[TowerTalk] 75 ohm - v - 50 ohm coax

[Steve Hunt]

The Op asked a simple question: "...  would I notice any serious 
difference in the aerial matching etc, between the use of 50 ohm and 75 
ohm coax at HF"

I gave him a factual answer: if he has an antenna providing a close 
match to 50 Ohms, any length of 75 Ohm line between 0.15 wavelength and 
0.35 wavelength long (or corresponding odd multiples) will result in a 
SWR(50) greater than 2:1 at the radio. That covers one third of all 
possible line lengths - hardly an isolated "worst case".

Of course, those lengths are readily easy to avoid on a monoband 
antenna; not so easy on a 5-band 20m thru 10m beam with a single feedline!

73,
Steve G3TXQ



On 19/02/2013 19:16, Jim Brown wrote:
> On 2/19/2013 10:54 AM, Steve Hunt wrote:
>> The SWR on the 75 Ohm line will be 1.5:1, but if the line is a 
>> quarter-wave or odd multiple long, the 50 Ohm load impedance is 
>> transformed to 75*1.5 or 112.5 Ohms - that's an SWR(50) of 2.25:1.
>>
>> In fact any line length between 0.15 wavelength and 0.35 wavelength 
>> will transform the load impedance to produce an SWR(50) greater than 
>> 2:1.
>
> My, my -- some of us do work pretty hard to find worst case scenarios.
>
> 73, Jim K9YC
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