[TowerTalk] 75 ohm coas on 50 circuits
Dale Jones K5MM
ddjones@nas.com
Sun, 6 Jul 1997 16:04:59 -0700 (PDT)
Greg, K9IG mentioned that he disconnected his 50-75 Ohm transformers
from his feedlines. I did precisely the same thing on 10 separate
runs of 75 ohm CATV hardline on my previous station.
I built the transformers...water got into the connectors...big mess.
After removing the transformers, and making RG-11U pigtails for
each end of the hardline, I just loaded my transmitters into them,
and drove a bunch of monoband yagis, a TH6DXX, and various wire
antennas for low frequencies. Lengths were 150 to 650 feet.
I was able to work everything I could hear, and for the most part
I think I could hear everything others in my area could. NO
discernable degredation in either transmit, receive, or SWR. This
was done in Oregon, where it is wet, so I was very much in favor
of low maintenance antennas and feedlines.
I mentioned this in a private note to WT9Q when he first posted his
querry on this subject, but obviously he has chosen to proceed along
the DIFFICULT path. Good luck to him, but I preferred to operate and
not fiddle with the difficulties of those clever transformers.
At HF Most transceivers will load well into the 75 ohm hardline
directly, and the east of doing this is considerably enhanced when
compared to the alternatives.
Greg, K9IG, you are right on from my perspective!!
73
Dale K5MM
***************************************************************************
At 04:36 PM 7/6/97 -0700, you wrote:
>Pete Smith wrote:
>>
>> At 12:23 PM 7/4/97 -0400, Bob Locher wrote:
>> >I don't understand the reluctance to use 50 to 75 ohm impedance
>> >transfomers. I built two of the ones featured in all recent ARRL
>> >Handbooks - took me about an evening, and total cost was about $30
>> >including the boxes. They work GREAT. When I put them into my system, I
>> >had one in backwards because of a stupid mistake, and the resulting SWR
>> >was 2:1. I figured out what was wrong and turned the transformer around
>> >and all was beautific.
>> >
>> >I run 1500 watts to them and they stay cool. I can't imagine why anyone
>> >would bother to do it any other way!
>>
>> Raises an interesting question, but one beyond my quantitative abilities.
>> Assuming the autotransformers are built well - adequate wire size, proper
>> core, big-enough box, etc. - what is the expected loss in such a 50:75
>> transformer? If I had that number, i think I could answer your question,
>> because according to TL (thanks, Dave), on 20m. my 230-odd feet of 75-ohm
>> hardline has an added loss of only 0.05 dB, when terminated on the antenna
>> end by a matched 50-ohm antenna or transmission line. I suspect that's
>> less than 2 very good toroid transformers could deliver.
>>
>>
>> 73, Pete Smith N4ZR
>> n4zr@contesting.com
>>
>
>It's been interesting following this thread for the past few days. In
>Pete's reply he almost said what I'm about to:
>
>Why bother with the transformers at all??
>
>I have many runs of 75ohm hardline running to 2 towers. They vary in
>length from 120ft to 300ft. I feed monobanders, tribanders, wire and
>shunt-feed a tower with it. I don't use transformers. Did I try
>them??...yep. Did I see any type of performance gain....no. So, they
>were done away with. One less thing to go wrong in the middle of a
>contest.
>
>As Pete stated....the loss in having a 75ohm feedline going to a 50ohm
>antenna is minimal. It would be interesting to see if the transformers
>in-line would cause more harm (be it a small amount) than good.
>
>73/Greg
>K9IG
>
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