[OFARC] question on coax cable

[Virgil Dean]

Ralph,

A 1/2 wave dipole has an impedance of 75ohms at the feedpoint. To my
knowledge, a 1/2 wave dipole with a feedpoint impedance of 75ohms isn't
critical. With a 75ohms feedpoint impedance will result in an theoretical
SWR of 2:1 at the antenna (aka the load). However, you typically don't
measure SWRs at the antenna, but what you do measure mainly is its
radiation efficiency or resonance. Now with a 75ohm feedpoint impedance for
a 1/2 wave dipole means that the currents are balanced between the two
radials. Which means theoretically that your entire signal is being
radiated by the antenna.

Now what is critical is your unbalanced (coaxial) transmission line
impedance. The reason why is that if the transmitter feedpoint impedance is
different from the transmission line impedance this will cause more
standing-waves due to the unbalanced impedances of the transmitter and
transmission line. So when you have more standing-waves on coaxial the
higher the losses. Which means that not all your signal will reach the
antenna. Lets say that the transmitter feedpoint and transmission line
impedances were the same. This will allow all of your signal to be
transported to the antenna without any loss theoretically. Yes, even if the
antenna feedpoint impedance is 75ohms. Now I'm sure you're thinking "but
what about the mismatch between the transmission line and the antenna
feedpoint".

Well I'm not 100% about this, but the transmission line doesn't see a
mismatch due to the antenna's current being balanced between the two
radials and each radial is connected to either the center conductor or the
shield causing a unbalanced line to become a balanced line. Like I said,
I'm not 100% sure that that statement is correct.

Now personally, I experimented on a 1/2 wave wire dipole I made using 75ohm
cable & then trying it out with the 50ohm cable. The 50ohm cable came out
on top due to less loss on the line.

I hope this information is useful to you Ralph.

P.S. - If anyone sees any statements that are in error please feel free to
comment.

73s,
Alex - KC5VEC

On Nov 21, 2016 11:17 PM, "Ralph (home)" <ke5hdf at sbcglobal.net> wrote:

> 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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