On Thu, 24 Dec 1998 11:03:35 -0500 "David A. Pruett" <k8cc@ix.netcom.com>
writes:
>
>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
Changing the dump load to 75 Ohms might have accomplished the same thing
Dave. However since 75 Ohm loads are not exactly popular your choice was
better.
Not sure what iteration Comtek you have but there were several changes
over the years and owners in the toroid and capacitor values to improve
phase balance.
I use a pair of 150 Ohm 400W monsters in parallel and have no problems on
160M but the price was right and I didnt have a Heath Cantenna handy. I
also have a rare 75 Ohm Bird and several 75 Ohm elements.
73 and Merry Christmas Carl KM1H
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