[OFARC] question on coax cable

[Hal]

I beleive that that 75 ohms is a theoretical free space value. Every 
antenna is part of its environment. For example, as you bring the antenna 
closer to the earth, the impedance tends to decrease. At typical heights 
(10-30ft), the impedance often comes pretty close to 50 ohms.

Even so, a 1.5:1 match is considered by many to be close enough that there 
won't be significant losses.


I can't speak much to the coax type, but I would note that voltages and 
currents are -much- higher when transmitting. I don't think a feedline 
intended for only reception would be my first choice. Not only is the 
impedance important, but the DC resistance per foot could be a significant 
factor.

hal kd5hw



On 11/19/2016 1:19 PM, Ralph wrote:
> i use mainly dipole antennas
>
> my understanding is that dipoles have a natural impedance near 75 ohms
>
> radios and coax for amateur radio are 50 ohm
>
> it is easy to find 75 ohm coax cable to TV
>
> is there a solid reason why we don't use a 1.5:1 transformer at the radio
> output and use 75 ohm coax??
>
> too much loss??
>
> what is the loss of 75 ohm cable at UHF and VHF frequencies?
>
> HF frequencies ??
>
>
> Ralph
>
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