[OFARC] question on coax cable

[Ralph (home)]

I agree 75 ohms is a theoretical value and I have let my dipoles sag in the
center so they approach 50 ohms impedance
So far I have found several articles on line that describe using TV coax for
antenna feed lines, some report using 1kw continuous power
The articles are contradictory regarding loss at HF and UHF frequencies, 
BUT I have not yet found specs that indicate the expected losses or power
tolerance, so I am still researching.

The cost advantage of 75 ohm cable is very attractive

KE5HDF
Ralph

-----Original Message-----
From: OFARC [mailto:ofarc-bounces at mailman.qth.net] On Behalf Of Hal
Sent: Saturday, November 19, 2016 19:50
To: ofarc at mailman.qth.net
Subject: Re: [OFARC] question on coax cable

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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