[ARC5] SCR-274N Antenna Matching

[Dennis Monticelli]

Ian,

Something is definitely not right.  Assuming you are getting adequate drive
to a decent set of 1625's, you should be seeing a lot higher plate
efficiency.

Not to promote one matching scheme over another, but just to give an
example:

On a T-22 I use a series cap of 100pF from the antenna post to a 50 ohm
antenna and a shunt 390pF from the secondary of the variometer to ground.
 The L setting is 23.  With a regulated 500V (exact) on the plates and 263V
regulated to the screens the Ip is exactly 100mA dipped.  I measure power
out as 30W on an accurate power meter.  That's a plate efficiency of 60%,
which is what I would expect from two parallel 807 types on a 500V rail
whose grids are properly driven.

Dennis AE6C


On Thu, Aug 21, 2014 at 8:05 PM, Ian Wilson <ianmwilson73 at gmail.com> wrote:

> 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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