[ARC5] SCR-274N Antenna Matching

Fri Aug 22 14:18:45 EDT 2014

I use scrapped CATV line splitters, couplers, amplifiers, power combiners, 
etc for a lot of indoor and outdoor "stuff". The  fittings are either F or 
Hardline connectors and the latter threads "just happen" to be the same 
5/8-28 threads as UHF connectors, barrels, etc. I wonder if some unheralded 
engineer did that on purpose (-;

Carl

----- Original Message ----- 
From: "Mike Feher" <n4fs at eozinc.com>
To: "'Mark K3MSB'" <mark.k3msb at gmail.com>; "'ARC5'" <arc5 at mailman.qth.net>
Sent: Thursday, August 21, 2014 11:37 PM
Subject: Re: [ARC5] SCR-274N Antenna Matching

>I built a toroidal 4:1 over 25 years ago to use with my WS19 set. Worked 
>great and made many contacts. I enclosed it in a milled out box with cover, 
>and with two binding posts and a BNC. Since, I have used it successfully 
>with a BC-474 and a TCS and many other similar rigs. Never bothered with a 
>capacitor, but, always had a resonant antenna. I remember when I was a kid 
>in my early teens, most everything I built worked. Was it maximally 
>efficient, more than likely not. Regardless, even with all the education I 
>have behind me now, and still use for my consulting work, I do not seem to 
>use a lot of it for my ham activities, other than common sense. 73 - Mike
>
> Mike B. Feher, N4FS
> 89 Arnold Blvd.
> Howell, NJ, 07731
> 732-886-5960
>
> -----Original Message-----
> From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Mark K3MSB
> Sent: Thursday, August 21, 2014 10:29 PM
> To: ARC5
> Subject: [ARC5] SCR-274N Antenna Matching
>
> 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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