[AMPS] 50:75 ohm matching

[David A. Pruett]

Carl, KM1H wrote:

>Why on earth do that? Any rig or amp worth using will load directly into
>75 Ohms with no problem. Not all the world has 50 Ohms as a standard and
>the mfg designs for the variations.

Up until I installed my 80M four-square using a ComTek switchbox, I would
have agreed.  My entire station (four towers and all monoband antennas)
uses
CATV hardline.  For most bands I use asynchronous transformers made of 50
and 75 ohm cable to calm any mismatch concerns.  With the 80 four-square,
I figured "why bother - its eighty meters, right?"  Well, with a properly
installed array centered on 3650 at the end of approximately 270' of 1/2"
CATV hardline, the SWR at the shack end was near 1:1 on in the phone DX
band
and 2:1 on CW.  At the load end it was near 1:1 in both places.

I suspect that the length of the feedline was approaching being a quarter-
wave multiple down near the CW end although I did not confirm this.
Installation of a pair of homebrew 50:75 toroidial transformers from N8CC
brought the SWR to near unity on both ends of the band.

(Note: for those who are not familiar with the ComTek box, it uses quadra-
ture feed and a dump load to drive the array.  While the resulting SWR on
the feedline is near unity, the normal SWR variations vs. frequency are
masked by the drive system.  I'm NOT claiming that the 50:75 transformers
made the antenna broadbanded.)

In my case, the 50:75 transformers allowed the system to function correctly
rather than forcing me to "tune" the feedline (a task with somewhat limited
flexibility seeing that the feedline is buried).

73,

Dave/K8CC


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