[Milsurplus] Fw: Pi- Matching Network: When? (More)

[mac]

Probably the first production transmitter using a Pi-/Collins PA  
network was the US Forest Service "SPF" AM transceiver. Production of  
the first 200 units took place in 1935, last produced in 1940, 1200  
units total.  Also, these sets were typically paired with a Windom  
antenna.  For a lot more on Forest Service radios contact me for an  
interesting DVD.

Dennis D.  W7QHO
Glendale, CA

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On Feb 9, 2012, at 11:01 AM, DeWitt Clay wrote:

> Sounds like the Windom Antenna and Pi Coupler grew up together.
> I experimented with the Windom antenna and it worked pretty well for  
> me. The tapped input impedance was given as around 500-600 Ohms and  
> worked well into a pi network tank circuit for the transmitter I  
> used on 40 meters. The single wire transmission line was also said  
> to be 500-600 Ohms.
> The Windom antenna descriptions and calculations I remember were for  
> single band use but Frank C. Jones' showed  it could be made into a  
> multiband antenna by moving the tap from the usual recommended point  
> and still get acceptable results. I haven't tried this multiband  
> Jones setup so I can't say how well it works. Feeding it with a pi  
> coupler/pi output network ought to reduce any potential harmonic  
> radiation problems though, hi.
> I liked the single wire feedline setup. It was simple, inexpensive,  
> and light weight. It is supposed to work against ground by  
> capacitive return to the transmitter thru the earth and ground line  
> to the rig. Some rf engineers have panned it because of this. But I  
> don't remember if I grounded  my transmitter in my old station. It  
> was a pretty basic setup.  Anyone know if it is really needed to get  
> the system to work?
> The Windom's transmission line could also be tapped on to the final  
> tank coil - adjustment being made until the proper loading was  
> achieved. I also tried this setup and it worked (may be prone to  
> harmonic radiation, hihi). Some of the older rig designs showed the  
> pi coupler tapped on to the final tank as W7QHO pointed out. This  
> makes for some interesting inital adjustments and tuneup, hi.
> 73, DeWitt  N4QNX
>