu, Aug 21, 2014 at 7:28 PM, Mark K3MSB <mark.k3msb at gmail.com> wrote:

> 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
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